Synthesis of the Upper Thurne Research andRecommendations for Management

Report to the Broads Authority

Holman IP and White SM

July 2008

Department of Natural ResourcesCranfield University

CranfieldBedfordshire

MK43 0ALTelephone: +44 (0) 1234 750111 Ext. 2764

Fax: +44 (0) 1234 752970

i

Executive Summary

The importance of the Upper Thurne (including Hickling Broad, Horsey Mere, Martham North andSouth Broads) for biodiversity is recognised under national and international conservation legislation.Appropriate water management (water resources, quality and flood defence) is fundamental toconservation in the broads. However, the incomplete understanding of the surface water system andtheir interactions with the wider catchment, particularly with respect to nutrient cycling, was recognisedby the Appropriate Assessment Team (Broads Authority, 1999). The resulting workshop, held inNorwich in November 2001, developed a framework for a research and monitoring programme whichintends to inform the ongoing activities which contribute towards the published 20 year Vision for theUpper Thurne water space.

The ecology of the Upper Thurne broads has gone through a number of Phases in response tochanging environmental conditions:

Phase 1 – until the early 20th

century – dominated by stoneworts (charophytes) and low-growing waterweeds, due to the low nutrient levels;

Phase 2 – early to mid 20th

century– luxuriant aquatic plant growth dominated by tallergrowing species better able to take advantage of enhanced nutrient levels;

Phase 3 – mid 20th

century to present – phytoplankton dominated system due to high nutrientand salinity levels.

The Upper Thurne research has aimed to help understand how a range of hydrological, chemical andecological factors contribute to achieving ‘clear water’ (Phase 1) conditions in Hickling Broad. Theresearch has included catchment modelling of water and nutrient movement, groundwater modelling,hydrodynamic modelling of water and salt movement from the sea to the Upper Thurne, mesocosm(small experimental ponds) experiments of salinity effects, the use of remote sensing, lake sedimentanalyses, laboratory experiments of stonewort response to a range of factors including watertemperature, cutting, establishment, pollutant concentrations.

The report has 4 sections:

1. How the Upper Thurne water spaces have changed;2. A description of the current status of the Upper Thurne waterways, and how these

compare to the Favourable Condition criteria under the EC Habitats Directive;3. A synthesis of the activities to identify the significant catchment water management

issues, which is focussed around salinity and ochre, biocides and heavy metals within thesediment and water column, point and diffuse sources of nutrients (nitrate and phosphate)under current and future climate, sea level rise and coastal protection, monitoring and thepopulation biology of charophytes;

4. Recommendations for management actions to address the significant issues previouslyidentified and thereby achieve Favourable Conservation Status.

The recommendations for management actions to achieve Favourable Conservation Status centrearound:

reducing salinity and ochre discharges from the land drainage pumps, principally the Brogravepump. An approach to identifying a solution is suggested based on principles of no significantchange in current flood risk; compatibility with a range of farm systems; being consistent withexisting agri-environment schemes; having a means of removing any seawater from a futurecoastal breach without discharging it through the Special Area of Conservation; and that thereis recognition that the Brograve sub-catchment is a system that is ‘naturally’ brackish andwhich produces limited ochre;

reducing diffuse source losses of nutrients from agriculture. Assuming that the farmingcommunity are following Codes of Good Agricultural Practice and Good Agricultural andEnvironment Condition requirements, a non-exhaustive list of practical measures to reducenutrient losses from agricultural activities are suggested.

ii

Key findings from the Upper Thurne studies

The importance of land drainage to salinity management Much of the salinity in the Brograve drainage system enters via the coastal marshes,

especially Hempstead marshes; Changes to the management of the land drainage systems have the potential to reduce the

salinity entering the rivers and broads from the pumps e.g. raising water levels in theHempstead Marshes by ~ 1 metre might lead to a 15% reduction in the amount of salt beingdischarged by drainage pumps;

Such changes need to be considered in conjunction with flood risk and ‘knock-on’ effects toneighbouring drainage systems and the River Thurne.

Sources and transport of nutrients from the catchment to the rivers and broads Total Phosphorous concentrations in the Upper Thurne broads regularly exceed the target

limit for favourable conditions, but point sources appear to contribute little; Agricultural drains and water from the drainage pumps have the highest nitrogen (N)

concentrations, but N concentrations in the broads are reduced through biological uptake orsedimentation;

Increased rainfall and higher temperatures through climate change will increase nutrient (Nand phosphorus (P) and sediment losses from the land;

Erosion control measures, on susceptible soils and slopes, should be employed as part ofgood agricultural practice to reduce sediment and nutrient losses.

The movement of water and salt within the rivers and broads Water and salt being discharged from the land drainage pumps form the main source of water

and salt entering the River Thurne from its catchment; Constrictions within the river system, principally at Potter Heigham old bridge, where the

narrow openings within the bridge impede both the downstream and upstream movement ofwater (depending upon tidal conditions);

The role of the land drainage pumps changes with the tides – reducing salinities duringextreme tides but increasing background salinity during normal tides.

Environmental needs of the Stoneworts Increased water temperature leads to considerably higher growth rates and seed production of

Stoneworts, suggesting that climate change may influence future growth patterns; While laboratory plant cutting experiments showed that cut stems had the ability to re-grow

and branch, uprooting of Stoneworts, which is a risk of weed harvesting on the broad, leads toa high rate of plant mortality;

Stoneworts are affected by a range of chemicals, particularly metals and boat antifoulingpaints, which have been found in sediment and water in Hickling Broad;

Stoneworts in the broads are likely to be able to withstand a salinity increase of up to around12.5 % of typical recent values for Hickling Broad, but such increases are likely to causechanges in community structure;

Replicated small ‘pond’ experiments have shown that modest reductions in salinity, to around1600–1800 mg Cl L

-1may have a substantial effect on Total Phosphorus and chlorophyll and

hence on the potential for plant growth.

Looking down from above (‘remote sensing’) ‘Remote sensing’ data can map the development of algal blooms in the broads, potentially

providing early warnings for water users such as sailing clubs; It is shown to distinguish the presence of potentially toxic blue-green algae from other non-

toxic species; The distribution and health of aquatic plants, both around the margins of the broads and, if the

water is clear, those submerged beneath the waters surface can be mapped.

iii

Table of Contents

Executive Summary ................................................................................................................i

Key findings from the Upper Thurne studies........................................................................... ii

Table of Contents.................................................................................................................. iii

Introduction ........................................................................................................................... 1

How the Upper Thurne water spaces have changed ............................................................. 2

A description of the current status of the Upper Thurne waterways; ...................................... 4Favourable Condition criteria and current condition ........................................................... 4

Synthesis of the research activities to identify the significant catchment water managementissues.................................................................................................................................... 4

Introduction........................................................................................................................ 4Aquatic plant monitoring programme ................................................................................. 6Salinity and ochre.............................................................................................................. 6

Introduction.................................................................................................................... 6Ecological effects of the salinity ..................................................................................... 7Causes of surface water salinity .................................................................................... 9Solutions to the ochre and salinity problems.................................................................10Hydrodynamic effects of changes to drainage management.........................................12

Biocides and heavy metals ...............................................................................................13Point and diffuse sources of nutrients...............................................................................15

Ecological effects of nutrients .......................................................................................15Point sources of P ........................................................................................................16Sediment sources.........................................................................................................16Diffuse sources of sediment, phosphorus (P) and nitrogen (N) .....................................17

Effects of climate change on water quality........................................................................18Sea level rise and coastal protection ................................................................................19Monitoring ........................................................................................................................19Population biology of charophytes ....................................................................................19

Recommendations for management actions to address the significant issues previouslyidentified and thereby achieve Favourable Conservation Status...........................................21

Reduction of salinity and ochre.........................................................................................21Reduction of diffuse source pollution (N, P, sediment and crop protection products) ........22

References...........................................................................................................................22

1

IntroductionThe low-lying River Thurne catchment is located in northeast Norfolk (TG 4020), adjacent to the NorthSea coast (Figure 1). The upper portion of the river system incorporates large, shallow ‘broads’connected directly, or via channels, to the main river. This includes Hickling Broad, Horsey Mere,Martham North and South Broads. The importance of the Upper Thurne for biodiversity is recognisedunder national and international conservation legislation. The Upper Thurne Broads and Marshes isdesignated nationally as a Site of Special Scientific Interest (SSSI) and internationally as part of theBroads Special Area of Conservation (SAC) and Broadland Special Protection Area (SPA) under theEU Habitat and Birds Directives, respectively. Hickling Broad and Horsey Mere were also designatedin 1976 as Wetlands of International Importance especially as a Waterfowl Habitat under the RamsarConvention, with the whole Upper Thurne SSSI subsequently designated as a Ramsar site in 1994.

Figure 1 The Thurne catchment, showing the (black) broads and watercourses and the (grey)main designated sites (The Broads SAC/Broadland SPA/Broadland Ramsar, Upper ThurneBroads & Marshes SSSI, Calthorpe Broad SSSI, Priory Meadows SSSI, Ludham & PotterHeigham Marshes SSSI, Shallam Dyke Marshes SSSI and the Winterton-Horsey Dunes SSSIand SAC.)

According to Hails (1996), the broads are “an example of the rich biological diversity and productivitywhich has arisen in an essentially cultural landscape. The wetland habitats and species now found inthe region are the result of centuries of manipulation by local communities for fuel production, wetlandplant products, and extensive summer grazing. Maintenance of the area's biological value depends onthe continuation of traditional management practices, combined with measures to restore damagedhabitats and to counter eutrophication from sewage and agricultural run-off, as well as the effects ofmass tourism, and, in the longer term, rising sea levels.”

