Current and predicted global change impacts of UVR, temperature and nutrient inputs on photosynthesis and respiration of key marine phytoplankton groups

Multiple stressors are altering primary production in coastal and estuarine systems; however, it is difficult to predict their combined impacts due to the scarcity of multifactorial experiments. Photosynthesis, respiration, and PSII photochemical performance of Alexandrium tamarense, Chaetoceros gracilis, Dunaliella salina and Isochrysis galbana were studied during daily cycles using a combination of two radiation treatments (UVR + PAR and PAR), two nutrient concentrations, and three temperatures (14, 17 and 20 °C). UVR exerted a negative impact in all species decreasing photosynthesis and quantum yield of PSII under low nutrient concentrations and temperatures up to 20 °C. At higher temperatures (global change scenario of 4 °C increase) and increased UVR and nutrients, C. gracilis and I. galbana reversed their responses by increasing photosynthesis and repair rates, respectively; they also showed a decrease in respiration rates. In contrast, A. tamarense and D. salina showed further decrease in photosynthesis and repair rates compared to present conditions. Our modeled responses to warming under a scenario of increased nutrients and UVR suggest that diatoms and haptophytes will benefit from these conditions and possibly will outcompete chlorophytes and dinoflagellates. If this is a generalized response, it might influence primary production and affect food web interactions in coastal ecosystems.