Marine Chemistry 167 (2014) 82–92

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Marine Chemistry

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Estuarine canal estate waters: Hotspots of CO2 outgassing driven byenhanced groundwater discharge?

Paul A. Macklin, Damien T. Maher ⁎, Isaac R. SantosCentre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, New South Wales, Australia

⁎ Corresponding author. Tel.: +61 2 66203577; fax: +E-mail address: damien.maher@scu.edu.au (D.T. Mahe

http://dx.doi.org/10.1016/j.marchem.2014.08.0020304-4203/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 December 2013Received in revised form 14 August 2014Accepted 18 August 2014Available online 30 August 2014

Keywords:Coastal hydrologyPermeable sedimentsFloodplainGreenhouse gasesMoreton BayMangrove

Increasedwater-to-air carbon dioxide fluxes are a potentially important, but as yet unquantified, consequence ofcanal estate developments in estuaries surrounded by coastalwetlands.Weused detailed pCO2 and radon (222Rn,a submarine groundwater discharge tracer) surveys to investigate whether water-to-air CO2 fluxes were en-hanced in residential canal systems, and whether groundwater exchange may drive pCO2 distribution. Observa-tions were performed along 300 km of canals, rivers, estuaries, and coastal embayments from the Gold Coast(Queensland, Australia), one of the largest estuarine canal systems globally. Overall, residential canal estate wa-ters were supersaturated in CO2 with pCO2 ranging from 372 to 3639 μatm and 434 to 3080 μatm in the dry andwet season surveys, respectively. pCO2 usually increased in areas of reduced connectivity (i.e., poorly flusheddead end canals). A stronger correlation between 222Rn and pCO2 than between dissolved oxygen and pCO2 im-plied that groundwater seepage (not pelagic respiration) was the major driver pCO2 supersaturation within thecanal system. Average area-weighted water-to-air CO2 fluxes within canals were 34 and 67 mmol C m−2 d−1

during the dry and wet seasons respectively. When upscaled to the entire Gold Coast estuarine system, residen-tial canal contributed 46% and 56% of the total flux of CO2 to the atmosphere during the dry and wet seasons, re-spectively. These results imply that areas that were previous atmospheric carbon sinks (i.e. coastal wetlands)have become sources of CO2 to the atmosphere since the development of residential canal estates.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

There is strong coupling between population growth and develop-ment in the world's estuaries and nearby low lying areas, with an esti-mated six billion people predicted to be living in coastal areasworldwide by 2025 (Hameedi, 1997). Extensive areas in the coastalzones have been lost by dredging, land reclamation and road construc-tion. Coastal development, hydrological disturbances, agriculture andpollution are among the dominant causes of coastal wetland distur-bances and can be responsible for the release of large amounts of CO2

to the atmosphere (Lovelock et al., 2011). Due to the vast quantities ofstored carbon in coastal wetlands, emission rates of CO2 resultingfrom anthropogenic disturbance of these carbon storage hotspots maybe significant (Adame et al., 2013; Donato et al., 2011).

Coastal systems play an important role in the global carbon cycle.Most estuaries are heterotrophic (respiration exceeds gross primaryproduction) resulting in CO2 supersaturation within the water column(Borges and Abril, 2011; Chen et al., 2012). As a result estuaries are asource of CO2 to the atmosphere (Cai, 2011). The terrestrial input oforganic carbon accounts for significant portions of CO2 producedwithin aquatic ecosystems (Abril et al., 2000; Cai, 2011). CO2 can enter

61 2 66212669.r).

waterways through organic matter degradation and root respirationwithin terrestrial systems (Kessler and Harvey, 2001). CO2 within thesoil may be either vertically diffused to the atmosphere or transportedthrough groundwater to surface waters, where it is lost to the atmo-sphere (Chen et al., 2012).

The balance between photosynthetic CO2 uptake and CO2 releasedduring respiration within the water column is often thought to deter-mine the magnitude of estuarine CO2 fluxes. However, recent studiessuggest that inputs of high pCO2 groundwater may also contribute tothe water-to-air flux of CO2 (Atkins et al., 2013; Cai, 2011; Cai et al.,2003). As groundwater is often highly enriched in CO2 compared to sur-facewaters, groundwater discharge can influence CO2 evasion from sur-face waters even when seepage rates are relatively low (Santos et al.,2012). With increased importance placed on constraining the globalcarbon budget, more studies on groundwater- surface water carbontransport are necessary (Moore, 2010). Site specific measurements areneeded as groundwater may enter the water column through multiplegroundwater and porewater pathways (Burnett et al., 2006).