Appropriate water management (water resources, water quality and flood defence) is fundamental toconservation in the broads. However, the incomplete understanding of the surface water system andtheir interactions with the wider catchment, particularly with respect to nutrient cycling, was recognisedby the Appropriate Assessment Team (Broads Authority, 1999). The resulting workshop, held inNorwich in November 2001, developed a framework for a research and monitoring programme.

The 20 year Vision for the Upper Thurne water space given in Broads Authority (2006) provides aconsensus-based view, produced by the Upper Thurne Working Group, of how the Upper Thurneshould operate in 2026. It provides a ‘target’ for ongoing activities which should be informed by therecent and ongoing research. The 20 year Vision encapsulates:

A reversion of drained marshland to low intensity agriculture, leading to a reduction in salinityand ochre loads into the broads;

‘Gin clear’ waterways with an aquatic plant dominated community, leading to an increase inthe richness and diversity of birds (including wintering waterfowl), fish and invertebrates;

2

Water-based recreation, particularly sailing/boating and fishing; A vibrant local economy providing high quality facilities, goods and services, with many local

people earning a livelihood in the protected landscape; Management of the water space based on mutual understanding and agreement, providing a

highly regarded example of what can be achieved in integrating different interests and usesinto a special landscape.

The overall aims of this current Synthesis proposal are to condense and 'translate' the understandingsgained from the research carried out since the November 2001 workshop on aspects of the UpperThurne catchment, as a basis for informing Partner organizations in the development and applicationof restoration options and/or revised management practices within the catchment(s).

How the Upper Thurne water spaces have changedThe ecology of the Upper Thurne broads has gone through a number of Phases (summarised in Table1) in response to changing environmental conditions:

Phase 1 – until the early 20th

century – dominated by stoneworts and low-growingwaterweeds, due to the low nutrient levels

Phase 2 – early to mid 20th

century (Photo 1 and 2)– luxuriant aquatic plant growth dominatedby taller growing species better able to take advantage of enhanced nutrient levels

Phase 3 – mid 20th

century to present – phytoplankton dominated system due to high nutrientand salinity levels

Table 1 Aquatic plant community phases in the Upper Thurne (from Broads Authority, 2006)Phase 1 Rivers and broads in their

pristine state.

Elements still present inMartham Broads.

Hickling Broad moved intothis Phase in the late 1990sinto the early 2000s

Low levels of phosphorous and moderate nitrogen withhigh calcium carbonate.

Water plant communities include Stoneworts (Charasp), Holly-leaved naiad (Najas marina) and low growingwaterweeds such as Reddish pondweed (Potamogetonalpinus) and bladderwort (Utricularia intermedia)

Phytoplankton virtually absent Wealth of invertebrate life Water crystal clear

Phase 2 Usual Phase for HorseyMere as at 2005/06.

.

Gradual rise in nutrient loading Robust, taller, nutrient demanding plants colonise with

competitive advantage over Phase 1 communities.Plants such as Horned Pondweed (Zanichelliapalustris), Fan-leaved Water-crowfoot (Ranunculuscircinatus), Hornwort (Ceratophyllum demersum),Greater bladderwort (Utricularis vulgaris), Yellow Water-lily (Nuphar lutea), White water-lily (Nymphaea alba),Spiked and Whorled Water-milfoils (Myriophyllumspicatum and M verticillatum) and Fennel-leavedpondweed (Potamogeton pectinatus)

Increase in periphyton (epiphytic algae) growing onplants, which Phase 2 species are able to withstandbetter than Phase 1

High biological productivity, exemplified by largepopulations of diverse fish species, a wealth ofinvertebrate species

Phase 3 Hickling Broad during the1970s and 1980s andcurrently (2008)

Phytoplankton dominance Major reduction in the biomass and diversity of the

aquatic flora Increased water turbidity Phosphorus levels greater than 100μg l-1 Accelerated rate of sediment deposition Loss of aquatic invertebrate diversity

3

Photo 1 “Gathering water lilies” (1886) by PH Emerson, showing Phase 2 emergent aquaticvegetation in a Norfolk Broad

Photo 2 The Brograve level, 1949, under traditional high water level management (Photo:Jocelyn Gardiner)

In the 1980s and 1990s, vegetation in Hickling Broad started to recover after the closure of theMartham landfill site and decline of the gull roost. Neomysis integer recovered, and helped to reducealgal crops (Moss, 2001). In 1998, the lake came back to the clear water Phase 2 state dominated bycharophytes, including the rare Chara intermedia, although salinity remained high. The charophytesgrowth caused problems in navigation, and the Broads Authority, who are responsible for navigationand conservation, implemented a series of cutting trials. However, in 2000 it was not necessary to cut,as the condition of the plants declined, which continued in 2001. The vegetation showed a slightrecovery to 2003, but has since declined (Table 2).

Table 2 Area and annual increase in dense C. Intermedia lawn coverage in Hickling Broad(source: Broads Authority)Year Area (ha) Annual Increase (ha) % of Broad1994199519961997199819992000200120022003200420052006

13.617.125.833.339.048.518.8No dataNo data31.220.511.00

3.58.77.55.79.5-29.7--12.4 (since 2000)-10.7-9.5-11.0

11.714.722.228.733.641.816.2--26.917.79.50

4

A description of the current status of the Upper Thurne waterways;

Favourable Condition criteria and current conditionTo achieve Favourable Condition in the Upper Thurne broads, the following criteria need to be met:

Good water quality:o annual mean concentration of Total Phosphorus < 30 μg/l (Broads Authority , 2006);o No targets currently set for nitrogen;o Upper limits for chloride set at 600 mg/l for Hickling Broad, and 1000 mg/l for Horsey

Mere and Martham North and South Broads Clear water - Secchi disc visible to bottom of water column throughout the water bodies Aquatic plant beds present across the whole of the water bodies, except in marked channels

(e.g. main channel across Hickling Broad and side channel connecting it to Catfield Dyke) Actively growing margins Disturbance free winter bird refuges (for feeding as well as resting/sleeping) over 50% of the

total area of the Upper Thurne

The current condition of the lake features are: Hickling Broad - Unfavourable declining Heigham Sound - Unfavourable recovering Horsey Mere - Unfavourable – no change Martham North and South Broads - Favourable

The above "Favourable Condition" criteria are taken from the Upper Thurne Water Spacemanagement plan (Broads Authority, 2006). The full Condition Tables produced by Natural Englandcontain the criteria for the terrestrial features of the SSSIs, which are not dealt with here.

Synthesis of the research activities to identify the significantcatchment water management issues

IntroductionThe Upper Thurne research has aimed to help understand how a range of hydrological, chemical andecological factors contribute to achieving ‘clear water’ conditions in Hickling Broad (Figure 2). Thesefactors are affected by a range of pressures exerted by activities by the human users within thecatchment and the wider environment which include drainage, pollution (point and diffuse sources),abstraction, coastal breaches The pressures exerted by these activities have been ordered by theperceived significance of their effect on the achievement of Favourable Conservation Status. This

section is completed by a synthesis of activities which provide underpinning knowledge of theecological requirements and population biology of charophyte species.

Figure 2 Schematic of factors contribute to achieving ‘clear water’ conditions in HicklingBroad.

A conceptual framework of the factors affecting chara growth in Hickling Broad was developed byServera-Martinez (2005) from literature review (Figure 3).

5

Figure 3 Conceptual framework linking factors affecting charophytes in Hickling Broad, Norfolk (from Servera-Martinez, 2005).

6

Aquatic plant monitoring programmeThe Chara intermedia lawns in Hickling Broad have been monitored since the mid 1990s(Table 2). The Chara lawns developed from the 1994 ‘foci’ in shallower water distant from thenavigation channel, and have expanded only onto silt sediment. Expansion of the lawnsappeared to be largely by vegetative spread, as no germinating oospores or seedlings wereobserved. Colonisation of bare sediment also occured by growth from plant fragments whichreadily produce new rhizoids and shoots from the branchlet nodes, and were constantly beinggenerated by bird grazing and recreational activity in the broad. In the later 1990s the denselawns increased annually not only in extent, but also in vertical height (Harris, 2000) through acycle of lawn overwintering and new summer growth at the top of the lawn.

Despite increasing grazing pressure, especially due to the gradual build-up in coot numbers,the dense lawns continued tom increase in height annually, reaching greatest heights in 1999(Harris 2000). However, following the dramatic decline in the Chara lawns from 2000 andsubsequent poor growth vigour attributable to other factors, new growth was not able to keeppace with removal by localised grazing by coot in Sailing Club Bay (Harris 2004).

Several factors may have been involved in the 1999/2000 die-back, the most obvious ofwhich were the release of nutrients available for algal growth via bird faeces during the periodof intense coot grazing in autumn 1999 and the observed widespread anoxia in the tall, denselawns which was inimical to plant growth. The gradual loss of the dense mono-dominantlawns of C. intermedia which covered 42% of the broad, resulted in the exposure of 48ha ofunvegetated, highly mobile sediment. The organic-rich sediment is easily disturbed,especially in shallow water, and lifted into the water column by wave action from where it issubsequently deposited onto aquatic plants (Harris, 2006). In the deeper parts of the broad,plant propagules are continually covered by a ‘rain’ of organic sediment which also reduceslight penetration through the already turbid water, and no species have been able tosuccessfully colonise the sediment (Harris, 2006).

