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 phytoplanktoncommunities isolated from the Neuse River Estuary in the springof 1998 and 1999 to assess the interactions between nutrientlimitation and ultraviolet (UV) radiation on photosyntheticparameters. Treatments consisted of the addition of differentforms of nutrients typically found in estuarine environmentsand the exclusion of ambient UV radiation (wavelengths <400 nm). The quantum yield of photochemistry (F_v/F_m), speciescomposition, and photosynthesis irradiance parameters were measuredrepeatedly over the course of 4 days. In spring 1998, duringa period of stratification and low run-off, nutrient limitationwas observed in the Neuse River. The F_v/F_m parameters in allthe 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 observableeffect. Conversely, during a period of high mixing in spring1999, there was no nutrient addition effect on F_v/F_m. Duringboth experiments F_v/F_m exhibited midday light-driven depressionsin response to high irradiances, with complete recovery at night.UV radiation accounted for a significant fraction of the middaydepression seen in F_v/F_m. Samples treated with the D1 proteinsynthesis inhibitor lincomycin showed that the midday decreasein photochemical efficiency was mostly due to photoinduced damageto the D1 protein. As an upper limit estimate, 80% of the decreasein F_v/F_m centered around local noon appears to be related tothis damage. The decrease in photochemical efficiency seen athigh light levels in both UV-exposed and UV-excluded treatmentswas not correlated with a decrease in carbon fixation parameters.