In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir

Abstract

Anoxic conditions within reservoirs related to thermal stratification and oxygen depletion lead to methylmercury (MeHg) production, a key process governing the uptake of mercury in aquatic food webs. Once formed within a reservoir, the timing and magnitude of the biological uptake of MeHg and the relative importance of MeHg export in water versus biological compartments remain poorly understood. We examined the relations between the reservoir stratification state, anoxia, and the concentrations and export loads of MeHg in aqueous and biological compartments at the outflow locations of two reservoirs of the Hells Canyon Complex (Snake River, Idaho-Oregon). Results show that (1) MeHg concentrations in filter-passing water, zooplankton, suspended particles, and detritus increased in response to reservoir destratification; (2) zooplankton MeHg strongly correlated with MeHg in filter-passing water during destratification; (3) reservoir anoxia appeared to be a key control on MeHg export; and (4) biological MeHg, primarily in zooplankton, accounted for only 5% of total MeHg export from the reservoirs (the remainder being aqueous compartments). These results improve our understanding of the role of biological incorporation of MeHg and the subsequent downstream release from seasonally stratified reservoirs and demonstrate that in-reservoir physical processes strongly influence MeHg incorporation at the base of the aquatic food web.

Biogeochemical and hydrologic synergy control mercury fate in an arid land river-reservoir system

“…, high vs. low flow years). 14,29,43 Only marginal MeHg accumulation was observed at depth at river mile 318, as f.MeHg and volumetric p.MeHg concentrations were uniformly low (avg ± std of 0.04 ± 0.03 and 0.09 ± 0.07 ng L −1 , respectively; n = 38) throughout the water column over the period sampled.…”

“…Dissolved oxygen concentration and temperature heat maps of Brownlee Reservoir that were collected during in-reservoir sampling are available in companion studies. 14,29…”

“…The post-August decreases in f.MeHg and p.MeHg concentrations at river mile 310 coincided temporally with interflow that eroded and flushed metalimnetic water out of the transition zone of Brownlee Reservoir. 14,29…”

“…S11†), with higher f.Hg( ii ) levels under elevated spring flow when terrestrial DOM and f.Hg( ii ) are delivered to the reservoir complex. Concurrent with thermal stratification, anoxic conditions developed first in the upgradient portion of Brownlee Reservoir 14,29 resulting in Hg( ii ) methylation in the metalimnion and hypolimnion (ESI Fig. S6–S7†).…”

“…7); (2) the organic matter content of particulate material increased (i) temporally and (ii) longitudinally in Brownlee Reservoir (Fig. 9a), presumably due to autochthonous organic carbon production; 30 and (3) cooler inflowing water that penetrated the metalimnion starting in ∼July–August at the transition zone (river miles ∼320–305), 14,29,45 transporting MeHg as both f.MeHg and p.MeHg downgradient at the depth of the metalimnion towards the penstocks of the Brownlee Dam. This anoxic, MeHg-rich water mixed with epilimnetic and metalimnetic water due to thermal destratification yielding particles with exceptionally high MeHg concentrations and % MeHg (upwards of 100 ng g −1 and 42% MeHg, respectively) within the upper layers of Brownlee Reservoir (Fig.…”