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Research Report

Influence of artificial mouth manipulation on thephysicochemical characteristics of Zandvlei

Estuary, Cape Town, South Africa.

Kyle Maurer

21221532

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1. Abstract

Recreational users, home owners and the biological component of Zandvlei Estuary haveconflicting requirements in terms of the physicochemical characteristics of the system.Artificial mouth manipulation is used in an attempt to satisfy the physicochemicalrequirements of the three stakeholders/ components of Zandvlei Estuary in a balancedmanner.

As a result of the importance of the physicochemical properties of the estuary to the threestakeholders, this study aims to further understand the influence of artificial mouthmanipulation on the physicochemical characteristics of Zandvlei Estuary. By gaining thisunderstanding artificial mouth manipulation can be used more effectively to satisfy thephysicochemical requirements of the different components of Zandvlei Estuary therebyensuring that the estuary operates as healthily as possible.

Sampling was conducted from March through to the end of September with six samplingdays being completed during mouth open conditions and six during mouth closed conditions.15 sampling stations located throughout the estuary were made use of. At all 15 samplingstations measurements of water depth, temperature, salinity, conductivity, pH, dissolvedoxygen, total dissolved solids and Secchi depth/ transparency were recorded.

Temperature and dissolved oxygen were found to be statistically significantly higher duringmouth closed state in comparison to mouth open state across the entire estuary, samplingstations and surface and bottom waters. Salinity and total dissolved solids were statisticallysignificantly higher during mouth open state in comparison to mouth closed state acrosssampling stations and bottom waters. Conductivity was found to be statistically significantlyhigher during mouth closed state in comparison to mouth open state across samplingstations and surface waters. Secchi depth was statistically significantly higher during mouthopen state in comparison to mouth closed state across the entire estuary, sampling stationsand bottom waters. pH displayed statistically significantly higher values during mouth openstate in comparison to mouth closed state across sampling stations and surface waters.Depth did not display a statistically significant difference between mouth open state andmouth closed state.

Therefore artificial mouth manipulation was found to have an influence on physicochemicalparameters at Zandvlei Estuary. As a result of the importance of these parameters tostakeholders, mouth manipulation is a key tool for management. Artificial mouthmanipulation should be used to manage the estuary in a manner that allows the system tofunction as naturally as possible without compromising the needs of recreational users andhome owners.

Contents Page

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Title page 11. Abstract2. Introduction

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43. Research Objectives4. Materials and Methods

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145. Results5.1 Mouth open state and mouth closed state combined5.2 Mouth open state and mouth closed state comparison

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226. Discussion6.1 Mouth open state and mouth closed state combined6.2 Mouth open state and mouth closed state comparison

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377. Conclusions8. Recommendations and Reflections

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439. Acknowledgements 4510. References cited 46

2. Introduction

Estuaries

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The term estuary refers to a body of water which forms the interface between a river and thesea where the mixing of fresh and saline water occurs (McQuaid, 2013). Estuaries may bepermanently or temporarily open to the sea. 75% of South Africa’s 250 estuaries aretemporarily open/ closed estuaries (Snow and Taljaard, 2007). When estuaries are open,water levels change as a result of tides and salinities change due to saline water inflow fromthe sea and fresh water inflow from the influent rivers (C.A.P.E., 2013). This makes estuarieshighly dynamic systems (McQuaid, 2013).

Estuaries are also generally highly productive and highly valuable in terms of biodiversity.Estuaries act as nursery areas for juvenile fish, habitat for migrant wading birds and offerrecreational opportunities for people (C.A.P.E., 2013). Their productivity, aesthetic beautyand the protection they provide means that estuaries are very sensitive and also veryvulnerable to development (C.A.P.E., 2013). Harmful activities in the catchment, estuaryitself or in the sea close to the estuary mouth all have negative impacts on an estuaryaccording to McQuaid (2013).

In South Africa there are about 250 estuaries, of which almost all have been impacted/modified as a result of human activity (McQuaid, 2013). The modifications to these estuarieshave had a large influence on deciding their characteristics. In South Africa one of the maincauses of modification by human activity has been a history of miss- management ofestuaries (C.A.P.E., 2013).

Zandvlei Estuary

History

Zandvlei Estuary can be seen on charts from as far back as 1700 (Morant and Grindley,1982) (Figure 2). In 1673 the Dutch East India Company created a cattle outpost on thebanks of the Zandvlei estuary and this signified the start of the development of the area(Morant and Grindley, 1982). In 1795 the Dutch lost to the British in the battle of Muizenbergand South Africa became a British Colony. As a result there was a significant increase invisitors to Cape Town. According to McQuaid (2013) the popularity of Muizenberg increasedso much that a direct railway line from Johannesburg Park Station to Muizenberg wasconstructed. This resulted in further development along the north-western edges of theestuary and additionally, the estuary became an increasingly popular spot for recreationalactivities (McQuaid, 2013). Muizenberg’s popularity as a holiday town dropped off afterWorld War II. However, the popularity of the area was once again raised due to theconstruction of Marina da Gama on Zandvlei Estuary in the 1970s (McQuaid, 2013).

Physical description

Zandvlei Estuary is located on the North West shore of False Bay, 20 kilometres (km) southof Cape Town (34°05' S: 18°28'E) (Quick and Harding, 1994) (Figure 2). The estuary is atemporarily open/ closed system which has been classified as eutrophic (McQuaid, 2013).

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Zandvlei Estuary is the largest estuary in False Bay and makes up about 80% of theestuarine area of False Bay (McQuaid, 2013). The estuary includes a wetland which covers60 hectares (ha), the main body covering 56 ha, Marina da Gama, 31 ha and an outletchannel of 9 ha (C.A.P.E., 2013). The main body of the Estuary is 2.6 kilometres (km) longand 0.5 km wide, at its widest point (Quick and Harding, 1994). Water levels are deepest inMarina da Gama at 2 meters (m) however the mean water level varies between 0.7-1.3 m(Morant and Grindley, 1982) (Figure 2).

Zandvlei Estuary’s main influent rivers/ streams include the Westlake Stream, Keysers Riverand the Sand River Canal (which includes the Diep River, Langvlei Canal and the LittlePrincess Vlei Stream) (C.A.P.E., 2013) (Figure 2). All influent rivers have been affected byhuman activity since the 1940’s and this results in low quality, high nutrient water enteringZandvlei estuary (McQuaid, 2013).

The Zandvlei estuary catchment lies entirely within the borders of the City of Cape Town.The catchment is made up of an area of approximately 9,200 ha (C.A.P.E., 2013). Accordingto Thornton et al (1995), the population of the catchment was thought to be as high as100,000, and is now likely to have increased significantly. Land-use activities in thecatchment vary from industry to housing, agriculture, forestry and conservation (C.A.P.E.,2013). Rainfall in the catchment occurs predominantly in winter, from May to September andsummers are hot and dry (McQuaid, 2013).

Conservation and management

Despite Zandvlei’s history it remains highly valued for its natural attributes and as an area ofregional importance for recreational activities. Recreational activities include various types ofboating as well as picnicking, birdwatching, hiking/ walking and fishing (C.A.P.E., 2013)

The first form of management came about in 1981 when a village management board wascreated and then in 1987 with the formation of a local municipality (McQuaid, 2013). Overthe years, there has been a heightened awareness of the need to maintain the natural healthof Zandvlei estuary in order to maximise the benefits of recreation and conservation(C.A.P.E., 2013). The first proper recognition of this was when the Cape Town City Councilcreated the Zandvlei Nature Reserve in 1977. The borders of the nature reserve wereexpanded in 2000 from 22ha to 204ha and in 2006 the reserve became the Greater ZandvleiEstuary Nature Reserve (GZENR) (C.A.P.E., 2013). In 1988 the Zandvlei Trust was createdwith the responsibility of conserving the indigenous fauna and flora of the Zandvlei Estuary(McQuaid, 2013). A variety of projects have been started to promote continuous restoration,monitoring and education focussed on Zandvlei Estuary (C.A.P.E., 2013).

Artificial mouth manipulation

Reasons for and timing of artificial mouth manipulation

The artificial opening/breaching of an estuary mouth is one of the most significanthydrodynamic management actions for an estuary to undergo (Whitfield et al, 2012). In

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South Africa, the main reason for artificial breaching is as a result of poor urban planning.The artificial opening of the mouth is most commonly carried out because of low-lyingdevelopments that are situated within the estuarine floodplain (Whitfield et al, 2012). Whenthe estuary (influent rivers) floods, the mouth needs to be artificially opened so that floodingof human infrastructure can be avoided (Whitfield et al, 2012).

The mouth of an estuary might also be artificially breached when the water level is high andthe estuarine water has been polluted as a result of overflow from septic tanks and sewagesystems (Whitfield et al, 2012). The mouth can be breached to flush the estuary of thepolluted water which could cause harmful algal blooms due to the excess nutrients. Artificialbreaching is also undertaken in order to maintain the estuaries functioning as a fish nursery(Whitfield et al, 2012). This often happens when the estuary mouth has not been open for along period of time (Whitfield et al, 2012). The need for opening the estuary mouth maybecome necessary if freshwater inflow into the estuary is reduced to the point where theoccurrence of natural breaching is unacceptably low and the estuary exhibits anoxicconditions, a loss of salinity and or the build-up of excess nutrients (Whitfield et al, 2012).Reduced freshwater inflow can also cause the need for artificial breaching of the mouthwhen evaporation rates are high and results in abnormally high salinities which can be adanger to the biota of the system (Whitfield et al, 2012). However, artificially breaching anestuary under these circumstances often results in poor scouring of sediment out of theestuary (Whitfield et al, 2012).

According to Whitfield et al (2012) the decision to open the mouth should only be madewhen there is enough river inflow into the estuary to maintain tidal exchange once theoutflow phase has ended. Barton and Sherwood (2004) stated that receding tides areregarded as a good time to create as large a mouth opening as possible which will allowsufficient drainage and tidal flushing. Estuary mouth breaching is generally not conducted ona rising tide or during stormy sea conditions coinciding with onshore winds (Barton andSherwood, 2004). Additionally breaching is generally not conducted when there is nofreshwater inflow into the estuary and if the outlet channel is too shallow or in the wrongplace (Barton and Sherwood, 2004).

Negative effects of artificial mouth manipulation

Artificial breaching may have negative effects on water quality and other ecologicalcomponents within an estuarine environment (Donald, 2013). Artificial breaching causesunnatural flushing conditions in an estuary, which may have negative effects on the ecologyof the ecosystem (Donald, 2013). For example sand bar skimming maintains the sand barheight at a specific level which prevents the estuary from reaching its natural water levelwhich in turn has a direct impact on the ecology of the ecosystem (Donald 2013). Accordingto Whitfield et al (2012) continuous low level artificial opening of the mouth will almostguarantee a slow shallowing of the estuary as a result of sediment accumulation, particularlyin the lower reaches of the system.

Under natural conditions, the sand bar may reach heights considerably higher than the highwater mark of the estuary (Donald, 2013). This would require natural water levels within theestuary to be higher than the sand bar height in order for natural breaching to take place. As

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a result of the greater difference in water levels between the ocean and the estuary undernatural conditions, a significant water gradient develops (Donald, 2013). The water gradientcauses high flow velocities throughout the estuary when the sand barrier is breached(Donald, 2013). The high flow velocities result in high scour rates of sediment and this actionreduces the build-up of sediment at the estuary mouth (Donald, 2013).

When an estuary can flush naturally, during a flood a large amount of freshwater enters theestuary and this is combined with the exit of estuarine water into the ocean (Donald, 2013).As a result of higher scour rates in the estuary, a large mouth area is created (Donald,2013). The estuary mouth stays open for long periods of time and this allows for themovement of saline water throughout the system (Donald, 2013). Donald (2013) thereforeconcluded that an increased flushing potential results in an increased range of salinitiesthroughout the estuary (Donald, 2013). This has an effect on the diversity of species in theestuary, as different species survive under different salinities (Donald, 2013).

Artificial mouth manipulation at Zandvlei Estuary

History

Attempts to control the amount of water in Zandvlei Estuary date back to 1866 when thesystem was shut off and drained so that it could be used for farming purposes (C.A.P.E.,2013). When the winter rain started the plan failed and subsequent manipulations to thesystem concentrated on keeping water levels constant for recreational activities and avoidingflooding in Marina da Gama. In the 1950’s the outlet channel was canalised to form a 20meter long concrete canal and this was followed by the construction of a rubble weir near themouth (McQuaid, 2013). The rubble weir serves to protect a sewer line that crosses beneaththe surface of the estuary (C.A.P.E., 2013). Other modifications include concreting theestuary shores to form steep embankments, the construction of a railway line whichseparated the Westlake wetlands from the rest of the estuary, the construction of the RoyalRoad Bridge over the outlet channel, the building of the Marina da Gama housingdevelopment and general urbanisation around the estuary and catchment (McQuaid, 2013).

Current mouth manipulation plan

The aims of the current management plan are summarised by C.A.P.E. (2013) which statesthat “Zandvlei should function optimally as an estuary with appropriate mouth conditions,tidal flows and salinity levels and with water levels showing sufficient variation to meet theneeds of biota without compromising socio-economic values.”

