Synergy of light and nutrients on the photosynthetic efficiency of phytoplankton populations from the Neuse River Estuary, North Carolina

A series of mesocosm studies were conducted using natural phytoplankton communities isolated from the Neuse River Estuary in the spring of 1998 and 1999 to assess the interactions between nutrient limitation and ultraviolet (UV) radiation on photosynthetic parameters. Treatments consisted of the addition of different forms of nutrients typically found in estuarine environments and the exclusion of ambient UV radiation (wavelengths < 400 nm). The quantum yield of photochemistry (F_v/F_m), species composition, and photosynthesis irradiance parameters were measured repeatedly over the course of 4 days. In spring 1998, during a period of stratification and low run-off, nutrient limitation was observed in the Neuse River. The F_v/F_m parameters in all the mesocosm treatments responded to the addition of nitrogen. The form of the nitrogen addition (nitrate, ammonium, or urea) was insignificant and the addition of phosphorus had no observable effect. Conversely, during a period of high mixing in spring 1999, there was no nutrient addition effect on F_v/F_m. During both experiments F_v/F_m exhibited midday light-driven depressions in response to high irradiances, with complete recovery at night. UV radiation accounted for a significant fraction of the midday depression seen in F_v/F_m. Samples treated with the D1 protein synthesis inhibitor lincomycin showed that the midday decrease in photochemical efficiency was mostly due to photoinduced damage to the D1 protein. As an upper limit estimate, 80% of the decrease in F_v/F_m centered around local noon appears to be related to this damage. The decrease in photochemical efficiency seen at high light levels in both UV-exposed and UV-excluded treatments was not correlated with a decrease in carbon fixation parameters.