PHYSIOLOGICAL-RESPONSES OF NATURAL PLANKTON COMMUNITIES TO ULTRAVIOLET-B RADIATION IN REDBERRY LAKE (SASKATCHEWAN, CANADA)

Damaging effects of ultraviolet radiation on the aquatic biota, relate d to anthropogenic modifications in the ozone layer, have been extensi vely described. However, most of the research has focused on marine en vironments, and information about the effects of ultraviolet radiation on saline prairie lakes of Canada is lacking. To test the deleterious effects of ultraviolet-B (UV-B) radiation on the planktonic community , two exposure experiments were performed in Redberry Lake, Saskatchew an. The responses of primary productivity, phytoplankton chlorophyll a and size, bacterial changes, and the electron transport system to nat ural UV-B fluxes reaching the surface of the lake were studied. No cle ar effects of UV-B on phytoplankton carbon assimilation and chlorophyl l a were observed. However, significant responses were found for the t wo phytoplankton size fractions studied (0.7-2 and >2 mu m), which wer e more related to the experimental conditions than to UV-B effects. Ba cteria presented a clear decrease in cell number under the highest UV- B doses, whereas the opposite was observed for the electron transport system activity.