Biomagnification of mercury through the food web of the Santos continental shelf, subtropical Brazil

This study was conducted on the continental shelf surrounding a large metropolitan region on the coast of Sao Paulo State, Southeast Brazil. This region harbours a large industrial plant and the largest port in Latin America, both of which release pollutants into the SantosSao Vicente estuarine system. High levels of Hg have been reported in sediments and fish from the estuaries and Santos Bay; however, data for the biota in offshore waters are scarce, and the bio- magnification of Hg across the food web here has never been assessed. In this study, the trophic structure of the Santos shelf was addressed through the carbon and nitrogen stable isotope compo- sitions of different species across a trophic gradient. We determined the total Hg levels (THg, dry weight) of invertebrates and fish to estimate the rate of biomagnification of this metal in the ben- thic and pelagic food webs. The lowest mean THg levels were found in zooplankton (0.006 µg g �1 ) and surface-depositivore polychaetes (0.011 µg g �1 ); the highest THg levels were found in the largest fishes: Patagonian flounder (0.825 µg g �1 ), fat snook (0.714 µg g �1 ), and lesser guitarfish (0.639 µg g �1 ). Overall, the Hg concentration in fish was below the recommended limit for human consumption. The THg and δ 15 N were positively correlated in both food webs; however, the rate of biomagnification was higher and the basal Hg was lower in the pelagic food web. These differ- ences may be related to the differing bioavailability of mercury in water and sediment, the higher diversity of prey and more complex feeding interactions in the benthic food web, and metabolic differences among different taxa.