Mercury in the St. Louis River Estuary

Several key fish species in the St. Louis River Estuary (SLRE) have elevated methylmercury levels relative to those found in the downstream St. Louis River. Surface water and sediment cores were taken from six SLRE sites using a gravity core.  The six sites were classified by habitat zone, and consisted of two Clay Bays, two Sheltered Bays, Upper Estuary Flats, and the main channel.  At each site three plots were taken.  The A plot was near the shore in shallow water, the C plot was in deep, open water, and the B plot was in intermediate water depth. Sediment cores were injected with a solution of methylmercury-201 and mercury-200(2+) every cm and incubated 5+ hours at approximately the temperature of the water at the sampling site.  Cores were then sectioned by depth (0-2 cm, 2-4 cm and 4-8 cm) and freeze dried. Methylmercury-201 and mercury-201(2+) formed during the incubation period were quantified by isotope dilution on the ICP-MS.  Methylation potential was calculated from  methylated to methylmercury-201 over time while demethylation potential was calculated from  methylmercury-201 demethylated to methylmercury-201(2+) over the incubation time. Preliminary methylation potential calculations suggest near shore zones have higher methylation potentials than zones in open water.  Three of the four available sets of methylation potential data have a distinct trend of decreasing methylation potential as the sampling plot is moved further from the shore. Plants release oxygen from their roots and may create micro environments that oxidize sulfur and provide energy to sulfate reducing bacteria that methylate mercury.  Near shore environments tend to be more vegetated, although SB2 had highly vegetated deep zones, providing an explanation for SB2’s deviation from the observed trend.