AN IN-SITU BIOLOGICAL WEIGHTING FUNCTION FOR UV INHIBITION OF PHYTOPLANKTON CARBON FIXATION IN THE SOUTHERN-OCEAN

A daily integrated in situ biological weighting function (BWF) for inh ibition of primary production by ultraviolet radiation (UVR, 280 to 40 0 nn) was determined for a natural community of Antarctic diatoms main tained under daylight conditions. The derived daily averaged BWF had a radiation amplification factor of 0.91 for the environmental radiatio n conditions under which it was determined, and displayed greater sens itivity to UV-B than BWFs determined for laboratory cultures of temper ate latitude phytoplankton (Cullen et al. 1992; Science 258:646-650). In addition, the function was shown to accurately predict the UVR-depe ndent in situ rates of primary production when the same community was under different stratospheric ozone (O-3) conditions. An error estimat e for the BWF is also provided and the predictive Limitations of the f unction are discussed briefly. In the early austral spring of 1993 nea r Palmer Station, Antarctica, surface samples were maintained in 6 spe ctrally distinct outdoor incubators over the course of a single day an d the spectral sensitivity of photosynthetic carbon fixation rates and phytoplankton pigmentation was quantified. The changes in spectral se nsitivity to O-3-dependent UV-B (280 to 320 nm radiation) and O-3-inde pendent UV-A (320 to 400 nm radiation) was resolved on time scale of 2 h intervals over the course of the 10 h incubation. Besides determini ng the daily peak of cell sensitivity to UVR damage, the derived short -term kinetics for the 6 different spectral light treatments provided the database for resolving a robust action spectrum for the UVR inhibi tion of in situ sates of primary production. For the diatom community being studied, daily exposure to ambient levels of UVR resulted in a 3 4% reduction in averaged carbon fixation without any significant effec t on the cellular pigment content. The UV-B portion of the solar spect rum photoinhibited daily rates of primary production by 15%, while UV- A was responsible for a 19% reduction in daily averaged rates of carbo n fixation. It appears that springtime diatom-dominated communities ar e equally or more sensitive to UV-B photoinhibition of daily primary p roduction than prymnesiophyte-dominated communities, analyzed during t he 1990 'Icecolors' expedition (Smith et al. 1992; Science 258:952-959 ).