A number of recent investigations have relied on natural geochemi-cal tracers to assess groundwater discharge into coastal ecosystems(Burnett et al., 2006;Moore, 2010). Radon (222Rn), a naturally occurringnoble gas part of the 236U decay chain, is an excellent groundwater trac-er because it is generally orders of magnitude higher in groundwaterthan surface waters (Swarzenski, 2007). It is now possible to produce

83P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

high resolution, continuous field observations of radon to obtain highprecision results, comparable or better than laboratory based techniques(Dulaiova et al., 2005). Using radon as a natural tracermay produce rapidand inexpensive results and allow one to qualitatively map the locationof groundwater discharge hotspots (Dulaiova et al., 2010; Santos et al.,2011; Stieglitz et al., 2010). By coupling 222Rn and pCO2 measurementsit can be determined if there is a link between groundwater input andsurface water pCO2 in coastal waters (Santos et al., 2012).

Residential canal estate construction is increasing in estuaries. As of2010 there was an estimated 4000 linear kilometres of residential canalestates worldwide (Waltham and Connolly, 2011). Waterfront livingdemands and increasing population have resulted in the modificationand extension of estuarine systems. Complex designs and the resultingdecreased connectivity to the main waterway often increases residencetimes in canal estate waters (Benfer et al., 2010) but as yet there is lim-ited information regarding the effects of residential canal systems onbiogeochemical processes such as CO2 evasion to the atmosphere.Some canal estate developments have grown by the connection ofolder canals systems to new estate developments. Restricted tidalexchange is optimum for canal construction as it allows landwarddevelopment to occur but also reduces hydraulic velocities which mayprevent erosion (Waltham and Connolly, 2011).

CO2 fluxes and drivers in residential canal estate waters have notbeen assessed. In this paper, we report high resolution observations ofsurface water pCO2,

222Rn, dissolved oxygen, pH, salinity, and tempera-ture throughout the artificial canals and estuary systems of the GoldCoast (Australia). We estimate the contribution of canal systems to

Fig. 1.Map of the Gold Coast waterways with survey tracks colour coded t

the total CO2 evasion from the estuary and discuss the potential driversof CO2 evasion with a focus on groundwater discharge.We hypothesizethat artificial canals are hotspots for groundwater discharge which en-hances the input of carbon-rich groundwater to surface waters andthe overall regional outgassing of CO2 to the atmosphere.

2. Material and methods

2.1. Study site

Field experiments were performed on the Gold Coast (Queensland,Australia) natural and modified estuarine systems (Fig. 1). The GoldCoast estuarine system is comprised of 11.9 km2 of unmodified rivers,16.4 km2 of canals and 22.4 km2 of enclosed coastal waters (hereaftertermed the Broadwater; Fig. 1). Australia has the largest expanse ofcanal systems worldwide with approximately 440 linear km of artificialurban waterways and canal estate construction with 90% of residentialcanal estates in the broader Gold Coast region (Waltham and Connolly,2007). In total there are ~300 km of canals in the study area (WalthamandConnolly, 2011). The artificial canals provide habitat to aquatic organ-isms, drainage of stormwater and flood mitigation (Waltham andConnolly, 2007).

The area under investigation receives the drainage from the Nerangand Coomera rivers. The Gold Coast study site comprises mostly ofmixed alluvial low lying coastal wetlands and mangroves less than10m above sea level while sedimentary beds in the region consist of ar-gillite, greywacke, quartzite, greenstone, chert, shale and conglomerate

o indicate categorised sampling areas (canals, Broadwater and rivers).

