Uptake of Sulfate from Ambient Water by Freshwater Animals

To better understand how the sulfate (SO(4)(2−)) anion may contribute to the adverse effects associated with elevated ionic strength or salinity in freshwaters, we measured the uptake and efflux of SO(4)(2−) 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 δ((34)S/(32)S) stable isotope ratios and the concentrations of S and SO(4)(2−), we measured the SO(4)(2−) influx rate (J(in)), net flux (J(net)), and efflux rate (J(out)) during a 24 h exposure period. For all four species, the means of J(in) for SO(4)(2−) were positive, and J(in) was significantly greater than 0 at both target SO(4)(2−) concentrations in the fish and mollusk and at the lower SO(4)(2−) concentration in the crayfish. The means of J(out) and J(net) were much more variable than those for J(in), but several species by target SO(4)(2−) concentration combinations for J(out) and J(net), were negative, which suggests the net excretion of SO(4)(2−) by the animals. The results of our experiments suggest a greater regulation of SO(4)(2−) in freshwater animals than has been previously reported.