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1985). The chemical form of the arsenic in solution varies both seasonally and along the axis of the Bay. Arsenic is present only as arsenate in winter, but substantial quantities of reduced and methylated forms are present in summer in different areas. The forms and distribution patterns of arsenic during the summer suggest that separate formation processes exist. Arsenite, present in low salinity regions, may have been formed by chemical reduction in anoxic, subsurface waters and then mixed into the surface layer. Methylated arsenicals are highly correlated with standing crops of algae. One particular form, methylarsonate, is significantly correlated with the dominant alga Chroomonas. Since both arsenic reactivity and toxicity are altered by transformation of chemical form, the observed variations in arsenic speciation have considerable geochemical and ecological significance (Sanders 1985). BIOLOGICAL SAMPLES Background arsenic concentrations in living organisms are usually <1 mg/kg fresh weight in terrestrial flora and fauna, birds, and freshwater biota. These levels are higher, sometimes markedly so, in biota collected from mine waste sites, arsenic-treated areas, near smelters and mining areas, near areas with high geothermal activity, and near manufacturers of arsenical defoliants and pesticides (Table 2). Marine organisms, however, normally contain arsenic residues of several to more than 100 mg/kg dry weight (Lunde 1977); as discussed later, however, these concentrations present little hazard to the organism or to its consumers. Shorebirds (seven species) wintering in the Corpus Christi, Texas, area contained an average of only 0.3 mg As/kg fresh weight in livers (maximum of 1.5 mg/kg), despite the presence of smelters and the heavy use of arsenical herbicides and defoliants; these values probably reflect normal background concentrations (White et al. 1980). Similar arsenic levels are reported in livers of brown pelicans (Pelecanus occidentalis) collected from South Carolina (Blus et al. 1977). The highest arsenic concentration recorded in seemingly unstressed coastal birds was 13.2 mg/kg fresh weight lipids (Table 2). This tends to corroborate the findings of others who demonstrated that arsenic concentrates in lipid fractions of marine plants, invertebrates, and higher organisms. An abnormal concentration of 16.7 mg As/kg fresh weight was recorded in liver of an osprey (Pandion haliaetus) from the Chesapeake Bay region (Wiemeyer et al. 1980). This bird was alive but weak, with serious histopathology including the absence of subcutaneous fat, and the presence of serous fluid in the pericardial sac and disorders of the lung and kidney. It died shortly after collection. Arsenic concentrations in liver from other ospreys collected in the same area usually were <1.5 mg As/kg fresh weight. 21