Table1

Dry

andwet

season

averag

es(±

stan

dard

error)

ofpC

O2,

222 R

n,pH

,salinity,

dissolve

dox

ygen

,and

tempe

rature.

pCO2(μatm)

222Rn

(Bqm

−3)

Salin

ity(p

pt)

Dissolved

oxyg

en(%

)pH

Tempe

rature

(°C)

Dry

season

Wet

season

Dry

season

Wet

season

Dry

season

Wet

season

Dry

season

Wet

season

Dry

season

Wet

season

Dry

season

Wet

season

Cana

ls78

3±

236

879±

276

32±

2937

±23

33.0

±2.6

22.6

±7.1

95.2

±5.3

94.4

±7.3

7.54

±0.08

7.43

±0.24

24.1

±1.1

21.3

±1.2

Rive

rs79

6±

369

763±

341

36±

3432

±48

33.7

±3.2

25.0

±7.0

93.3

±6.9

94.5

±6.4

7.52

±0.12

7.41

±0.29

24.9

±1.5

19.8

±1.3

Broa

dwater

468±

100

460±

7714

±10

11±

836

.0±

0.3

33.1

±1.4

100.6±

5.2

100.7±

3.0

7.73

±0.08

7.63

±0.15

22.8

±1.8

20.5

±1.1

Who

leestuary

647±

235

667±

231

25±

2424

±26

34.5

±2.0

27.8

±5.2

97±

5.8

97±

5.6

7.62

±0.09

7.51

±0.2

23.7

±1.5

20.6

±1.2

84 P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

which have weathered to form moderately fertile soils (Franks, 1971).The subtropical climate is characterised by dry (May–October) andwet (November–April) seasonswithmost rainfall in the summer periodfrom December to February (Dunn et al., 2014). The average rainfallfrom 1994 to 2013 was 1294 mm per year, with the dry season record-ing 50–100mmand thewet season 400–600mm (Australian Bureau ofMeteorology; www.bom.gov.au). Tidal range in the Gold Coast Seawayis approximately 2meters. The city of the Gold Coast has a population of~540,000 with about 15,000 people living in waterfront residentialcanal state properties.

2.2. Approach and methods

Two spatial surveys covering approximately 300 km of waterwayswere conducted on Gold Coast canals, rivers, and estuaries. The firstsurvey was conducted in the dry season from 17 to 25 October, 2012.The second survey was conducted in the wet season from 14 to 22May, 2013. Rainfall in themonth prior to the dry season andwet seasonsurveys was 34 mm and 149 mm respectively.

Instrumentation was installed on a small research vessel that wasdriven at 4–6 km/h. The boat was stopped or slowed down at sites ofhigh interest (i.e., canal dead ends). The vessel location was logged con-tinuously by a Garmin GPS. Sampling of temperature, salinity, conduc-tivity, dissolved oxygen, and pH (National Bureau of Standards scale)was undertaken using a calibrated Hydrolab multiparameter waterquality sonde at 5 min intervals. pCO2 (1 min intervals) and222Rn(10 min intervals) were measured underway using a shower head gasequilibration device coupled to a Licor 7000 CO2 monitor and RAD7radon-in-air monitor (Durridge) as described by Santos et al., 2012.The dissolved gas concentrations in the source water was determinedby the gas solubility calculated as a function of temperature and salinityas described in Santos et al. (2012) and references therein.

The CO2 flux across thewater–air interface was calculated accordingto Wanninkhof (1992):

FCO2 ¼ k�KH�ΔpCO2 ð1Þ

where k is the CO2 gas transfer velocity, KH is the solubility of CO2 (Weiss,1974) and ΔpCO2 is the difference between sea and air (pCO2sea −pCO2air). k was calculated using the wind-speed based parameterizationmethod of 4 different authors to provide a reasonable range in evasionrates. We used k parameterizations from Wanninkhof and McGillis(1999), Wanninkhof (1992), Raymond and Cole (2001), and Borgeset al. (2004), respectively:

k ¼ 0:0283u3 Sc=660ð Þ‐1=2 ð2Þ

k ¼ 0:31u2 Sc=660ð Þ‐1=2 ð3Þ

k ¼ 1:91e0:35u Sc=600ð Þ‐1=2 ð4Þ

k ¼ 5:141u0:758 Sc=600ð Þ‐1=2 ð5Þ

where k is the transfer velocity (cmh−1), u is thewind speed (m s−1) at aheight of 10mand Sc is the Schmidt number of CO2 at in situ temperatureand salinity (Wanninkhof, 1992). Positive fluxes represent the net CO2

exchange from water-to-air and a negative value represents the net ex-change from air-to-water. Estimation ofwater-to-air CO2 fluxeswere cal-culated by using one minute sampling times for measured pCO2 data.Wind speed datawas sourced from the Australian Bureau ofMeteorology(www.bom.gov.au) for a weather station located at the Gold CoastSeaway. Water-to-air CO2 fluxes were calculated using the average

Fig. 2. Spatial distribution of pCO2 and associated parameters in dry (top) and wet (bottom) surveys on the Gold Coast waterways. Note the different colour scales for the wet and dry season.

