Climate induced thermocline change has an effect on the methyl mercury cycle in small boreal lakes


Weconducted awhole-lake experiment by manipulating the stratification pattern (thermocline depth) of a small polyhumic, boreal lake (Halsjärvi) in southern Finland and studying the impacts on lakemercury chemistry. The experimental lake was compared to a nearby reference site (Valkea-Kotinen Lake). During the first phase of the experiment the thermocline of Halsjärvi was lowered in order to simulate the estimated increase in wind speed and in total lake heat content (high-change climate scenario). The rate of methyl mercury (MeHg) production during summer stagnation (May-August)was calculated fromwater profiles before the treatment (2004), during treatment (2005, 2006) and after treatment (2007). We also calculated fluxes of MeHg from the epilimnion and from the hypolimnion to the sediments using sediment traps. Experimental mixing with a submerged propeller caused a 1.5-2 mdeepening of the thermocline and oxycline.Methylmercury production occurred mostly in the oxygen free layers in both lakes. In the experimental lake there was no net increase in MeHg during the experiment and following year; whereas the reference lake showed net production for all years.Weconclude that thenewexposedepilimnetic sediments causedby a lowering of the thermoclinewere amajor sink forMeHg in the epilimnion. The results demonstrate that in-lakeMeHg production can bemanipulated in small lakeswith anoxic hypolimnia during summer. The climate change induced changes in small boreal lakes most probably affect methyl mercury production and depend on the lake characteristics and stratification pattern. The results support the hypothesis that possible oxygen related changes caused by climate change are more important than possible temperature changes in small polyhumic lakes with regularly occurring oxygen deficiency in the hypolimnion

Medarbetare: John Munthe

Nyckelord: mercury, methylmercury, boreal, lake, climate

Typ: Artikel

År: 2010

Rapportnummer: A1819

Författare: Matti Verta, Simo Salo, Markku Korhonen, Petri Porvari, Anna Paloheim, John Munthe

Publicerad i: Science of the Total Environment, 2010. Issue 408, page 3639-3647