Bioaccumulation of Lead and Arsenic in Gastropods

Water Air Soil Pollut (2016) 227:75 DOI 10.1007/s11270-016-2774-6

Bioaccumulation of Lead and Arsenic in Gastropods Inhabiting Salt Marsh Ponds in Coastal Bay of Fundy, Canada Amanda L. Loder & Mark L. Mallory & Ian Spooner & Christine McLauchlan & Patrick O. Englehardt & Nic McLellan & Chris White

Received: 25 October 2015 / Accepted: 31 January 2016 # Springer International Publishing Switzerland 2016

Abstract The Cumberland Marsh Region (CMR), located on the coast of the Bay of Fundy, is a major feeding ground for waterfowl and contains significant coastal wetland systems. In this study, concentrations of lead (Pb) and arsenic (As) were assessed in the bottom sediments of various open water wetlands across the CMR, and gastropods were sampled from the same wetlands to assess bioaccumulation of these nonessential trace elements and the potential for transfer to higher trophic level species. It was predicted that gastropods would have higher concentrations of Pb and As from wetlands with higher concentrations of these elements in sediments. Although wetland sediments and gastropods had elevated Pb and As concentrations, in some cases above the Canadian Sediment Quality Guidelines for the protection of aquatic life, there were no significant correlations between sediment and A. L. Loder (*) : M. L. Mallory : C. McLauchlan Department of Biology, Acadia University, 33 Westwood Avenue, Wolfville, Nova Scotia B4P 2R6, Canada e-mail: [email protected] I. Spooner : P. O. Englehardt Department of Earth and Environmental Science, Acadia University, 12 University Avenue, HSH 327, Wolfville, Nova Scotia B4P 2R6, Canada N. McLellan Ducks Unlimited Canada, Atlantic Region Office, 64 Hwy 6, P.O. Box 430, Amherst, Nova Scotia B4H 3Z5, Canada C. White Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia B3J 2T9, Canada

gastropod trace element concentrations. Gastropod to sediment ratios of Pb and As concentrations were highest in the brackish wetlands, but overall, levels were not of toxicological concern. Wetland chemistries and gastropod physiologies are hypothesized to be driving factors in determining the level to which Pb and As will bioaccumulate and merit careful consideration when developing wetland management strategies. Keywords Bioaccumulation . Lead . Arsenic . Wetlands . Gastropods

1 Introduction Coastal wetlands support complex ecosystems and an abundance of food and are preferred breeding and brood-rearing habitat for many waterfowl and migratory birds (Mallory et al. 1994; Euliss et al. 1999; Sharitz and Batzer 1999; Scott et al. 2014; Mitsch and Gosselink 2015). Although wetlands are a major source of food, non-essential trace elements can accumulate in local vegetation and soils and then be sequestered in bottom sediments that are at the sediment-water interface; these may be altered into bioavailable forms that enter and bioaccumulate in aquatic food chains and can potentially be consumed at levels that are toxic to wildlife (Simonyi-Poirier et al. 2003; Hamilton 2004; Zhou et al. 2007; Rubio-Franchini et al. 2016). Upper trophic level consumers (like waterfowl) are attracted to wetlands with high aquatic invertebrate abundance (e.g., Krapu and Reinecke 1992), particularly during the

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breeding season when birds require a significant supply of calcium, protein, and lipids (Scheuhammer et al. 1997; Euliss et al. 1999). However, if aquatic food chains contain substantial contamination by nonessential trace elements, they may pose a risk to feeding birds (Kennish 1994; Zhou et al. 2007; SchuhaimiOthman et al. 2012). The Cumberland Marsh Region (CMR), located in the upper Bay of Fundy, is a macrotidal estuarine environment influenced by some of the largest recorded tides in the world (Gordon et al. 1985; Shaw et al. 2010; Scott et al. 2014). It has been the focus of significant wildlife conservation and wetland restoration efforts in eastern Canada because of its productive waterfowl habitat comprised of salt marshes, brackish wetlands, constructed wetlands and lakes, and its long history of anthropogenic alteration (Gordon et al. 1985; Davis and Browne 1996; Scott et al. 2014). Little is known on the sequestration and accumulation of toxic metals in macrotidal mid-latitudinal coastal marshes such as the CMR, how these processes might be influenced by wetland morphology, and how anthropogenic alteration might influence wetland chemistries. Lead (Pb) and arsenic (As) are naturally occurring nonessential trace elements that bioaccumulate and can have serious repercussions on waterfowl at sufficiently high levels (Adler 1944; Hall and Fisher 1985; Fedynich et al. 2007). Elevated Pb concentrations have been recognized in both bedrock and till in the CMR, and Pb is also present in forest fire ash (Boyle 1977; White 2012). High levels of As in the Maritime provinces are commonly associated with slate, black shale, coal, and highFe sedimentary rock and have a deleterious impact on soil and water quality in Nova Scotia and New Brunswick (Klassen et al. 2009; Dummer et al. 2015). Former industrial activities (significantly gold mining) have also made As more available in the environment of certain areas (Walker et al. 2009; Meunier et al. 2010). The CMR is a traditional waterfowl hunting area, major transportation corridor, and former coal production zone. Traffic-related pollutants and particulates from emissions (particularly Pb) are capable of accumulating in topsoil near major roads and traveling long distances (Heintzman et al. 2015), and residual lead shot pellets are capable of persisting in soil for hundreds of years (Scheuhammer and Norris 1996; Hui 2002). Thus, contamination from these sources could have deleterious effects on local aquatic food chains (Franson and Pain 2011).

