Interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton

Photosynthesis of Antarctic phytoplankton is inhibited by ambient ultraviolet (UV) radiation during incubations(1-4), and the inhibition is worse in regions beneath the Antarctic ozone 'hole'(4). But to evaluate such effects, experimental results on, and existing models of, photosynthesis(5-7) cannot be extrapolated directly to the conditions of the open waters of the Antarctic because vertical mixing of phytoplankton alters UV exposure and has significant effects on the integrated inhibition through the water column(2,8,9). Here we present a model of UV-influenced photosynthesis in the presence of vertical mixing, which we constrain with comprehensive measurements from the Weddell-Scotia Confluence during the austral spring of 1993. Our calculations of photosynthesis integrated through the water column (denoted PT) show that photosynthesis is strongly inhibited by near-surface UV radiation, This inhibition can be either enhanced or decreased by vertical mixing, depending on the depth of the mixed layer, Predicted inhibition is most severe when mixing is rapid, extending to the lower part of the photic zone. Our analysis reveals that an abrupt 50% reduction in stratospheric ozone could, in the worst case, lower P-T by as much as 8.5%, However, stronger influences on inhibition can come from realistic changes in vertical mixing (maximum effect on P-T of about +/-37%), measured differences in the sensitivity of phytoplankton to UV radiation (+/-46%) and cloudiness (+/-15%).