Poor water clarity and sediment disturbance by the strong winds experienced in late summer2004 are both likely to have contributed to a lack of establishment and further loss of Chara.The winds were strong enough to uproot large amounts of milfoil, and sediment disturbancemay have been sufficient to dislodge or smother small plants of Chara in areas with patchycover (Harris, 2004). In the early 1990s, when milfoil was already abundant in Hickling, plantsof C. intermedia colonised the sediment surface below the milfoil canopy, often using thestems for support to grow vertically. It is highly likely that milfoil acted as a ‘nurse’ at thisstage, protecting the stonewort from sediment disturbance. In the absence of milfoil, there hasbeen no recolonisation of the mobile sediment by C. intermedia (Harris, 2006). If thisscenario is correct, with M. spicatum the primary and C. intermedia the secondary coloniser,Harris (2006) suggests that it may be many years until there is sufficient stonewort biomass inHickling to bring about a return to clear water.

Salinity and ochre

Introduction

The unusual saline nature of the surface waters in the River Thurne catchment has beenrecorded since at least the 1892 High Court (Chancery Division) case of Micklethwait vVincent (1892), the 'Hickling Broad Case' (Innes, 1911). The source of the salinity wasthought to be "probably due to salt springs" within Hickling Broad and Horsey Mere (Gurney,1904), a view supported by Innes (1911). Following the work of Pallis (1911), it is generallyrecognised that the source of the salinity within the Brograve sub-catchment is by directunderground communication between the sea and the dykes.

The Thurne valley has the highest proportion of acidified soils in Broadland; occurring mainlyon drained land, which produce both acidic drainage water and the brown, orange brown oryellow `ochre', Fe(OH)3. The lowering of water levels as a consequence of the drainageimprovement to the Brograve and Somerton Level, which lead to increased salinity, alsoincreased the formation of ochre significantly. White et al. (2005) estimated an ochre-derivedsediment load of 800 t year

-1from the Brograve and Somerton New Pumps.

7

Ecological effects of the salinity

Replicated laboratory experiments by Lambert (2007) to investigate the effects on growth ofChara connivens, C. intermedia and N. obtusa to salinity variations in Hickling Broad showedthat Chara connivens and Nitellopsis obtusa growth was significantly inhibited (p< 0.01) atmedian salinities between 1996 and 2006 of 6.1 mS cm

-1. Chara intermedia growth rate was

significantly higher than these two species at this salinity range and appeared to showaccelerated growth rate at (as shown by mean relative growth rate) at 12.5 – 50% addition ofseawater to Hickling water. However, the increased growth response of the Chara intermediawas not statistically significant across the range due to wide variation in individual plantresponse. The experiments also demonstrated that all three species would suffer stress, asindicated by a reduction of photosynthetic efficiency at 25% addition of seawater to recentmedian Hickling Broad salinities. The experiment results suggest that changes in salinitywithin the Broad are likely to change the charophyte species assemblage.

Lambert also carried out a three-year survey (2004-06) of 26 environmental variables at 124historical chara-holding water bodies which supported 18 charophyte species. The purpose ofwhich was to evaluate the environmental ranges of charophytes in the field. The electricalconductivity data appears to have an almost trimodal distribution with the Hickling Broad dataexhibiting a distribution with a lower conductivity limit of around 3600 µS cm

-1, although

another statistical test (Waller-Duncan ‘k’ test) placed the charophyte species recorded inHickling Broad into a brackish range of 5456 - 6112µS cm

-1. Certain chara species show

clear salinity tolerance with Chara hispida being recorded growing over the widest chloriderange, and Chara intermedia at the highest mean and median concentration. However, thechloride ion concentration clearly shows the Thurne sites (Hickling Broad and Horsey Mere)as outliers within all charophyte species’ chloride data.

Barker et al (2007) describes a two-year mesocosm experiment, based on 48 tanks of 3 mdiameter containing around 3m

3of water when full. The experiment had four salinity

treatments, of around 600 mg Cl L-1

(as in the early years of the 20th century), 1000 mg L-1

(which is about half the current value), 1600 mg L-1

and around 2500mg L-1

, the latter twospanning current salinity values in Hickling Broad. To each pond were added three packageseach (25-50 g wet weight) of Chara intermedia, Chara hispida L, Chara globularis Thuill.Myriophyllum spicatum, Potamogeton pectinatus, Hippuris vulgaris, Callitriche sp., Elodeacanadensis Michx., Ranunculus circinatus Sibth., Lemna trisulca L. and Ceratophyllumdemersum L.; mixed innocula of zooplankton from Hickling Broad and Daphnia species andtwo male stickleback. Nitrate and Phosphate were added in excess of current loadings inHickling Broad to ensure that responses seen were not due to nutrient shortage. In thesecond year, two further males and two female sticklebacks were added and reproductionallowed the fish community to rise to the carrying capacity.

Although Barker et al. report statistical analyses from the first year of data, they acknowledgethat the first year was not strictly intended to be an experimental year, and that the effects oflow versus normal populations of fish in the two years are statistically not comparable.Therefore focusing on the results from the second year of the experiment, a number offindings were made:

1. In the presence of fish predation, cladocera declined greatly and copepodsincreased in proportion as salinities increased. Daphnia generallydisappeared in the mesocosms above Low (600 mg Cl L

-1) salinities, despite

laboratory tests showing greater resilience to higher salinities;2. Salinity was significantly positively correlated with both Total Phosphorus

(TP) and phytoplankton chlorophyll a (Figure 4). The highest salinity levelwas associated with a marked increase in the TP, due to the reduction ofsulphate to sulphides which remove iron, allowing phosphate to move fromthe sediment into the overlying water. Due to the loss of cladocera at thishighest salinity, the remaining grazing community (which was dominated bycopepods) could not effectively control the increase in algal populations,measured as chlorophyll a. The soluble phosphorus released from sedimentwas taken up rapidly by phytoplankton, measured as chlorophyll a, keepingSRP low, but allowing TP, which includes the P incorporated into living [algal]

8

cells to increase. The uptake of SRP was greater with increasing salinity assalinity decreased the production of the more efficient zooplankton grazers(daphnids first, then other Cladocera, followed by calanoid copepods, whichare relatively inefficient grazers, but more tolerant of salinity)- reducedgrazing resulted in more algae and more efficient use of available SRPreleased from the sediment;

3. Salinity did not influence periphyton chlorophyll a (which would affect the lightclimate at the plant surface) but did have some effects on the invertebrateslikely to graze periphyton, as Gammarus duebeni became very abundant,suggesting that potential increased periphyton growth (due to higher P) wascounterbalanced by a salinity-induced increased grazing pressure.

4. In addition to effects of salinity manifested through phosphorus availabilityand effects on the zooplankton community, it was also associated withreduced submerged plant species richness and a reduction in macrophytePVI (Figure 5);

Figure 4 Changes in (left) Total Phosphorus and (right) phytoplankton chlorophyll a inrelation to salinity treatment in a mesocosm experiment. Light dotted line, low salinity;light dashed line, moderate salinity; heavy dashed line, sub-present salinity; heavyline, high salinity [Aquatic conservation: marine and freshwater ecosystems, Control of ecosystem state in ashallow, brackish lake: implications for the conservation of stonewort communities, Barker T, Hatton K, O’Connor M,Connor L, Bagnell L and Moss B, Copyright © 2007. John Wiley & Sons Limited. Reproduced with permission]

Figure 5 Total plant abundance in relation to salinity in a mesocosm experiment.Dotted line, low salinity; light continuous line, moderate salinity; heavy dashed line,sub-present salinity; heavy line, high salinity. [Aquatic conservation: marine and freshwaterecosystems, Control of ecosystem state in a shallow, brackish lake: implications for the conservation of stonewortcommunities, Barker T, Hatton K, O’Connor M, Connor L, Bagnell L and Moss B, Copyright © 2007. John Wiley &Sons Limited. Reproduced with permission]

Barker et al propose a framework for the relationships linking salinity to plant performance inthe mesocosms (Figure 6). Increasing salinity inhibits reproduction of different zooplanktersat different concentrations, but the threshold of salinity on reproduction depends on thepredation by fish - more predation, leading to greater vulnerability to salinity. A normal fish

9

population will reduce the numbers of the bigger zooplankters first, such that the large, visibleand vulnerable Daphnia magna cannot coexist with fish. Fish will thus eliminate D. magna,the most efficient potential grazer, leaving the smaller, less efficient species. Although thesesmaller daphnids can co-exist with fish, given the availability of refuges such as plantstructures or dark water, they are much more susceptible to increased salinity. Hence thecombination of salinity plus predation leads to lower zooplankton biomass, which coupled withthe chemical effect of salinity on phosphorus release from the sediment (which enablesincreased potential phytoplankton growth) produces more actual phytoplankton growth.Whilst the effect of salinity-induced increased phytoplanktyon chlorophyll a causing reducedlight availability is clear, the case for increased periphyton shading is less apparent in theexperimental data, but the overall effect of salinity on macrophyte dry weight is a statisticallysignificant reduction.

