Modeling Mercury Transport and Bioaccumulation in the Carson River and Lahontan Reservoir System, Nevada
AuthorCarroll, Rosemary Woods-Hart
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A fully dynamic transport model is developed to capture loading mechanisms for total and dissolved mercury (Hg) species for all flow regimes in the Carson River Lahontan Reservoir (CRLR) system. The conversion of inorganic Hg into the toxic, and highly bioavailable, methylmercury (MeHg), is defined with methylation and demethylation (M/D) ratios. M/D occurs in the channel and reservoir bed sediments as well as in channel bank sediments, with bank M/D adjusted for moisture history. Monte Carlo, generalized likelihood uncertainty estimates and first-order second-moment analysis help define what components in the model are well understood and what components remain highly uncertain. Uncertainty in dissolved species in the downstream reservoir is dominated by uncertainty in higher flow loading mechanisms upstream and not from uncertainty related to reservoir benthic fluxes. The CRLR system affords the opportunity to test sensitivity of predicted body burdens in the planktivorous Sacramento blackfish (Orthodon microlepidotus) to a variable water quality signal in the context of uncertainty in that signal. A bioenergetic and mercury mass balance model (BMMBM) is developed and shows that dynamic and constant loading scenarios produce statistically similar results in predicted MeHg body burden at any given location in the reservoir. However, results suggest that coupling of peak dissolved MeHg loads with periods of maximum plankton growth and maximum fish consumption rates can account for large burdens in the planktivore. Lags in downstream transport can decouple processes and body burden estimates decrease. Sensitivity to the timing of fluvial inputs to the reservoir is greatest in the upstream receiving basin where there is little attenuation in fluvial loads.