85P.A

.Macklin

etal./Marine

Chemistry

167(2014)

82–92

Fig. 3. Runaway Bay residential canal estate section showing pCO2 increasing towardscanal end-members in the dry (A) and wet (B) seasons. Note the different colour scalesfor the wet and dry season, and compared to Fig. 2.

86 P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

wind-speed in the week prior to field experiments. This approach waschosen to allowmore realistic spatial comparisons and increase the inte-grative power of our interpretation. Using instantaneous wind speed ob-servations would create a potential bias because different parts of thecanal system were sampled during different times of the day and windspeeds are subject to natural diel cycles. In addition, sampling duringeach season took several days, therefore using a weekly average windspeed prevented day to day wind speed induced biases in estimatedflux calculations.

The data were categorised into four classes: canal estate waters,Broadwater (i.e., large marine-dominated embayment), river, and allsamples (Fig. 1). The surface area of the study site and the individualclasses were calculated using ARC GIS. Areal CO2 fluxes were calculatedby interpolating point data using the splinewith barriers tool in ARC GIS(Maher and Eyre, 2012). Averages and standard deviations of auxiliaryparameters within each estuary section were also calculated using thesame method. To compare dry and wet season differences in pCO2,data was normalised to the average temperature between the two sea-sons of 22.2 °C. pCO2 was normalised according to the equation ofTakahashi et al. (2002):

NpCO2 ¼ in situpCO2ð Þ � exp 0:0423 Taveg‐ Tin situ

� �h i;

whereNpCO2 is the temperature normalised pCO2, T is temperature (°C)and the subscripts are average and in situ values respectively. All thepCO2 values reported here represent normalized concentrations evenif “N” is not used for simplicity.

3. Results

The average salinity in the dry season was 34.5 ppt and during thewet was 27.8 ppt (Table 1) reflecting the 5-fold greater rainfall inthe month proceeding the wet season than in the dry season survey.The highest dissolved oxygen (DO) and pH values were found inthe Broadwater (close to the ocean entrance) in both seasons and thelowest were recorded in the upstream areas of the rivers and canalend-members (Fig. 2).

Area-weighted average 222Rn (groundwater tracer) concentrationswere similar between seasons in each section of the estuary (Table 1).During each season, 222Rn followed a similar trend with the lowestconcentrations recorded in the Broadwater where seawater enters thesystem and highest concentrations in the canals and upper river areas(Fig. 2). The highest concentrations observed were in the upstreamreaches of the Nerang River (439 Bq/m3).

pCO2 ranged from 312 to 5928 μatm and 342 to 3672 μatm in thedry and wet seasons respectively and increased upriver and towardscanal end-members in both seasons (Figs. 2, 3 and 4). Overall, area-weighted estuary-wide average temperature normalized pCO2 wasslightly higher in thewet season (667 μatm) compared to thedry season(647 μatm) (Table 1). Aswith 222Rn, pCO2was lowest in the Broadwater,particularly the seaway entrance (down to 342 μatm) with the highestpCO2 in rivers (up to 5928 μatm) and canal dead-ends (Figs. 2, 3, and 4).

The estimated water-atmosphere CO2 fluxes within the canalsranged from 13 mmol C m−2 d−1 to 60 mmol C m−2 d−1 and40mmol Cm−2 d−1 to 83mmol Cm−2 d−1 during the dry andwet sea-son respectively depending on the piston velocities used (Table 2). Theoverallwater to air CO2flux from theGold Coast estuarine system rangedbetween 4.5 × 105 mols CO2 day−1 and 2.1 × 106 mols CO2 day−1 in thedry season and 1.2 × 106mols CO2 day−1 and 2.5x 106mols CO2 day−1 inthewet season (Table 2). The river systems had ~ 1.5 fold higher fluxes inthewet seasonwhile canals had ~ 2 fold higherfluxes during thewet. TheBroadwater had similar CO2 flux rates during both seasons (Table 2).These fluxes were calculated based on the average wind speed in theweek preceding observations (5.0 and 7.2 m s−1 in the dry and wet sea-son, respectively).