Water Air Soil Pollut (2016) 227:75

Although North America banned the use of lead shot pellets (for hunting near water) and leaded gasoline in on-road vehicles during the 1990s, and Pb emissions have decreased by 89 % in Canada between 1988 and 2009 (Health Canada 2013), significant levels of Pb from anthropogenic activities remain in wetlands (Kennish 1994; Hui 2002; Fedynich et al. 2007). Collectively, these conditions indicate that metals from several sources could have an impact on a variety of birds (Franson and Pain 2011). Lead and As in particular warrant investigation for bioaccumulation in the CMR. Invertebrates have been used in previous studies to assess environmental conditions in wetland ecosystems because of their linkage to water quality, hydrology, primary productivity, and migratory and resident vertebrates (Hershey et al. 1999; Sharitz and Batzer 1999). Gastropods are considered good bioindicators of metal accumulation because of their abundance, wide distribution in brackish and freshwater environments, tolerance to high metal concentrations, and ability to be easily collected (Kim and Kim 2006; Zhou et al. 2007). They may ingest metals by consuming aquatic sediment, suspended particles, detritus, organic matter, macrophytes, periphyton, other mollusks, and/or the water they live in (Rainbow 2002; Simonyi-Poirier et al. 2003; Zhou et al. 2007; Penha-Lopes et al. 2010; Bates and Hall 2011). Breeding birds require high levels of calcium to produce their eggs and to replace calcium mobilized from their bones after producing eggs; their chicks also require protein for growth (Krapu and Reinecke 1992; Scheuhammer et al. 1997). As a consequence, breeding birds often select sites where gastropods are abundant and frequently prey upon gastropods due to their highly calcareous shells (Krapu and Reinecke 1992; Scheuhammer et al. 1997). Lead and As concentrations were examined in the aquatic sediment and gastropods of CMR wetlands, and specific wetland environmental conditions that might influence non-essential trace element bioaccumulation were measured. Whole gastropods (unclean and not dissected) were analyzed to determine the concentrations of Pb and As that birds may intake while foraging upon gastropods. It was predicted that gastropods inhabiting wetlands with higher sediment concentrations of Pb and As would have relatively higher concentrations of these elements relative to gastropods inhabiting wetlands with low Pb and As.