Artificial mouth manipulation at Zandvlei Estuary is under the control of the City of CapeTown (C.A.P.E, 2013). Bodies that have input include Catchment, Stormwater and RiverManagement, under the Transport, Roads and Stormwater department as well as the SandRiver Catchment Forum, Zandvlei Trust, Zandvlei Nature Reserve management, ScientificServices, Zandvlei Environmental Monitoring Program (ZIMP) volunteers, city councillors,scouts, yacht and canoe club members as well as environmental and home owner groups(C.A.P.E, 2013; Zandvlei Trust, 2006).

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The management plan for Zandvlei Estuary makes use of a rubble weir together with themanipulation of a sand bar across the estuary mouth to control water levels in the estuary(C.A.P.E, 2013). The latest mouth manipulation plan was implemented in 2001 after theprevious plan was done away with because it was leading to decreased salinities andincreased sedimentation in the estuary. There was concern about the associated negativeeffects on biodiversity and the estuary’s ability to act as a fish nursery (C.A.P.E., 2013). Thechanges made to the previous plan included decreasing the height of the weir from 0.9meters to approximately 0.7 meters (in 2010/2011 it was decreased further to 0.6 meters)and the manipulation of the sand bar at the estuary mouth was given more emphasis(C.A.P.E., 2013; McQuaid, 2013).

According to the current management plan during the wet winter months the estuary mouth(and therefore the sand bar) is kept open to prevent flooding of the houses in Marina daGama but also to allow marine migrant fish to move into and out of the system (C.A.P.E.,2013). During the dry summer months the estuary mouth is kept closed to maintain thewater level for recreational activities (summer is when recreational activities take place mostcommonly on the estuary) (C.A.P.E., 2013). The mouth remains closed except for whenthere is a high spring tide which happens on five to six occasions every summer. In this casethe mouth is opened to allow the estuary to be flushed by the sea and increase salinity,improve circulation and allow marine migrant fish to move into and out of the system(C.A.P.E., 2013). The mouth will also open in summer if water levels remain high for longperiods of time as this could threaten houses in Marina da Gama (C.A.P.E., 2013).

Physicochemical characteristics of estuaries

Temperature: Water temperature is an important parameter as it has an effect on variouschemical and biological processes acting within an estuary (Kaselowski, 2012). Furthermorethe majority of aquatic organisms have a specific temperature range at which optimalgrowth, reproduction and general health occur (Mabaso, 2002). An organism is able toadapt its optimal temperature to subtle temperature changes, however rapid changes mayresult in negative effects on the organism (Kaselowski, 2012). Long term temperaturechanges can affect the overall distribution and abundance of estuarine organisms(Kaselowski, 2012).

Salinity: Salinity is a measure of the quantity of dissolved salts in the water (Mabaso, 2002).According to Kaselowski (2012) salinity is the most important parameter that controls thehabitat preference of the biota of an estuary. The majority of estuarine biota occur withinprecise salinity ranges and variations in these ranges will directly affect estuarine organismsdistribution and life history cycles (Kaselowski, 2012).

Conductivity: According to Mabaso (2002) “the conductivity of water refers to its ability toconduct an electrical current and is measured as the total amount of dissolved anions andcations in water”.

pH: The pH of water is a measure of the acidity or alkalinity of a solution and is an importantindicator in evaluating water quality (Kaselowski, 2012; Mabaso, 2002). In addition the pH ofwater has a very important influence on the survival of estuarine biota (Kaselowski, 2012).When pH drops below 5 or increases above 9 many species become stressed (Kaselowski,

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2012). According to Mabaso (2002) increased pH creates more suitable conditions for algalblooms and increased aquatic weed growth and is therefore a concern in estuaries thatexperience nutrient enrichment. Furthermore variations in pH levels can change key aspectsof an estuary’s water chemistry which in turn can negatively affect the indigenous biota of anestuary (Kaselowski, 2012).

Dissolved Oxygen: Dissolved oxygen is a measure of the amount of oxygen present in waterand thus available for respiration (Mabaso, 2002). Dissolved oxygen is an essentialrequirement of all aquatic biota and is one of the most accurate indicators of an estuary’shealth (Kaselowski, 2012; Mabaso, 2002). According to Kaselowski (2012) most estuarineorganisms need dissolved oxygen levels of greater than 3 mg/l, however hypoxic (< 3mg/l)and anoxic conditions (< 0.5mg/l) do occur in estuaries. If dissolved oxygen levels remainbelow 3mg/l for an extended period of time, estuarine biota can become negatively affectedwhich in turn would decrease the productivity and, ultimately, the ecological health of theestuary (Kaselowski, 2012).

Total Dissolved Solids: Total dissolved solids is the total amount of inorganic salts andorganic matter present in a given volume of water (Mabaso, 2002).

Secchi Depth: Secchi depth reflects water transparency. As a result Secchi depth affectshow deeply light can penetrate the water column which is important for photosynthesis andoxygen production (Kaselowski, 2012).

Classification of estuaries according to salinity structure

The mixing of salt water from the ocean and freshwater from influent rivers determines thesalinity of an estuary. This mixture is known as brackish water and the salinity can varybetween 0.5 – 35 parts per thousand (ppt) (sometimes higher in areas where evaporation ishigh) (Donald, 2013). The salinity of an estuary can change regularly as a result of rainfall,tides and whether the estuary mouth is opened or closed amongst other factors (Donald,2013). Estuaries are classified according to salinity structure as follows.

Highly stratified (salt wedge) estuary: In a highly stratified estuary, mixing is poor and a layerof lower salinity water lies above a denser layer of higher salinity (Barton and Sherwood,2004). Between these layers is a thin mixing area with an obvious halocline that exhibitsquick salinity changes (Barton and Sherwood, 2004). As a result of the water bodies beingmostly separate from each other, vertical differences may be seen in other water propertiesincluding temperature and dissolved oxygen (Barton and Sherwood, 2004). Stratifiedestuaries are located where the ocean tidal range is small (less than 1 m) and there isinadequate energy to properly mix the two water layers (Barton and Sherwood, 2004). Themore saline layer at the bottom is pushed into a wedge shape as a result of the frictionbetween the out flowing surface layer and the inflowing bottom water (Barton and Sherwood,2004). After the closure of the estuary mouth, this stratification can stay constant for longperiods of time (Barton and Sherwood, 2004).

Moderately stratified (partially mixed) estuaries: Moderately stratified estuaries exhibitincreased turbulent mixing because of greater tidal range (Barton and Sherwood, 2004). Thevertical salinity gradients along this type of estuary are lower than those most often found ina salt wedge estuary (Barton and Sherwood, 2004).

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Well mixed estuaries: Well mixed estuaries show a high degree of turbulence which resultsin the absence of a vertical salinity gradient (water is mixed) (Barton and Sherwood, 2004).In well mixed estuaries, salinity increases in proximity to the mouth (Barton and Sherwood,2004). Mixing can also take place in closed estuaries with little marine or freshwater inflow,for example as a result of wind (Barton and Sherwood, 2004). According to Barton andSherwood (2004) one issue with classifying estuaries according to salinity structure is thatestuaries do not always exhibit the same type of behaviour either over time or at differentlocations along the estuary. At any time an estuary’s mixing behaviour can be changed byfactors including the magnitude of tides or river discharge (Barton and Sherwood, 2004).

Physicochemical characteristics of Zandvlei Estuary

Temperature: Zandvlei Estuary shows similar temperature values throughout including whenmoving from the estuary mouth to the estuary head as well as from shallow waters to deepwaters (Morant and Grindley, 1982). The presence of thermal stratification is rare as a resultof the shallow depth of the estuary and the high winds in the area which cause mixing(Morant and Grindley, 1982). Surface temperatures are significantly different to bottomtemperatures only during calm conditions which occur from May to June (late autumn)(Morant and Grindley, 1982).

Salinity: Noble and Hemens (1978) stated that Zandvlei is “fresh to saline and shallow withno vertical salinity stratification” (Morant and Grindley, 1982). In contradiction to this,Benkenstein (1982) showed that a salt wedge salinity stratification was present at the“northern end of the main basin”. Furness (1978) stated that salinity stratification is presentin Zandvlei and occurs most often in winter when the estuary mouth has been breached.Under these conditions fresh water moves out of the estuary and saline water moves inunder the freshwater (Morant and Grindley, 1982).

McQuaid (2013) analysed the salinity records of Zandvlei Estuary for the period between1989 and 2012. No significant trends were seen. The salinity of Zandvlei Estuary after 2000was higher than in years before 2000 but this pattern was not significant (McQuaid, 2013).Statistical analysis showed a significant increase in salinity values for the entire estuarywhen making a comparison of data for 1989 and 2011 only (McQuaid, 2013). The highestsalinity value that was recorded in 1989 was at the mouth of the estuary. In 2011 this samesalinity value was now found in the middle of the Estuary, with the value at the mouth beingeven higher (McQuaid, 2013). This demonstrates that higher salinity values are being foundhigher up the system.

pH: According to Morant and Grindley (1982), Zandvlei Estuary exhibits wide pH rangeswhich are due to the inflow of saline water, from the ocean and freshwater, from the influentrivers. The estuary’s wide pH ranges can also be as a result of photosynthesis. When plantsphotosynthesise they remove carbon from the water which can raise pH levels in the water(Morant and Grindley, 1982). Additionally, the estuary itself generally shows higher alkalinitythan the rivers feeding into it (Morant and Grindley, 1982). These conditions arerepresentative of an estuary that is being put under stress and this may well contribute to areduced number of species in the estuary (Morant and Grindley, 1982).

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Dissolved Oxygen: According to Morant and Grindley (1982) Zandvlei Estuary is eutrophic.Dissolved oxygen values at the water surface have been found to be similar over the entireestuary whilst bottom readings of zero, which indicate anoxic conditions, have been seen inboth the main body of the estuary and in the Marina da Gama canals (Morant and Grindley,1982). The reason for the anoxic bottom conditions in the main body of the estuary was mostlikely as a result of large quantities of organic matter building up on the bottom due to thewinter die back of pondweed (Stuckenia pectinatus) and phytoplankton (Morant andGrindley, 1982). On the other hand, the anoxic bottom conditions in the canals were relatedto salinity stratification (explained further in the section “wind” below) according to Morantand Grindley (1982). The macrophyte, Stuckenia helps oxygenate the bottom waters of thecanals but it needs to be kept under control before it affects recreational activities in theestuary (Quick and Harding, 1994). Dissolved oxygen readings are distinctively lower afterStuckenia has been cut in the canals according to Morant and Grindley, 1982.

Transparency: Secchi disk water transparency was shown to range from 0.2 meters to 1.8meters with an average of 0.7 meters for the entire system (Morant and Grindley, 1982).

Wind: The wind patterns at Zandvlei are a critical physical factor that influences estuaryfunctioning (Morant and Grindley, 1982). The predominant winds at Zandvlei Estuary areSoutherly winds during summer and Northerly winds during winter (Morant and Grindley,1982). Due to the windiness of the area (mean wind speed for 1992 was 6 m s -1) and thealignment of the estuary parallel to prevailing winds, the main body of the estuary is usuallywell mixed for most of the year (Harding, 1994). However the canals at Marina da Gama arepositioned east to west so that the houses block the wind and provide shelter to the peopleliving on the canals and using them for recreation (Morant and Grindley, 1982). As a result ofthe calm conditions very little mixing occurs and a halocline forms between denser salinebottom water and less dense fresh water on top of it (salinity stratification) (Morant andGrindley, 1982). Anoxic waters quickly become apparent below the halocline and this leadsto the formation of hydrogen sulphide gas, H2S (originates from sea water as it is high insulphate) (Morant and Grindley, 1982). When windy conditions and subsequent mixingoccurs, the halocline is disrupted and the H2S gas is released (Morant and Grindley, 1982).

Physicochemical characteristics- management and targets

Salinity: The salinity of the Zandvlei estuary should be at a level that affords fish and bottomdwelling communities the opportunity of re-establishment in the areas in which they lived(C.A.P.E., 2013). Fish populations in Zandvlei are quite capable of handling low salinities butthey generally do not cope with abnormally high salinity levels (hypersaline conditions)(C.A.P.E., 2013). Invertebrates and plants are not as tolerant to changes in salinity (C.A.P.E.,2013). An example is the invertebrate Sandprawn, Callianassa kraussi, which stops breedingwhen salinity levels drop below 25 ppt (van Niekerk et al, 2005). Pondweed, Stuckeniapectinata is known to be able to handle a salinity range of 5 to 20 ppt and at salinitiesbetween 5 and 10 ppt has an ecological advantage over other macrophytes andphytoplankton in the system (C.A.P.E., 2013). On the other hand another macrophyte,Phragmites, will die if it is exposed to salinities greater than 16 ppt for more than twelveweeks (C.A.P.E., 2013).

Zandvlei Estuary management sets ambient salinity targets for surface and bottom waters inthe outlet channel (extends upstream to a point parallel to the downstream end of the

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marina) and main body of the estuary for both summer and winter (C.A.P.E., 2013). Thecurrent salinity targets are; for the main body of the estuary; in winter salinity must bebetween 5 ppt for surface waters and 7ppt for bottom waters and in summer; 10 pptthroughout the water column (C.A.P.E., 2013). For the outlet channel; in winter salinity mustbe between 6 ppt for surface waters and 18 ppt for bottom waters and in summer; between11 ppt for surface waters and 13 ppt for bottom waters (C.A.P.E., 2013).