4. Discussion

4.1. Canals as a source of CO2 to the atmosphere

Our observations revealed supersaturated pCO2 values in canal sys-tems that have replaced coastal wetlands and floodplains. Gold Coastresidential canal estates showed upper estuarine characteristics ofpCO2 in spite of relatively high salinity (Fig. 2) and were in general asource of CO2 to the atmosphere. A clear example of this is shown inRunaway Bay canal estates (Fig. 3) where pCO2 increased towardscanal end-members in both dry and wet season sampling periods.Several other disconnected canals displayed similar patterns (Fig. 4).The high pCO2 in the upper estuarine areas may be due to inputof high pCO2 waters from upstream and as the stream flow reachesmore saline and flushed areas CO2 is increasingly buffered while thede-gassing of upstream CO2 has occurred (Cai and Wang, 1998).

By categorising the Gold Coast estuarine system into the three clas-ses (i.e., rivers, canals and Broadwater), more effective comparisonswith selected estuaries worldwide could be made (Fig. 5). The GoldCoast residential canal system had pCO2 values and CO2 evasion rateswithin the broad range observed in other estuaries (Chen et al., 2013).For example, it was found that the Piauí River (Brazil) had evasion ratesof 35 mmol C m−2 d−1 (Raymond et al., 2000) while studies in theRanders Fjord, Denmark recorded ranges from −5 mmol C m−2 d−1

in April and 53 mmol C m−2 d−1 in August (Gazeau et al., 2005). TheCochin Estuary on the west coast of India had dry season fluxes of65 mmol C m−2 d−1 while the wet season was 287 mmol C m−2 d−1

(Gupta et al., 2009) The Pearl River, (Hong Kong) exhibited fluxes of

Fig. 4. Nerang River residential canal estate section showing pCO2 increasing towards canal end-members and the Nerang River while decreasing towards the Broadwater in the dry(A) and wet (B) seasons. Note the different colour scales for the wet and dry season, and compared to Figs. 2 and 3.

87P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

24 mmol C m−2 d−1 (Yuan et al., 2011). Most studied estuaries are riverdominated and influenced by human settlement (Cai, 2011). Due to thenet heterotrophic nature of estuaries worldwidemost emit CO2 to the at-mosphere however there are systems that are net autotrophic and atmo-spheric CO2 sinks (Borges and Abril, 2011; Maher and Eyre, 2012).

The overall range ofwater-to-airfluxeswere 3 to 86mmol Cm−2 d−1

in the different sections during the two seasons (Table 2). Canalsaccounted for ~50% of the total flux. This is significant considering resi-dential canal estates occupy 32% of the total waterway area with theBroadwater occupying 44% and the rivers 24% (Table 2; Fig. 6). Whilethe Broadwater represents the largest area, water-to-air atmosphericfluxes were relatively small whereas the natural river and estuarinesystems (which originally would have been the main source of water-to-air CO2) now represent a relatively smaller proportion of the totalarea than the artificial canal system. Although the averageflux rate is sim-ilar or higher in rivers compared to canals, when the CO2 fluxes areweighted by area (see Methods) the canals were larger sources of CO2

and had higher water-to-air CO2 fluxes than the river systems, i.e., the

Table 2Area, area-weighted CO2 fluxes using the 4 different transfer velocity parameterizations (Eqs. (and Broadwater in dry and wet seasons.

Area Weighted CO2 Flux (mmol m

Area (km2) Eq. (2) Eq. (3)

Dry SeasonBroadwater 22.4 2.9 6.4Canals 16.4 12.7 27.8Rivers 11.9 15.1 33Total 50.7 8.9 19.6

Wet SeasonBroadwater 22.4 5.1 7.8Canals 16.4 40.0 60.8Rivers 11.9 32.9 50.1Total 50.7 22.9 34.9

highest fluxes in rivers are confined to the upper rivers where the areais smallest and the canal systems have higher fluxes throughout. There-fore, residential canals may represent a significant new regional atmo-spheric source of CO2 and potentially other greenhouse gases. Inaddition, thehigh carbonburial rates associatedwith the coastalwetlandsthat occupied these areas prior to canal development may not be occur-ring under current canal conditions.

