Spectral modeling of UV inhibition of in situ Antarctic primary production using a field-derived biological weighting function

Our major aim is to illustrate an approach for hindcasting or forecasting UV radiation (UVR, 280-400 nm) effects on in situ rates of aquatic primary production when field measurements do not include estimates of UVR effects, A composite of spectral field measurements is employed to model UVR-dependent rates of photosynthesis in diatom-dominated waters in a coastal region of the Southern Ocean, Assumptions, caveats and limitations of the modeling are discussed, Calculations begin with the 1991 Palmer Long Term Ecological Research (LTER) primary production and optical databases, from which daily integrated rates of carbon fixation in the absence of UVR are calculated as a function of depth for a 140 km transect line sampled between dawn and dusk of a single day (14 November 1991), The UVR measurements from the nearby NSF/OPP Polar Network at Palmer Station are used to determine ozone (O-3) concentration on the day of the transect, which is then employed in Madronich's On W-B Radiation and Ozone Depletion (Edited by M, Tevini), pp, 17-68, Lewis, Boca Raton, FL, 1993) spectral code to model daytime variations in surface spectral irradiances under clear sky conditions. These data are corrected for cloudiness and then combined with estimates of in-water UVR spectral attenuation coefficients, derived from Icecolors '90 data, to estimate in situ light exposure for phytoplankton collected at different depths and locations, An absolute chlorophyll-specific biological weighting function (BWF), determined under natural solar light fields for Antarctic diatom communities and shown to be reproducible while differing from a laboratory diatom BWF and other in situ BWF determined for other phytoplankton assemblages, is combined with estimates of in situ UVR exposure to derive in situ estimates of chlorophyll-specific losses of carbon fixation due to WR inhibition, By repeating calculations for every sampling site along the transect, we derive a spatial map of estimated UVR effects on primary production across the region, We repeat calculations for different O-3 concentrations expected during the austral spring over Antarctica and illustrate the O-3 dependency of UVB (280-320 mm) inhibition effects in near surface waters, We estimate ambient UVR reduced carbon fixation rates up to 65% in surface waters, depending upon location, down to undetectable levels at 36 m, Reducing stratospheric O-3 concentrations by 50% further inhibits ;near surface primary production by less than or equal to 8% and integrated primary production by less than or equal to 5%. Primary production was forced to subsurface maxima across the entire transect line in the presence of UVR.