Suppression of hypolimnetic methylmercury accumulation by liquid calcium nitrate amendment: redox dynamics and fate of nitrate

Anoxia, specifically sulfate (SO42-) reduction, in the hypolimnia of stratified lakes is a prerequisite for monomethylmercury (HgCH3+) formation in sediments and subsequent efflux into the water column. A wholelake pilot project used liquid calcium nitrate (LCN) to suppress formation of HgCH3+ by raising hypolimnetic redox in a dimictic, mesotrophic lake in Minnesota. A nearby lake of similar morphology and trophic status served as a reference. In the experimental lake, the HgCH3+ concentration at the onset of LCN application was 0.34 ng/L and 0.30 ng/L at the point of nitrate (NO3-) depletion. The NO3- concentration peaked at 17.7 mg/L (as ion) 16 days after application and declined to 0.5 mg/L at day 64. A principal NO3- sink was anaerobic oxidation of sulfidic compounds, but dissimilatory NO3- reduction to ammonium (NH4+) may also have been a major sink. SO42- at the time of the LCN dose was 0.99 mg/L and rose to 4.0 mg/L at the point of NO3- depletion (<0.5 mg/L). Thereafter, NO3- induced SO42- enrichment of the hypolimnion induced HgCH3+ to increase to 1.09 ng/L by SO42- reduction. In both lakes, we observed strong redox functional relationships in the hypolimnia with unambiguous breakpoints (thresholds) for SO42- reduction and sediment efflux of iron, manganese, orthophosphate, NH4+, and HgCH3+. Study results suggest that LCN application be done in frequent, small doses with redox monitoring to prevent SO42- reduction until turnover. This method would minimize downstream NO3- loading at turnover while avoiding inadvertent SO42- enrichment that can boost HgCH3+ production in SO42- limited lakes.