Response of Antarctic phytoplankton to solar UVR exposure: inhibition and recovery of photosynthesis in coastal and pelagic assemblages

We examined ultraviolet radiation (UVR)-induced inhibition and recovery of Antarctic phytoplankton photosynthesis during the austral spring in surface coastal waters near Palmer Stn and in the open ocean waters of the Weddell-Scotia Confluence from 1997 to 1999. Primary productivity was measured in 12 h time-course experiments using enclosures that transmitted either full-spectrum solar UVR or only long-wavelength UVA. Recovery from inhibition was determined by transferring samples from high to low UVR at intervals during the incubation. Biological weighting functions for UVR inhibition of photosynthesis were also determined for each experiment. Photosynthesis measurements were compared to model predictions using 3 exposure-response relationships: an irradiance-based (E) model that assumes rapid repair, a cumulative-exposure (H) model that assumes no repair, and a model that incorporated slow repair (R model). Open-ocean phytoplankton were generally much more sensitive to UVR inhibition than coastal assemblages, which primarily had high rates of repair (E model). In contrast, open-ocean assemblages generally recovered from inhibition more slowly (R model). Some recovery was evident in all cases, so the Hmodel was not applied to any assemblage. Our previous view of repair being either simply present or absent is therefore revised to recognize that repair rates range from slow to fast and should be taken into account, together with spectral weight, in assessments of photosynthetic response to UVR in the Southern Ocean, including the effect of ozone depletion. Information on repair rate is particularly important for simulations of production in vertically mixed surface layers.