Seabirds as biomonitors of mercury inputs to epipelagic and mesopelagic marine food chains

Mercury concentrations are presented for time-series of feather samples from seabirds feeding near the apex of epipelagic and mesopelagic marine food chains in the south-east and north-east North Atlantic. These quantify historical increases in surface and deep water mercury concentrations due to mercury pollution. Our results highlight the importance of mercury methylation in mesopelagic water as the factor responsible for the greatest increases in seabird mercury concentrations, which occur in seabirds feeding primarily on mesopelagic prey. Seabirds which feed on prey occupying the epipelagic zone exhibited more modest increases in mercury concentration than mesopelagic feeders, but the rates of increase in the epipelagic-feeding birds are in agreement with recent model predictions regarding anthropogenic inputs of mercury to the environment. We investigated the importance of mesopelagic prey in the diet of seabirds, as opposed to trophic status, in determining mercury concentrations. Analysis of stable isotope ratios of nitrogen revealed that trophic status was not the principal factor in determining mercury concentration and that the presence of mesopelagic prey in the diet was likely to contribute more to the patterns of mercury burdens.