Water levels: In its natural, undisturbed state, Zandvlei Estuary’s water levels would havevaried between 0 and 2.5 to 3 meters above mean sea level (AMSL) (C.A.P.E., 2013).However, currently water levels range from 0.7 to 1.4 m AMSL in order to avoid flooding inMarina da Gama (houses in danger at 1.4 m) as well as to protect the revetments in themarina (designed for water levels of 0.7 m) (C.A.P.E., 2013). Additionally recreationalactivities in the estuary require a depth of 1 m and the pondweed harvester requires a depthof 0.8 m to operate (C.A.P.E., 2013).

The targets/goals for the management of water levels in the estuary include; reducing theheight of the rubble weir by 10 to 20 cm in the winter months as a trial, changing the mouthmanipulation protocol so that note is taken of predicted wave and wind conditions as well asallowing marginally higher water levels (C.A.P.E., 2013).

Dissolved oxygen: The Water Quality Index project recently created guidelines which advisethat dissolved oxygen values ranging from 6 to 8 mg/l (milligrams per litre) are preferred(C.A.P.E., 2013). It has been proposed by C.A.P.E. (2013) that these values be used astargets for Zandvlei estuary.

Zandvlei Estuary- overview

Zandvlei Estuary is an important recreational space in South Africa (Quick and Harding,1994) (Figure 1). The estimated recreational value of the estuary is between one and fivemillion rand per year according to C.A.P.E. (2013). Recreational activities taking place atZandvlei Estuary include canoeing/kayaking, yachting/sailing and windsurfing/kitesurfing witha number of other activities taking place on the banks of the estuary (C.A.P.E., 2013).Recreational users require that the water level be maintained at a depth that allowsrecreational activities to be practically possible and allows the estuary to be aestheticallydesirable (Quick and Harding, 1994).

In addition to recreational users, home owners of Marina da Gama (a housing developmentlocated on a canal system joined to the main body of the estuary) have a vested interest inthe water levels of Zandvlei Estuary. If water levels rise too high in the estuary, homes inMarina da Gama will flood. In contrast if water levels drop too low it is aestheticallyundesirable to home owners (C.A.P.E., 2013).

Zandvlei Estuary is highly valuable in terms of biodiversity and conservation (C.A.P.E.,2013). The estuary has a diversity of fauna and flora including 25 species of fish, more than150 species of bird, 18 reptile species and 210 plant species (open Green Map, 2014).Zandvlei is also the only estuary of significance as a fish nursery on the False Bay coastlinewhich is very important to juvenile marine migrant fish including the critically endangeredwhite steenbras Lithognathus lithognathus (Quick and Harding, 1994; C.A.P.E., 2013). The

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estuary forms part of the Greater Zandvlei Estuary Nature Reserve (GZENR), furtheremphasising its natural value (C.A.P.E., 2013). However, in order to conserve thebiodiversity of the estuary, it needs to be managed in a way that allows the estuary and themouth to function as naturally as possible year round (Quick and Harding, 1994). The naturalfunctioning of the estuary entails seasonal fluctuations in water levels, high salinities tosupport indigenous estuarine species, good circulation to prevent the build-up of polluted,anoxic water and opportunities for marine migrant fish to move into and out of the system(C.A.P.E., 2013).

Recreational users of Zandvlei Estuary, home owners in Marina da Gama and the biologicalcomponent of the estuary have conflicting requirements in terms of the physicochemicalcharacteristics of the system. Artificial mouth manipulation is used in an attempt to satisfy thephysicochemical requirements of the three stakeholders/ components of Zandvlei Estuary ina balanced manner.

As a result of the importance of the physicochemical properties of the estuary to the threestakeholders, this study aims to further understand the influence of artificial mouthmanipulation on the physicochemical characteristics of Zandvlei Estuary. By gaining thisunderstanding artificial mouth manipulation can be used more effectively to satisfy thephysicochemical requirements of the different components of Zandvlei Estuary (in abalanced manner) thereby ensuring that the estuary operates as healthily as possible evenin its current disturbed state.

3. Research Objectives

To quantify water depth, temperature, salinity, conductivity, pH, dissolved oxygen,total dissolved solids and Secchi depth of the waters of Zandvlei Estuary duringmouth open state and mouth closed state

o To determine if there are statistically significant differences (95% confidenceinterval) in the above mentioned parameters between surface waters andbottom waters, between zones, between the main body of Zandvlei Estuaryand the Marina da Gama canals.

To quantify the influence of artificial mouth manipulation on water depth, temperature,salinity, conductivity, pH, dissolved oxygen, total dissolved solids and Secchi depth ofthe waters of Zandvlei estuary during mouth open state and mouth closed state

o To determine if there are statistically significant differences (95% confidenceinterval) between mouth open state and mouth closed state for the abovementioned parameters across the entire estuary, sampling stations, surfacewaters and bottom waters.

To understand whether there are factors other than mouth manipulation influencingsalinity of the waters of Zandvlei Estuary

o To determine if salinity readings demonstrate a statistically significantcorrelation (95% confidence interval) with rainfall.

4. Materials and Methods

13

Study site

A detailed description of the study site is given under the headings “Zandvlei Estuary” and“Physical Description”.

Sample collection

Sampling frequency varied from week to week and month to month and took between 3 to 5hours to complete. Sampling was conducted on twelve occasions from March through to theend of September. Six sampling days were completed during mouth open conditions and sixduring mouth closed conditions. Sampling was not conducted at specific times of the day,specific tides, specific seasons (although emphasis was placed on sampling when the mouthwas being actively opened or closed, mainly in summer, autumn and spring) nor at a specificnumber of days after mouth opening and closing.

Samples were taken throughout the estuary including in the main body of the estuary and inthe canals which form part of the Marina da Gama housing development (Figure 2). In total15 sampling stations were used (Figure 2). Stations 1 to 9 were located in the main body ofthe estuary and stations 10 to 15 in the Marina da Gama canals (Figure 2). Of the sixstations in the canals, stations 10, 12 and 14 were positioned at the mouth of the canals andstations 11, 13 and 15 at the head of the canals (Figure 2). At all 15 sampling stations

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Figure 1: Zandvlei Estuary in relation to the rest of South Africa and False Bay (close up) (source: GoogleEarth; last accessed 22/10/15).

measurements of water depth (meters- m), temperature (degrees Celsius- C), salinity (partsper thousand- ppt), conductivity (micro Siemens per centimetre- µS/cm), pH, dissolvedoxygen or DO (milligrams per litre- mg/L), total dissolved solids or TDS (milligrams per litre-mg/L) and transparency or Secchi depth (meters- m) were recorded. Samples were takenthrough the water column. If the water depth at a particular sampling station was less than orequal to 0.5 meters (m) only a surface reading was taken. If however the water depth wasgreater than 0.5 m but less than 2 m then both a bottom reading and a surface reading wastaken. If the water depth was 2 m or greater a bottom, middle and surface reading wastaken. Surface readings were taken at a depth of 0.1 m.

Figure 2: The study site including sampling stations 1 to 15 (source: Google Earth; lastaccessed 22/10/15).

A Secchi disk with a diameter of 0.2 m was used to measure Secchi depth/ watertransparency and water depth at each sampling station. Secchi depth was measured bylowering a Secchi disk into the water column and waiting for it to disappear from sight. Whenthe Secchi disk disappeared a depth reading would be taken and the disk would be raiseduntil it was visible again. At this point a second depth reading would be recorded and theaverage of the two depth values would produce the Secchi depth. Water depth wasmeasured by making use of the depth markers on the Secchi disk cord. A YSI multimeter(professional plus model) was used to measure temperature, salinity, conductivity, pH,

15

dissolved oxygen and total dissolved solids at each of the sampling stations. A canoe wasneeded to access the sampling stations which were located with the use of physicalmarkers.

Sampling station 2’s depth never exceeded 0.5 m and therefore only surface readings weretaken at this station with the exception being depth which was taken at the bottom of thewater column. Therefore for each of the parameters measured, 29 samples were taken perparameter on each sampling day for twelve sampling days. This resulted in a total of 348samples being taken per parameter. Fewer samples were taken for depth (179 samples),dissolved oxygen (205 samples) and Secchi depth (119 samples); Depth was not recordedat the surface as the depth at which readings were taken was standardised at 0.1 m into thewater column. Fewer dissolved oxygen readings were utilised in the study due to anincorrect sampling technique. This resulted in data being removed (143 readings) and onlythe data collected using the correct technique was retained. Secchi depth could not bemeasured at some sampling stations due to the Secchi disk being visible at the bottom ofthe water column and on rare occasions due to pondweed, Stuckenia pectinata (aquaticmacrophyte) covering over the Secchi disk and causing readings to be inaccurate.

Data Analyses

Data was organised in columns so that analyses could be conducted for mouth open stateand mouth closed state combined as well as mouth open state and mouth closed statecomparison. Each column represented one of the eight physicochemical parameters thathad been measured. Columns were also created for mouth state, surface/ bottom waters,sampling station, zone, main body/ canals, sampling date and sampling time so that themeasured parameters could be analysed across these variables. Sampling stations weregrouped into zones. Sampling station 1 to 4 were grouped as the lower zone (closest to themouth of the estuary), station 5 to 7 as the middle zone, station 8 and 9 as the upper zone(closest to the head of the estuary), station 10, 12 and 14 as the canal mouth zone andstation 11, 13 and 15 as the canal head zone. In order to compare the main body of theestuary to the canals, station 1 to 9 were grouped together as main body and station 10 to15 as the canals. Data was organised in Microsoft Excel 2010 and then imported into IBMSPSS Statistics 23.

Graphs and tables were created using IBM SPSS Statistics 23. The data was then checkedfor normality using the Kolmogorov- Smirnov test in combination with reviewing theskewness and kurtosis values. Normality testing was done for each parameter across mouthstate, surface/ bottom waters, zone, main body/ canals and sampling station. The data wasfound to be not normally distributed and therefore non parametric analyses had to be carriedout on the data.

A Mann-Whitney U test was used to determine whether there were statistically significantdifferences (95% confidence interval) between surface and bottom waters, between zones(canal mouth zone and canal head zone), between the main body of the estuary and thecanals, between mouth open and mouth closed state and between mouth open and mouthclosed state across surface and bottom waters. Analyses were carried out on all parametersmeasured. A Kruskal- Wallis H test was used to determine whether there were statisticallysignificant differences (95% confidence interval) between zones (lower, middle and upper

16

zones). Analyses were carried out on all parameters measured. A Wilcoxon Signed Rankstest was used to determine whether there were statistically significant differences (95%confidence interval) between mouth open and mouth closed state across sampling stationsas well as between mouth open and mouth closed state across sampling stations andsurface and bottom waters. Analyses were carried out on all parameters measured. Theaforementioned statistical analyses were conducted using IBM SPSS Statistics 23.

Environmental data

Correlation between salinity and rainfall: Rainfall data was sourced from the Zandvlei Trustinternet website, Zandvlei Inventory and Monitoring Program (ZIMP). Citizen scientistscollect rainfall data using a standard rain gauge (a funnel that is approximately 35cm longand 12.8cm wide at the top) and make it available on the ZIMP website. Five citizenscientist’s rainfall data was used which came from a number of different locationssurrounding the Zandvlei Estuary. The rainfall data was averaged to get the bestrepresentation of the rainfall that fell in the areas surrounding the estuary. The rainfall from aparticular sampling day was then added to the rainfall that fell both one day and two daysprior to the sampling day. This produced the sum rainfall for a particular sampling day withthe method being repeated for each sampling day. An average salinity per zone wascalculated for each sampling day using the salinity data collected during the study.Spearman’s Rank Order Correlation was used to determine whether there was a statisticallysignificant relationship (95% confidence interval) between the mean salinity per zone persampling day and the sum rainfall per sampling day. Statistical analyses were conductedusing IBM SPSS Statistics 23.

5. Results

Mouth open state and mouth closed state combined

Descriptive statistics for the entire estuary

A maximum value for temperature of 22.70 ºC was measured at the canal head zone, at thesurface waters during mouth closed state (Table 1). A minimum value of 2.72 ppt (measuredat the upper zone, at the surface waters during mouth open state) and a maximum value of31.62 ppt (measured at the lower zone, at the bottom waters during mouth open state) wererecorded for salinity (Table 1). A minimum value for dissolved oxygen of 0.09 mg/l wasrecorded at the canal head zone, at the bottom waters during mouth open state (Table 1). Aminimum value for Secchi depth of 0.2 m was measured at both the canal mouth and canalhead zones at the bottom waters during mouth closed state (Table 1). Both conductivity andtotal dissolved solids displayed large differences (range) between minimum and maximumvalues and this contributed to high variance values of 41214532.94 µS/cm and 24979379.18mg/l respectively (Table 1). Mean salinity for Zandvlei Estuary was found to be 13.55 ppt,with a standard deviation of 5.10 ppt and a variance of 26.03 ppt (Table 1). Mean depth forthe estuary was 1.24 m, temperature, 17.32 ºC, conductivity, 19007.69 µS/cm, pH, 8.65,dissolved oxygen, 8.34 mg/l, total dissolved solids, 14464.63 mg/l and Secchi depth, 0.76 m(Table 1).

