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ug/l of As +5 or various methylated arsenicals, mortality+las 10% to 32% in amphipods (Gammarus pseudolimnaeus) in 28 days; at 9?5ug As /1, marine red alga failed to reproduce sexually; and at 100 ug As /1, marine copepods died and goldfish behavior was impaired ??able 4). Rainbow trout (Salmo gairdneri) fed diets containing up to 90 mg As /kg were only sliq?tly affected, but those given diets containing 120 mg As/kg (as As or As ), and higher, grew poorly, avoided food, and failed to metabolize food efficiently; no toxic effects were reported over 8 weeks of exposure to diets containing 1,600 mg/kg, as methylated arsenicals (Table 4). In bluegills (Lepomis macrochirus), tissue residues of 1.35 mg As/kg fresh weight in juveniles and 5 mg/kg in adults are considered elevated and potentially hazardous (NRCC 1978). Toxic and other effects of arsenicals to aquatic life are significantly modified by numerous biological and abiotic factors (Woolson 1975; NAS 1977; NRCC 1978; EPA 1980, 1985; Howard et al. 1984; Michnowicz and Weaks 1984; Bryant et al. 1985; Sanders 1986). The LC-50 values, for example, are markedly affected by water temperature, pH, Eh, organic content, phosphate concentration, suspended solids, and presence of other substances and toxicants, as well as arsenic speciation, and duration of exposure. In general, inorganic arsenicals are more toxic than organoarsenicals to aquatic biota, and trivalent species are more toxic than pentavalent species. Early life stages are most sensitive, and large interspecies differences are recorded, even among those closely related taxonomically. Arsenic is accumulated from the water by a variety of organisms; however, there is no evidence of magnification along the aquatic food chain (Woolson 1975; NAS 1977; NRCC 1978; Hallacher et al. 1985; Hood 1985). In a marine ecosystem based on the alga Fucus vesiculosus, arsenate (7.5 ug As+5/1) was accumulated by all biota. After 3 months, arsenic was concentrated most efficiently by Fucus (120 mg/kg dry weight in apical fronds) and filamentous algal species (30 mg/kg dry weight); little or no bioaccumulation occurred in invertebrates, although arsenic seemed to be retained by gastropods and mussels (Rosemarin et al. 1985). In a freshwater food chain composed of algae, daphnids, and fish, water concentrations of 0.1 mg cacodylic acid/l produced residues (mg As/kg dry weight), after 48 hours of 4.5 in algae and 3.9 in daphnids, but only 0.09 in fish (NAS 1977). Microcosms of a Delaware co+ggrass (Spartina alterniflora) salt marsh exposed to elevated levels of As showed that virtually all arsenic was incorporated into plant tissue or strongly sorbed to cell surfaces (Sanders and Osman 1985). Studies with radioarsenic and mussels (M ty ilus galloprovincialis) showed that accumulation varied with nominal arsenic concentrations, tissues, age of the mussel, and temperature and salinity of the medium (Unlu and Fowler 1979). Arsenate uptake increased with increasing arsenic concentration in the medium, but the response was not linear, accumulation being suppressed at higher external arsenic concentrations. Smaller mussels took up more arsenic than larger ones. In both size groups, arsenic was concentrated in the byssus and digestive gland. In general, arsenic uptake and loss increased at increasing 53