This study tests the effects of elevated CO 2 and ultraviolet radiation (UVR) on phytoplankton photosynthesis through in situ incubations in Lake Giles, Pennsylvania. In a first experiment, CO 2 was supplied from a tank to simulate atmospheric CO 2 concentrations predicted in scenarios of future global change. In a second experiment, elevated CO 2 conditions were obtained by the mineralization of added colored dissolved organic matter (CDOM) of terrestrial origin (400 mmol L 21 final concentration). The results demonstrated that for natural assemblages from Lake Giles, atmospheric CO 2 concentrations similar to those predicted for the end of the century can increase primary productivity up to 23% in the absence of photoinhibition. However, elevated CO 2 concentrations also increased sensitivity of phytoplankton to UVR, making cells more susceptible and increasing photoinhibition. Increased sensitivity was observed in samples incubated with the artificial CO 2 supply as well as with the CDOM addition, the latter resulting in CO 2 partial pressures close to three times present atmospheric levels. Photosynthetic rate modeled for elevated CO 2 and midday solar exposure just below the lake surface was 17% of potential production compared with 21% under usual CO 2 levels in the lake, resulting in similar rates between phytoplankton assemblages grown under high and low CO 2 levels. Understanding the effect on primary productivity of the interaction between factors that may be affected by global change is essential to predict future changes in ecosystems and climate.

The relationship of photosynthesis ( 14 C 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 BWF Rmax /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.

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