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Table 1: Descriptive statistics for the entire estuary

Descriptive Statistics

Depth

(m)

Temperature

(°C)

Salinity

(ppt)

Conductivity

(µS/cm) pH

Dissolved

Oxygen

(mg/l)

Total

Dissolved

Solids (mg/l)

Secchi

Depth

(m)

Valid N 179 348 348 348 348 205 348 119

Minimum .20 11.10 2.72 3935.00 7.09 .09 3282.50 .20

Maximum 1.95 22.70 31.62 39318.00 9.84 16.78 31479.50 1.69

Mean 1.24 17.32 13.55 19007.69 8.65 8.34 14464.63 .76

Median 1.30 17.20 13.20 18924.00 8.70 8.43 14189.50 .65

Standard Deviation .42 2.11 5.10 6419.85 .61 3.21 4997.94 .41

Standard Error of Mean .03 .11 .27 344.14 .03 .22 267.92 .04

Variance .18 4.47 26.03 41214532.94 .37 10.29 24979379.18 .17

Surface waters and bottom waters comparison

Mean pH and dissolved oxygen were found to have statistically significantly higher values atthe surface waters when compared to readings from the bottom waters, whilst meantemperature was not statistically significantly higher. Mean temperature was 17.37 ºC at thesurface and 17.27 ºC at the bottom (U= 14311, Z= -.86, p= .388), mean pH, 8.76 and 8.54respectively (U= 12500, Z= -2.79, p= .005) and mean dissolved oxygen, 9.64 mg/l and 6.54mg/l respectively (U= 2402.5, Z= -6.48, p< .001). In contrast mean salinity, conductivity and total dissolved solids values were statisticallysignificantly higher at the bottom waters in comparison to the surface waters. Mean salinitywas 12.40 ppt at the surface and 14.78 ppt at the bottom (U= 11422.5, Z= -3.94, p< .001),mean conductivity, 17606.56 µS/cm and 20508.89 µS/cm (U= 11503, Z= -3.86, p< .001) andmean total dissolved solids, 13321.34 mg/l and 15689.59 mg/l respectively (U= 11369.5, Z=-4, p< .001).

Zone comparison

Main body: An increase from the lower zone through the middle zone to the upper zone wasapparent for both mean depth and pH, with the difference between zones being statisticallysignificant for mean depth (H(2)= 23.54, p< .001) and pH (H(2)= 52.16, p< .001). A decreasefrom the lower zone through the middle zone to the upper zone was apparent for meansalinity, conductivity and total dissolved solids, with the difference between zones beingstatistically significant for mean salinity (H(2)= 86.37, p< .001), conductivity (H(2)= 75.52,p< .001) and total dissolved solids (H(2)= 84.31, p< .001). Mean dissolved oxygen wasfound to be highest at the middle zone, lowest at the lower zone and in between at the upperzone, with the difference between zones being statistically significant (H(2)= 13.37, p= .001).Mean Secchi depth was also highest at the middle zone, lowest at the lower zone and inbetween at the upper zone but the difference between zones was not statistically significant(H(2)= 0.6, p= .741). Mean temperature was found to be highest at the upper zone, lowest at

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the middle zone and in between at the lower zone with the difference between zones beingnot statistically significant (H(2)= 2.59, p= .274).

Canals: Mean depth, salinity, conductivity, pH, dissolved oxygen, total dissolved solids andSecchi depth did not display statistically significantly higher values for canal mouth zone incomparison to canal head zone. Mean depth was 1.62 m at the canal mouth and 1.53 m atthe canal head (U= 470, Z= -1.89, p= .059), mean salinity, 11.96 ppt and 11.71 ppt (U= 2450,Z= -.57, p= .57), mean conductivity, 17317.85 µS/cm and 17056.94 µS/cm (U= 2481, Z=-.44, p= .657), mean pH, 8.93 and 8.84 (U= 2539.5, Z= -.21, p= .834), mean dissolvedoxygen, 8.87 mg/l and 7.66 mg/l (U= 830.5, Z= -1.13, p= .258), mean total dissolved solids,12954.01 mg/l and 12711.19 mg/l (U= 2444.5, Z= -.59, p= .556) and mean Secchi depth,0.94 m and 0.75 m respectively (U= 324, Z= -1.68, p= .092). Mean temperature did notexhibit a statistically significantly higher value for canal head zone in comparison to canalmouth zone. Mean temperature was 17.99 ºC at the canal mouth and 18.10 ºC at the canalhead (U= 2479.5, Z= -.45, p= .653).

Main body and canal comparison

Mean depth, temperature and pH were recorded to have statistically significantly highervalues at the canals in comparison to the main body of Zandvlei Estuary, whilst mean Secchidepth was not statistically significantly higher. Mean depth was 1.57 m at the canals and1.03 m at the main body (U= 834, Z= -8.88, p< .001), mean temperature, 18.04 ºC and16.81 ºC (U= 8966, Z= -6.19, p< .001), mean pH, 8.88 and 8.49 (U= 8552, Z= -6.64, p< .001) and mean Secchi depth, 0.85 m and 0.66 m respectively (U= 1438.5, Z= -1.76, p= .078).

In contrast mean salinity, conductivity and total dissolved solids showed statisticallysignificantly higher values at the main body of the estuary in comparison to the canals, whilstmean dissolved oxygen was not statistically significantly higher. Mean salinity was found tobe 11.84 ppt at the canals and 14.76 ppt at the main body (U= 9569.5, Z= -5.54, p< .001),mean conductivity, 17187.40 µS/cm and 20292.60 µS/cm (U= 10833, Z= -4.17, p< .001),mean dissolved oxygen, 8.29 mg/l and 8.38 mg/l (U= 4867, Z= -.67, p= .504) and mean totaldissolved solids, 12832.60 mg/l and 15616.65 mg/l respectively (U= 9579.5, Z= -5.53, p< .001).

Surface/ bottom waters and zone comparison

Zone- main body: Mean salinity, conductivity and total dissolved solids decreased from thelower zone through the middle zone to the upper zone both at the surface and bottom waters(Figure 3, Table 2). Mean pH followed the opposite trend increasing from the lower zonethrough the middle zone to the upper zone both at the surface and bottom waters (Figure 5,Table 2). Mean depth, temperature and dissolved oxygen also increased from the lower zonethrough the middle zone to the upper zone but only at the bottom waters for depth andtemperature and only at the surface waters for dissolved oxygen (Figure 4, Table 2).

Zone- canals: Mean temperature was higher at the canal head zone in comparison to thecanal mouth zone for both surface and bottom waters (Table 2). For mean salinity,conductivity, dissolved oxygen and total dissolved solids higher values were witnessed at thecanal head zone (in comparison to the canal mouth) for surface waters and at the canalmouth zone (in comparison to the canal head) for bottom waters (Figures 3 and 4, Table 2).

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Mean pH was homogenous over canal mouth zone and canal head zone for surface waters(Figure 5, Table 2).

Water column- main body: Mean salinity, conductivity and total dissolved solids displayedhigher values at the bottom of the water column in comparison to the surface across allzones (Figure 3, Table 2). In direct contrast, mean pH and dissolved oxygen had highervalues at the surface of the water column in comparison to the bottom across all zones(Figures 4 and 5, Table 2). Mean temperature also exhibited higher values at the surfacewaters (in comparison to the bottom waters) across all zones with the exception of themiddle zone (Table 2).

Water column- canals: Mean temperature, pH and dissolved oxygen were higher at thesurface waters in comparison to bottom waters for both canal mouth zone and canal headzone (Figures 4 and 5, Table 2). In contrast mean salinity, conductivity and total dissolvedsolids were higher at the bottom waters in comparison to surface waters for both canalmouth zone and canal head zone (Figure 3, Table 2).

Table 2: Mean surface and bottom readings across zones

Lower Middle Upper Canal (mouth) Canal (head)

Depth (m) Bottom .83 1.18 1.21 1.62 1.53

Temperature (°C) Surface 16.77 16.57 17.15 18.23 18.25

Bottom 16.64 16.84 17.14 17.75 17.94

Salinity (ppt) Surface 16.49 11.73 8.89 11.12 11.21

Bottom 20.89 15.87 10.83 12.79 12.22

Conductivity (µS/cm) Surface 22567.79 16566.08 12889.79 16331.42 16451.72

Bottom 27730.92 21719.69 15436.67 18304.28 17662.17

pH Surface 8.25 8.79 8.86 9.04 9.04

Bottom 8.10 8.49 8.73 8.82 8.64

Dissolved Oxygen (mg/l) Surface 7.89 9.89 10.04 10.42 10.71

Bottom 6.56 8.02 7.55 7.17 3.97

Total Dissolved Solids (mg/l) Surface 17283.58 12712.10 9801.42 12114.68 12200.85

Bottom 21533.58 16803.39 11799.35 13793.35 13221.53

Secchi Depth (m) Bottom .62 .71 .64 .94 .75

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Mouth open state and mouth closed state comparison

Descriptive statistics for mouth open state and mouth closed state comparison

For both mouth open and closed states 174 samples (Valid N) were taken for all parameterswith the exceptions of depth, dissolved oxygen and Secchi depth for which less sampleswere taken (Table 3). Maximum and minimum values for salinity, conductivity, pH and totaldissolved solids were higher and lower respectively (greater range) for mouth open state incomparison to mouth closed state (Table 3). As a result standard deviation, standard error ofthe mean and variance for salinity, conductivity, pH and total dissolved solids were higher formouth open state in comparison to mouth closed state (Table 3). Conductivity and totaldissolved solids displayed very high variance for both mouth open and closed states (Table3).

Mean temperature and dissolved oxygen were found to have statistically significantly highervalues during mouth closed state in comparison to mouth open state, whilst meanconductivity was not statistically significantly higher (Table 3). Mean temperature was 18.57ºC during mouth closed state and 16.08 ºC during mouth open state (U= 4770.5, Z= -11.05,p< .001), mean conductivity, 19185.93 µS/cm and 18829.45 µS/cm (U= 13969, Z= -1.25,p= .213) and mean dissolved oxygen, 9.21 mg/l and 7.36 mg/l respectively (U= 3372, Z=-4.4, p< .001).

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In contrast mean Secchi depth was found to have statistically significantly higher valuesduring mouth open state in comparison to mouth closed state, whilst mean depth, salinity,pH and total dissolved solids were not statistically significantly higher (Table 3). Mean depthwas 1.24 m during mouth closed state and 1.25 m during mouth open state (U= 3947.5, Z=-.166, p= .868), mean salinity, 13.21 ppt and 13.88 ppt (U= 13968.5, Z= -1.25, p= .213),mean pH, 8.62 and 8.69 (U= 13740.5, Z= -1.49, p= .136), mean total dissolved solids,14188.92 mg/l and 14740.34 mg/l (U= 14072, Z= -1.14, p= .256) and mean Secchi depth,0.63 m and 0.91 m respectively (U= 998, Z= -4.04, p< .001).

Table 3: Descriptive Statistics for mouth open state and mouth closed state comparison

Mouth State Descriptive Statistics

Depth

(m)

Temperature

(°C)

Salinity

(ppt)

Conductivity

(µS/cm) pH

Dissolved

Oxygen

(mg/l)

Total

Dissolved

Solids (mg/l)

Secchi

Depth (m)

Closed Valid N 90 174 174 174 174 108 174 65

Minimum .30 15.20 3.54 5352.00 7.17 .52 4179.50 .20

Maximum 1.95 22.70 25.55 34552.00 9.55 16.78 25954.50 1.55

Mean 1.24 18.57 13.21 19185.93 8.62 9.21 14188.92 .63

Median 1.29 18.50 12.93 19372.50 8.68 9.24 13968.50 .40



Variance .18 3.31 14.89 28097162.66 .26 11.04 14582419.70 .16

Open Valid N 89 174 174 174 174 97 174 54

Minimum .20 11.10 2.72 3935.00 7.09 .09 3282.50 .20

Maximum 1.90 18.60 31.62 39318.00 9.84 13.21 31479.50 1.69

Mean 1.25 16.08 13.88 18829.45 8.69 7.36 14740.34 .91

Median 1.30 16.40 13.29 18694.50 8.75 7.68 14306.50 .88



Variance .17 2.54 37.09 54506232.32 .48 7.75 35367812.81 .13

Mouth open state and mouth closed state comparison across surface waters and bottomwaters

Mean temperature and dissolved oxygen at both surface and bottom waters displayedstatistically significantly higher values during mouth closed state in comparison to mouthopen state. For surface waters mean temperature was 18.79 ºC during mouth closed stateand 15.94 ºC during mouth open state (U= 878, Z= -9.08, p< .001) and mean dissolvedoxygen, 10.65 mg/l and 8.60 mg/l respectively (U= 936, Z= -4.43, p< .001). For bottomwaters mean temperature was 18.32 ºC and 16.22 ºC (U= 1549.5, Z= -6.28, p< .001) andmean dissolved oxygen, 7.42 mg/l and 5.44 mg/l respectively (U= 581, Z= -2.88, p= .004).Statistically significantly higher mean conductivity values were recorded during mouth closedstate for surface waters, whilst mean salinity and total dissolved solids were not statisticallysignificantly higher. Mean salinity was 12.79 ppt during mouth closed state and 12.00 ppt

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during mouth open state (U= 3926, Z= -.36, p= .723), mean conductivity 18696.76 µS/cmand 16516.37 µS/cm (U= 3254, Z= -2.28, p= .023) and mean total dissolved solids,13759.91 mg/l and 12882.77 mg/l respectively (U= 3890, Z= -.46, p= .647).

