Resumen
To better understand how the sulfate (SO42-) anion may contribute to the adverse effects associated with elevated ionic strength or salinity in freshwaters, we measured the uptake and efflux of SO42- in four freshwater species: the fathead minnow (Pimephales promelas, Teleostei: Cyprinidae), paper pondshell (Utterbackia imbecillis, Bivalvia: Unionidae), red swamp crayfish (Procambarus clarkii, Crustacea: Cambaridae), and two-lined mayfly (Hexagenia bilineata, Insecta: Ephemeridae). Using d(34S/32S) stable isotope ratios and the concentrations of S and SO42-, we measured the SO42- influx rate (Jin), net flux (Jnet), and efflux rate (Jout) during a 24 h exposure period. For all four species, the means of Jin for SO42- were positive, and Jin was significantly greater than 0 at both target SO42- concentrations in the fish and mollusk and at the lower SO42- concentration in the crayfish. The means of Jout and Jnet were much more variable than those for Jin, but several species by target SO42- concentration combinations for Jout and Jnet, were negative, which suggests the net excretion of SO42- by the animals. The results of our experiments suggest a greater regulation of SO42- in freshwater animals than has been previously reported.