Environ. Sci. Technol. 2000, 34, 3878-3884
Bioaccumulation and Diminution of Arsenic and Lead in a Freshwater Food Web
human health hazard (2-5). Yet, elevated concentrations in water have also been measured in lakes where the fish have relatively low burdens. The lack of a consistent relationship between metals in water and fish is partly due to the way that different metals are transferred trophically through food webs (e.g., whether they biomagnify or biodiminish).
CELIA Y. CHEN* AND CAROL L. FOLT Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire, 03755
Trophic transfer rates of metals may change between metals as well as between seasons due to changes in the taxonomic composition of different trophic levels and in the physical and chemical conditions. Therefore, differences in metal burdens of taxa in different lakes (even with similar aqueous levels) may be partly due to temporal differences in field conditions among sites. Variation in mercury has been measured seasonally in a single trophic level (7-10), yet few studies have examined the temporal differences of metal transfer among several trophic levels, particularly for metals other than Hg (11). This information is fundamental to understanding metal uptake processes under natural conditions.
This study provides strong evidence for biotic accumulation of two metals in a contaminated watershed and diminution of both metals from lower trophic levels to fish. Bioaccumulation of As and Pb in water and four food web components (particulates, two size fractions of zooplankton, and six species of fish) were measured on three dates in Upper Mystic Lake (UML), MA, which is located in the Ascontaminated Aberjona Watershed. Arsenic and Pb levels in small and large plankton and fish biodiminished with increasing trophic level, but only As was elevated in lower trophic levels relative to uncontaminated food webs. Metal levels in water and biota differed by date and were lowest in the spring and, in most cases, highest in summer samples. Variation in metal accumulation in zooplankton across dates may be due to changes in metal concentrations in the aqueous and particulate phase over time. Metal burdens in fish with different feeding strategies were also compared. We found the highest As in planktivorous species that feed directly on the metalenriched zooplankton, but no differences were observed for Pb concentrations between fish groups. Finally, we compared the levels of As and Pb in food web components in UML relative to 20 uncontaminated lakes in New England and found that As levels but not Pb in particulates and zooplankton were higher in UML. This provides the first evidence that As contamination in the Aberjona Watershed is being transferred to the biota at lower trophic levels. Nevertheless, despite elevated As in zooplankton, pronounced diminution between zooplankton and fish in UML appears to result in concentrations of As in fish that do not differ from uncontaminated systems.
Introduction Recent studies demonstrating high levels of metals such as Hg in fish from seemingly pristine lakes have focused attention on the widespread exposure of humans and wildlife to metals via consumption of fish (1-5). These findings have identified a need to determine the variables that could be most effective in predicting metal burdens in taxa in a range of aquatic ecosystems. The extent to which metals are bioaccumulated by different organisms depends on their exposure to metals via food and water. However, it appears that aqueous concentrations of metals in lakes are often not good predictors of metal concentrations in aquatic organisms (6). For example, fish from lakes with metal levels below detection in water can carry metal burdens that present a * Corresponding author e-mail:
[email protected]; phone: (603)646-2376; fax: (603)646-1347. 3878
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In this study, we compared the trophic transfer of two metals, As and Pb, on three dates in a metal-contaminated lake. We investigated As and Pb because they exhibit similar biogeochemical characteristics and pathways of trophic transfer (6). Our prior work showed that lakes containing elevated concentrations of As and Pb tended to be nutrient enriched and located in agricultural watersheds (6). Furthermore, both As and Pb have been shown to biodiminish in some systems (i.e., the mass-specific body burden decreases at higher trophic levels). For As, this information is largely derived from studies on marine organisms (1214), while the diminution of Pb has been studied in streams and marine systems (14-16). Neither have been examined in particulates (algae), zooplankton, and fish in intact freshwater food webs. Seasonal differences for either metal in freshwater food webs also have not been addressed previously. Our investigation took place in Upper Mystic Lake (UML), which is located in the Aberjona Watershed, MA. Past leather and chemical manufacturing upstream left an area of metal and organic contamination severe enough to warrant its designation as a U.S. Superfund site (17, 18). Despite the migration of contaminants from the Superfund site to UML, it is a recreational lake used for swimming, fishing, and other activities. Fish are known to be an important exposure pathway of metals to humans, so it is essential to determine whether plankton and fish from lakes with high levels of As or Pb contamination carry unusually high metal burdens. However, both metals are known to biodiminish as they move up the food chain, so the composition of the