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2 Methodology

2.2 Water and Sediment Chemistries

2.1 Study Site

Between 24 May and 2 August 2012, the following water quality parameters were measured on a weekly basis at the study sites: pH (EXTECH ExStikII pH/ Conductivity EC 500 probe), dissolved oxygen (YSI 550A DO meter), temperature, salinity (Oakton Salt 6 Acorn Series salinity meter), and total dissolved solids (TDSs; EXTECH ExStikII pH/Conductivity EC 500 probe). Modern bottom sediment was collected using a Glew Gravity Corer (60 and 6.5 cm diameter; Glew et al. 2001), which was undisturbed at the sediment-water interface and in some cases, permitted the collection of the underlying minerogenic (salt marsh or till) sediment. After collection, the sediment cores were transported to the lab and the sediment was extruded for analysis. The top 3 cm and bottom 3 cm of each core were extruded at 1-cm intervals, and duplicate samples were extracted with a modified syringe (Glew 1988; Glew et al. 2001). Samples were dried in a Cole Palmer StableTemp© oven for 24 h at 60 °C (Mallory et al. 2015). Excess sediment was stored in plastic bags and placed in a refrigerator. In preparation for elemental analyses, samples were ground with a mortar and pestle to maximize homogenization (White 2012; Mallory et al. 2015). An Innov-X X5000© X-Ray fluorescence (XRF) elemental analyzer was used to determine trace element concentrations in the bottom (gyttja) and shore (till or salt marsh) sediment. XRF analysis is a rapid, inexpensive method to effectively measure elemental trends (with acceptable accuracy). This approach has been used extensively in bedrock and soil surveys as well as paleolimnological studies (e.g., Boyle 2000; Franz et al. 2006; Liu et al. 2013; Mallory et al. 2015). The XRF unit measures 38 trace elements in samples but not all of those were relevant to our research, and for many trace elements, more than 75 % of detections were below limits of the XRF. Consequently, Pb and As were the primary focus for this study. However, there is an interference of Pb on As (Pb Lα at 10.55 keV on As Kα at 10.54 keV), but extensive studies by the Nova Scotia Department of Natural Resources on several hundred rock samples comparing certified lab results (e.g., ACME and ACT Labs) to the portable XRF machine have shown no significant difference for levels below 250 ppm. Nonetheless,

The CMR is an expanse of low land situated on the coast of the Bay of Fundy that separates the provinces of Nova Scotia and New Brunswick (Chmura et al. 2001; Hung and Chmura 2007), and the region supports four nationally protected wildlife habitat areas. The CMR is underlain by stratified Carboniferous bedrock, Wisconsin glacial deposits, and modern wetland soil. Shoreline sediment in the CMR can be either till (glacially deposited sediment of terrestrial origin) or salt marsh sediment (silts and clays derived predominantly from coastal erosion around the Bay of Fundy; White 2012). Unaltered and constructed open water wetlands overlay previously deposited salt marsh sediment, whereas lakes almost entirely overlay till (allochthonous sediment types; White 2012). Both open water wetlands and lakes are accumulating autochthonous and allochthonous bottom sediment that is primarily organic (gyttja; White 2012). Unaltered and constructed wetlands in the CMR accumulate predominantly autochthonous sediment, much of it a result of the primary production of biomass by plants and algae, whereas the lakes also accumulate significant allochthonous biogenic and clastic sediment. Bedrock exposure in the CMR is minimal and does not directly influence any of the study sites. Open water wetlands in the CMR are unsheltered and generally 0.05).

Table 2 Mean (SD) water quality measurements among the CMR sites measured between 24 May and 2 August 2012 (n = 11) Site

pH

DO (mg/L)

Temperature (°C)

Salinity (mg/L)

TDS (mg/L)

CMR1

9.2 (1.0)

7.8 (4.1)

18.2 (6.0)

1.99 (0.53)

>1990

CMR2

8.8 (0.6)

9.3 (2.7)

19.7 (4.1)

1.82 (0.60)

>1990

CMR3

6.7 (0.4)

6.1 (2.6)

19.7 (3.9)

0

126 (48)

CMR4

6.6 (0.5)

6.2 (1.9)

20.8 (4.1)

0

74 (40)

CMR5

8.3 (1.3)

9.0 (3.5)

20.0 (5.3)

0

103 (28)

CMR6

7.2 (0.5)

6.8 (2.9)

18.9 (4.4)

0

376 (52)

CMR7

7.2 (1.3)

8.3 (1.6)

19.8 (5.9)

0

247 (117)

CMR8

7.4 (0.7)

8.0 (2.6)

17.5 (4.2)

0.01 (0)

1515 (534)

CMR9

7.1 (0.5)

6.9 (2.0)

20.5 (4.5)

0

116 (32)

CMR10

7.1 (0.7)

7.1 (1.9)

19.5 (4.3)

0

438 (90)

CMR11

6.8 (0.6)

6.4 (1.3)

19.9 (3.9)

0

89 (40)

