INHIBITION OF MARINE PHOTOSYNTHESIS BY ULTRAVIOLET-RADIATION - VARIABLE SENSITIVITY OF PHYTOPLANKTON IN THE WEDDELL-SCOTIA CONFLUENCE DURING THE AUSTRAL SPRING

To assess the potential impacts of ozone depletion on photosynthesis i n the Southern Ocean, we need to know more about effects of ultraviole t radiation (UV) on phytoplankton in Antarctic waters, where, in addit ion to variable stratospheric ozone, temporal and regional differences in vertical mixing might influence photosynthesis and photoacclimatio n of phytoplankton assemblages. Toward this end, we quantified the res ponses to UV of Antarctic phytoplankton in the Weddell-Scotia Confluen ce during the austral spring of 1993. Experimental results on spectral sensitivity of photosynthesis were fit statistically to a model-that incorporated uninhibited photosynthesis as a function of photosyntheti cally available radiation (PAR), wavelength-dependence of inhibition, and the kinetics of photosynthesis during exposure to UV. In contrast to previous results on UV-induced photoinhibition in a diatom culture at 20 degrees C, natural phytoplankton from open waters of the Antarct ic showed no ability to counter UV-induced inhibition of photosynthesi s during exposures of 0.5-4 h: the rate of photosynthesis declined exp onentially as a function of cumulative exposure, and inhibition was no t reversed during incubations for up to 3.5 h under benign conditions. The results suggest that nonlinear exposure-response relationships ar e necessary for modeling UV-dependent photosynthesis in the surface mi xed layer of the springtime Weddell-Scotia Confluence. Consequently, w e modified our laboratory-based model of photosynthesis and photoinhib ition to describe photoinhibition as a nonlinear function of biologica lly weighted cumulative exposure to damaging irradiance. The model des cribed similar to 90% of the spectrally dependent experimental variati on in photosynthetic rate, and yielded six biological weighting functi ons (BWFs) for phytoplankton in the Weddell-Scotia Confluence. Assembl ages from different stations showed substantial variability in sensiti vity to UV. Tolerance of UV was generally highest in assemblages from shallower mixed layers, which presumably had experienced higher irradi ance, including W, prior to sampling. The BWFs of assemblages that see med acclimated to low irradiance showed the highest sensitivity to UV yet seen for Southern Ocean phytoplankton. The pattern of UV sensitivi ty was consistent with acclimation, but also with selection against le ss tolerant species.