food web could influence the amount of metal carried by particular taxa. Moreover, even within particular trophic groups (i.e., among the zooplankton or among the fish) animals with different foraging behaviors are expected to have different burdens. For instance, for a metal that biodiminishes, animals at the top of the food chain are predicted to carry lower burdens than animals at the bottom (e.g., predators < prey; piscivores < planktivores). To understand the trophic transfer of metals to fish in UML, it is necessary to investigate metal burdens at the lower trophic levels (water to particulates to zooplankton). To explore metal movement among trophic levels, we measured metal burdens in four food web components: particulates, two size fractions of zooplankton (three dates), and six species of fish (fall only). Fish included planktivores, omnivores, and piscivores. Our hypotheses were as follows: 10.1021/es991070c CCC: $19.00
2000 American Chemical Society Published on Web 08/17/2000
(1) As and Pb biodiminish with increasing trophic levels (i.e., metal levels in small plankton > macrozooplankton > fish). (2) As and Pb burdens in water and the planktonic food web differ in spring, summer, and fall. (3) As and Pb burdens in fish vary with feeding strategy (piscivores < planktivores). (4) Heavy metal contamination in Upper Mystic Lake has resulted in increased metal burdens in the biota.
Methods Site Description. UML lies approximately 10 km downstream of the main areas of contamination in the Aberjona Watershed. It is a 51-ha dimictic, eutrophic kettle hole lake (19). Metal contaminants including As, Pb, Cd, Cu, Zn, and Cr have migrated downstream to UML and beyond to Lower Mystic Lake (LML) (18). Arsenic deposition from contaminant transport has been found in the sediments of both UML and LML (18); it is estimated that 10 t of As has been transported to Upper Mystic Lake alone (17). Sediment cores from the deepest part of the lake show As levels well above natural background with concentrations as high as 2000 mg/kg dry weight (DW) (18). The As peaks in the sediment record of UML correspond to periods of arsenical pesticide and sulfuric acid manufacturing from 1890 to 1930 and later remobilization of the previously deposited contaminants in the 1960s (18). Seasonal and year-to-year variation in arsenite and arsenate in the water column have been measured and depend on the redox conditions and stratification of the lake (19). Pb contamination in the Aberjona Watershed derived mainly from the leather tanning industry and the manufacture of lead arsenate. Early combustion of fossil fuels and use of tetraethyl lead as a gasoline additive also resulted in lead deposition to UML sediments (18). Unlike As, Pb speciation and mobility in the watershed has not been extensively investigated. Water, Particulates, and Zooplankton. With respect to these ecosystem components in UML, our objectives were to determine whether metals biodiminished from lower to upper trophic levels and to determine whether these patterns were consistent across sampling dates representative of different seasons for both As and Pb. To accomplish these goals, metal concentrations in water, particulates (0.4-45 µm), and two size fractions of zooplankton (45-202 µm and >202 µm) were measured in Upper Mystic Lake in June, August, and October 1997. Environmental variables were also measured (dissolved oxygen (DO), temperature, pH), and phytoplankton and zooplankton taxonomy were identified. Throughout the remainder of the text, metal concentrations in biota will also be referred to as mass-specific concentrations and metal burdens. All water, particulate, and zooplankton samples were taken from a fiberglass rowboat using trace metal clean technique and nonmetallic sampling gear. Prior to field sampling, Teflon storage containers were acid-cleaned in sequential concentrated nitric acid, dilute HCl, and trace metal grade dilute nitric acid baths. Field sampling was conducted with great care to minimize contamination using previously established protocols (20). Dissolved aqueous samples were taken using a peristaltic pump to draw water through acid-cleaned LDPE tubing from 1 m depth. The water samples were then filtered through acid-cleaned 0.45 µm Gelman filters. A blank aqueous sample was also taken at each location using ultra clean water pumped and filtered through the same apparatus. Aqueous samples were acidified in the field to pH 1 with ultrapure nitric acid and analyzed directly without further digestion. Particulate samples were collected by passing water through 45 µm mesh and then filtering a known volume of whole water sample onto 0.45 µm Teflon filters. Biovolumes of phytoplankton were calculated from cell counts and