4.2. Drivers of pCO2 distribution

A simple correlation analysis was used to gain insight into the driversof CO2 in the modified estuarine system under investigation. We assumesalinity represents a proxy of freshwater discharge (i.e., direct rainfall, up-stream river inputs, urban runoff, and groundwater discharge), dissolvedoxygen represents a proxy of photosynthesis and respiration, and radon isa proxy of fresh and saline groundwater discharge. Radon sources otherthan groundwater discharge (molecular diffusion frombottom sedimentsanddecay of dissolved 226Ra)were not quantified inGold Coast canals but

2)–(5)) and minimum andmaximum upscaled flux estimates for Gold Coast rivers, canals

−2 d−1) Upscaled fluxes (mol CO2 d−1)

Eq. (4) Eq. (5) Average Minimum Maximum

8.7 13.7 7.9 6.5 × 104 3.1 × 105

37.6 59.6 34.4 2.1 × 105 9.8 × 105

44.6 70.7 40.8 1.8 × 105 8.4 × 105

26.5 41.9 24.2 4.5 × 105 2.1 × 106

11.0 10.6 8.6 1.1 × 105 2.5 × 105

85.6 82.8 67.3 6.6 × 105 1.4 × 106

70.6 68.2 55.5 3.9 × 105 8.4 × 105

49.1 47.5 38.6 1.2 × 106 2.5 × 106

Fig. 5. Examples of pCO2 ranges in some estuaries worldwide including the present Gold Coast highlighted in red.

88 P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

were demonstrated to beminor sources of radon to other shallow coastalsystems locatedwithin 100 km from the Gold Coast (Gleeson et al., 2013;Makings et al., 2014) and overseas (Berelson et al., 1982; Cable et al.,2004). Therefore, we treat radon as an unambiguous qualitative tracerfor groundwater exchange which likely includes a combination of freshgroundwater discharge and seawater recirculation in canal beach sedi-ments (i.e., tidal pumping).

Fig. 6. Spatial distribution of CO2 evasion rates in thedry andwet season. The inset shows the relunder investigation.

pCO2 was significantly correlated to several of the parameters inves-tigated (Fig. 7 and 8). There was a significant correlation between 222Rnand pCO2, and salinity and pCO2. In the dry season, pCO2 had strongercorrelations to 222Rn than salinitywithin canals, while in thewet seasoncorrelations to salinity were stronger than with 222Rn. This may reflectan enhanced relative contribution of groundwater seepage to canalwater budgets and pCO2 when there is no or little surface runoff

ative contribution of rivers, canals andBroadwater to the total evasion CO2 from the system

Fig. 7.Dry season relationships between pCO2 and 222Rn (GW), salinity, dissolved oxygen, and pH showing (from left to right) all samples, canals, Broadwater and rivers. Interpolated andduplicate samples (n) are included in ‘total’ (utilised for Arcmap GIS integration) but removed in canals, Broadwater and rivers sections.

89P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

(Santos and Eyre, 2011). The negative relationship between salinity andradon (Fig. 9) implies that groundwater dischargewas amajor source offreshwater to the Gold Coast estuarine system. While in the dry seasonthere was a stronger relationship between pCO2 and 222Rn in rivers andcanals (Fig. 7), the wet season still had significant CO2/222Rn relation-ships in rivers and canals (Fig. 8). These patterns support our initial hy-pothesis that canals are acting as conduits for CO2 to the atmosphere byfacilitating the inputs of carbon-rich groundwater to surfacewaters. Ar-tificial canals and drains may cut through aquitards and function as awindow to the aquifer (de Weys et al., 2011; Santos et al., 2008). As aresult, canals and drains may be considered groundwater dischargehotspots. Canals also increase the total shoreline length which extendsthe total area where regional groundwater may interact with surfacewaters. Lower and more dynamic groundwater levels are often foundnear artificial drains and canals as a result of intensified groundwaterrecharge and discharge cycles (Harvey et al., 2006; Johnston et al.,2005).

