Increased metal bioavailability following alteration of freshwater dissolved organic carbon by ultraviolet B radiation exposure.

Dissolved organic carbon (DOC) is critically important in the chemistry of freshwater. It complexes heavy metals, making them less bioavailable to aquatic organisms, and absorbs and attenuates UVB radiation, undergoing degradation and alteration in the process. This study examined changes in metal toxicity to the freshwater green alga Pseudokirchneriella subcapitata in high- and low-DOC natural water samples after exposure to UVB radiation. Brown-water and clear-water samples were irradiated for 0, 5, and 10 days. The DOC concentrations of the samples were measured, and they were subsequently spiked with Cu, Ni, Zn, Cd, Co, and Pb and used in algal bioassays to measure changes in metal toxicity following irradiation. DOC concentrations declined only 20% with UVB irradiation in both samples, although DOC concentration was much higher in the brown-water sample than in the clear-water sample. In the brown-water sample metal toxicity increased up to 78% after 10 days of UVB irradiation for Cu, Zn, Co and Pb, but not for Ni and Cd. Changes were less evident in the clear-water sample. The differences observed between IC(50) values for relatively fresh, high-DOC water from the headwaters of the Raisin River and much "older," low-DOC water from Lake Simcoe point to the likelihood of the effects observed in this study being many times greater in the natural environment because of very long exposure to solar radiation. Alteration of DOC by UVB irradiation may influence primary productivity and species composition, especially in waters in which metal concentrations are high.