Factors controlling temporal variation in methyl mercury levels in sediment and water in a seasonally stratified lake

Abstract

Total mercury (total Hg) levels were low in the water (2.5–7.5 pM) and in the sediment (150–300 pmol g (DW)−1), indicating an absence of significant local sources of Hg. During summer stratification, methyl mercury (MeHg) levels increased below the thermocline, reaching 2.5 pM in the anoxic hypolimnion, whereas in the epilimnion levels remained low (0.25–0.6 pM) throughout the study period (late April–late October 1993). On October 27 (the last sampling date), the lake was totally mixed and MeHg in the entire water column had returned to low levels. Within the part of the basin turning anoxic during summer stratification, the MeHg levels in the surficial sediment (0–0.5 cm) and those in the overlying water were negatively correlated (r = −0.87, P < 0.05). Sediment MeHg decreased during summer stratification until August and then increased. In the sediment, total organic carbon was significantly correlated with total Hg (r = 0.86, P < 0.05), MeHg (r = 0.90, P < 0.01), and total Mn (r = 0.88, P < 0.01). A mechanism for the partitioning of MeHg between water and sediment is discussed, involving the Mn redox cycle. Budget calculations indicated that exchanges of a fixed amount of MeHg between the water and sediment could not explain the observed seasonal variation in MeHg levels. Other processes that may have contributed to the variation in MeHg levels in the sediment and water are methylation, demcthylation, mass dilution, and uptake of MeHg in the biota.