Dissolved CO2 above atmospheric equilibrium within estuaries mayoriginate from inputs of organic carbon or free CO2 through four mainpathways: in situ respiration, the soil system, the hyporheic zone or in-puts of high CO2 water from upstream or the ocean (Borges and Abril,2011; Jones and Mulholland, 1998; Schindler and Krabbenhoft,1998). Groundwater and shallow porewater are often enriched in CO2.

For example, a recent study revealed average pCO2 of 78,000 μatm inshallow, acidic groundwaters in a coastal floodplain wetland locatedabout 100 km south of the Gold Coast canals (Atkins et al., 2013).Production of CO2 in residential canals may be further enhanced byanthropogenic impacts as organic pollutants are introduced to the soilzone and waterways, potentially resulting in increased respirationrates (Zhai et al., 2005). When high pCO2 groundwater reaches surfacewaters, CO2 will eventually equilibrate with the atmosphere (Stuteand Schlosser, 2000).

Several previous studies suggest that high CO2 evasion to the atmo-sphere in estuaries may be driven primarily by the degradation of river-ine organic carbon within the water column (Ahad et al., 2008; Cai,2011). We can use dissolved oxygen observations to get insight intowhether CO2 supersaturation was potentially driven by high in siturespiration rates rather than groundwater seepage. The Gold Coastcanal waters were usually slightly undersaturated in oxygen (Fig. 2).However, several canal segments were oversaturated in both oxygenand CO2. The oxygen supersaturation implies high photosyntheticrates which would be associated with low CO2 unless externalsources of CO2 such as groundwater seepage can sustain high pCO2.

In addition, correlations between pCO2 and DO within canals wereeither insignificant (wet season; Fig. 8), or weaker than radon (dryseason; Fig. 7). Therefore, we suggest that groundwater discharge

Fig. 8.Wet season relationships between pCO2 and 222Rn (GW), salinity, dissolved oxygen, and pH showing (from left to right) all samples, canals, Broadwater and rivers. Interpolated andduplicate samples (n) are included in ‘total’ (utilised for Arcmap GIS integration) but removed in canals, Broadwater and rivers sections.

90 P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

exerts a stronger control than pelagic respiration on CO2 supersatu-ration within Gold Coast canals.

Would the discharge of groundwater be enough to drive a concurrentdecrease of dissolved oxygen in surface waters? While we have no

Fig. 9. The relationship between salinity and 222Rn in dry (left) and wet (right) seasons, indica

information on the chemical composition of groundwater discharginginto Gold Coast canals, samples taken from nearby coastal floodplainsrevealed DO approaching 0% saturation, 222Rn of ~1000 Bq/m3, dis-solved inorganic carbon (DIC) ~ 3195 umol L−1 and alkalinity of 121

ting that groundwater is a major source of freshwater to the Gold Coast estuarine system.

91P.A. Macklin et al. / Marine Chemistry 167 (2014) 82–92

umol L−1 (Atkins et al., 2013; deWeys et al., 2011; Gleeson et al., 2013;Maher et al., 2013; Makings et al., 2014). Simply dividing average canalradon concentrations (Table 1) by the expected concentration in re-gional groundwater (see Peterson et al., 2010 for reasoning) impliesthat at least 3% of the canal water volume was groundwater inthe last few days (time scales of radon decay; t1/2 = 3.84 days).Mixing 3% of this groundwater with surface water with a composi-tion similar to seawater (DIC ~2100 umol L−1 and 2405 umol L−1

alkalinity) would decrease DO concentrations by only 3%, whilepCO2 would increase ~150%. This simple exercise demonstratesthat the input of small volumes of low DO, high CO2 groundwaterdisproportionally increases CO2 in surface waters relative to theDO decrease which is consistent with our observations in the GoldCoast Canals. Sampling local groundwater would be required to val-idate this interpretation.

