Mercury contamination higher in the middle of lakes

Posted on September 24, 2012

While mercury occurs naturally in aquatic environments, mercury inputs resulting from human activities increase its concentration.  Inorganic mercury settles at the bottom of water bodies and is converted to methylmercury, which is taken up by organisms.  Concentrations of methylmercury increase as it passes up the food chain, and can reach hazardous levels in predatory fish that are consumed by humans.  It’s been found that mercury concentrations tend to be higher in fish that feed in the open waters of lakes than in those living closer to the shore, but the reasons for this difference have been unclear.  Potential causes include habitat-related disparities in factors such as the supply of mercury, the taxonomic status of invertebrate fish prey, or the position of prey in the food chain.  Methylmercury concentrations in nearshore invertebrates and open water zooplankton were compared by reference to data from 52 mid-latitude lakes in Canada and the U.S.A.  Mercury concentrations were significantly higher in midwater zooplankton than in nearshore primary consumers, but there were no strong differences between the two types of prey in terms of their taxonomic affinities or their position in the food chain.  There was also no significant difference between mercury levels in water samples from inshore and offshore.  Interestingly, of all the water and sediment measures that were examined, the mercury concentration in mid-lake sediment was the best predictor of zooplankton mercury levels, even though the lake bottom wasn’t the main habitat for plankton .  These results suggested that mercury accumulation was more efficient in midwater than nearshore food webs, possibly because in shallow waters at the lake edge, mercury uptake is dampened because organic nutrient inputs are dominated more by non-living detritus, which doesn’t take up the metal.  Midwater mercury levels may also be increased by the regular vertical migrations of zooplankton towards the lake bottom.   More work is required to better understand habitat-related differences in mercury uptake in lakes.

Reference:  Chetelat, J., Amyot, M. & Garcia, E.  2011.  Habitat-specific bioaccumulation of methylmercury in invertebrates of small mid-latitude lakes in North America.   Environmental Pollution 159, 10-17.