7. Measuring the Rate of Mercury Loss in Amphipod Gut and Tissues after Exposure to Contaminated Sediments

Mercury (Hg) contamination of the St. Lawrence River along the Cornwall waterfront is the result of over a century of industrial inputs. Yellow perch (Perca flavescens) are contaminated above the consumption guidelines deemed safe by Health Canada in one of three contaminated depositional zones. Amphipods are crustaceans that play an important role in aquatic food webs, and a recent study of the diet of yellow perch showed that amphipods were the primary food source of yellow perch in these zones and that amphipods showed similar patterns of Hg contamination. However, not all Hg taken up by amphipods is bioavailable. That is, not all Hg is incorporated into the tissues and available for uptake by yellow perch. To determine if Hg analyses of amphipods are biased by Hg present in their gut contents, the rate of Hg loss was measured from the gut and tissues. Amphipods were collected in the field using artificial substrates. A sample was frozen immediately upon retrieval, and the remaining amphipods were kept in a sieve in a basin of filtered river water. Amphipods were then sampled over 16 days to compare Hg concentrations in gut contents and tissues before and after they were removed from the Hg source in the field. The data were used to estimate the portion of Hg bioavailable to yellow perch. This enables us to more accurately estimate the extent of Hg contamination that is moving through the Cornwall food web, leading to elevated concentrations in top trophic fish species.