Mean Secchi depth was statistically significantly higher at the bottom waters during mouthopen state, whilst mean depth was not statistically significantly higher. Mean depth was 1.24m during mouth closed state and 1.25 m during mouth open state (U= 3947.5, Z= -.17, p= .868) and mean Secchi depth, 0.63 m and 0.91 m respectively (U= 998, Z= -4.04, p< .001).Statistically significantly higher mean salinity and total dissolved solids values were seenduring mouth open state for bottom waters, whilst mean conductivity was not statisticallysignificantly higher. Mean salinity was 13.67 ppt during mouth closed state and 15.90 pptduring mouth open state (U= 2840, Z= -2.18, p= .029), mean conductivity 19710.04 µS/cmand 21307.75 µS/cm (U= 3355, Z= -.55, p= .583) and mean total dissolved solids, 14648.57mg/l and 16730.61 mg/l respectively (U= 2863.5, Z= -2.11, p= .035). Mean pH was notstatistically significantly higher at both surface and bottom waters during mouth open state incomparison to mouth closed state. For surface waters mean pH was 8.70 during mouthclosed state and 8.81 during mouth open state (U= 3575.5, Z= -1.36, p= .175). For bottomwaters mean pH was 8.53 and 8.55 respectively (U= 3404.5, Z= -.39, p= .695).

Mouth open state and mouth closed state comparison across sampling stations

Mouth state: Mean temperature (Z= -3.41, p= .001), dissolved oxygen (Z= -3.01, p= .003)and conductivity (Z= -2.16, p= .031) were found to be statistically significantly higher duringmouth closed state in comparison to mouth open state across sampling stations (Figure 6and 9, Table 4). Mean pH did not display statistically significantly higher values during mouthclosed state in comparison to mouth open state across sampling stations (Z= -1.19, p= .234)(Figure 8, Table 4).

Mean salinity (Z= -2.27, p= .023), total dissolved solids (Z= -2.1, p= .036) and Secchi depth(Z= -2.61, p= .009) displayed statistically significantly higher values during mouth open statein comparison to mouth closed state across sampling stations (Figure 7 and 10, Table 4).Mean depth (Z= -1.11, p= .267) and pH (Z= -1.19, p= .234) were found to be not statisticallysignificantly higher during mouth open state in comparison to mouth closed state acrosssampling stations (Figure 8 and 11, Table 4).

Sampling stations- main body: Mean depth was found to increase from sampling station 2 to6 (lower and middle zones) during mouth open state (Figure 11, Table 4). Mean temperatureincreased from station 6 to 9 (middle and upper zones) during both mouth open and closedstates (Figure 6, Table 4). Mean pH increased from station 1 to 8 (lower, middle and upperzones) during mouth closed state and increased from station 3 to 8 (lower, middle and upperzones) during mouth open state (Figure 8, Table 4). Mean salinity and total dissolved solidswere recorded to decrease from station 1 to 9 (lower, middle and upper zones) during mouthclosed state and decrease from station 3 to 9 (lower, middle and upper zones) during mouthopen state (Figure 7, Table 4). Mean conductivity decreased from station 2 to 9 (lower,middle and upper zones) during mouth closed state and decreased from station 3 to 9(lower, middle and upper zones) during mouth open state (Table 4). Mean Secchi depth wasfound to decrease from station 1 to 5 (lower and middle zones) during mouth closed stateand decrease from station 6 to 9 (middle and upper zones) during both mouth open and

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closed states (Figure 10, Table 4). Dissolved oxygen did not display any obvious trendsacross the sampling stations during mouth closed state or mouth open state.

Sampling stations- canals: Temperature, conductivity and dissolved oxygen at the canalmouth and canal head were higher during mouth closed state in comparison to mouth openstate for station 10, 12, 14 and station 11, 13, 15 respectively. Depth at the canal head washigher during mouth closed state in comparison to mouth open state for station 13 and 15.

Secchi depth at the canal mouth and canal head was higher during mouth open state incomparison to mouth closed state for station 10, 12, 14 and station 11, 13, 15 respectively.Salinity and total dissolved solids at the canal mouth were higher during mouth open state incomparison to mouth closed state for station 10, 12 and 14. Salinity and total dissolvedsolids at the canal head were higher during mouth open state in comparison to mouth closedstate for station 13 and 15. Depth at the canal mouth was higher during mouth open state incomparison to mouth closed state for station 12 and 14. pH at the canal mouth was higherduring mouth open state in comparison to mouth closed state for station 10 and 12. pH atthe canal head was higher during mouth open state in comparison to mouth closed state forstation 11 and 13.

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Table 4: Mouth open and closed state comparison across sampling stations (significantly different at p< 0.05 level, Wilcoxon

Signed Ranks test)

Station Zone

Depth (m) Temperature (°C) Salinity (ppt) Conductivity (µS/cm)

Closed Open Closed Open Closed Open Closed Open

1 Lower 1.35 1.34 17.99 15.33 19.59 24.46 27176.42 30703.58

2 Lower .33 .32 19.70 15.93 19.05 16.88 27514.67 22637.67

3 Lower .85 .78 17.65 15.12 16.74 17.39 23477.08 22765.08

4 Lower .79 .86 17.82 15.28 15.98 16.51 22528.08 21737.50

5 Middle .78 .88 17.79 15.28 14.16 15.23 20266.25 20202.17

6 Middle 1.65 1.61 17.72 15.66 13.63 14.49 19419.83 19469.50

7 Middle 1.04 1.09 18.03 15.75 12.36 12.92 17903.58 17596.00

8 Upper 1.17 1.20 18.07 15.91 11.06 11.48 16188.58 15764.50

9 Upper 1.20 1.26 18.38 16.24 8.07 8.83 12117.42 12582.42

10 Canal (mouth) 1.58 1.54 19.33 16.73 12.71 12.91 18881.33 18035.92

11 Canal (head) 1.53 1.48 19.39 16.71 12.84 12.36 19068.33 17386.67

12 Canal (mouth) 1.54 1.63 19.43 16.63 12.13 12.44 18124.92 17411.33

13 Canal (head) 1.47 1.46 19.62 16.86 11.91 12.02 17879.33 16982.75

14 Canal (mouth) 1.70 1.71 19.16 16.66 10.49 11.08 15784.83 15668.75

15 Canal (head) 1.57 1.65 18.99 17.02 10.41 10.75 15622.58 15402.00

Significance p= .267 p= .001 p= .023 p= .031

Table 4: Mouth open and closed state comparison across sampling stations continued (significantly different at p< 0.05 level,

Wilcoxon Signed Ranks test)

Station Zone

pH Dissolved Oxygen (mg/l) Total Dissolved Solids (mg/l) Secchi Depth (m)

Closed Open Closed Open Closed Open Closed Open

1 Lower 7.87 8.35 5.89 7.42 20372.54 24550.96 1.13 .78

2 Lower 8.02 8.39 7.97 7.26 19860.58 17679.92 . .

3 Lower 8.19 8.05 7.79 7.06 17704.42 18149.67 .51 .45

4 Lower 8.36 8.28 8.73 7.42 16907.04 17280.21 .48 .60

5 Middle 8.55 8.40 10.14 7.91 15213.96 16082.63 .34 .65

6 Middle 8.64 8.57 8.79 8.21 14629.33 15374.13 .66 1.03

7 Middle 8.87 8.80 11.72 7.77 13371.04 13875.38 .62 .71

8 Upper 8.93 8.93 11.15 7.78 12067.25 12407.96 .60 .80

9 Upper 8.65 8.68 9.24 6.97 8945.50 9780.83 .51 .64

10 Canal (mouth) 8.85 9.03 9.59 7.71 13736.67 13879.67 .88 1.13

11 Canal (head) 8.74 9.01 7.75 6.93 13855.75 13346.67 .63 .96

12 Canal (mouth) 8.74 9.01 8.58 8.35 13153.13 13397.58 .75 1.19

13 Canal (head) 8.85 8.94 9.19 6.86 12939.88 12995.67 .54 1.00

14 Canal (mouth) 8.98 8.95 11.08 7.67 11490.92 12066.13 .68 1.16

15 Canal (head) 8.75 8.75 9.58 5.05 11421.58 11707.58 .61 .90

Significance p= .234 p= .003 p= .036 p= .009

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Mouth open state and mouth closed state comparison across sampling stations and surface/bottom waters

Mean temperature at both the surface waters (Z= -3.41, p= .001) and bottom waters (Z=-3.3, p= .001) was found to be statistically significantly higher during mouth closed state incomparison to mouth open state across sampling stations (Table 5). Mean dissolved oxygenat the surface waters (Z= -3.29, p= .001) and bottom waters (Z= -2.29, p= .022) displayedstatistically significantly higher values during mouth closed state in comparison to mouthopen state across sampling stations (Table 5). Mean salinity (Z= -2.16, p= .031), conductivity(Z= -3.29, p= .001) and total dissolved solids (Z= -2.39, p= .017) at the surface waters werestatistically significantly higher during mouth closed state in comparison to mouth open stateacross sampling stations (Table 5). Mean pH at the bottom waters was not statisticallysignificantly higher during mouth closed state in comparison to mouth open state acrosssampling stations (Z= -.09, p= .925) (Table 5).

Mean salinity (Z= -3.3, p= .001), conductivity (Z= -2.48, p= .013) and total dissolved solids(Z= -3.3, p= .001) at the bottom waters displayed statistically significantly higher valuesduring mouth open state in comparison to mouth closed state across sampling stations(Table 5). Mean pH at the surface waters displayed statistically significantly higher valuesduring mouth open state in comparison to mouth closed state across sampling stations (Z=-2.1, p= .035) (table 5).

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Table 5: Mouth open state and mouth closed state comparison across sampling stations and surface/ bottom waters (significantly

different at p< 0.05 level, Wilcoxon Signed Ranks test)

Station Zone

Depth (m) Temperature (°C) Salinity (ppt) Conductivity (µS/cm)

Bottom Surface Bottom Surface Bottom Surface Bottom

Closed Open Closed Open Closed Open Closed Open Closed Open Closed Open Closed Open

1 Lower 1.35 1.34 18.38 15.35 17.60 15.32 19.08 21.18 20.11 27.74 26780.50 26495.17 27572.33 34912.00

2 Lower .33 .32 19.70 15.93 . . 19.05 16.88 . . 27514.67 22637.67 . .

3 Lower .85 .78 17.47 14.72 17.83 15.52 16.03 12.82 17.45 21.96 22376.33 17420.50 24577.83 28109.67

4 Lower .79 .86 17.63 14.98 18.00 15.58 14.70 12.20 17.26 20.82 20775.33 16542.17 24280.83 26932.83

5 Middle .78 .88 17.82 15.15 17.77 15.40 13.07 11.58 15.26 18.88 18807.67 15853.50 21724.83 24550.83

6 Middle 1.65 1.61 17.70 15.43 17.73 15.88 12.54 11.26 14.71 17.71 17991.17 15529.83 20848.50 23409.17

7 Middle 1.04 1.09 18.00 15.30 18.07 16.20 11.65 10.29 13.08 15.56 16936.00 14278.33 18871.17 20913.67

8 Upper 1.17 1.20 18.25 15.72 17.88 16.10 10.49 9.65 11.63 13.31 15459.33 13452.17 16917.83 18076.83

9 Upper 1.20 1.26 18.60 16.05 18.15 16.43 7.20 8.21 8.95 9.44 10958.67 11689.00 13276.17 13475.83

10 Canal (mouth) 1.58 1.54 19.63 16.77 19.02 16.70 12.36 11.57 13.06 14.25 18525.67 16413.50 19237.00 19658.33

11 Canal (head) 1.53 1.48 19.68 16.65 19.10 16.77 12.52 11.52 13.15 13.21 18759.50 16322.33 19377.17 18451.00

12 Canal (mouth) 1.54 1.63 19.73 16.58 19.12 16.67 11.23 11.20 13.03 13.67 16989.83 15888.33 19260.00 18934.33

13 Canal (head) 1.47 1.46 19.85 16.85 19.38 16.87 11.30 11.45 12.51 12.60 17120.33 16233.50 18638.33 17732.00

14 Canal (mouth) 1.70 1.71 19.97 16.68 18.35 16.63 10.31 10.08 10.66 12.08 15787.00 14384.17 15782.67 16953.33

15 Canal (head) 1.57 1.65 19.50 16.98 18.48 17.05 10.33 10.15 10.49 11.35 15669.33 14605.33 15575.83 16198.67

Significance p= .267 p= .001 p= .001 p= .031 p= .001 p= .001 p= .013

Table 5: Mouth open state and mouth closed state comparison across sampling stations and surface/bottom waters continued

(significantly different at p< 0.05 level, Wilcoxon Signed Ranks test)

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Station Zone

pH Dissolved Oxygen (mg/l) Total Dissolved Solids (mg/l) Secchi Depth (m)

Surface Bottom Surface Bottom Surface Bottom Bottom

Closed Open Closed Open Closed Open Closed Open Closed Open Closed Open Closed Open

