Effects of lake oxygenation on mercury accumulation in zooplankton in Twin Lakes, Washington

Brandon Reed

Mercury is an important global pollutant due to its mobility in the environment, high toxicity, and ability to bioaccumulate in aquatic food webs. This study examined total mercury and methylmercury in water and in zooplankton at oxygenated North Twin Lake and nonoxygenated South Twin Lake in 2009 and 2010. Hypolimnetic oxygenation is a lake treatment strategy that was hypothesized to limit the amount of methylmercury in lake water and subsequent uptake into zooplankton. Water and zooplankton were collected using trace metal sampling and handling techniques, and are analyzed for THg and MeHg with ultrasensitive DMA-80 and the MERX-M autoanalyzers. In both 2009 and 2010, the hypolimnion of South Twin Lake was anoxic, while the oxygenation of North Twin Lake maintained dissolved oxygen levels generally above 5 mg/L. In 2009 bottom waters of South Twin Lake accumulated MeHg up to 0.5 ng/L and MeHg concentration was negatively correlated with dissolved oxygen concentration (ANOVA, p = 0.023). Hypolimnetic MeHg concentrations in North Twin were below 0.05 ng/L, likely as a result of oxygenation. In contrast to water column results, THg and MeHg in zooplankton from North Twin were consistently higher than in South Twin. Average zooplankton MeHg concentrations in North Twin were 63.8 and 127.9 µg/kg in 2009 and 2010, while in South Twin they were 46.0 and 40.3 µg/kg. Potential explanations behind these perplexing results included: difference in pH and dissolved organic carbon, key water quality parameters known to control mercury accumulation in zooplankton; biodilution in the form of both zooplankton density dilution and bloom dilution; and enhancement of mercury uptake by zooplankton due to the presence of metal oxides in the water column. The first two explanations were discounted based on a review of lake water quality. The third explanation is thought to be the most plausible and suggests that the presence of metal oxides is more important than the level of MeHg in the water column in controlling mercury uptake into zooplankton. In addition, results suggest that to limit mercury uptake into zooplankton, hypolimnetic oxygenation systems must inhibit the release of metal oxides by maintaining a well-oxygenated sediment-water interface.

Effects of lake oxygenation on mercury accumulation in zooplankton in Twin Lakes, Washington

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