Mercury and Drought Along the Lower Carson River, Nevada: I. Snowy Egret and Black-Crowned Night-Heron Annual Exposure to Mercury, 1997–2006

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The dynamic nature of the annual volume of water discharged down the Carson River over a 10-year period, which included a century flood and drought, was examined in order to gain a better understanding of mercury movement, biological availability, and exposure to waterbirds nesting at Lahontan Reservoir. Total annual water discharge directly influenced total mercury (THg) in unfiltered water above the reservoir and downstream of a mining area, whereas methyl mercury (MeHg) at the same site was negatively related to annual discharge. Annual water storage at Lahontan Reservoir in the spring and early summer, as expected, was directly related to annual Carson River discharge. In contrast to the findings from above the reservoir, annual MeHg concentrations in water sampled below the reservoir were positively correlated with the total discharge and the amount of water stored in the reservoir on 1 July; that is, the reservoir is an important location for mercury methylation, which agrees with earlier findings. However, unfiltered water MeHg concentrations were about 10-fold higher above than below the reservoir, which indicated that much MeHg that entered as well as that produced in the reservoir settled out in the reservoir. Avian exposure to mercury at Lahontan Reservoir was evaluated in both eggs and blood of young snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax). Annual MeHg concentrations in unfiltered water below the reservoir, during the time period (Julian Days 90-190) when birds were present, correlated significantly with mercury concentrations in night-heron blood (r(2 )= 0.461, p = 0.027), snowy egret blood (r(2 )= 0.474, p = 0.024), and night-heron eggs (r(2) = 0.447, p = 0.029), but not snowy egret eggs. A possible reason for lack of an MeHg water correlation with snowy egret eggs is discussed and relates to potential exposure differences associated with the food habits of both species. THg concentrations in water collected below the reservoir were not related to egg or blood mercury concentrations for either species.