1 Lower 7.93 8.31 7.82 8.38 6.31 6.94 5.32 7.90 19902.92 21136.83 20842.17 27965.08 1.13 .78

2 Lower 8.02 8.39 . . 7.97 7.26 . . 19860.58 17679.92 . . . .

3 Lower 8.20 8.23 8.19 7.87 8.55 7.98 6.79 5.23 16971.58 13893.75 18437.25 22405.58 .51 .45

4 Lower 8.46 8.48 8.25 8.09 9.78 8.32 7.32 5.61 15676.83 13146.25 18137.25 21414.17 .48 .60

5 Middle 8.70 8.57 8.40 8.23 10.34 8.55 9.87 6.65 14073.00 12520.08 16354.92 19645.17 .34 .65

6 Middle 8.84 8.78 8.45 8.37 10.87 8.91 6.02 5.40 13556.83 12190.75 15701.83 18557.50 .66 1.03

7 Middle 8.90 8.96 8.83 8.65 11.37 9.33 12.19 4.68 12664.17 11267.75 14077.92 16483.00 .62 .71

8 Upper 8.97 9.00 8.89 8.86 11.87 8.91 10.42 6.27 11492.00 10575.50 12642.50 14240.42 .60 .80

9 Upper 8.73 8.76 8.57 8.61 11.33 8.05 7.16 5.52 8016.50 9121.67 9874.50 10440.00 .51 .64

10 Canal (mouth) 8.89 9.17 8.82 8.89 10.70 9.03 8.49 6.39 13395.42 12539.58 14077.92 15219.75 .88 1.13

11 Canal (head) 8.94 9.29 8.55 8.72 10.64 9.94 3.89 2.91 13539.42 12493.00 14172.08 14200.33 .63 .96

12 Canal (mouth) 8.93 9.08 8.55 8.95 11.89 9.19 5.27 7.22 12242.50 12165.83 14063.75 14629.33 .75 1.19

13 Canal (head) 9.03 9.11 8.68 8.78 12.48 9.45 5.90 3.41 12342.42 12402.00 13537.33 13589.33 .54 1.00

14 Canal (mouth) 9.01 9.13 8.95 8.76 12.47 9.24 9.69 5.58 11314.33 11030.42 11667.50 13101.83 .68 1.16

15 Canal (head) 8.98 8.90 8.52 8.60 13.26 7.70 4.68 2.40 11350.08 11078.17 11493.08 12337.00 .61 .90

Significance p= .035 p= .925 p= .001 p= .022 p= .017 p= .001 p= .009

Environmental data

Correlation between salinity and rainfall

Mean salinity per sampling day did not correlate significantly with the sum of the rainfall thatfell on the sampling day and two days prior (r(60)= .12, p= .345). When the correlation wascarried out per zone, no significant correlation was achieved for the lower (r(12)= .01, p= .974), middle (r(12)= .27, p= .389), upper (r(12)= .49, p= .108) (Figure 12), canal mouth(r(12)= .14, p= .667) or canal head zones (r(12)= .22, p= .484). Spearman correlationcoefficient was a positive number for each zone indicating a positive relationship betweenmean salinity per sampling day and the sum rainfall per sampling day. As the correlationcoefficient increased (more positive relationship) from the lower to the upper zone, the pvalue decreased (closer to significance). Canal head displayed a higher correlationcoefficient and a lower p value in comparison to canal mouth.

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6. Discussion

Mouth open state and mouth closed state combined

Descriptive statistics for the entire estuary

Mean depth for the Zandvlei Estuary was found to be 1.24 m, temperature, 17.32 ºC, salinity,13.55 ppt, pH, 8.65, dissolved oxygen, 8.34 mg/l and Secchi depth, 0.76 m (Table 1).Harding (1994) sampled various physicochemical parameters across the entire ZandvleiEstuary between 1978 and 1991 (13 year period). Harding (1994) found that mean depthwas 0.89m, mean pH, 8.6, mean dissolved oxygen, 8.6 mg/l and mean secchi depth, 0.54m. Quick and Harding (1994) monitored a number of physicochemical parameters inZandvlei Estuary during 1992 to 1993 and stated that mean temperature was 18 ºC, meansalinity, 3 ppt, mean pH, 8.3 and mean sechi depth, 0.4 m (Quick and Harding, 1994).Furthermore Muhl et al (2004) analysed salinity records for Zandvlei Estuary for the period

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between 1978 and 2003 (25 year period) and found that mean salinity was 7 ppt. Thereforewith the exceptions of depth, salinity and secchi depth values (which were higher incomparsison to previous studies) results for the physicochemical parameters measuredwere similar between the current study and past studies at Zandvlei Estuary.

Harding (1994) stated that the 1992 mean secchi depth was very low at 0.36 m andaccording to Muhl et al (2004) mean salinity levels between 1990 and 1992 were low incomparison to other years (1978 to 2003). Perhaps this could explain why secchi depth andsalinity values were lower in comparison to the current study. In addition Muhl et al (2004)stated that since january 2000 salinity peaks have been rising in Zandvlei Estuary. Perhapsmore recently (since 2000) the mouth has been artificially opened more frequently and forlonger time periods and this has caused certain physicochemical parameters such as salinityto be higher in the current study in comparison to previous studies. According to Snow andTaljaard (2007) the influx of seawater under mouth open conditions has been documented toraise salinity in an estuary, particularly at the mouth. Throughout the current study totaldissolved solids was found to track salinity and any reasoning’s or trends related to salinitycould be expected to apply to total dissolved solids.

In the current study minimum and maximum values for temperature were found to be, 11.10ºC and 22.70 ºC respectively, salinity, 2.72 ppt and 31.62 ppt respectively, conductivity, 3935µS/cm and 39318 µS/cm respectively, pH, 7.09 and 9.84 respectively and secchi depth, 0.20m and 1.69 m respectively (Table 1). Harding (1994) found that minimum and maximumvalues for temperature were 9 ºC and 25.4 ºC respectively, salinity, <1 ppt and 22 pptrespectively, conductivity, 410 µS/cm and 29000 µS/cm respectively, pH, 5.8 and 9.9respectively, dissolved oxygen, 0.09 mg/l and 16.78 mg/l respectively and secchi depth, 0.09m and 1.2 m respectively. According to Quick and Harding (1994) temperature exhibited aminimum value of 12 ºC and a maximum value of 22.5 ºC, salinity, <1 ppt and 14 pptrespectively, pH, 7.1 and 9.00 respectively and secchi depth, 0.01 m and 1.05 mrespectively. Furthermore Muhl et al (2004) found that minimum salinity for the estaury was 0ppt and maximum salinity, 25 ppt. Therefore salinity, conductivity and secchi depth in thecurrent study displayed higher minimum and higher maximum values (greater range) incomparsion to previous studies at Zandvlei Estuary. As mentioned above this could be dueto recent changes in the mouth manipulation strategy to allow the mouth to stay open morefrequently and for longer time periods. A minimum value of 0.09 mg/l for dissolved oxygenindicated anoxic conditions (<0.5 mg/l) which can negatively affect biota if values remain atthis level for extended periods of time (by looking at the mean dissolved oxygen value thiswas most probably not the case) (Kaselowski, 2012).

Surface waters and bottom waters comparison

The current study recorded mean temperature to be 17.37 ºC at the surface waters and17.27 ºC at the bottom waters (U= 14311, Z= -.86, p= .388). By looking at the similarity inmean values and the lack of statistical significance one can see that temperature was mostlikely relatively homogenous between surface and bottom waters. Morant and Grindley(1982) analysed physicochemical data from Zandvlei Estuary for the period from 1973 to1982. Morant and Grindley (1982) found that mean temperature was similar between surfaceand bottom waters due to the shallowness of the Zandvlei Estuary and the wind inducedmixing that occurs in the system.

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Mean salinity was found to be 12.40 ppt at the surface waters and 14.78 ppt at the bottomwaters (U= 11422.5, Z= -3.94, p< .001) and mean conductivity, 17606.56 µS/cm at thesurface waters and 20508.89 µS/cm at the bottom waters (U= 11503, Z= -3.86, p< .001).Harding (1994) found a comparable result whereby bottom waters exhibited 5 ppt highermean salinity and 13- 20% higher mean conductivity in comparison to surface waters for theentire estuary. Harding (1994) mentioned that differences between surface and bottomwaters were most obvious after the artificial opening of the estuary mouth which resulted indenser sea water moving into the estuary underneath (at the bottom waters) the outflowingfresh water (at the surface waters).

The current study found that mean pH was 8.76 at the surface waters and 8.54 at the bottomwaters with the difference being statistically significantly different (U= 12500, Z= -2.79, p= .005). Morant and Grindley (1982) mentioned that wide pH ranges exist at Zandvlei Estuary.pH differences in the system can be caused by a number of factors including seawaterintrusion during mouth open state, freshwater inflow from rivers as well as stormwater drainsand the photosynthetic activity of aquatic macrophytes and phytoplankton (Morant andGrindley, 1982).

The current study found that mean dissolved oxygen was 9.64 mg/l at the surface watersand 6.54 mg/l at the bottom waters with the difference being statistically significantly different(U= 2402.5, Z= -6.48, p< .001). The minimum dissolved oxygen value recorded was 0.09mg/l and was measured at the bottom waters in the canals of the estuary. Morant andGrindley (1982) stated that very low values for dissolved oxygen (as low as 0 mg/l) havebeen recorded at the bottom waters in the Zandvlei Estuary (in comparison to surfacewaters). Morant and Grindley (1982) went on to say that low dissolved oxygen values at thebottom waters are due to large quantities of organic matter collecting on the bottom undercalm weather conditions (and subsequently being broken down by bacteria) for exampleduring the die back of Stuckenia pectinata and phytoplankton in winter as well as when S.pectinata has been harvested in the canals. Another reason could be as a result of theorientation of the canals so that they are protected from the wind. This results in salinitystratification in the canals which in turn causes anoxic conditions to build up below thehalocline.

Zone comparison

Harding’s (1994) study to understand the physicochemical parameters of Zandvlei Estuarybetween 1978 and 1991 made use of 11 sampling sites. The results for sampling station 1from Harding’s (1994) study were compared to the upper zone in this study, station 2 and 3were combined and compared to the middle zone, station 4 and 5 were combined andcompared to the lower zone, station 6 and 7 were combined and compared to the canalmouth zone and station 8 was compared to the canal head zone. Three other stations (A, B,C) were not used as they were located in the influent rivers.

Main Body: Mean temperatures were lower at the lower zone (16.72 ºC and 17.80 ºCrespectively), middle zone (16.70 ºC and 17.75 ºC respectively) and upper zone (17.15 ºCand 17.80 ºC respectively) when compared to the study by Harding (1994). In addition,mean temperature was found to be highest at the upper zone, lowest at the middle zone andin between at the lower zone, with the difference between zones being not statistically

34

significantly different (H(2)= 2.59, p= .274). No trend was found for mean temperature acrosszones in Harding’s (1994) data.

Mean pH was higher at the middle zone (8.64 and 8.60 respectively) and upper zone (8.80and 8.50) when compared to the study by Harding (1994). In contrast, mean pH was lower atthe lower zone (8.19 and 8.30 respectively) when compared to the study by Harding (1994).Furthermore, mean pH was found to increase from the lower zone through the middle zoneto the upper zone, with the difference between zones being statistically significant (H(2)=52.16, p< .001). No trend was found for mean pH across zones in Harding’s (1994) data.

Mean salinity and conductivity were higher at the lower zone (18.38 ppt and 10.00 pptrespectively, 24780.56 µS/cm and 14645.00 µS/cm respectively), middle zone (13.80 pptand 7.00 ppt respectively, 19142.89 µS/cm and 10985.00 µS/cm respectively) and upperzone (9.86 ppt and 6.00 ppt respectively, 14163.23 µS/cm and 10250.00 µS/cm respectively)in comparison to the study by Harding (1994). Furthermore, a decrease from the lower zonethrough the middle zone to the upper zone was apparent for mean salinity and conductivity,with the difference between zones being statistically significant for mean salinity (H(2)=86.37, p< .001) and conductivity (H(2)= 75.52, p< .001). The same trend was found in thestudy by Harding (1994) for mean salinity and conductivity across zones. Morant andGrindley (1982) stated that salinity and conductivity were highest nearest the mouth of theestuary and decreased moving towards the head of the estuary as a result of sea waterintrusion at the mouth and fresh water intrusion at the head.

Mean dissolved oxygen was higher at the lower zone (7.43 mg/l and 7.05 mg/l respectively),middle zone (9.20 mg/l and 8.55 mg/l respectively) and upper zone (8.88 mg/l and 8.4 mg/lrespectively) in comparison to the study by Harding (1994). In addition, mean dissolvedoxygen was highest at the middle zone, lowest at the lower zone and in between at theupper zone, with the difference between zones being statistically significant (H(2)= 13.37, p=.001). The same result was witnessed in the study by Harding (1994) for mean dissolvedoxygen across zones.