measurements of cells sizes in taxonomy samples. Metal concentrations in the particulates (referred to as mass-specific concentrations) were therefore reported as nanogram of As or Pb per biovolume (mm3) of phytoplankton. Zooplankton were collected with vertical tows in the deepest portion of the lake (approximately 20 m depth) from 0.5 m above of the bottom to the surface using a cone net (202 µm nylon mesh) for macrozooplankton (>202 µm) and a Wisconsin net (45 µm nylon mesh) for smaller plankton (45-202 µm). The 45-202 µm fraction was additionally filtered through a 202 µm mesh filter to remove the larger organisms. Tows that were dragged through the anoxic zone at the bottom were not collected if they contained any bottom sediment. Zooplankton taxonomy samples were anaesthetized for 1-3 min in CO2-charged water and preserved in buffered formalin sucrose (21). Taxonomic groups found in the 45-202 µm fraction included phytoplankton, rotifers, and small crustaceans, and macrozooplankton (>202 µm) included cladocerans and calanoid and cyclopoid copepods. Metal concentrations per biomass of zooplankton (µg of metal/g of zooplankton dry weight) were calculated from separate metal samples and biomass samples because the weight could not be measured directly without contaminating the metal samples. Zooplankton biomass was estimated from three replicate tows, which were filtered onto preweighed Whatman filters, dried at 60 °C, and weighed. Zooplankton metal samples were filtered in situ onto acid-cleaned Teflon filters, and filter blanks were taken by filtering ultra clean water through the same-sized Teflon filters. Particulate and zooplankton metal samples were digested for analysis with an aqua regia solution (2:1 Seastar nitric acid and hydrochloric acid) and heated to 70 °C for 8-10 h. After digestion, Teflon filters were removed, and the sample was centrifuged to separate undigested material from the metal in solution. Finally, the sample was split into glass and Teflon vials for analysis of As (glass) and Pb (Teflon) and stored at 4 °C. Fish. Our objectives with respect to fish were to compare the metal burdens in fish with different feeding strategies and to determine whether metal burdens in fish biodiminished from metal burdens in plankton. Fish were collected in October at multiple sites in the littoral zone of the lake using seines, fyke nets, and minnow traps. Five individuals were obtained for each of six species of fish (alewife, bluegill sunfish, black crappie, killifish, largemouth bass, and yellow perch), and fish lengths and weights were measured. In the field, fish were handled using procedures to minimize contamination and were transported back to the lab on dry ice where they were kept frozen at -20 °C until digestion and analysis. Each replicate fish was thawed and ground individually in a Cuisinart with ultra clean water. Three 1.0-mL subsamples from each fish were measured into separate Teflon vials. The Cuisinart was cleaned thoroughly with Citranox and ultra clean water between fish samples. Samples were digested in aqua regia solution using procedures as described above for zooplankton samples. Metal Analysis. All metal samples were analyzed in the Dartmouth Superfund Trace Metal Core Facility using a magnetic sector inductively coupled plasma-mass spectrometer (ICP-MS ELEMENT, Finnigan MAT). Pb was determined with a standard liquid sample introduction system (microconcentric nebulizer MCN-2, CETAC, and cooled Scott-type spray chamber). Relative determination limits (10 s blank ( SD) were 0.005-0.01 µg/L for Pb. Arsenic was analyzed using cold vapor/hydride generation ICP-MS (22) with determination limits of 0.3-0.5 ng/L (10 s blank ( SD). All samples were quantified with matrix-matched NISTtraceable standard solutions (HPS, Charleston; VHG, Manchester). External quality control was achieved by digesting and analyzing identical amounts of rehydrated (90% water) standard reference materials (DORM-2, NRC-CNRC VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Canada and Prawn CRM, China). Recovery rates ranged from 95 to 100% for Pb and from 85 to 90% for As. The lower recovery rates for As with hydride generation can be explained by incomplete mineralization of the dominant arsenobetaine component in the reference materials, which was expected to be negligible for the zooplankton samples. Redigestion of the reference materials with perchloric acid at 120 °C for 4 h yielded recovery rates of 95-100%. Zooplankton subsamples were also redigested and reanalyzed to ensure that all the As was extracted, but no significant differences from the initial analysis in As concentrations were observed (6). Data Analysis. Statistical analyses were conducted to compare seasonal and species differences in metal concentrations of zooplankton and fish (µg of metal/g of animal dry weight). Most measured variables were log (x + 1) transformed to correct for nonnormal distributions of the data and unequal variance and to adjust for 0 values. Two-way ANOVAs were calculated to compare metal concentrations by season and size fraction and to determine interactions. One-way ANOVA was used to compare the As and Pb burdens in different species of fish. T-tests were used to compare metal burdens in UML to uncontaminated sites (see below). All analyses were conducted using the statistical program JMP (23). Comparison with “Uncontaminated” Lakes. Our final objective was to compare the As and Pb burdens of organisms in the presumably contaminated UML to lakes in New England considered to be uncontaminated by direct human activity. To do this, we took advantage of a data set we have been gathering as part of the NIEHS-Superfund Program (6). In this program, we sampled particulate and zooplankton concentrations in 20 lakes throughout New England, and fish metal data for those lakes were obtained from the U.S. EPA EMAP-SW Program (6, 24). Methods for sampling and analysis were the same as presented above, except that survey lakes were sampled one time in summer or fall of 1995 or 1996 (one lake was sampled in both years). We compared the average burden of As and Pb in particulate, zooplankton, and fish from the survey lakes (hereafter, NIEHS lakes) to the metal concentrations in UML from the months of August and October which were most comparable to the sampling time of the NIEHS lakes.