Figure 6 Summary of main relationships linking salinity to plant performance in amesocosm experiment on the Hickling Broad ecosystem [Aquatic conservation: marine andfreshwater ecosystems, Control of ecosystem state in a shallow, brackish lake: implications for the conservation ofstonewort communities, Barker T, Hatton K, O’Connor M, Connor L, Bagnell L and Moss B, Copyright © 2007. JohnWiley & Sons Limited. Reproduced with permission]

Causes of surface water salinity

Pallis (1911) mapped salinities in parts of the Upper Thurne long before the drainageimprovements of the 1950s to 1980s. The Brograve Level is shown as having a high salinityof 15-24% that of seawater, as the freshwater from the Lessingham Valley was not providingdilution to the Brograve Level at this time (as occurs now) but was being discharged byIngham Mill into the head of the Waxham Cut.

Driscoll (1984) showed that drainage improvement in the West Somerton Level in the 1980'slead to a significant increase in dyke water salinity, as dyke deepening penetrated theunderlying clay, removing the impediment to saline seepage (Holman, 1994; Holman &Hiscock, 1998). Within all of the brackish drainage systems (e.g. Brograve, Somerton,Horsey, Eastfield and Stubb), the distribution of salinities is not uniform. Holman (1994)showed that the salinity within individual ditches varied from essentially zero to over 60%seawater-equivalent (Figure 7), dependent on a number of topographical, geological andhydrogeological factors. The progressive drainage improvements and associated lowering ofwater levels have caused an increase in the salt load being discharged into the Thurne riverand Broad system (Moss, 2001).

10

Figure 7 Distribution of surface water chloride concentration in spring 1991 (fromHolman 1994)

Solutions to the ochre and salinity problems

Because ochre production and dyke water salinity are both a consequence of land drainageand low water levels, it is appropriate to consider the studies which have looked at solutionstogether. Harding and Smith (2002) assessed solutions to the generation of ochre in theBrograve system and recommended:

Comprehensive remedial solutions– based on:o raised regional water levels to within 30 cm or 45 cm of the marsh level;o re-sealing the bed of the main drains within the coastal marshes (Hempstead

Marshes, Great Moss Fen etc) with clay;o Construction of a coastal interceptor Spine Drain, running from Eccles-on-

Sea to near Horsey Corner. Local schemes or piecemeal remedial solution- based on:

o Conifer bark filters, installed in pairs within the drains, to remove ochre;o A settling lagoon near the Brograve pump;o Clay lining of the coastal main drains;o Discharging drainage water from the Brograve system directly to sea;o Raising water levels locally;o Dyke widening to provide improved aquatic vegetation habitat.

Subsequently, ELP and Cranfield University (2005) reviewed the results from Harding andSmith (2002) in the context of delivering a partnership vision for the Brograve catchment to:

“Identify a preferred, sustainable solution to address Ochre and salinity problems inthe Brograve catchment, within the context of all the existing formats of farming in thisarea. Such a solution needs as far as possible to be balanced to meet theenvironmental requirements of the European Wildlife sites, and aims to beacceptable, and where possible beneficial, to people living and working in thecatchment. Opportunities for additional environmental enhancement will beconsidered with rate payers and other stakeholders”

ELP and Cranfield University (2005) identified a range of scenarios of measures that aimed toprovide comprehensive catchment wide solutions. Given the potential impacts on land,property and people, the selection of a preferred option was not possible. In addition, it wasrecognised that a solution to the high priority objective of solving the water quality problems

11

for the international sites, could be resolved with solutions which did not meet the full visionbut which had lesser impacts on the catchment. The final scenarios investigated were:

Scenario 1-1 – included:o regional water level increases, whereby the water level in the drains would be

maintained within 35cm of ground level during the summer, April-Octoberinclusive,

o construction of new shallower ditches in coastal marshes, and infilling oforiginal deep ditches;

o restoration of past aquatic and floodplain habitats Scenario 1-2 – included:

o regional water level increases, whereby water levels in the drains would bemaintained at whatever levels are needed to maintain average water tableswithin 35cm of ground level

o construction of new shallower ditches in coastal marshes, and infilling oforiginal deep ditches;

o restoration of past aquatic and floodplain habitats Scenario 2 – included:

o Splitting the Brograve drainage system, such that: a new drain is installed to collect the saline water from the coastal

marshes (Hempstead, Great Moss Fen, Waxham, Poplar Farm andFir Tree Farm sub-catchements) and to discharge it directly to seathrough the Hempstead marshes

The good quality water from the Lessingham Valley enters theBrograve Level, to provide dilution, to be discharged by the existingpump

o Water level increases in the Brograve, Lessingham and Calthorpe sub-catchments, as in either Scenario 1-1 or 1-2

Scenario 3 – similar to Scenario 2, except that the new coastal drain is deepened toact as a (uncertain) sacrificial salinity line.

The appraisal of the above scenarios provides the following conclusions:

The only scenario which is likely to fulfil the Vision is Scenario 1-2, but this scenarioalso has maximum impacts upon land use, livelihoods and potentially, on property.

If the primary objective is resolving water quality issues for the SACs, then Scenario 2combined with the higher water level in the Brograve, Lessingham and Calthorpesub-catchments, provides the maximum benefit as it removes the water with highestsalinity and ochre while retaining the fresh water for discharge to Horsey Mere.

Scenarios 2 and 3 provide considerable flexibility to vary water level managementwithin the non-coastal sub-catchments, depending on the balance between waterquality benefit compared to impact on land use and flooding.

There is considerable uncertainty with Scenario 3 and very high and increasingrunning costs over time.

Both Scenarios 2 and 3 require discharge to sea, for which a discharge consent maybe required from the Agency. In addition, the design of the outfall will need carefulengineering design given the dynamic nature of the coast.

It is imperative that any proposed solution to the water quality problems in theBrograve catchment does not lead to increased flood risks. In taking any proposedsolution forward, it was recommended by ELP and Cranfield University (2005) that, inaddition to creating additional flood storage capacity within the Main Drains andfloodplain, alternative control mechanisms to better manage Lessingham water levelsare investigated

For Scenarios 2 or 3, it is suggested that there is a managed hydraulic connectionbetween the Lessingham Valley and Hempstead Marshes to provide a means ofevacuating seawater following a coastal breach from the Brograve catchment, ratherthan via the Brograve pump.

12

Simpson (2007) used a numerical groundwater model to simulate three previously proposedmanagement or engineering remedial measures to assess their affect on saline inflows. Hefound that:

raising the water levels in the drains of the Hempstead Marshes from around -1.1 to -1.8 mOD to -0.4 mOD will reduce the saline inflow into the Brograve sub-catchmentby around 15%. This will decrease the overall saline inflow into the whole Thurnecatchment from 3081 m

3/day to 2822 m

3/day;

(ii) lining the main drain of the Hempstead Marshes with low permeability materialproduces a reduction in the saline inflow into the Brograve sub-catchment by around7%. The saline inflow into the whole catchment decreases from 3,081 m

3/day to

2,958 m3/day;

(iii) The construction of a coastal interceptor drain could in theory prevent the inflow ofsaline groundwater into the Brograve system. However, such a drain would increasethe saline inflow across the coastal boundary by around six times (from 3,081 m

3/day

to 19,750 m3/day), remove large quantities of fresh groundwater from the Pleistocene

Crag aquifer and lead to high energy and pumping costs.

Simpson (2007) has shown that there are partial solutions to reducing the saline inflow intothe drainage systems in this lowland coastal catchment, but stresses that any intendedalterations must consider other potential impacts, such as changes to flood risk, landmanagement restrictions or hydrodynamic effects on the Thurne river.

Hydrodynamic effects of changes to drainage management

White et al. (2008) applied the ISIS hydrodynamic and water quality model to the Thurnesystem. They improved upon the original model set-up developed by Halcrows for theBroadland Flood Alleviation Project. It is apparent that the difficulty of calibrating thesimulated river levels and salinities at the two locations within the Lower Bure and Thurnewith good data (Acle Bridge and Repps) indicates that the factors controlling thehydrodynamics of this low hydraulic gradient system are variable in space and time.Nevertheless a reasonable match has been achieved at Repps and Acle Bridge across adiverse range of tide and river flow events which provides a degree of confidence in therobustness of the model.

Analysis of measured tide and salinity data, and model results show that: In a high tide and low flow situation, the saline water comes up to Acle Bridge and

sometimes further upstream to Repps and beyond. If the pumps are operating thesalinity is reduced at Acle Bridge, Repps and Hickling as the salinity is diluted by theextra flow. so therefore: If salinity is from tidal influence, then turning off pumps hasimpact of raising salinity as the dilution of the tidal saline water from downstream isless.

In the situation of normal tides and with a variety of river flows, higher saline values atRepps can be caused by the water being pumped from upstream. The impact isdiluted by higher river flows. The impact has generally disappated by the time itreaches Acle Bridge. If the pumps are tuned off the salinity levels drop therefore: Ifsalinity is from pumps then turning off pumps causes a drop in salinity as pumps areno longer pumping saline water into the system.

If there is a sustained period of high tide levels with a low tidal range, the saline watergets trapped and salinity levels at Acle Bridge can remain high until the tide levelsdrop or the tidal range increases. The sustained levels of salinity seen at Acle Bridgeunder these circumstances are not repeated at Repps as the saline water does notget trapped as high up the system. The peak levels of salinity are seen at Repps forvery high tides of over 2.5m ODN (at Great Yarmouth) but they are not sustained in aperiod of higher tide levels.

The extreme scenarios of high tides (1 in 200 year tide) and high (1 in 100 year flow) and lowriver show similar patterns in Hickling Broad (Figure 8) and Horsey Mere:

The high tide levels cause high levels of salinity even with a high flow

13

At around 50 hours the impact of the high tide levels causes high levels of salinityespecially when the river flow is low. This reduces as the tidal levels reduce.