Mean Secchi depth was higher at the middle zone (0.71 m and 0.59 m respectively) andupper zone (0.64 m and 0.53 m respectively) in comparison to the study by Harding (1994).In contrast, mean Secchi depth was lower at the lower zone (0.62 m and 0.89 mrespectively) in comparison to the study by Harding (1994). Furthermore, mean Secchidepth was highest at the middle zone, lowest at the lower zone and in between at the upperzone but the difference between zones was not statistically significant (H(2)= 0.6, p= .741).In contrast, both Harding (1994) and Morant and Grindley (1982) found that mean Secchidepth decreased from the lower zone through the middle zone to the upper zone.

Canals: Mean pH (8.93 and 8.65 respectively, 8.84 and 8.70 respectively), salinity (11.96 pptand 7.00 ppt respectively, 11.71 ppt and 7.00 ppt respectively) and conductivity (17317.85µS/cm and 10840 µS/cm respectively, 17056.94 µS/cm and 11730 µS/cm respectively) werehigher at both the canal mouth zone and canal head zone in comparison to the study byHarding (1994). Mean dissolved oxygen (8.87 mg/l and 8.55 mg/l respectively, 7.66 mg/l and8.50 mg/l respectively) and Secchi depth (0.94 m and 0.68 m respectively, 0.75 m and 1.01m respectively) were higher at the canal mouth zone and lower at the canal head zone incomparison to the study by Harding (1994). Mean temperature was lower at the canal mouth

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zone (17.99 ºC and 18.05 ºC respectively) and canal head zone (18.10 ºC and 18.30 ºCrespectively) in comparison to the study by Harding (1994).

Mean salinity (U= 2450, Z= -.57, p= .57), conductivity (U= 2481, Z= -.44, p= .657), pH (U=2539.5, Z= -.21, p= .834), dissolved oxygen (U= 830.5, Z= -1.13, p= .258) and Secchi depth(U= 324, Z= -1.68, p= .092) displayed higher values for canal mouth zone in comparison tocanal head zone (all of differences however were not statistically significant). The sametrend was found by Harding (1994) for dissolved oxygen. In contrast, Harding (1994) foundthat mean pH, conductivity and Secchi depth exhibited lower values for canal mouth zone incomparison to canal head zone (salinity was homogenous). Mean temperature (U= 2479.5,Z= -.45, p= .653) was lower at the canal mouth zone in comparison to the canal head zone(the difference however was not statistically significant). The same trend was found byHarding (1994).

Interestingly none of the sampled parameters displayed a statistically significant differencebetween canal mouth zone and canal head zone. This could be due to the proximity of thecanal mouth zone to the canal head zone and as a result the two zones are affected equallyby factors such as wind mixing and stratification.

Main body and canal comparison

In order to compare results from this study to the study by Harding (1994) station 1 to 5 inthe study by Harding (1994) were combined and compared to the main body in the currentstudy and station 6 to 8 were grouped together and compared to the canals in this study.

Mean salinity (14.76 ppt and 8.00 ppt respectively, 11.84 ppt and 7.00 ppt respectively)conductivity (20292.60 µS/cm and 12302.00 µS/cm respectively, 17187.40 µS/cm and11136.67 µS/cm respectively), pH (8.49 and 8.46 respectively, 8.88 and 8.67 respectively)and Secchi depth (0.66 m and 0.65 m respectively, 0.85 m and 0.79 m respectively)displayed higher values at both the main body of the estuary and the canals when comparedto the study conducted by Harding (1994). Mean dissolved oxygen was higher at the mainbody (8.38 mg/l and 7.92 mg/l respectively) and lower at the canals (8.29 mg/l and 8.53 mg/lrespectively) when compared to the study by Harding (1994). Mean temperature was lowerat both the main body (16.81 ºC and 17.78 ºC respectively) and canals (18.04 ºC and 18.13ºC respectively) when compared to the study by Harding (1994).

Mean salinity (U= 9569.5, Z= -5.54, p< .001), conductivity (U= 10833, Z= -4.17, p< .001) anddissolved oxygen (U= 4867, Z= -.67, p= .504) displayed higher values for the main body ofthe estuary in comparison to the canals (the difference for dissolved oxygen however wasnot statistically significant). The same trend was found by Harding (1994) for mean salinityand conductivity. Mean dissolved oxygen however was lower for the main body incomparison to the canals in the study by Harding (1994). Mean temperature (U= 8966, Z=-6.19, p< .001), pH (U= 8552, Z= -6.64, p< .001) and Secchi depth (U= 1438.5, Z= -1.76,p= .078) were recorded to have lower values for the main body in comparison to the canals(the difference for Secchi depth however was not statistically significant). The same trendwas found by Harding (1994). Morant and Grindley (1982) reported that the depth of thecanals was greater than the depth of the main body of the estuary. The same result wasfound in the current study whereby the mean depth of the main body was 1.03 m and themean depth of the canals was 1.57 m (U= 834, Z= -8.88, p< .001).

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Interestingly looking at the comparisons made with Harding’s (1994) data for “zonecomparison” and “main body and canal comparison” mean salinity and conductivity wereconsiderably higher in the current study in comparison to Harding’s (1994) study. Asmentioned earlier this could be due to changes in the mouth manipulation strategy to allowmore marine influence on the system by breaching the mouth more often and for longer timeperiods.

Mouth open state and mouth closed state comparison

The current study found that mean temperature was statistically significantly higher duringmouth closed state in comparison to mouth open state (18.57 ºC and 16.08 ºC respectively)(U= 4770.5, Z= -11.05, p< .001) (Table 3). In the study by C.A.P.E. (2013) two probes thatrecorded depth, temperature and salinity were positioned in the main body of the ZandvleiEstuary, one adjacent the yacht club (the same location as station 7 in this study) and theother near the mouth of the estuary. Data was recorded for the period between September2012 and January 2013 (C.A.P.E., 2013). Temperature data collected by C.A.P.E. (2013)demonstrated the same result as the current study whereby mean temperature was higherduring mouth closed state in comparison to mouth open state. C.A.P.E. (2013) stated thatthe reason for the lower temperature values during mouth open state (in comparison tomouth closed state) was as a result of the influx of cold seawater into the system.

In the current study mean salinity displayed a greater range during mouth open state incomparison to mouth closed state (28.9 ppt and 22.01 ppt respectively). The same resultwas found by Riddin and Adams (2008), who studied the influence of mouth state and waterlevel on macrophytes in the small temporarily open/closed East Kleinemonde Estuary,Whitfield et al (2008), who studied the influence of mouth state on the ecology of the EastKleinemonde Estuary and Kaselowski (2012), who studied physicochemical and micro algalcharacteristics of the Goukamma Estuary.

Snow and Taljaard (2007) developed a conceptual model for water quality characteristics intemporarily open/closed estuaries. Snow and Taljaard (2007) compared the model to resultsfrom various temporarily open/closed estuaries including the Diep Estuary (sampled in 1988and 1989) and the Palmiet Estuary (sampled between 1986 and 2000).

Mean dissolved oxygen in the current study was found to have a statistically significantlyhigher value during mouth closed state in comparison to mouth open state (U= 3372, Z=-4.4, p< .001) which is in direct contrast to the conceptual model for temporarily open/closedestuaries and the result found by Whitfield et al (2008). However, the Groot Brak Estuarywas often found to display low dissolved oxygen values at the bottom waters of the middleand upper reaches (deep sections of the estuary) during mouth open state (Snow andTaljaard, 2007). The explanation given was that the Groot Brak Estuary is a long, largeestuary (in comparison to other temporarily open/closed estuaries) and therefore theeffectiveness of tidal flushing on bottom waters was reduced (Snow and Taljaard, 2007).Perhaps at Zandvlei Estuary the reduction in tidal flushing is not due to its size but ratherdue to the physical modifications that have taken place at its mouth which reduce sea waterintrusion. Additionally, high dissolved oxygen readings during mouth closed state at ZandvleiEstuary could be as a result of wind mixing the water column and maintaining aeratedconditions as was witnessed in the Diep Estuary (Snow and Taljaard, 2007).

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Zandvlei Estuary exhibited a statistically significantly higher mean Secchi depth readingduring mouth open state in comparison to mouth closed state (U= 998, Z= -4.04, p< .001).According to Snow and Taljaard (2007) the “turbidity levels in seawater entering estuariesalong the cool and warm temperate regions of South Africa are relatively low” (indicatingrelatively high water transparency/ Secchi depth).

Mouth open state and mouth closed state comparison across surface waters and bottomwaters

Mean temperature was statistically significantly higher at both the surface (U= 878, Z= -9.08,p< .001) and bottom waters (U= 1549.5, Z= -6.28, p< .001) during mouth closed state incomparison to mouth open state. This could be as a result of the influx of cold seawater intothe estuary during mouth open conditions (C.A.P.E., 2013: Snow and Taljaard, 2007).

Mean dissolved oxygen was statistically significantly higher at both the surface (U= 936, Z=-4.43, p< .001) and bottom waters (U= 581, Z= -2.88, p= .004) during mouth closed state incomparison to mouth open state. Higher dissolved oxygen readings during mouth closedstate for surface and bottom waters could be as a result of wind causing mixing andtherefore aerating the water column (Snow and Taljaard, 2007). Lower dissolved oxygenreadings during mouth open state for surface and bottom waters could be as a consequenceof reduced tidal flushing due to the physical modifications to the mouth of Zandvlei Estuary.

A statistically significantly higher mean salinity value was seen during mouth open state incomparison to mouth closed state for bottom waters (U= 2840, Z= -2.18, p= .029). Thisoccurrence could be as a result of the influx of seawater into the system during mouth openconditions (C.A.P.E., 2013; Snow and Taljaard, 2007).

Mouth open state and mouth closed state comparison across sampling stations

When mouth open state and mouth closed state data was analysed across sampling stationsa number of parameters that were not statistically significantly different for “mouth open stateand mouth closed state comparison” were then found to be statistically significantly differentand these parameters are discussed below.

Mouth state: The current study found that there was no statistically significant difference inmean depth between mouth open state and mouth closed state across sampling stations (Z=-1.11, p= .267) (Figure 8, Table 4). In contrast, the research conducted by C.A.P.E. (2013)demonstrated that the depth of the estuary was higher when the mouth was closed incomparison to periods when the mouth was open. C.A.P.E. (2013) explained that the reasonfor this occurrence was that when the mouth is closed, water levels build up in the estuarydue to the inflow of freshwater from influent rivers.

Mean temperature was found to be statistically significantly higher during mouth closed statein comparison to mouth open state across sampling stations (Z= -3.41, p= .001) (Figure 6,Table 4). In temporarily open/closed estuaries temperature is usually related to seasonaltrends in atmospheric temperature as was the case with the Diep Estuary (Snow andTaljaard, 2007). The current study did not look at trends in physicochemical parameters overseasons and therefore a more plausible explanation mentioned by Snow and Taljaard (2007)is that temperature in a temporarily open/closed estuary is affected by the temperature of theseawater that moves into the estuary under mouth open conditions (as was seen in the

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Palmiet Estuary). If the temperature of the ocean water is low enough it can lower thetemperature in the estuary. This would explain why at Zandvlei Estuary temperatures arestatistically significantly lower during mouth open state in comparison to mouth closed state.

Mean salinity displayed statistically significantly higher values during mouth open state incomparison to mouth closed state across sampling stations (Z= -2.27, p= .023) (Figure 7,Table 4). The same result was found by C.A.P.E. (2013) at Zandvlei Estuary and Kaselowski(2012) at the Goukamma Estuary. According to C.A.P.E. (2013) the influx of saline waterfrom the ocean during mouth open conditions can increase salinity in the estuary, inparticular at the estuary mouth. Snow and Taljaard (2007) commented that in temporarilyopen/ closed estuaries salinity is also influenced by freshwater inflow at the head (mainlyduring winter) as well as evaporation (mainly during summer). Fresh water inflow could havelowered salinity levels during mouth closed state at Zandvlei Estuary as was seen in thestudy by Kaselowski (2012) at the Goukamma Estuary.

No statistically significant difference was found in mean pH between mouth open state andmouth closed state across sampling stations (Z= -1.19, p= .234) (Figure 11, Table 4). Inagreement with this result, Whitfield et al (2008) stated that the East Kleinemonde Estuarydid not show any considerable variation between mouth open and closed states for pH.According to Snow and Taljaard (2007) high freshwater inflow lowers pH and high saltwaterinflow increases pH, however pH generally ranges between 7 and 8.5 in temporarilyopen/closed estuaries (this result was found in the Diep and Palmiet Estuaries).

Mean temperature (Z= -3.41, p= .001) and dissolved oxygen (Z= -3.01, p= .003) were foundto be statistically significantly higher during mouth closed state in comparison to mouth openstate across sampling stations (Figure 6 and 9, Table 4). Mean Secchi depth displayedstatistically significantly higher values during mouth open state in comparison to mouthclosed state across sampling stations (Z= -2.61, p= .009) (Figure 10, Table 4). As a result thesame reasoning’s used in the section “mouth open state and mouth closed statecomparison” for temperature, dissolved oxygen and Secchi depth can also be applied here.

Sampling stations- main body: Mean depth was found to increase from sampling station 2 to6 (lower and middle zones) during mouth open state (Figure 11, Table 4). This is as a resultof bathymetry of the estuary which displays shallow depths close to the mouth gettingdeeper towards the middle reaches. Dissolved oxygen did not display any obvious trendsacross the sampling stations during mouth closed state or mouth open state. In contrastKaselowski (2012) stated that dissolved oxygen was negatively correlated with distance fromthe mouth during both mouth open and mouth closed state.