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3.2 Sediment Data Sediment cores from the unaltered and constructed open water wetlands (that overlay salt marsh) contained both gyttja and underlying salt marsh sediment indicating that the complete, post-construction sediment stratigraphy was recovered. The sediment cores retrieved from the shallow lake (that overlays till) contained 60 cm of gyttja; the underlying till was not recovered. Bottom sediment (modern) concentrations of As, Pb, Ti, V, Zr, Nb, Ba, and Rb varied among the sampling sites (Table 3). Core-top gyttja from CMR11 had the greatest concentration of Pb (52 ppm; Fig. 2), while Pb was undetected in CMR4. CMR6 had the greatest concentration of As detected in core-top gyttja (124 ppm; Fig. 2), and the brackish sites had the lowest concentrations (21 and 20 ppm). Lead concentrations were less than As concentrations, thus minimizing interference. Lead and As concentrations were most variable (87 and 48 % coefficients of variation [CV], respectively), and Ti and V varied the least (19 and 13 % CV, respectively) among the sites. Concentrations of these eight trace elements were not significantly correlated with each other, with the exceptions that Nb was higher in sediments with high As (rs11 = 0.79, p < 0.05) and Nb was also higher in sediments with high Zr (rs11 = 0.63, p < 0.05). A principal component analysis (PCA) conducted on trace elements in bottom sediment and water pH indicated that the three types of wetland systems (unaltered brackish wetlands, constructed freshwater wetlands, shallow freshwater lake) had distinct chemical

characteristics (Fig. 3). The first two factors explained 63 % of the variation in bottom sediment and water chemistries. Factor 1 was dominated by negative loadings of pH and positive loadings of As, Nb, Zr, and V; this distinguished the brackish wetlands (negative) from the constructed wetlands and the shallow lake site (positive; Fig. 3). The dominance of pH was expected because the brackish sites were saline and had the highest TDS concentrations, resulting in more alkaline water column pH than in the freshwater sites. Factor 2 was loaded positively by Rb and Ti and negatively by Pb and Ba; this further separated the lake (negative) from the constructed wetlands (positive). The shallow (freshwater) lake is underlain by till, and thus, trace element concentrations are expected to be significantly different from concentrations in the wetlands underlain by salt marsh sediment that is primarily derived from marine sources. 3.3 Gastropod Data Concentrations of As, Pb, Ti, V, Zr, Nb, Ba, and Rb in gastropods also varied among the study sites (Table 3). Lead was detected in gastropods from the lake and unaltered brackish wetlands but was not detected in gastropods from the constructed freshwater wetlands (Fig. 2). The ratio of Pbgastropod:Pbsediment was greatest in the unaltered brackish wetlands (0.16 and 0.25) compared to the lake (0.038). Arsenic was detected in gastropods from all CMR sites except CMR3 and CMR4. The ratios of

Table 3 Maximum concentrations (ppm) of As, Pb, Ti, V, Zr, Nb, Ba, and Rb detected in gastropods and core-top sediment among the CMR sites CMR1

CMR2

CMR3

CMR4

CMR5

CMR6

CMR7

CMR8

CMR9

CMR10

CMR11

As gastropods

17

18

ND

ND

2

10

26

11

15

15

22

As sediment

21

20

37

92

90

124

48

71

73

79

71

Pb gastropods

3

4

ND

ND

ND

ND

ND

ND

ND

ND

2

Pb sediment

19

16

5

ND

14

14

28

20

1

14

52

Ti gastropods

239

272

108

203

226

129

218

214

463

381

442 4463

Ti sediment

5139

4532

2570

6381

5640

4822

4787

4773

5467

4982

V gastropods

7

8

8

5

6

15

9

5

10

6

35

V sediment

100

89

77

109

98

118

95

88

95

99

122

Zr gastropods

23

33

10

ND

33

7

8

16

32

22

22

Zr sediment

313

232

206

469

591

352

449

357

467

269

587

Nb gastropods

5

6

14

26

15

6

7

4

10

10

20

Nb sediment

24

25

38

46

44

46

41

34

41

33

39


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Fig. 2 Lead and As concentrations (ppm) detected during XRF analysis, using core-top sediment and an amalgamation of gastropods from each study site. CMR7 and CMR10 have duplicate

samples, and CMR1 and CMR7 were analyzed twice with the XRF spectrometer. CMR2 has a duplicate sample, which was analyzed twice with the XRF spectrometer

Asgastropod:Assediment were greatest in the unaltered brackish wetlands (0.81 and 0.90). Maximum As concentrations were significantly greater in gastropods from four newly constructed freshwater wetlands (16.8 ± 6.4 ppm) than in four older (at least 30 years of age) constructed freshwater wetlands (3.5 ± 4.3 ppm; Mann– Whitney U = 16, p = 0.03). Lead interference was likely negligible in the gastropods as Pb concentrations were not detected in the newly constructed wetlands, and Pb concentrations were at the detection limit and less than As concentrations in the brackish ponds and freshwater lake. Among trace elements tested in gastropods, Ba and Pb concentrations were most variable (31 and 87 % CV, respectively) and Ti was least variable (19 % CV). Titanium and V concentrations did not vary greatly in gastropods across the Cumberland Marsh Region, with the exception of V in gastropods from CMR11. Concentrations in gastropods were about ×10 lower than concentrations in the sediment. There were generally no strong correlations between trace element concentrations in the sediment and gastropods, with the exception that gastropods had significantly higher As in sites with high sediment Pb (rs = 0.757), and conversely, gastropods had high Pb in sites with low As in the sediment (rs = −0.640; both p < 0.05). Thus, Pb uptake by gastropods is not predicted well by the sediment concentration of Pb, and As uptake by gastropods is not predicted well by the sediment concentration of As.

