Phytoplankton photosynthetic response to solar ultraviolet irradiance in the Ross Sea Polynya: Development and evaluation of a time-dependent model with limited repair

The relationship of photosynthesis (C-14 incorporation) to ultraviolet (UVR) and photosynthetically active radiation (PAR) was measured over the course of the late-spring to early-summer phytoplankton bloom in the Ross Sea Polynya (Southern Ocean). Experiments were conducted in November 2005 to determine PAR-only photosynthesis-irradiance (P-E) curves, biological weighting functions (BWFs) using a new version of the photoinhibitron laboratory spectral incubator, and variation in photosynthesis under high vs. low solar UVR treatments in on-deck incubations. These observations were incorporated into a new spectral model of photosynthetic response to UVR + PAR with time-dependent repair rates. The distinguishing feature of this model is that repair scales with inhibition up to a maximum absolute repair rate (r(max)). Once repair is limited at the maximum rate, additional exposure has a more severe inhibitory effect on photosynthesis, consistent with measured exposure response curves. Parameters for the BWFRmax/P-E model were determined for 10 sampling locations ranging from mixed diatom and Phaeocystis antarctica assemblages at the beginning of the bloom to assemblages dominated by P. antarctica at the peak of the bloom. The model explained 86-97% of the measured spectral variation with BWFs severalfold higher (more inhibitory) than those previously measured in the Weddell-Scotia Confluence and coastal waters near the Antarctic Peninsula. Predicted relative productivity (ratio of modeled photosynthesis under high vs. low UVR) was close to observed relative productivity, but absolute photosynthetic rates were higher in the on-deck incubations than in the photoinhibitron.