For the option of the1 in 100 year flow, at 50 hours the salinity impact from the hightide is less than the options with the lows flow as there is more dilution of the salinewater. As the flow subsides but the tide levels are still high, the salinity rises as thedilution effect from the freshwater is less.

When the pumps are on, the peak salinity levels from the tidal saline intrusion arereduced as the water from the pumps dilutes the impact of the tidal salinity. Switchingthe pumps off has the effect of reducing dilution and increases peak salinityconcentrations

-2000

0

2000

4000

6000

8000

10000

12000

0 10 20 30 40 50 60 70 80

Time (hrs)

Sali

nit

ym

g/l

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

Le

ve

lm

AO

D

200yr tide 100yr flow pumps on 200yr tide 100yr flow pumps off 200year tide low flow pumps full

200year tide low flow pumps off Tide levels

Figure 8 Simulated salinity levels at Hickling Broad for the tide, flow and pumpscenarios (from White et al., 2008)

Overall it is apparent that under normal tidal conditions, the input of saline drainage water bythe pumps leads to an increase in salinity in Hickling Broad, which is consistent with previousstudies into the origin of the salinity of the Broad. However, the modelling has demonstratedthat under more extreme tidal situation in which saline water moves up the Thurne, the pumpsact so as to dilute salinities. The drainage pumps therefore have a role in the salinity ‘story’ ofthe Upper Thurne which varies with the tides which must be taken into account in their futuremanagement.

Biocides and heavy metals

The use of tri-butyl tin (TBT) as an active ingredient in boat antifouling paints was banned in1987 owing to widespread evidence of negative ecological effects, but has been replaced bycopper based compounds (which act as a molluscicide). As copper is not as effective indeterring algal growth herbicidal “booster biocides” have been added to improve performance.A number of workers have looked at the potential threat to charophytes posed by boatantifouling paints and associated compounds.

Sayer et al. (2006) investigated TBT concentrations within a core extracted from HicklingBroad and relationships with biostratigraphical data. TBT was present in the core down to 12cm depth (dated 1971 ± 4 years), which is coincident with the shift from a plant-dominated toa phytoplankton-dominated ecosystem. Stratigraphic changes in the diatom, charophyte and

14

zooplankton data and sediment lithology coincided with the TBT contamination. As a result,Sayer et al. (2006) postulate that TBT caused a chronic reduction of periphyton- andphytoplankton-grazing invertebrates, particularly molluscs and zooplankton, breaking down astrong feedback loop reinforcing plant dominance, thereby precipitating (with otherenvironmental stressors) a regime shift and the substantial loss of aquatic plants.

Smith (2003) looked at the effect of the newer biocides directly in field and laboratory studies,and indirectly through investigating the presence of copper in sediment within Hickling Broad.Smith (2003) found that total copper content of the top 5 cm of sediment in Hickling Broadexponentially increased towards the boat channel, reaching around 20 mg/kg and aporewater concentration of 100 µg/l nearest to the channel. Evans (2004), cited by Lambert(2007) recorded maximum copper concentrations of filtered water drained from sedimentsamples taken near the boat channel of 27 mg/l, sufficiently high to affect molluscs (LaBrecheet al., 2002). The chlorophyll fluorescence ratio (Fv/Fm), which is used as a measure ofphotosynthetic function and therefore stress, had a negative relationship with the total coppercontent of the sediment and was also depressed by Irgarol 1051 and Diuron in field andlaboratory tests. The temperature effects of Irgarol 1051 on C. vulgaris in laboratory testswere significant as freshly painted boats would have been entering the broads around thetime of greatest apparent temperature sensitivity to the active ingredient;

Lambert et al. (2006) studied the effects of Irgarol 1051 and Diuron on UK freshwatermacrophytes. Within a survey of rivers and broads of East Anglia in May, June andSeptember 2001, Irgarol 1051 was detected in the Hickling area at concentrations from belowdetectable limits to 2430 ng/l; GS26575 (the principle metabolite of Irgarol) from belowdetectable limits to 36 ng/l, and Diurone from below detectable limits to 86 ng/l. Theconcentrations detected by Lambert in the Hickling area, with the exception of Irgarol (forwhich the maximum observed concentration was double the previous highest recorded levelin UK freshwaters) were generally in the middle range of the data range. Laboratory-basedtoxicity tests on Apium. nodiflorum, Myriophyllum spicatum and Chara. vulgaris showed thatMeasured Environmental Concentrations were consistently significantly greater than thecalculated No Observed Effect Concentration and that Chara vulgaris was the most sensitiveof the species tested.

Barker et al. (2007) dismiss the effects of boat antifouling paints on the grounds of the“innocuous use of the same presumably TBT-contaminated sediments in the mesocosms”.However, while the previous studies have demonstrated the high concentrations of copperand TBT-contaminated sediment at shallow depth, the methodology used by Barker et al.(2007) to collect and prepare the sediment had the potential to significantly affect thesediment chemistry, mixing sediment of different depth/ages, changing the redox condition ofthe sediment (by introducing oxygen) and removing contaminants that might have beenmobilised from the sediment or porewater.

Lambert (2007) surveyed 70 sample points throughout Hickling Broad in August 2005investigating the relationship between macrophyte distribution and abundance and thechemical properties of the interstitial water from the top 2 cm of sediment and the shallow andslightly deeper sediment (5 cm depth). A key finding was that copper was ubiquitousthroughout the samples of interstitial water. Only one site contained an MEC of less than 50µgl

-1, 50 sites contained an MEC of between 50 µgl

-1and 90 µgl

-1, 13 sites contained an MEC

of between 90 and 200 µgl-1

, four sites contained an MEC of between 200 µgl-1

and 300 µgl-1

,and one site contained an MEC in excess of 300 µgl

-1. The two highest concentrations of

copper were ascribed by Lambert (2007) to locations in the proximity of the two inflow pointsto the Broad at the Hickling drainage mill (>300 µgl

-1) and the Catfield dyke (>200 µgl

-1),

although there were three samples near Catfield dyke and one sample within it which hadmuch lower MEC of between 50 µgl

-1and 90 µgl

-1, while other samples from around the

Hickling drainage mill had MEC of less than 150 µgl-1

. Two further high concentrations werefound in the vicinity of the boat house (>150 µgl

-1), in addition to a larger number of lower

concentrations. Lambert suggests that the highest copper values located close to CatfieldDyke, Hickling Mill and the boat house might be caused by either bioaccumulation anddeposition by algae from the water column or anthropogenic sources, and states that further

15

research of a similar fashion is required of other broads for comparison before firmconclusions may be made.

Further observations from the survey data and a time series (1996-2006) analysis ofEnvironment Agency water quality data were

1. A statistical analysis (PCA of element associations) indicated that copper, chromium,cadmium and zinc were distributed in similar patterns and concentrations within theBroad, suggestive of similar sources.

2. The canonical RDA analysis by Lambert indicated that while water hardness was thestrongest correlate with charophyte abundance (both total and C. intermedia), nitrite,copper and sulphate were lesser positive correlates.

3. There is a highly significant (p<0.01) positive relationship between mean copperconcentration in Hickling Broad (based upon Environment Agency sampling) andcharophyte canopy and percentage coverage between 1999 and 2006.

Lambert followed this survey work with extensive replicated laboratory experimentsdemonstrating that in an artificial Hickling growth media, the mean concentration of filterablecopper of approximately 100μgl

-1as recorded in the interstitial waters of Hickling Broad in

August 2006 caused rhizoid loss and inhibited growth of photosynthetic shoots of Charaintermedia plants grown from apical cuttings.

TBT is banned and Irgarol 1051 and Diuron are no longer licensed for use on boats under 25m in inland waters. The historically high recorded concentrations and the long termpersistence of these compounds and/or their metabolites in the environment provides somecause for concern. Whilst their use may have contributed to the original regime shift of theUpper Thurne system, it seems unlikely that their continued presence is a serious factor in thefailure to achieve clear water, given the recovery of the chara lawns in the later 1990’s andearly 2000’s. However, the presence of these contaminants (including copper) in thesediment, porewater and water column (Figure 9) may make the chara less resilient to otherstressors.

0

2

4

6

8

10

12

14

10

-Ma

y-9

7

10

-Ma

y-9

8

10

-Ma

y-9

9

09

-Ma

y-0

0

09

-Ma

y-0

1

09

-Ma

y-0

2

09

-Ma

y-0

3

08

-Ma

y-0

4

08

-Ma

y-0

5

08

-Ma

y-0

6

08

-Ma

y-0

7

Co

pp

er

co

ncen

trati

on

s(μ

g/l

)

Cu Filtered

Copper - Cu

Figure 9 Monthly open water copper concentrations measured within Hickling Broadfrom May 1997- February 2008 (source: Environment Agency)

Point and diffuse sources of nutrients

Ecological effects of nutrients

Analysis of the Lambert (2007) dataset of 124 water bodies supporting 18 charophyte speciesshowed that there was a statistically significant difference (p<0.01) between the mean nitrateion concentrations at sites where charophytes were present and absent, with the upper

16

confidence limit for the presence of charophytes being < 3 mgl-1

nitrate although outlying datapoints showed chara presence at individual sites at concentrations up to 19 mgl

-1.

