Uptake of Sulfate from Ambient Water by Freshwater Animals

To better understand how the sulfate (SO₄ ^2-) anion may contribute to the adverse effects associated with elevated ionic strength or salinity in freshwaters, we measured the uptake and efflux of SO₄ ^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 SO₄ ^2-, we measured the SO₄ ^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 SO₄ ^2- were positive, and J _in was significantly greater than 0 at both target SO₄ ^2- concentrations in the fish and mollusk and at the lower SO₄ ^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 SO₄ ^2- concentration combinations for J _out and J _net , were negative, which suggests the net excretion of SO₄ ^2- by the animals. The results of our experiments suggest a greater regulation of SO₄ ^2- in freshwater animals than has been previously reported.