Does lake thermocline depth affect methyl mercury concentrations in fish?

Climate change is projected to increase mean temperature of northern lakes by the end of the twenty-first century. To simulate this scenario, during 2004-2007 we imposed artificial mixing in Halsjarvi, a small, polyhumic, boreal lake in southern Finland, to increase the depth of the thermocline by similar to 1.5 m. The aims of the experiment were to evaluate potential effects of climate change on biogeochemical cycles, especially of mercury, and on food web structure, productivity and biodiversity in dystrophic lake ecosystems. Following the initial depression of the thermocline in the experimental lake, the methyl mercury (MeHg) concentration in small European perch (Perca fluviatlis L.) decreased and remained lower throughout the study. In contrast, perch in a nearby reference lake exhibited increased MeHg during the same period. The delta(15)N values of the muscle tissue of perch were similar in both lakes over the study period, suggesting that the trophic position of perch remained unchanged. However, delta(13)C values of perch became clearly more negative in Halsjarvi, probably due to the greater mixing of the water column that resulted in changes in the carbon sources for the food web. A marked decrease of epilimnetic MeHg concentrations recorded during the experiment was considered to be the main reason for the decreased MeHg concentrations in perch. The results suggest that oxygen-related changes induced by climate change will be more important than direct temperature changes for MeHg accumulation in fish in small humic lakes with persistent oxygen deficiency in the hypolimnion.