Results and Discussion Our results demonstrate that not only is there biotic accumulation of metal contaminants in the UML food web but also that trophic transfer processes appear to have an important role in determining the burden of metals that bioaccumulate in fish. Before testing the hypotheses however, we confirmed that UML was contaminated with As. Our total aqueous concentrations of As (0.6-1.2 µg/kg) were comparable to those measured previously (19). Moreover, based on our comparisons of water chemistry in UML with the more pristine NIEHS lakes, we determined that As but not Pb was elevated. Arsenic levels in summer were well above the maximum concentrations (0.59 µg/L) measured in uncontaminated NIEHS lakes (6), whereas Pb concentrations were comparable to concentrations in the NIEHS lakes (maximum levels ) 0.060 µg/L). The high aqueous levels of total As we measured in UML relative to NIEHS lakes confirm previous findings that dissolved As is elevated above pristine levels in the water column (19). This As contamination is probably due to industrial As sources upstream and the remobilization of As from the sediments of UML (17-19). Tests of the four hypotheses and the implications of our findings are discussed below: Hypothesis 1: As and Pb Biodiminish with Increasing Trophic Levels (i.e., Metal Levels in Small Plankton > Macrozooplankton > Fish). The evidence was compelling for diminution of both metals from lower trophic levels to 3880
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FIGURE 1. Comparison of uncontaminated lakes (NIEHS survey lakes) to UML in summer and fall: (a) Arsenic concentrations (µg/g dry weight ( SE) in small (45-202 µm) and large (>202 µm) zooplankton size fractions and in fish (average of six species). (b) Pb concentrations (µg/g dry weight ( SE) in small (45-202 µm) and large (>202 µm) zooplankton size fractions and in fish (average of six species). fish in UML, i.e., small plankton (45-202 µm) < macrozooplankton (>202 µm) < fish (Figure 1a,b). Although As and Pb burdens were detectable in all species of fish, the burdens for all fish were 10-20 times lower than burdens in zooplankton. Similarly, as expected for diminution, As and Pb burdens in the larger planktonic fraction (>202 µm) were considerably less than in the small fraction (45-202 µm; Figure 2a,b) in both summer and fall. Our findings in UML and in the NIEHS lakes (6; Figure 1a,b), are the first to demonstrate diminution of As in lake ecosystems. Diminution of As has only been reported previously in marine food webs and in simple laboratory food chains (25-28). Total As in laboratory studies biodiminishes with increasing trophic level, but methylated forms of As, which are less toxic, increase (26-28). In marine systems, an intriguing pattern has been observed where As biomagnified at the highest trophic levels, despite diminution at lower levels (14). To test for this effect in UML would require examining As in older, larger piscivorous fish. Pb concentrations have also been shown to biodiminish in marine and freshwater stream organisms (14, 15). Hypothesis 2: As and Pb Burdens in Water and the Planktonic Food Web Differ in Spring, Summer, and Fall. Water Chemistry. Our results show that aqueous As but not Pb concentrations differed among sampling dates in UML even though there were no major changes in oxygen and temperature stratification on the same dates (Figure 3a). Surface temperatures and DO measurements ranged from a maximum of 27 °C and 10 mg/L to minimum values at depth of 5.6 °C and 0.06 mg/L across the three dates. Hypolimnetic DO was extremely low over all dates as measured in past studies of UML (19, 29). Nevertheless, concentrations of dissolved As in the epilimnion increased on the summer sampling date. Past studies have shown that the concentration and speciation of As in UML is dependent on redox conditions in the hypolimnion and surface sediments where there is greater release of As with increased
FIGURE 2. (a) Arsenic concentrations (µg/g dry weight ( SE) across sampling dates in small (45-202 µm) and large (>202 µm) zooplankton size fractions. (b) Pb concentrations (µg/g dry weight ( SE) across seasons in small (45-202 µm) and large (>202 µm) zooplankton size fractions.