All chara species growing at concentrations of inorganic phosphorus of > 80 µg/l were ineither Hickling Broad or Horsey Mere, whilst those at >150µg l

-1were in Hickling Broad and

died within in 12 months following recording, making interpretation of this data uncertain!There was a statistically significant difference (p<0.01) between the mean phosphate MECsof sites where charophytes were present and absent, with the upper confidence limit for thepresence of charophytes being < 60 µg l

-1phosphate. There is a wide variation within the

MECs of TP of many of the species recorded, with several being found at extremely highMECs occurring at a variety of sites. However, subsequent laboratory studies on Characonnivens, C. virgata and Nitella opaca in an artificial Hickling Broad growth media showedthat growth of all three species was significantly reduced above 100 µg/l filterable inorganicphosphate.

Point sources of P

Holman and Deeks (2007) updated the Soil and Water Assessment Tool (or SWAT)(Gassman et al., 2007) model of Whitehead (2006) to assess the contribution of point sourcedischarges to TP concentrations in the Upper Thurne. Results suggest that:

Point source discharges have a very small (<5 μg l-1

Pl) effect on annualaverage TP concentration in Horsey Mere, Heigham Sound and HicklingBroad;

Point source discharges may have a significant (10-30 μg l-1

P) effect onannual average TP concentration in Martham Broad, although this may partlyreflect an under-prediction of summer discharge from the Somerton pumps;

Reduced flows from the Somerton South pump, to represent the effects ofgroundwater abstraction, lead to a small simulated increase (<5 μg l

-1P) in

annual average TP concentration in Martham Broad;

Diffuse source control of P losses from agriculture may be more effective inreducing surface water P concentrations.

Sediment sources

Understanding sediment dynamics is important, due to the role of sediment as a transportmedia for nutrients (principally phosphorus) and for changing channel dimensions, withconsequent effects on navigation and floods. The most comprehensive investigation ofsediment sources and transport processes is given in the desk-based study of White et al.(2005), although the availability of data on sediment sources and sediment supply processesare patchy.

The headwater catchment upstream of the Brograve Mill (Management Unit T1) wasestimated to contribute 6-10 t year

-1to the Thurne system, which is equivalent to an erosion

rate across the headwater catchment of 0.11 – 0.19 mm/year. This estimate was based onmodelled erosion rates for a 1 in 10 years return period event (McHugh et al., 2002) andrepresents the lowest rate in the Broadland headwater catchments. The Internal lowerThurne catchment unit from Brograve Mill to Thurne Mouth (T2) was estimated by White et al.(2005) to contribute between 29-39 t year

-1, equivalent to an erosion rate across the

Management Unit of 0.24 – 0.33 mm/year. The sediment supply associated with ditchmanagement was not quantified (although this is the subject of a current MSc project atCranfield University)

Sediment inputs from bank erosion in T2 were estimated as a function of water level range,flow velocity, boat pressure, channel curvature and bank protection at 40.33 t/year, althoughthis may be an under-estimate, as it does not include erosion of the whole bank profile byslumping or undercutting. The only significant industrial sediment source is the LudhamSewage Treatment Works, which provides an estimated annual sediment load of 1.4 t/year.

17

Plants and algae provide a substantial load of organic detritus to the broads system. Usingdata from Moss (1977), the phytoplankton numbers in Unit T2 were estimate at around 53000per ml in 1973, which has been classified as Excessive. White et al. (2005) found itimpossible to produce a sediment budget or to relate sediment inputs to sedimentaccumulation, but:

Hickling Broad was reported to have reduced in depth between the 1930s and 1990sby approximately 25-30 cm (Defra, 2001)

Rose and Appleby (2005) report sedimentation rates in Hickling Broad of0.014g/cm

2/yr in 1922 and 0.055 g/cm

2/yr in 2002

Sayer et al. (2006) reported that sedimentation rates since 1950, within a core takenin Hickling Broad, have fluctuated between 0.031 and 0.055 g/cm

2/yr, with a mean

value of 0.041 ± 0.009 g/cm2/yr, equivalent to 0.31 cm/yr

Diffuse sources of sediment, P and N

ADAS have applied both the Environment Agency’s spatial toolkit modeling and the PSYCHICmodel to the much of Thurne catchment (as part of the Catchment Sensitive Farming initiativefor the Bure, Ant and Muckfleet catchments) to assess erosion and P delivery (Figure 10).Neither model takes account of the presence of the pumped drainage system. The outputsfrom the two models of sediment risk are quite different, with the highest risk areas from thespatial toolkit modeling being usually in the lower risk categories of the PSYCHIC model. ThePSYCHIC model output shows high risk of surface sediment and P along the Waxham Cut,Hickling Marshes. The Environment Agency’s spatial toolkit modeling shows high sedimentrisk on the Brograve Level, Eastfield marshes and along the River Thurne. So while Figure10provides useful information on potential source areas of sediment of P, there are significantlimitations in how that can be translated into a delivery into the surface water bodies. TheEnvironment Agency’s spatial toolkit modeling for nitrogen shows that most areas of theThurne catchment have a uniformly high risk.

Figure 10 Extract of (left) EA spatial toolkit modelling and (right) Psychic modelling forthe Thurne showing surface sediment risk [darker hue = greater risk] from the CSF inthe Bure and Muckfleet

A combination of simple modelling techniques, field work and laboratory analysis is beingused in an ongoing research project by Faye Horne at UEA to understand the dynamics ofnitrogen (N) in the Upper Thurne catchment in terms of sources, fluxes and seasonalvariations. All types of dissolved N are being measured including nitrate (NO3

-), nitrite (NO2

-),

ammonium (NH4+), dissolved organic nitrogen (DON) and total nitrogen (TN). Nitrous oxide

(N2O), a potent greenhouse gas, is also being studied to determine whether there are anysignificant sources from the waterbodies or field surfaces within the catchment.

18

A simple GIS model has been used by Faye Horne to calculate the amount of N potentiallyavailable for run-off from different types of land use, according to cropping, fertiliserapplication rates, stock numbers and export coefficients from Johnes (1996). Cerealproduction is the biggest source of N from land use, which potentially contributes 81904 kg Nyear

-1, followed by cattle and then sugar beet which contribute 49106 and 33394 kg N year

-1

respectively. This technique can be used to identify the areas most at risk from N enrichment.

The field work has shown that in winter NO3-- is the predominant form of N within the river and

broads due to higher run-off during this season, although maximum concentrations foundwere less than 30 mg NO3

-l-1

which is below the 50 mg NO3-l-1

limit set by the EC NitratesDirective. In contrast in summer, DON, such as urea and amino acids, predominates as thisis when decomposers of organic matter are most active, although proportionallyconcentrations of DON remain fairly constant throughout the year, as this is largely abiologically unavailable pool of N. All of the sample sites chosen throughout the catchmentwere found to be sources of N2O; concentrations were lowest in the broads and highest in thepump water. N2O concentrations correlated well with both NO3

-and NH4

+concentrations.

This suggests that both denitrification and nitrification are sources of N2O in the UpperThurne, so that if N loadings to the water were to increase so too would the N2O flux. Theconcentration data collected for each type of N will be used to calculate flux values for thewhole catchment, allowing a mass balance to be created.

Effects of climate change on water qualityWhitehead (2006) used the SWAT model to help understand current and future nutrientdynamics within the Thurne, Bure and Ant catchments. SWAT was used to assess theaffects of climate change (using the UKCIP02 scenarios) with and without future socio-economic (landuse) change (Holman et al., 2005).

The simulation results showed that both changes in climate and land use affect future diffusesource pollutant losses and TP concentrations in Hickling Broad (Table 3). However, thedistribution of nutrient and sediment losses within the catchment is not spatially uniform,demonstrating the effects of soil type, land use and crop rotation.

Whitehead (2006) simulated the effects of a limited range of soil conservation practices (covercrops, no till, no till with cover crops and conversion of arable to pasture) on average TP andnitrate-N concentrations in Hickling Broad (Table 3). These demonstrated the potentialefficacy of soil conservation practices in reducing sediment losses (and thus P losses) and Nleaching in the catchment.

Table 3: SWAT results for management practices in the Thurne model (from Whitehead,2006) [GS – Global Sustainability; RE – Regional Enterprise]

Average TP (mg l-1

) Hickling Broad

ScenarioCurrent

Management Cover Cover & No till No Till Pasture

Baseline 0.1 0.03 0.03 0.03 0.02

2050s Low 0.11 0.05 0.05 0.05 0.04

2050s High 0.12 0.06 0.06 0.06 0.05

2050s Low GS 0.11 0.04 0.04 0.04 0.03

2050s High RE 0.13 0.07 0.07 0.07 0.06

Average NO3-N (mg l-1

) Hickling Broad

ScenarioCurrent

Management Cover Cover & No till No Till Pasture

Baseline 0.85 0.8 0.49 0.54 0.54

2050s Low 0.98 0.51 0.76 0.77 0.49

2050s High 1.01 0.96 0.59 0.81 0.59

2050s Low GS 0.83 0.64 0.64 0.67 0.43

2050s High RE 1.32 0.96 0.79 0.83 0.6

19

Sea level rise and coastal protectionThe Thurne catchment lies within the Kelling to Lowestoft Shoreline Management

Plan (sub-cell 3b). A Shoreline Management Plan (SMP) is a non-statutory, policy documentfor coastal defence management which provides a large-scale assessment for addressing therisks to people and environment (historic, natural and developed) in a sustainable way. Aconsultation period was held during 2005, but was not fully adopted by April 2007.

This coastline has a rich diversity of features including soft cliffs at the north of thecatchment and low-lying plains fronted by dunes and beaches. The structures currentlyproviding protection for the dunes, sand banks and beaches against erosion, are 14km ofconcrete sea wall, as well as timber and rock groynes, and a series of rock reefs at SeaPalling. In addition the beach is recharged periodically.

