Elevated CO2 increases sensitivity to ultraviolet radiation in lacustrine phytoplankton assemblages

This study tests the effects of elevated CO2 and ultraviolet radiation (UVR) on phytoplankton photosynthesis through in situ incubations in Lake Giles, Pennsylvania. In a first experiment, CO2 was supplied from a tank to simulate atmospheric CO2 concentrations predicted in scenarios of future global change. In a second experiment, elevated CO2 conditions were obtained by the mineralization of added colored dissolved organic matter (CDOM) of terrestrial origin (400 mmol L-1 final concentration). The results demonstrated that for natural assemblages from Lake Giles, atmospheric CO2 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 CO2 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 CO2 supply as well as with the CDOM addition, the latter resulting in CO2 partial pressures close to three times present atmospheric levels. Photosynthetic rate modeled for elevated CO2 and midday solar exposure just below the lake surface was 17% of potential production compared with 21% under usual CO2 levels in the lake, resulting in similar rates between phytoplankton assemblages grown under high and low CO2 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.