FIGURE 3. (a) Aqueous As concentrations across sampling dates (µg/kg). (b) Mean As and Pb concentrations (ng/mm3 ( SE) in the particulate fraction (0.4-45 µm) across sampling dates. anoxia (19, 29). Those studies found the highest levels of aqueous As(III) and As(V) occurring in the late fall, but there was also significant year-to-year variation in As speciation and concentration (29). The difference in timing of elevated aqueous As between our results and those of previous UML studies may be due to interannual variation and/or our focus on the epilimnion rather than the entire water column for aqueous and particulate samples. However, similar to our
findings, studies of other aquatic systems have measured maximum concentrations of aqueous As in the summer (30, 31). In this study, aqueous Pb concentrations were extremely low (0.046 µg/L) in spring and lower than our sample blanks in summer and fall. Particulates. This study provides strong evidence for temporal differences in the bioaccumulation of both As and Pb in phytoplankton. Mass-specific As and Pb concentrations in the phytoplankton (i.e., metal concentrations in the particulate fraction) were highest on spring and summer sampling dates and lowest in the fall (Figure 3b). In contrast, the particulate biomass was lowest in the spring and summer and highest in the fall. There were no major temporal changes in phytoplankton species composition, with the phytoplankton assemblage dominated by Chrysophytes in all three seasons (Figure 4). However, there were temporal differences in the phytoplankton size distribution. The assemblage comprised smaller cells in the summer relative to spring or fall, due to greater relative abundances of small microflagellates. There are several different mechanisms involving interactions between the metals in the aqueous and particulate phases that could drive temporal differences in particulate metal concentrations. Three likely mechanisms are changes (1) in aqueous concentrations that directly affect phytoplankton burdens, (2) in the total abundance or biomass of phytoplankton that result in different levels of metal per cell, and (3) in the size distributions of the phytoplankton to species/sizes with relatively greater or lesser mass-specific metal burdens. Our data could be explained at least in part by each of these mechanisms on some dates. For example, as expected (mechanism 1) the greatest mass-specific As concentrations in phytoplankton were measured in the summer when dissolved As levels were also greatest. A similar relationship was observed in the hypolimnion of Canadian Shield Lakes where Hg increased in phytoplankton as water concentrations increased (10). However, low dissolved levels of As in the spring were also associated with high massspecific levels, which indicates that a different mechanism might also be operating at this time. Aqueous data from all three sampling dates are not available for us to evaluate this mechanism for Pb. Changes in phytoplankton density may also be associated with changes in mass-specific As and Pb burdens in phytoplankton (mechanism 2). Most commonly, “biodilution” of contaminants in plankton has been observed, where an increase in phytoplankton abundance results in a reduction in the concentration of contaminant per cell (32-34). This phenomenon has been observed most often with organic contaminants and less so with metals. As predicted by biodilution, the mass-specific metal levels of As and Pb in the phytoplankton of UML were lowest in fall (relative to spring and summer) when total biomass of phytoplankton was greatest. An increase in phytoplankton density has also been shown to result in an decrease in aqueous metal concentrations in some cases. For example, Cd, Ni, and Zn were found to decline during a phytoplankton bloom in San Francisco Bay (35). This process may explain at least in part our observation in UML in the fall that high phytoplankton densities were associated with low aqueous As levels. Finally, changes in the size distribution of phytoplankton could also drive changes in phytoplankton mass-specific metal concentration (mechanism 3). For example, smaller algal cells have higher bioconcentration factors for metals than larger cells due to their higher surface:volume ratios (36). Hence, the small microflagellates that dominated the summer sample in UML may have had greater metal uptake per mass of individual cell resulting in overall higher mass-specific metal burdens in the particulate fraction on that date. In summary, all three of these mechanisms could explain temporal differences in metal burdens in the particulates. VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Species composition and biomass of phytoplankton (as biovolume × 100 µm3) across sampling dates. Taken together, it appears in UML that a combination of changes in cell size and density