Mean temperature at Zandvlei Estuary increased from station 6 to 9 (middle and upperzones) during both mouth open and closed states (Figure 6, Table 4). Snow and Taljaard(2007) commented that during mouth open state a longitudinal temperature gradient cansometimes occur in temporarily open/closed estuaries with the lowest values beingwitnessed at the estuary mouth and increasing towards the estuary head (as was seen in thePalmiet Estuary). Furthermore Kaselowski (2012) found that temperature was positivelycorrelated with distance from the mouth but only for mouth closed state.

Mean salinity was recorded to decrease from station 1 to 9 (lower, middle and upper zones)during mouth closed state and decrease from station 3 to 9 (lower, middle and upper zones)during mouth open state (Figure 7, Table 4). A similar result was found by Kaselowski

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(2012), whereby salinity was significantly negatively correlated with distance from the mouthduring both mouth open and mouth closed state. Snow and Taljaard (2007) mentioned thatduring mouth open state a longitudinal salinity gradient is present in temporarily open/closedestuaries with the highest values being witnessed at the estuary mouth and decreasingtowards the estuary head (as was witnessed at the Diep Estuary during periods when freshwater inflow was low). When the mouth is closed salinity is homogenous, however somelongitudinal stratification may be apparent as was the case with the Palmiet Estuary (Snowand Taljaard, 2007).

Mean pH increased from station 1 to 8 (lower, middle and upper zones) during mouth closedstate and increased from station 3 to 8 (lower, middle and upper zones) during mouth openstate (Figure 8, Table 4). According to Kaselowski (2012) pH was negatively correlated withdistance from mouth at the Goukamma Estuary. Furthermore Snow and Taljaard (2007)stated that high freshwater inflow lowers pH and high saltwater inflow increases pH. Thefindings from both Kaselowski (2012) and Snow and Taljaard (2007) disagree with thecurrent studies results regarding pH. Perhaps the photosynthetic activity of aquaticmacrophytes and phytoplankton is having an effect on pH in Zandvlei Estuary. According toMorant and Grindley (1982) when plants photosynthesise they remove carbon from thewater which can raise pH levels in the water.

Mean Secchi depth (transparency) at Zandvlei Estuary was found to decrease from station 1to 5 (lower and middle zones) during mouth closed state and decrease from station 6 to 9(middle and upper zones) during both mouth open and closed state (Figure 10, Table 4).Kaselowski (2012) found that transparency was significantly negatively correlated withdistance from the mouth and therefore decreased from the estuary mouth to the estuaryhead. The decrease from station 6 to 9 could be as a result of river water with lowtransparency (in comparison to estuarine water) flowing into the system at the head of theestuary (near station 9).

Mouth open state and mouth closed state comparison across sampling stations and surface/bottom waters

When mouth open state and mouth closed state data was analysed across sampling stationsand surface/ bottom waters a number of parameters that were not statistically significantlydifferent for “mouth open state and mouth closed state comparison across surface watersand bottom waters” were then found to be statistically significantly different and theseparameters are discussed below.

Mean salinity at the surface waters were statistically significantly higher during mouth closedstate in comparison to mouth open state across sampling stations (Z= -2.16, p= .031) (Table5). According to Snow and Taljaard (2007) salinity in an estuary is affected by seawaterintrusion, fresh water intrusion and evaporation. Under mouth closed conditions salinitylevels would be affected by fresh water intrusion and evaporation. The presence or absenceof any of these factors could have resulted in salinity at the surface waters being statisticallysignificantly higher during mouth closed state in comparison to mouth open state acrosssampling stations.

Mean pH at the surface waters displayed statistically significantly higher values during mouthopen state in comparison to mouth closed state across sampling stations (Z= -2.1, p= .035)(table 5). According to Snow and Taljaard (2007), high freshwater inflow lowers pH and high

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saltwater inflow increases pH. Perhaps the inflow of saltwater from the sea into the estuaryduring mouth open conditions caused the elevated pH levels witnessed at Zandvlei Estuaryin the current study.

Mean pH at the bottom waters was not statistically significantly higher during mouth closedstate in comparison to mouth open state across sampling stations (Z= -.09, p= .925) (Table5). The lack of significance would indicate that pH at the bottom waters was similar for bothmouth open and mouth closed states. Whitfield et al (2008) found a similar result wherebypH did not display any considerable variation between mouth open and mouth closed stateat the East Kleinemonde Estuary. Snow and Taljaard (2007) commented that pH generallyranges between 7 and 8.5 in temporarily open/closed estuaries whether the mouth is openor closed.

Mean temperature at both the surface waters (Z= -3.41, p= .001) and bottom waters (Z=-3.3, p= .001) was found to be statistically significantly higher during mouth closed state incomparison to mouth open state across sampling stations (Table 5). Mean dissolved oxygenat the surface waters (Z= -3.29, p= .001) and bottom waters (Z= -2.29, p= .022) displayedstatistically significantly higher values during mouth closed state in comparison to mouthopen state across sampling stations (Table 5). Mean salinity (Z= -3.3, p= .001) at the bottomwaters displayed statistically significantly higher values during mouth open state incomparison to mouth closed state across sampling stations (Table 5). As a result the samereasoning’s used in the section “mouth open state and mouth closed state comparisonacross surface waters and bottom waters” for temperature (at the surface and bottomwaters), dissolved oxygen (at the surface and bottom waters) and salinity (at the bottomwaters) can also be applied here.

Physicochemical targets for Zandvlei Estuary

Targets were outlined by C.A.P.E. (2013) for salinity and dissolved oxygen at ZandvleiEstuary. Targets for salinity were separated according to season (summer and winter),according to zone (main body and outlet channel) and according to surface and bottomwaters. Therefore in order to find out whether the current study’s data met the targets thelower zone in the current study was compared to the outlet channel in the study by (C.A.P.E.,2013) and the middle and upper zones in the current study were combined and compared tothe main body in the study by (C.A.P.E., 2013). The current study was not conducted overspecific seasons and therefore the data could not be compared to specific seasonal targets.

The current study met the winter targets of 5 ppt (surface) and 7 ppt (bottom) targets withvalues of 10.31 ppt (surface) and 13.35 ppt (bottom) for the main body of the estuary as wellthe summer target of 10 ppt (throughout the water column) with a value of 11.83 ppt(throughout the water column) (Table 2). The current study also met the winter targets of 6ppt (surface) and 18 ppt (bottom) with values of 16.49 ppt (surface) and 20.89 ppt (bottom)as well as the summer targets of 11 ppt (surface) and 13 ppt (bottom) with values of 16.49ppt (surface) and 20.89 ppt (bottom) (Table 2). C.A.P.E. (2013) set a target for dissolvedoxygen of 6 to 8 mg/l for the entire estuary. The current study exceeded the target with avalue of 8.34 mg/l for the entire estuary (Table 1). The mean depth for the entire estuarywas found to be 1.24 m (Table 1). This depth is sufficient for recreation activities to bepractically possible, it allows the pond weed harvester to operate and does not put thehouses of Marina da Gama in danger.

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Environmental data

Correlation between salinity and rainfall

Muhl et al (2004) made use of rainfall records to calculate the mean rainfall per month.Monthly salinity was calculated using the mean salinity of the first measure of each month forfour sampling stations in Zandvlei Estuary. Mean salinity was then correlated with meanmonthly rainfall from the previous month to determine the relationship between rainfall andsalinity for the period between April 1978 and March 2003. Muhl et al (2004) found thatsalinity values were significantly negatively related to rainfall. This meant that when rainfallwas high (winter) salinity was low and when rainfall was low (summer) salinity was high.

In contrast the current study found that mean salinity per sampling day did not correlatesignificantly with the sum of the rainfall that fell on the sampling day and two days prior(r(60)= .12, p= .345). When the correlation was carried out per zone, no significantcorrelation was achieved for the lower, middle, upper, canal mouth or canal head zones.Interestingly Spearman correlation coefficient was a positive number for each zoneindicating a positive relationship between mean salinity per sampling day and rainfall sum.This means that when rainfall was high, salinity was high and vice versa. An explanation forthis could be that there are other factors besides rainfall that are affecting salinity values.One such factor could be mouth state. Currently the mouth of Zandvlei Estuary remainsopen during the winter months. Perhaps this is causing high salinity values (due to salt waterintrusion) in winter which is also the time of year when rainfall is expected to be high in thearea.

7. Conclusions

Salinity, conductivity, pH, dissolved oxygen and total dissolved solids were found to bestatistically significantly different between surface and bottom waters. Depth, salinity,conductivity, pH, dissolved oxygen and total dissolved solids displayed a statisticallysignificant difference between lower, middle and upper zones. No parameter exhibited astatistically significant difference between canal mouth and canal head zones. Depth,temperature, pH, salinity, conductivity and total dissolved solids were found to be statisticallysignificantly different between the main body of Zandvlei Estuary and the Marina da Gamacanals. When the sampled parameters were compared to previous literature from ZandvleiEstuary, salinity and conductivity displayed considerably higher values across zones andacross the main body and canals whilst the other parameters were similar.

Temperature and dissolved oxygen were found to be statistically significantly higher duringmouth closed state in comparison to mouth open state across the entire estuary, samplingstations and surface and bottom waters. Salinity and total dissolved solids were statisticallysignificantly higher during mouth open state in comparison to mouth closed state acrosssampling stations and bottom waters. Conductivity was found to be statistically significantlyhigher during mouth closed state in comparison to mouth open state across sampling

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stations and surface waters. Secchi depth was statistically significantly higher during mouthopen state in comparison to mouth closed state across the entire estuary, sampling stationsand bottom waters. pH displayed statistically significantly higher values during mouth openstate in comparison to mouth closed state across sampling stations and surface waters.Depth did not display a statistically significant difference between mouth open state andmouth closed state. In addition mean salinity per sampling day did not correlate significantlywith the sum of the rainfall that fell on the sampling day and two days prior.

Therefore artificial mouth manipulation has an effect on physicochemical parameters atZandvlei Estuary. As a result of the importance of these parameters to recreational users ofthe estuary, home owners in Marina da Gama and the biological component of ZandvleiEstuary, artificial mouth manipulation is a key tool for management. Artificial mouthmanipulation should be used to manage the estuary in a manner that allows the system tofunction as naturally as possible without compromising the needs of recreational users andhome owners. Furthermore the current study found that physicochemical targets for ZandvleiEstuary are being met and therefore the current mouth manipulation strategy should becontinued.

8. Recommendations and Reflections

The current study adds to the existing body of knowledge regarding the physicochemicalproperties of Zandvlei Estuary by providing new/ current data in this area. As no priorresearch had been conducted at Zandvlei Estuary dedicated to understanding the influenceof artificial mouth manipulation on the physicochemical characteristics of the system, thecurrent research provides valuable baseline data in this regard. This data can be used toassist the GZENR and the bodies’ manging mouth manipulation to make informed decisionsregarding the artificial mouth manipulation plan for Zandvlei Estuary. If the artificial mouthmanipulation plan is correctly managed then the estuary will be able to function in a way thatbenefits all stakeholders equally. Furthermore the results provided add to the existingknowledge on the influence of artificial mouth manipulation on the physicochemicalproperties of temporarily open/closed estuaries in South Africa.

The results gathered and the conclusions drawn by this study give the opportunity forexpansion whereby further research and analyses can be done on the physicochemicalcharacteristics of Zandvlei Estuary. This could include:

conducting a seasonal study to quantify if physicochemical parameters change over

seasons during mouth open state and mouth closed state; correlating tidal height as well as the number of days after mouth opening/closing

with mean salinity levels in the estuary. This would explain whether there are otherfactors besides mouth manipulation influencing salinity levels;

studying the effects of the physicochemical characteristics of Zandvlei Estuary on the

flora and fauna of the system This could be done for example by correlating salinitylevels to the abundance of a particular species;

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Conducting an inter correlation between physicochemical parameters during mouth

open state and mouth closed state; developing salinity contour maps for better data representation; and studying the physicochemical characteristics of the influent rivers. The inflowing

water will most likely have an influence on the physicochemical characteristics of theestuary.

The current study found that physicochemical targets set for Zandvlei Estuary are being metby the current mouth manipulation plan and therefore the plan should be maintained. It isimportant to note that ongoing monitoring of physicochemical parameters must continue sothat management (of mouth manipulation) is kept up to date with this data. This will avoid asituation whereby changes to the system (for example a drop in salinity or dissolved oxygenwhich could threaten biota) go unnoticed and have a drastic and irreversible negative effecton the system. By noticing negative physicochemical changes in the system, the mouthmanipulation plan can be altered by management so as to keep the estuary functioning in ahealthy manner; in a way that takes all stakeholders physicochemical requirements intoconsideration.

9. Acknowledgements

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Thank you to my supervisor, Dr Walker for assisting me throughout the study, to RussellMaurer and Torey Westgarth- Taylor for helping me with sampling, the Cape PeninsulaUniversity of Technology for giving me permission to use their YSI multimeter and to thecitizen scientists, Timm Hoffman, Lucia Rodrigues, Gerrard Wigram, Bert Bron and JohnFowkes who’s rainfall data I made use of from the Zandvlei Trust website. I greatlyappreciate the advice, assistance and support I have received over the duration of the study.

10. References cited

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