The issue of coastal protection is already emotive in the catchment, given thesignificant erosion near Happisburgh following the damage to the coastal defenses. Sea levelrise due to climate change and isostatic rebound of the land from the last ice age poses aparticular risk to the Thurne catchment, given the soft coastline and the low-lying land.Although there are no published studies regarding this area, there has been significantresearch activity in sub-cell 3b as it is the test case for the Tyndall Centre’s Coastal Simulator.Given the current uncertainty over future coastal defence in the catchment, this issuerepresents a concern of over-riding importance to many of the local inhabitants.

MonitoringShallow lakes and wetlands are, by their very nature, complex environments. This can oftenresult in conventional field-based approaches, which are based on monitoring conditions at alocation at a point in time, proving ineffective when attempts are made to monitor theecological status of these habitats. The collection of data by sensors mounted on aircraft andsatellites, in combination with conventional approaches, offers an alternative means ofmonitoring important wetland habitats such as the Norfolk Broads.

The research carried out by Hunter (2007) investigated the potential contribution thatdata from remote sensing instruments (mounted on aircraft during the project) may make tomonitoring programs in shallow lake and wetland environments such as the Norfolk Broads.In particular for the assessment of (i) phytoplankton abundance and species composition and(ii) aquatic vegetation distribution and ecophysiological status in shallow lakes.

Using high resolution in-situ and airborne remote sensing data, Hunter demonstratesthat semi-empirical algorithms could be formulated and used to provide accurate and robustestimations of the concentration of chlorophyll-a. It was further shown that it was possible todifferentiate and quantify the abundance of potentially toxic blue-green algae (cyanobacteria)using the biomarker pigment C-phycocyanin, such that diurnal and seasonal regional-scaletime-series of phytoplankton dynamics in the Norfolk Broads could be constructed.

It was further shown that remote sensing can be used to map the distribution ofaquatic plants in shallow lakes, both around the margins of the broads and, perhaps moreimportantly, those submerged beneath the waters surface that are of high conservationinterest. Hunter also shows that remote sensing metrics could be constructed for thequantification of plant vigor, in particular related to the ecophysiological response of CommonReed (Phragmites australis) to lake nutrient enrichment.

Population biology of charophytesSmith (2003) mostly used highly replicated laboratory-based studies to investigate thepopulation biology of charophytes. In the context of the Upper Thurne his important findingsmostly relate to restoration and climate change impacts. In particular, Smith (2003) foundthat:

1. C. intermedia propagation in experimental laboratory conditions could only beachieved by vegetative growth and not from germination of oospores (i.e. seeds). Thevegetative source of early shoots indicates that over wintered vegetative materialcould have a competitive advantage over growth from germinating oospores. Therewas a higher density of shoots in samples where the source of growth was from

20

oospores than that in samples where the source of growth was from nodes. However,germination did not occur in oospores from those samples until the vegetative matterwas removed.

2. Water temperature is critical for Chara growth and oospore production, thus climatechange may influence future growth patterns. Temperatures of c. 10

oC, which are

generally found in Hickling Broad between March-April, result in little growth; whilst20

oC, which is characteristic of July and August, results in considerably higher growth

rates.3. Analysis of grouped data from a number of broads (including Upper Thurne broads)

showed that the percentage cover of charophytes was related to degree days > 15oC

and 20oC in the previous year, possibly due to the development of either the oospore

bank, or a higher density of vegetative individuals. Analysis of Hickling Broad dataafter 1993 showed that day-degrees > 15 ºC prior to the end of June is negativelyassociated with charophyte % cover, which appears to conflict with the experimentalresults

4. Plant cutting experiments showed that cutting encouraged branching and increasedoverall production, which has obvious implications for future chara management inHickling Broad. Mortality was low- no plants of C. intermedia or C. hispida were lostand only 2/95 plants of C. aspera died. Cutting of single stems showed that C.intermedia established 25% more branches after the first cut, while C. hispida formedmore branches after the second cut. There was some evidence suggesting that C.intermedia may suffer when cut early in the season, as early cutting made asignificant negative difference to dry mass, but a second cut appeared to increase drymass formation. However these experiments must be interpreted with caution ascutting was done cleanly without the uprooting force typical of cutting by machinery infield conditions.

5. Establishment experiments showed that charophyte stem sections with as few as twonodes (which are dispersed naturally through physical disturbance by herbivores andmechanical cutting) are equally capable of growing into independent plants as longerstem sections. Stem sections were shown to be capable of re-growth whether theyare buried, planted, or merely resting on the substrate surface, suggesting that theywould be tolerant of planting in, or being covered by a range of sediment depths ifused for re-introduction during restoration of shallow lakes.

6. Chara vulgaris would be the best species to re-introduce into a lake with softsediments, as it produced shoots that were longer and had greater plan-form areathan C. intermedia or C. aspera within establishment experiments.

7. Based on a survey of 29 chara sites in Norfolk (although none were in the Thurnesystem) Smith (2003) suggests that sediments with higher shear strength are morelikely to be colonised by charophytes, rather than implying that charophytes increasethe shear strength of sediments that are unstable. Smith also considers that theexperimental evidence shows that charophytes are not physiologically better adaptedto develop in very firm sediments than in soft ones, which suggests that there may bean environmental factor, not present in his controlled experiments that precludescolonization of softer sediments. This may relate to oospore germination as, insediments with a low shear strength of 0.0098 kPa (typical of eutrophic broads withno re-growth of charophytes), the majority of oospores sank below 3 cm (Smith 2003)with important implications for germination and emergence.

Based on his water body survey data, Lambert used 95% confidence intervals to summarisepredicted ranges for key variables for which there were found to be a significant difference inrange between where charophytes were found and where they were absent. The data werebe grouped into:

Water quality variables which are required at minimum concentrations – calcium,chloride, magnesium, sodium, sulphate and water redox;

Water quality variables which are predicted to limit the probability of charophytesbeing present above predicted concentrations- cobalt, copper, manganese, nitrate,inorganic phosphate and silica;

Physical factors which are predicted to limit the probability of charophyte existence –sediment shear strength of the top 2 cm, canopy % cover and light extinctioncoefficient.

21

The Hickling data were frequently outliers in the distribution of sites in Lambert’s study. Inaddition the charophytes died within in 12 months following data collection, hence the Hicklingdata was not incorporated into the calculation of general tolerance limits for charophytes.However, the fact that Hickling when included was frequently an outlier in predicting forcharophyte presence, combined with the complete vegetative loss of the charophytecommunity in Hickling Broad in 2006, tends to support the conclusion by Lambert that undercurrent water and sediment chemistry conditions, we should not expect charophytes tosuccessfully re-germinate from the oospore bank.

Lambert suggests that the environmental variables which emerge from his data as significantpredictors for charophyte presence or absence may represent unexplored factors affectingcharophyte communities, directly as toxic agents (in the case of cobalt, copper andmanganese) or as limiting macro- and micro-nutrients which remedial measures such asnutrient reduction, lake bio-manipulation of fish communities, or sediment removal have notaddressed.

Recommendations for management actions to address thesignificant issues previously identified and thereby achieveFavourable Conservation Status

It is apparent from the previous sections that there are a number of issues that need to beaddressed in order to achieve Favourable Conservation Status. Given the many diverseinterests in this catchment, the following recommendations are based on generic principlesand activities, rather than ‘on the ground measures’, but provide the framework for futurestakeholder engagement activities. However, where possible, they should be linked toexisting schemes (including the current agri-environmental schemes) to maximise credibilityand increase the potential for future compliance: Examples of relevant schemes mightinclude:

Proposed capital grant scheme for farm-scale mitigation under the CatchmentSensitive Farming scheme

DEFRA codes of Good Agricultural Practice; Defra / ADAS Fertilizer recommendations; DEFRA / HGCA guide to Arable cropping and the environment; NSRI Guide to Good Soil Structure Environment Agency guide to waterway bank protection DEFRA soil strategy

Reduction of salinity and ochre

Given the evidence that increased salinity has had a detrimental impact on the ecology of theUpper Thurne broads, and is hindering the stable recovery of the clear water conditions, thereis a need to reduce chloride concentrations. However, Barker et al. suggest that a quitemodest reduction in salinity, to around 1600–1800 mg Cl L-1 may have a substantial effect onTP and chlorophyll and hence on the potential for plant growth, as the sub-present salinitymesocosm treatment were not greatly different from the lower salinities. Significant ecologicalbenefits may be obtainable without reaching the Favourable Condition chloride concentrationcriteria of 600 mg/l for Hickling Broad.