could have driven most of the observed differences in mass-specific levels of As and Pb in the phytoplankton on the three sampling dates. However, a more detailed investigation of the seasonal dynamics of phytoplankton densities and metal uptake of different phytoplankton taxa and morphologies is needed to elucidate the relative importance of these mechanisms. Zooplankton. We also found strong evidence for temporal differences in the concentration of both metals in the two zooplankton fractions (Figure 2a,b). Metal concentrations in both size fractions were lowest in the spring and increased in summer. However, the temporal patterns for the two metals in summer and fall differed somewhat. Arsenic in the small fraction declined from summer to fall but remained constant in the large fraction. Two-way ANOVAs comparing As concentrations in the two plankton size fractions across dates confirmed that there were significant differences between dates (p ) 0.0003) and an interaction between date and size fraction (p ) 0.0005). In contrast, Pb in both size groups increased in summer and remained high in the fall. For Pb burdens in zooplankton, there were significant effects of date (p < 0.0001) and size fraction (p ) 0.0496) but no interaction. The species composition of the smaller zooplankton fraction also differed between sampling dates. Several summer dominant taxa including Ascomorpha, Gastropus, and Hexartha were not present in the spring and declined in the fall. In contrast, the community composition of the macrozooplankton remained fairly constant over time and was dominated by the same species of cladocerans (Daphnia galeata mendota, Daphnia ambigua, Bosmina longirostris) and cyclopoid copepods (e.g., copepodites, Mesocyclops edax) from the spring through the fall. The densities of small and large zooplankton also differed over dates with the highest abundances of both size fractions present in the summer. Densities of small zooplankton in the summer were slightly greater than densities in the spring and fall (∼20%) whereas macrozoopankton densities in summer were nearly twice as high as densities in the spring and fall. Temporal differences in metal bioaccumulation of zooplankton could result from several mechanisms, such as changes (i) in food composition that alter ingestion, exposure, and/or assimilation of metal; (ii) in the taxonomic composition of the zooplankton community to species with relatively greater or lesser metal burdens; or (iii) in aqueous concentrations that directly affect zooplankton burdens. Our data suggest that changes in the availability of high-quality food were not responsible for the observed seasonality of As or 3882
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Pb (mechanism 1). There was an abundance of high-quality food comprising diatoms, dinoflagellates, and cryptomonads throughout the three sampling months. Moreover, the composition of the phytoplankton community remained fairly constant during the sampling period, although dinoflagellates and cryptomonads did increase slightly in the fall. However, ingestion of the phytoplankton containing higher metal burdens in the summer could explain the increase in zooplankton metal burdens at that time. Zooplankton were likely to have ingested a higher concentration of metal per biomass of phytoplankton in the summer when smaller phytoplankton cells were more abundant and massspecific burdens of As and Pb were high. In the spring, lower metal concentrations in larger cells (colonial diatoms, Crytpomonas, Ceratium, Peridinium) may have contributed to the lower macrozooplankton concentrations of As and Pb. However, in fall, low mass-specific metal concentrations in algae appeared to affect only As in macrozooplankton, which was also low. In contrast, Pb burdens in macrozooplankton remained high at that time. The mechanisms controlling Pb concentrations in macrozooplankton may be less related to their uptake from food. The community composition of the microzooplankton also differed in the summer and fall (mechanism 2). Summer taxa included more illoricate species that have a thin flexible body wall, whereas fall taxa were mostly loricate. It is possible that loricate taxa whose body walls are more rigid and thickened have poorer metal absorption properties. Organism size is also known to determine Pb concentrations in small plankton (37). Changes in the taxonomic composition of the small zooplankton fraction resulting in changes in membrane characteristics or size frequency distributions could affect adsorption of either metal and retention in this trophic level. However, greater knowledge of bioaccumulation of metal by microzooplankton is needed to fully investigate this mechanism. Direct uptake of metal from water by zooplankton (mechanism 3) also may have contributed to seasonal changes in zooplankton metal concentration. Aqueous concentrations of As have been shown to be predictive of macrozooplankton concentrations (6), and zooplankton are known to take up other metals directly from water (38, 39). In UML, both aqueous and zooplankton concentrations of As were elevated in summer and lower in spring and fall. This result suggests that zooplankton may take As directly from water. In contrast, aqueous Pb concentrations have been shown to be unrelated to zooplankton burdens (6); therefore, direct uptake from water may not be an important uptake pathway for Pb in
