INTERACTIVE EFFECTS OF OZONE DEPLETION AND VERTICAL MIXING ON PHOTOSYNTHESIS OF ANTARCTIC PHYTOPLANKTON

Photosynthesis of Antarctic phytoplankton is inhibited by ambient ultr aviolet (UV) radiation during incubations(1-4), and the inhibition is worse in regions beneath the Antarctic ozone 'hole'(4). But to evaluat e such effects, experimental results on, and existing models of, photo synthesis(5-7) cannot be extrapolated directly to the conditions of th e open waters of the Antarctic because vertical mixing of phytoplankto n alters UV exposure and has significant effects on the integrated inh ibition through the water column(2,8,9). Here we present a model of UV -influenced photosynthesis in the presence of vertical mixing, which w e constrain with comprehensive measurements from the Weddell-Scotia Co nfluence during the austral spring of 1993. Our calculations of photos ynthesis integrated through the water column (denoted PT) show that ph otosynthesis is strongly inhibited by near-surface UV radiation, This inhibition can be either enhanced or decreased by vertical mixing, dep ending on the depth of the mixed layer, Predicted inhibition is most s evere when mixing is rapid, extending to the lower part of the photic zone. Our analysis reveals that an abrupt 50% reduction in stratospher ic ozone could, in the worst case, lower P-T by as much as 8.5%, Howev er, stronger influences on inhibition can come from realistic changes in vertical mixing (maximum effect on P-T of about +/-37%), measured d ifferences in the sensitivity of phytoplankton to UV radiation (+/-46% ) and cloudiness (+/-15%).