As the largest pump in the catchment, the Brograve pump has been a focus of attention. Oneof the Nature Conservation Objectives of the Water Level Management Plan (WLMP)produced by the Kings Lynn Consortium of Drainage Boards (now the Water ManagementAlliance) in March 2001 to cover the Brograve drainage district was to ensure that dischargesfrom the Brograve pump do not compromise the water quality of the receiving waters. Asolution to the salinity and ochre issues in the Brograve catchment will not be easily gainedthrough consensus, given the range of potential impacts on individuals and businesses.However, as a starting point and building upon the work of ELP and Cranfield University(2006), it is recommended that such a solution is designed on the following principles:

22

1. No significant change in current flood risk (though recognising that flood risk islikely to increase regardless as a result of climate change) – which might beachieved by re-profiling drains, providing additional flood storage and better waterlevel management control than the current water level trigger at the BrogravePump;

2. A range of farm systems should be able to continue within the marshes of thecatchment – this may be based on the splitting of the drainage system or theinstallation of private pumps (as are present in other parts of the catchment) toallow localised lower drainage levels;

3. Opportunities should be afforded for farm businesses to voluntarily implementmeasures consistent with existing agri-environment schemes (Entry Level andHigher Level, as appropriate) or to enter into habitat creation partnerships withconservation NGOs or charities;

4. In the event of a coastal breach that effects the rivers and broads, as well as themarshes, (as in 1937 or 1953), there is a means of removing the seawater from thecatchment without discharging it through the SAC with the minimum delay. Thismight be achieved by the installation of an auxiliary coastal pumping station (assuggested in ELP and Cranfield University, 2006, for near Hempstead) or theBroads IDB having immediate access to a mobile pump and generator.

5. That there is recognition that the Brograve sub-catchment is a ‘naturally’ brackishsystem, and that management changes should not aim to achieve freshwaterconditions. Pallis (1911) showed that dyke salinities were brackish or saline(10,000-16,000 μS/cm) when the Brograve Level would mostly have been underhigh water level management.

6. Similarly, that ochre production in the Brograve sub-catchment is unlikely to bestopped. It should be recognised that limited production of ochre may be beneficialto surface water quality, given its propensity for removing P from solution.

Reduction of diffuse source pollution (N, P, sediment and crop protection products)The study by Holman and Deeks (2007) suggests that point sources (e.g. consented smallsewage treatment works) contribute little to nutrient levels in the Upper Thurne broads.Activities should therefore be focussed around reducing diffuse source losses fromagriculture. Assuming that the farming community are following CoGAP and GoodAgricultural and Environment Condition (GAEC) requirements, it is recommended that:

The nutrient content of manures, where used, are recognised within nutrient budgetsto avoid application;

The Catchment Sensitive Farming iniative is rolled out to the Thurne and that Ptesting is carried out on soils and manures to target applications and to assesswhether fertiliser applications can be reduced;

Care is taken to avoid excessive mud on roads during the autumn/winter sugar beetharvesting season, particularly where road runoff is likely to enter drains andwatercourses and awareness is raised of the Environment Agency ‘Mud on Road’campaign;

Voluntary Initiative measures are followed to minimise losses of agrochemicalproducts (pesticide and herbicides) to the environment. This is particularly importantwith regard to application in under-drained fields and those with wet ditch margins,and to the disposal of sprayer washings, where incentives for installing Biobedsshould be sought;

Increased use of extensive buffer strips, headlands and reversion to semi-naturalvegetation next to watercourses and minimum tillage on soils which are susceptible toerosion.

ReferencesBarker, T., Hatton, K., O’Connor, M., Connor L., Bagnell, L and Moss B (2008). Control of

ecosystem state in a shallow, brackish lake: implications for the conservation ofstonewort communities. Aquatic conservation: marine and freshwater ecosystems,18(3), 221-240.

Broads Authority (1999) Assessment of the proposed cutting of aquatic plants in HicklingBroad : final report by the Assessment team. Broads Authority, Norwich.

Broads Authority (2006) Upper Thurne water space. Broads Authority, Norwich.

23

Defra (2001). The future of the Broads National Park. Defra review, 19 Nov 2001.www.broads-society.org.uk/defrareview.html

Driscoll, R.J. (1984). Chloride ion concentrations in dyke water in the Thurne catchment areain 1974 and 1983. Unpublished Report for NCC, Norwich, 62pp.

ELP and Cranfield University (2005). Feasibility study for solutions to the salinity and ochreissues in the Brograve catchment. Unpubl. Rep for the Broads Internal Drainage Board

Evans, L.C. (2004). Factors affecting the distribution and abundance of the submergedmacrophyte community in Hickling Broad, Norfolk: is boating activity having an adverseimpact. Unpubl. MSc thesis, University College London

Gassman P.W., Reyes M.R., Green C.H.. Arnold J.G. (2007). The Soil and WaterAssessment Tool: Historical Development, Applications, and Future ResearchDirections. Transactions of the ASABE, 50(4): 1211-1250.(http://www.card.iastate.edu/environment/items/asabe_swat.pdf)

Gurney, R (1904). The fresh and brackish water crustacean of east Norfolk. Trans. Nor. Nor.Nat. Soc., 7, 637-660

Hails, A. J. (ed) (1996). Wetlands, Biodiversity and the Ramsar Convention: the role of theConvention on Wetlands in the Conservation and Wise Use of Biodiversity. RamsarConvention Bureau, Gland, Switzerland (http://www.ramsar.org/lib/lib_bio_1.htm )

Harding, M., and Smith, K. (2002) Ochre In The Brograve Catchment: Causes and Cures.Happisburgh-Winterton Internal Drainage Board

Harris, J. (2000). Hickling Broad Aquatic Plant Cutting and Monitoring Programme- Summaryreport of results from 1994 to 1999. Unpublished report for the Broads Authority

Harris, J. (2004). Hickling Broad Aquatic Plant Cutting and Monitoring Programme- Summaryreport of results from May 2004 to September 2004. Unpublished report for the BroadsAuthority

Harris, J. (2007). Hickling Broad Aquatic Plant Cutting and Monitoring Programme- Summaryreport of results from 2000 to 2006. Unpublished report for the Broads Authority

Holman, I.P. (1994). Controls on Saline Intrusion in the Crag aquifer of north-east Norfolk.Unpubl. Thesis, University of East Anglia, Norwich.

Holman, I.P. and Deeks, L.K. (2007). Upper Thurne modelling for Stage 4of the Review ofConsents for the Habitats Directive. Unpublished report for the Environment Agency

Holman, I.P. & Hiscock, K.M. (1998). Land drainage and saline intrusion in the coastalmarshes of north east Norfolk. Quarterly Journal of Engineering Geology, 31, 47-62.

Holman I.P., Rounsevell M.D.A., Shackley S., Harrison P.A., Nicholls R.J., Berry P.M. andAudsley E. (2005). A regional, multi-sectoral and integrated assessment of the impactsof climate and socio-economic change in the UK: I Methodology. Climatic Change, 71,9-41.

Hunter, P.D. (2007). Remote Sensing in Shallow Lake Ecology. Unpublished Ph.D. Thesis,University of Stirling

Innes, A.G. (1911). Tidal actions in the Bure and its tributaries. Trans. Norf. Nor. Nat. Soc.,9(2), 244-262.

Johnes, P. (1996) Nutrient export modelling - River Bure, Norfolk. Aquatic EnvironmentsResearch Centre, University of Reading.

LaBreche, TMC, Dietrich AM, Gallagher DL and Shepherd N (2002). Copper toxicity to larvalMercenaria mercenaria (hard clam). Environmental Toxicology and Chemistry 21, 760-766

Lambert, S.J., Thomas K.V and Davy A.J (2006). Assessment of the risk posed by theantifouling booster biocides Irgarol 1051 and Diuron to freshwater macrophytes.Chemosphere 63(5), 734-743

Lambert, S.J. (2007). The environmental range and tolerance limits of British stoneworts(Charophytes). Unpubl. PhD thesis, University of East Anglia, Norwich, UK

McHugh M, Wood G, Walling D, Morgan R, Zhang Y, Anthony S and Hutchins M (2002).Prediction of sediment delivery to watercourses from land. Phase II R&D TechnicalReport No. P2-209, Environment Agency, Bristol.

Moss, B. (1977) Conservation problems in the Norfolk Broads and rivers of East Anglia –phytoplankton, boats and the causes of turbidity. Biol. Cons. 12, 95-114.

Moss, B (2001). The Broads; the people’s wetland. HarperCollins Publishers, London.Pallis, M. (1911). Salinity in the Norfolk Broads. I. On the cause of the salinity of the Broads of

the River Thurne. Geograph. J., 37(3), 284-291.

24

Rose, N.L. and Appleby PG (2005). Sediment accumulation in the Broads. A report to theBroads Authority. Research Report No. 101. Environment Change Research Centre,University College London

Sayer, C.D, Hoare DJ, Simpson GL, Henderson ACG, Liptrot ER, Jackson MJ, Appleby PG,Boyle JF, Jones JI and Waldock MJ (2006). TBT causes regime shift in shallow lakes.Environmental Science and Technology 40, 5269-5275

Servera-Martinez, E (2005). Ecological failure criteria for Hickling Board, Norfolk. Unpubl.MSc thesis, Cranfield University

Simpson, TB (2007). Understanding the groundwater system of a heavily drained coastalcatchment and the implications for salinity management. Unpubl. PhD thesis. CranfieldUniversity

Smith, D.C.S. (2003). The population biology of charophytes in the context of shallow lakerestoration. Unpubl. PhD thesis, University of East Anglia, Norwich, UK

White, SM, Fisher KR and Holman IP (2008). Modelling the dynamics of the Upper Thurneriver system and its influence on Hickling Broad. Unpubl report for the BroadsAuthority.

White, S, Deeks L, Apitz SE, Holden A and Freeman M (2005). Desk based study of thesediment inputs to the Broads catchment, with the identification of key inputs andrecommendations for further targeted research and management to minimise inputs.Unpubl. Final report: Phases I and II for the Broads Authority Report, Norwich

Whitehead, J. 2006. Integrated catchment scale model of a lowland eutrophic lake and riversystem: Norfolk, UK. Unpubl. PhD thesis. Cranfield University.