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

Total mercury (total Hg) levels were low in the water (2.5-7.5 pM) and in the sediment (150-300 pmol g (DW) I), 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 (O-O.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 (Y = 0.86, P < 0.05), MeHg (r = 0.90, P < O.OJ), and total Mn (Y = 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. Methyl mercury (MeHg), owing to its remarkable ability to pass biological membranes, its high chemical stability, and its slow excretion from most organisms (Lofroth 1968), is biomagnified in aquatic food chains (Cabana and Rasmussen 1994). Its toxicity makes it a potential threat to regular consumers of fish. Furthermore, epidemiological studies and experiments with various mammals suggest that the sensitivity of the human fetus to MeHg may be much higher than that of human adults (Clarkson 1990). Even in seemingly pristine lakes, the fish surprisingly often contain high enough levels of MeHg to call for fish consumption advisories (Driscoll et al. 1994). This is explained by long-range atmospheric transport and deposition of both methylated and inorganic Hg species (Lee and Hultberg 1990; Fitzgerald et al. 1991). Some of the deposited inorganic Hg(IT) will be converted to MeHg in lake water, lake sediments, soils, and wetlands, which probably is the main reason for the buildup of MeHg in most lakes (St. Louis et al. 1994; Watras et al. 1995) If the aim is to reduce MeHg levels in freshwater fish, it is of vital importance to know the relative contributions from different sources to the MeHg burden of lakes. An important question is whether most of the MeHg in a lake is produced within the lake itself or originates from external sources. It is relatively easy to quantify the MeHg that is entering a lake through direct precipitation and runoff water. The difficult part, however, is to determine the fate of MeHg within a lake. Some of it could be demethylated in the water or assimilated by the sediment, from which it may or may not be released back to the water. As long as these processes are poorly understood, it is not possible to draw any firm conclusions concerning the relative importance of internal and external sources in the supply of MeHg to lakes. Studies of the partitioning of Hg between sediment and water in microcosms, carried out in our laboratory, have indicated MeHg release from sediments to be a redox-controlled process (Regnell and Tunlid 199 1; Regnell 1994; Regnell et al. 1996). Here we report results from a lake study that support this finding. We think that this study is the first to demonstrate an inverse relationship between MeHg concentrations in surficial sediment and those in the overlying water, suggesting internal cycling of MeHg between water and sediment.