The role of phytoplankton in the accumulation of organic and inorganic mercury in freshwater zooplankton

Understanding the processes that underlie mercury concentrations in freshwater fish species has been difficult because fish derive mercury primarily from their fish and invertebrate prey. Zooplankton herbivores are major prey items for zooplanktivores and important contributors of dietary mercury to fish. The accumulation dynamics of mercury in freshwater zooplankton are not well understood, yet they are critical to establishing dietary mercury intake by their fish predators. Furthermore, processes that affect algal abundance and composition are likely to affect the mercury concentrations of zooplankton herbivores and initiate the dietary accumulation of mercury to subsequent trophic levels. This dissertation tests hypotheses that assert that concentrations of mercury in freshwater zooplankton are dictated by algal food abundances and types. Furthermore, the accumulation of mercury in zooplankton was predicted to be taxon specific, so ubiquitous crustacean zooplankton taxa were contrasted.; To conduct experiments at the low concentrations of mercury typical of natural systems we employed enriched stable isotopes of mercury. We tracked the accumulation of organic (CH₃200Hg+) and inorganic (201Hg2+) mercury from water, to algae, and crustacean zooplankton. We conducted experiments in 550 liter mesocosms and at smaller scales in the laboratory to test our hypotheses on the accumulation of mercury in zooplankton. Mercury accumulation in zooplankton was affected significantly by algal abundance and composition, important factors across lakes of different trophic status and across seasons. Results from the mesocosms demonstrated that concentrations of CH₃Hg + in Daphnia, and two species of copepods declined significantly with increased algal abundance. Laboratory experiments corroborated the effect of decreased CH₃Hg+ accumulation in zooplankton with increased algal abundance for Daphnia pulex, and two diaptomid copepods Leptodiaptomus minutus and Skistodiaptomus pygmaeus. Furthermore, we found that D. pulex and S. pygmaeus fed the green alga Chlamydomonas reinhardtii had significantly higher concentrations of CH₃Hg + relative to zooplankton fed the cryptomonad, Cryptomonas erosa. Critical to the trophic transfer of mercury to higher trophic levels, we found that D. pulex fed either alga accumulated higher CH₃Hg+ relative to Hg2+ across all food levels. Moreover, concentrations of CH₃Hg+ were significantly higher in Daphnia sp. relative to copepod species in mesocosm and laboratory experiments. Many freshwater bodies across North America and Europe currently contain fish with high concentrations of mercury. Our results indicate that future research should examine how algal abundance and composition throughout the annual cycle are interconnected with mercury accumulation in zooplankton and their fish predators.