Assessing element-specific patterns of bioaccumulation across New England lakes

Abstract

Little is known about differences among trace elements in patterns of bioaccumulation in freshwater food webs. Our goal was to identify patterns in bioaccumulation of different elements that are large and consistent enough to discern despite variation across lakes. We measured methylmercury (MeHg) and trace element (As, Cd, Hg, Pb, and Zn) concentrations in food web components of seven New England lakes on 3–5 dates per lake, and contrasted patterns of bioaccumulation across lakes, metals and seasons. In each lake, trace element concentrations were compared across trophic levels, including three size fractions of zooplankton, planktivorous fish, and piscivorous fish. The trophic position of each food web component was estimated from N isotope analysis. Trace element concentrations varied widely among taxa, lakes and sampling dates. Yet, we identified four consistent patterns of bioaccumulation that were consistent across lakes: (1) MeHg concentration increased (i.e., was biomagnified) and Pb concentration decreased (i.e., was biodiminished) with increased trophic position. (2) Zinc concentration (as with MeHg) was higher in fish than in zooplankton, but overall variation in Zn concentration (unlike MeHg) was low. (3) Arsenic and Cd concentrations (as with Pb) were lower in fish than in zooplankton, but (unlike Pb) were not significantly correlated with trophic position within zooplankton or fish groups. (4) Average summer concentrations of As, Pb, Hg, and MeHg in zooplankton significantly predicted their concentrations in either planktivorous or piscivorous fish. Our secondary goal was to review sampling approaches in forty-five published studies to determine the extent to which current sampling programs facilitate cross-lake and cross-study comparisons of bioaccumulation. We found that studies include different components of the food web and sample too infrequently to enable strong cross-lake and cross-study comparisons. We discuss sampling strategies that would improve our capacity to identify consistent patterns of bioaccumulation and drivers of elevated trace element concentrations under naturally high levels of variability.

Comparing nearshore benthic and pelagic prey as mercury sources to lake fish: the importance of prey quality and mercury content

“…However, zooplankton and benthic invertebrates shared similar Hg concentrations in Post Pond. These same patterns were consistent when comparing with other size fractions of zooplankton (45–100, 100–202 μm Ward et al, 2012). Among the benthic taxa, relative Hg concentrations were variable across lakes.…”

“…We chose lakes that ranged from oligotrophic to eutrophic (Gregg Lake (NH), Post Pond (NH), Canobie Lake (NH), Weatherhead Hollow Pond (VT) and Horseshoe Pond (NH)), to include variation in Hg concentrations across lake types. Detailed lake characteristics are provided elsewhere (Chen et al, 2000; Ward et al, 2012). Briefly, all sites have no known point sources of Hg pollution and had no major land use changes between sampling periods.…”

“…Briefly, all sites have no known point sources of Hg pollution and had no major land use changes between sampling periods. For the multi-lake study, we used Hg concentrations in zooplankton collected from 3 of the 5 lakes (Gregg Lake, Post Pond and Horseshoe Pond) in 2001–2002 from a previous study (Ward et al, 2012). We examined the potential for, and found no evidence of, interannual differences in prey Hg content (see 2.4 Statistical analysis of field data).…”

“…Bulk zooplankton samples were collected using trace metal clean techniques. In the multi-lake study, zooplankton were collected as in Chen et al (2000) and Ward et al (2012). Briefly, zooplankton were collected from 1 to 5 vertical tows in the deepest part of the lake using a >202 μm plankton net.…”

“…To evaluate differences between nearshore benthic and pelagic prey across lakes in the multi-lake study, we compared Hg concentrations in benthic invertebrates (collected in 2003) to Hg concentrations in macrozooplankton (>202 μm) collected in August 2001 (Gregg Lake), and 2002 (Horseshoe Pond and Post Pond) (Ward et al, 2012). To minimize effects of seasonal differences in the multi-lake study, we analyzed zooplankton sampled on particular dates in August (2001 in August (2002) that most closely matched dates for benthic invertebrate sampling (August 2003).…”