An Investigation of Elevated Fish Mercury Levels In Isle Royale National Park, USA

Recent data have shown that six of the inland lakes of Isle Royale National Park contain northern pike with mercury (Hg) concentrations exceeding health consumption advisory levels (>500 ng/g wet weight).  Standard length (550 mm) pike in these advisory lakes have Hg concentrations that are approximately twice that of non-advisory lakes.  The purpose of this study was to assess Hg concentration in various components of the aquatic ecosystem.  Our working hypothesis is that elevated Hg concentrations in water, sediments, or organisms of the lower food chain could account for differences in Hg concentrations observed in northern pike between advisory and non-advisory lakes.

In 1998 and 1999, using Hg-clean techniques, we collected water, sediment, zooplankton, macro invertebrates, and fishes from one advisory lake, Sargent Lake, for total (Hg_T) and methyl mercury (MeHg) analysis.  Comparison samples were also collected from a non-advisory lake, Lake Richie.  The lakes have similar physical and watershed characteristics and both lakes develop anoxic hypolimnia during the summer with associated increases in MeHg (from below detection to 0.3 ng/L).  Counter to expectations, Lake Richie had slightly higher average Hg concentrations (both Hg_T and MeHg) in open water samples, and also higher suspended particulate matter (SPM, p=0.05), dissolved organic carbon (DOC, p<0.05), chlorophyll a (p<0.01) and blue-green algal pigment zeaxanthin concentrations (p<0.01).  Streams flowing into both lakes contained significantly higher concentrations of DOC and MeHg than the lake water but were often dry.  Zooplankton in Sargent Lake had higher average concentrations of Hg_T and MeHg than in Lake Richie, while Hg concentrations in macro invertebrate taxa were similar. Age-1 yellow perch contained similar Hg_T levels in Sargent Lake (22.0 ng/g wet weight) and Lake Richie (19.0 ng/g wet weight).  Concentrations of Hg_T in adult yellow perch and northern pike increased significantly with length and age in both lakes, and adult perch and pike had significantly higher Hg_T concentrations in Sargent than Richie (p<0.05).  The largest pike in Sargent had above-advisory Hg concentrations. Analysis of stable isotopes (d¹³C and d¹⁵N) from biota showed that pike from the two lakes are positioned at the same trophic level (4.2 and 4.3) and that the food chain is more pelagic-based in Sargent and benthic-based in Richie.  Sediment cores for both lakes exhibited recent increases in Hg_T, with concentrations increasing from about 120 (background) to 320 ng/g dry weight (present).  Surface sediments MeHg concentrations were higher in Sargent (26 ng/g dry weight) than Richie (17 ng/g dry weight).  A diatom community analysis of sediment cores suggests increasing total phosphorus concentration in lake water of about 2 mg/L in Lake Richie, whereas Sargent Lake has remained constant. 

Both lakes have Hg concentrations similar to other lakes in the Upper Midwest, except for those directly affected by industrial contamination.  The historical record provided by sediment cores suggests atmospheric inputs have been the main external source of Hg to both lakes.  Relatively lower levels of SPM, DOC, and algae may enhance the bioavailability of MeHg in Sargent Lake, allowing more MeHg to reach zooplankton and accumulate in the food chain.  This is evidenced by higher Hg concentrations in zooplankton and higher BAFs and BMFs in Sargent Lake than Lake Richie.  Differences in Hg between the two lakes become more pronounced at the large pike level, suggesting unique processes occurring at the pike trophic level.  Elevated Hg levels in Sargent Lake pike are not merely due to elevated Hg concentrations in water, sediments, or food items.

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