MERCURY ACCUMULATION IN THE BURROWING MAYFLY HEXAGENIA-RIGIDA (EPHEMEROPTERA) EXPOSED TO CH3HGCL OR HGCL2 IN WATER AND SEDIMENT

The accumulation of Hg and its distribution within nymphs of the burro wing mayfly Hexagenia rigida (Ephemeroptera) were investigated experim entally by exposure of nymphs to radiolabelled Hg in laboratory microc osms containing water and sediment. Mercury was introduced into the ex perimental units either in the overlying water (twice-daily additions) or in the sediment (single addition prior to introduction of the nymp hs). At the whole animal level, Hg accumulation varied according to th e chemical form of the Hg added and the contamination source. When add ed in an organic form (CH3HgCl), Hg was accumulated to a greater exten t than when added in an inorganic form (HgCl2); the ratio of accumulat ion between the two compounds exceeded 60 (organic/inorganic exposure) when Hg was introduced via the sediment but was only about 2 when Hg was added to the overlying water. The distribution of Hg among the var ious body parts of the nymphs indicated that the Hg burdens in two maj or target organs-the gills and the gut-depended strongly on the initia l contamination source. When Hg was added via the sediment, the contri bution of the gut to the total body burden (43% for inorganic Hg, 18% for methyl-Hg) was much greater than that of the gills (3 and 5%, resp ectively). In contrast, when Hg was added to the overlying water, the contribution of the gills to the total body burden (49% for inorganic Hg, 20% for methyl-Hg) exceeded that of the gut (8 and 17%, for the tw o forms, respectively).