Toxicity of major cations and anions (Na+, K+, Ca2+, Cl-, and SO4(2-)) to a macrophyte and an alga.

In many freshwater systems around the world, the concentrations of major ions (Na(+), K(+), Ca(2+), Mg(2+), Cl(-), HCO(3)(-), CO(3)(2-), and SO(4)(2-)) are exhibiting increasing trends, approaching the concentrations historically found mainly in estuaries. The objectives of the present study were to determine at what concentrations these salts are toxic to an aquatic plant and a green alga, to investigate two potential mechanisms of toxicity, and to determine the usefulness of conductivity as an indicator of salt toxicity. In a series of laboratory trials, Lemna minor and Pseudokirchneriella subcapitata were exposed to a range of concentrations of five different salts. Conductivity levels that caused 10 or 50% reductions in growth-related traits (EC10 and EC50, respectively) were determined, using conductivity of the test solutions as the independent variable. The EC10 values ranged from 0.44 to 2.67 mS/cm for P. subcapitata and from 1.3 to >19 mS/cm for L. minor. The EC50 values ranged from 1.7 to 5.8 mS/cm for P. subcapitata and from 4.2 to >27 mS/cm for L. minor. For both species the EC values varied dramatically among the salts. Pseudokirchneriella subcapitata was most sensitive to KCl and NaCl, whereas L. minor was most sensitive to Na(2)SO(4). The mechanism of toxicity does not appear to be related to production of reactive oxygen species, nor to reduction in chlorophyll concentrations. Because toxicity was strongly influenced by salt composition, regulation and management of specific ions may be preferable to conductivity.