ULTRAVIOLET-RADIATION AND PHOTOSYNTHESIS BY GEORGIAN BAY PHYTOPLANKTON OF VARYING NUTRIENT AND PHOTOADAPTIVE STATUS

The photosynthesis-irradiance responses of phytoplankton from Georgian Bay (Laurentian Great Lakes) were measured in 1993 to determine the i nfluence of ultraviolet-B (UV-B, 290-320 nm) radiation and the interac ting effects of natural changes in nutrient status and photoadaptive s tate. Silicate concentrations and phosphorus availability (inferred fr om P-32 incorporation) varied widely but had little direct influence o n photosynthetic performance. W-B caused up to 80% loss of photosynthe tic capacity in 1-day simulated surface exposures (artificial light) a nd up to 40% loss in half-day exposures (natural light), quantitativel y consistent with reported responses of marine phytoplankton. The wide ly varying nutrient status and photoadaptive state (as inferred from p hotosynthetic light responses) of the phytoplankton had no significant influence on their sensitivity to UV-B radiation, and midsummer commu nities were as sensitive as spring or fall communities. To a first app roximation, the impact of UV-B was proportional to the seasonal variat ion in surface UV-B irradiance being maximal near the summer solstice.