Groundwater studies have been limited in the past as groundwaterflows may be difficult to investigate particularly in heterogeneouscoastal aquifers (Burnett et al., 2006). Radon is an effective naturalgroundwater tracer and several studies have demonstrated its applica-tion to hydrological problems. For example, a recent study by Atkinset al. (2013) in an Australian floodplain creek found radon to be signif-icantly correlatedwith pCO2 similarly to thepresentGold Coast study. Ina mangrove tidal creek located about 30 km north of the Gold Coast, aradon mass balance revealed that groundwater exchange in crab bur-rows accounted for nearly 100% of dissolved inorganic carbon exports(Maher et al., 2013). Combined to the recent literature, our investigationon the Gold Coast modified estuary reinforces a potential link betweenpCO2 distribution and groundwater discharge as traced by radon.

4.3. Implications

Estuarine CO2 emissions have been identified to be an importantcomponent of the global carbon cycle (Borges and Abril, 2011;Frankignoulle et al., 1998). CO2 emissions from estuaries worldwideare comparable to the uptake of the shelf waters despite only covering~4% of the area (Cai, 2011), while coastal wetland management isbecoming crucial for maintaining carbon burial stocks and reducingemissions from the loss of wetlands (Lawrence et al., 2012). Thereforeit is critical to quantify estuarine CO2 and evasion rates for global carbonbudgets. There are very few estimates of air-water CO2 fluxes fromsouthern hemisphere subtropical estuarine systems (Maher and Eyre,2012). To our knowledge, there are no previous assessments onthe magnitude of the air-water CO2 fluxes from estuarine canalsystems, and no studies quantifying how estuarine canals mayalter groundwater-surface water exchange. The few previous biogeo-chemical studies in canals revealed frequent eutrophication andde-oxygenation events that were suggested to be related to long waterresidence times within canals (Waltham and Connolly, 2011). Our re-sults add to the recent canal estate literature by demonstrating a relatedincrease in pCO2 in most canals investigated, and hypothesizing that apoorly investigated mechanism (i.e., groundwater discharge) was amajor driver of CO2 supersaturation.

The addition of artificial canals to estuarine carbon budgets may besignificant considering the increasing demand for waterfront living incoastal areas and the related worldwide expansion of canal estatewaters (Waltham and Connolly, 2011). We thus hypothesize thatdraining of, and canal constructionwithin, coastal wetlandsmay changethese ecosystems from net carbon sinks to net carbon sources to the at-mosphere. More comprehensive carbon budgets quantifying carbonburial rates, groundwater discharge, greenhouse gas evasion and carbonexports to the ocean are necessary to address this hypothesis.

5. Conclusions

By simultaneously measuring 222Rn and pCO2, site specific ground-water discharge hotspots (e.g. dead end canals) were located and

found to be major drivers of estuarine pCO2 distribution and contribu-tors to CO2 water-to-air fluxes. Residential estate canal systems were anet source of CO2 to the atmosphere in both dry andwet season surveyswith average CO2 evasion rates ranging from 13 to 86mmol Cm−2 d−1,respectively (Table 2). The significant correlations between CO2 and222Rn (Fig. 7 and 8) imply that inputs of carbon-rich groundwater ratherthan pelagic respiration (as traced by dissolved oxygen) were a majordriver of CO2 evasion within canals. While the highest fluxes were con-fined to the upper rivers, where the area is smallest, the canals occupieda larger area and had higher fluxes throughout. In the study area, CO2

evasion was increased by about 2 fold relative to the natural estuarinearea through the construction of canals.

Our observations are used to develop three related hypotheses:(1) Residential canals enhance groundwater seepage to surface watersand represent a new window to coastal aquifers, (2) organic carbonwhich may have previously been sequestered in coastal wetlands isnow released to the atmosphere through increased discharge ofcarbon-rich groundwater to surface waters; and (3) canals increasethe contribution of modified estuaries to regional CO2 budgets. Addi-tional studies are needed to confirm and refine these hypotheses.

Acknowledgements

Richard Boulton, Jackie Gatland, Jennifer Taylor, Christian Sanders,Luciana Sanders Mitch Call and Mahmood Sadat-Noori helped withfield investigations. A grant from the WH Gladstones Populationand Environment Fund from the Australian Academy of Sciences pro-vided funding for field investigations, while the Australian ResearchCouncil (DP120101645 and LE120100156) funded instrumentation.DTM is supported by an SCU Postdoctoral Fellowship. We thank theAssociate Editor and 2 anonymous reviewers for insightful com-ments that helped improve this manuscript.

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