UML. Moreover, we lack the aqueous Pb data from UML to be able to evaluate this process. Thus, upon the basis of our results, temporal patterns of As and Pb burdens in zooplankton appear to be most likely explained by the massspecific level of each metal in their algal food, and for As, direct uptake of metal from water as well. Hypothesis 3: As and Pb Burdens in Fish Vary with Feeding Strategy (Piscivores < Planktivores). As predicted, metal burdens in individual fish taxa differed by functional feeding group. Lower trophic feeders had higher metal burdens, suggesting diminution of both metals. There were significantly higher As concentrations in planktivores (e.g., alewife and killifish) than in omnivores and piscivores (black crappie, yellow perch, and largemouth bass; p ) 0.0369). Arsenic concentrations in planktivorous alewife were also significantly greater than in omnivorous bluegill. While the qualitative trends for Pb were similar to As (with planktivore > piscivore burdens), the differences between species were not significant. The differences between the two metals in terms of their concentrations across functional feeding groups could be due to trophic transfer in the food web or to the nature of their food sources (32, 36). The fish species (alewives and killifish) exhibiting highest As concentrations were strictly planktivorous and are expected to have acquired higher As levels from the elevated levels in the zooplankton. The species with the lower As burdens were omnivorous (bluegill, yellow perch, and black crappie) or piscivorous (largemouth bass). They do not feed exclusively on the As-enriched zooplankton but also feed on prey (e.g., fish) with lower metal concentrations. This feeding difference is also expected to result in lower burdens as we found. In contrast to As burdens, Pb concentrations were not elevated in the zooplankton, nor were there significant differences in burdens among fish species. Therefore, the lack of a measurable difference in Pb levels between fish of different functional groups could have been due to the similarities in metal burdens across their food sources. Hypothesis 4: Heavy Metal Contamination in Upper Mystic Lake Has Resulted in Increased Metal Burdens in the Biota. Particulates and Zooplankton. Comparisons of As and Pb burdens in particulates and zooplankton between UML and NIEHS survey lakes clearly show that As contamination in the Aberjona Watershed has accumulated in the UML food web. In contrast, Pb concentrations are not different from uncontaminated sites. These UML and NIEHS lake data are among the only published for As and Pb in lower trophic levels of lake food webs (6). Aqueous and particulate concentrations of As in UML in summer and fall were significantly greater than the range of concentrations measured in the NIEHS survey lakes (Table 1, Figures 3 and 6). Arsenic in the zooplankton fractions in both summer and fall were also significantly greater than the range of concentrations measured in the survey lakes (Table 1, Figure 1a). For example, As levels in the microzooplankton of UML were more than 5× greater than in the microzooplankton in the NIEHS lakes. Pb concentrations in particulate and zooplankton fractions were also generally higher but not significantly different from those of the NIEHS survey lakes (Table 1, Figure 1b). Overall results of the comparison between NIEHS and UML data were the same regardless of whether summer and fall UML data were analyzed individually or pooled. Thus, As contamination in UML is being transferred to lower trophic levels whereas the relative lack of Pb contamination has resulted in water, particulates, and zooplankton burdens comparable to more pristine systems. Fish. One of the most important results of this study is that despite quite elevated levels of As in particulates and zooplankton, concentrations of As in fish from UML lie within the range of concentrations measured in fish in the uncontaminated NIEHS survey lakes. Moreover, the As concentra-
TABLE 1. Results of T-Tests (p Values and n ) NIEHS, UML) Comparing NIEHS and UML (Summer and Fall) Metal Concentrations in Different Trophic Levels metal and trophic level As particulate 45-202 µm zooplankton >202 µm zooplankton fish Pb particulate 45-202 µm zooplankton >202 µm zooplankton fish a
summer
fall