The PCA of trace elements in gastropod and water pH differentiated the brackish sites from the constructed wetlands from the lake (Fig. 3). The first two factors explained 68 % of the variation in wetland chemistries. Factor 1 was dominated by negative loadings of pH and Rb and positive loadings of Nb; like the sediment chemistries, this separated the brackish wetlands (negative) from the lake and constructed wetlands (positive). Factor 2 was loaded negatively by V, Ba, Ti, and As and essentially separated the lake gastropods (negative) from the gastropods sampled from brackish and constructed wetlands (positive).

4 Discussion Waterfowl use wetlands, such as those in the CMR, as preferred breeding and brood-rearing habitat and for staging during migration (Mallory et al. 1994; Euliss et al. 1999). At these locations, they consume both plant and animal matter to meet their energy and nutrient requirements (Krapu and Reinecke 1992). Aquatic macroinvertebrates (significantly mollusks) are an important food source, particularly in the breeding season, because females require calcium for eggshell production and protein for egg formation (Krapu and Reinecke 1992; Scheuhammer et al. 1997). However, if those macroinvertebrates and their aquatic ecosystems are contaminated, then they can potentially pose a risk to the health of feeding birds.

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Fig. 3 Principal component analyses of As, Pb, Ti, V, Zr, Nb, Ba, and Rb in bottom sediment and gastropods. Upper panels represent loadings of individual trace elements on PCA scores, and lower panels represent loadings of individual wetland sediment

chemistry or gastropods relative to others from the region. PCA effectively separated sediment chemistry and gastropod chemistry from unaltered brackish wetlands, constructed freshwater wetlands, and a shallow freshwater lake

Concentrations of trace elements in the shallow (freshwater) lake, underlain by till, were expected to be significantly different from concentrations in the wetlands underlain by salt marsh sediment (primarily derived from marine sources). Based on the results from the CMR, it was suggested that constructing wetlands in salt marsh substrate will likely reduce access to Pb as concentrations were significantly lower in comparison to substrate from lake till. It was observed that bottom sediment concentrations of Pb for all sites (average 16.6 ppm) were under the probable effect level (PEL; 91.3 ppm for freshwater sites and

112 ppm for marine/estuarine sites) according to the Canadian Council of Ministers of the Environment (1999a, b). Concentrations of As in bottom sediment were under the PEL in the brackish wetlands (average sediment concentrations 20.5 ppm and PEL 41.6 ppm marine/estuaries) but exceeded the PEL in the constructed wetlands and lake (average sediment concentrations 76.1 ppm and PEL 17.0 ppm for freshwater sites). However, the potential for Pb and As bioaccumulation may be reduced regardless of bottom sediment concentrations, if Pb and As are in a state that is unavailable for bioaccumulation and biomagnification.


Gastropods with greater concentrations of Pb and As in their tissues did not necessarily come from wetlands with higher levels of these elements in bottom sediment, and Pb and As were not detected in many gastropod samples where it was detected in bottom sediment. Even though whole gastropods were used in this study, these observations suggest that the majority of sample used in the XRF analyses of the gastropods contained minimal sediment and debris. However, the relatively consistent concentrations of Ti and V in gastropods from all study sites suggest that the gastropods were ingesting some sediment and likely had similar amounts in their digestive tract and on their shell (Duan et al. 2013). Therefore, the apparent bioaccumulation and retention of varying levels of Pb and As in gastropod tissues was not likely related to sediment accumulation on the samples or ingestion by gastropods. Collectively, these results support the interpretation that gastropods from the brackish wetlands, constructed freshwater wetlands, and freshwater lake were differentiated by water and/or sediment chemistries in their environment. Despite the proximity of highways, local industrial activity, and urban and rural development to the study sites, it was observed that Pb and As concentrations detected in gastropods were not at levels considered a toxic risk to birds (Adler 1944; Stanley et al. 1994; Franson and Pain 2011). Nonetheless, Pb and As detections and trends indicated that accumulation is occurring in the CMR wetlands, and waterfowl and other birds may be ingesting and retaining these deleterious trace elements during foraging. Unexpectedly, it was observed that newly constructed wetlands may be more efficient than older constructed wetlands at biotransferring As because concentrations in gastropods from the constructed freshwater wetlands significantly increased with a decrease in site age. To the best of the authors’ knowledge, there has been no research on bioaccumulation trends of As in newly constructed wetlands; therefore, this observation must be considered preliminary. Clearly, additional research is required to examine the effects of wetland chemistry, the effects of varying habitat conditions on gastropods, and varying metabolic characteristics of different gastropod species in determining factors influencing bioaccumulation (Gupta 1997; Rainbow 2002). Environmental conditions in the CMR wetlands appear to promote bioavailability of As, as it was detected in most gastropod samples, but restrict bioavailability of Pb in the constructed wetlands; thus, As is of primary

