Role of bubbling from aquatic sediments in mercury transfer to a benthic invertebrate in the St. Lawrence River, Cornwall, Ontario
Razavi, Neguysa Roxanna
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Benthic uptake of mercury (Hg) governs bioavailability to fish yet there are still large gaps in our knowledge of what mediates this process. Without this information it is difficult to ascertain where Hg accumulation in the foodweb will be greatest. In the St. Lawrence River Area of Concern (AOC) at Cornwall, one contaminated zone (Zone 1) shows elevated Hg in yellow perch (Perca flavescens) and their prey items compared to those from other zones in the AOC. Greater availability of Hg to benthos due to unique physical features (large deposition of woodfibre deposits) of Zone 1 is hypothesized to account for this observation. In this study, amphipods (Gammarus fasciatus) and (Echinogammarus ischnus) were collected in Zone 1 using artificial substrates between June-September 2007, and Hg concentrations compared to those obtained in sediments and porewaters of surficial sediments, as well as methane gas evasion rates. Methylmercury (MeHg) concentrations in amphipods were significantly related to porewater total Hg (THg) and MeHg concentrations. No parallel relationship was found for sediment Hg concentrations or methane bubbling rates from sediments. Spatial and temporal trends in Hg bioavailability were evident from significant relationships with water column depth and temperature. Water column depth was associated with higher MeHg concentrations in amphipods and porewaters. Concentrations of porewater MeHg were above the detection limit in all of the June samples, the month which also coincided with highest amphipod MeHg concentrations. Finally, sediment organic matter may be influencing patterns of MeHg availability in Zone 1, and displayed a negative relationship to amphipod MeHg. Although bubbling from contaminated sediments did not directly correlate with amphipod Hg uptake, future studies should look at the influence of bubbling on the redistribution of contaminated sediment particles within the zone.