Uptake of Sulfate from Ambient Water by Freshwater Animals.

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