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concern for accumulation in waterfowl and birds. The dominant influence of pH as a factor was expected because the brackish sites were saline and had the highest TDS concentrations, resulting in more alkaline water column pH than in the freshwater sites. Water chemistry strongly influences the mobility of trace elements in aquatic systems and is likely differentiating the species and availabilities of Pb and As in the CMR wetlands despite their close proximity (Scheuhammer et al. 1997). Parameters including pH and alkalinity, dissolved oxygen, temperature, salinity, and water hardness influence all bioavailability of non-essential trace elements when they are within the optimal range to trigger metal releases from the bottom sediment into the sediment-water interface (Gupta 1997; Hung and Chmura 2007; Zhou et al. 2007) but also may directly affect metal accumulation rates and absorption in gastropod tissues (Albers and Camardese 1993; Weis and Weis 2004; Penha-Lopes et al. 2010; Charriau et al. 2011). Thus, adverse effects are possible if there are significant levels of Pb and As in these wetlands, and demonstrated environmental conditions and biomagnification vectors exist that are conducive to uptake and accumulation in waterfowl. It was observed that gastropod to sediment ratios of both Pb and As in the brackish wetlands were greater than those in the freshwater sites; thus, the brackish wetlands appear to have greater potential for transfer to foraging waterfowl. The CMR brackish wetlands are saline and contained the highest TDS concentrations. These characteristics can affect dissolved metal speciation and partitioning and produce complexes that are available for accumulation (Speelmans et al. 2007; Laing et al. 2008, 2009). The CMR brackish wetlands also experience greater sediment mobility than the freshwater sites because they are tidally inundated. Tidal inundation events may create turbidity and/or alter the redox potential in the bottom sediment and release sediment particles into the sediment-water interface where they could alter media and/or become more available for gastropods to ingest (Speelmans et al. 2007). Thus, these trace elements may be less available in constructed freshwater wetlands as they absorb to bottom sediment in static environments and should be considered in management practices (Laing et al. 2009). Studies have shown that various uptake; accumulation; detoxification and excretion mechanisms; and physiological attributes including life spans, body sizes, and feeding habits may affect Pb and As concentrations in various gastropod species (Karouna-Renier and

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Sparling 2001; Rainbow 2002). This may affect gastropod concentrations among all CMR wetlands but particularly, the differentiation between brackish and freshwater sites because different taxa were sampled from each. Additional sampling and taxon-specific analyses may help refine our interpretation of which gastropod and wetland features influence Pb and As concentrations (Catsiki et al. 1994; Martin 1999; Karouna-Renier and Sparling 2001). Lead and As concentrations were detected in whole gastropods from the CMR but were not at levels toxic to birds and did not correlate strongly to metal concentrations in bottom sediment. Differential wetland chemistry and/or gastropod metabolic processes likely regulate Pb and As uptake and accumulation in gastropods, despite bottom sediment concentrations and wetland proximity, and merit consideration when constructing wetlands. Importantly, our results demonstrate that upper trophic level predators, such as waterfowl, may forage in managed wetlands within a few hundred meters of each other and be exposed to markedly different trace element concentrations from similar prey. Environmental conditions in the CMR wetlands appear to promote bioavailability of As but restrict Pb. Higher gastropod to sediment ratios of Pb and As in the brackish sites, compared to the freshwater sites, may be a result of greater conductivity and/or redox reactions. Further research is required to determine the chemical forms of Pb and As and mechanisms regulating their transfer in various wetlands and develop management practices that limit metal availability. Acknowledgments Financial support for this project was provided by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chairs program, Ducks Unlimited Canada, and Acadia University.

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