Effects of acidification and warming on a coastal mediterranean plankton food web: the WARMACIDWEB mesocosm experiment.

Coastal and continental shelf regions constitute the most diverse and productive parts of the world's oceans providing substantial ecosystem services. Hence, understanding the effects of climate change on the functioning of coastal productive ecosystems is a main ecological and economical challenge. The consequences of the projected climate change on the structure and functioning of a plankton community were investigated during a mesocosm experiment in the coastal Mediterranean Thau lagoon. This lagoon is an important European shellfish farming area sustaining 10% of the French oyster production. Twelve mesocosms (about 2300 L each, 2 m depth) were deployed in the lagoon and filled with natural surrounding waters to realize four treatments each in triplicates: i) water pCO2 increase, ii) water temperature increase, iii) both water pCO2 and temperature increases, and iv) natural water pCO2 and temperature conditions. Water temperature was increased by 3°C using submersible heating elements, and pCO2 was increased to approximately 900 µatm via CO2 enriched seawater additions. Dissolved inorganic nutrients were added to all of the mesocosms in the middle of the experiment to mimic nutrient input in the water column which frequently occurs in this system due to wind events. Water temperature, salinity, pH, oxygen and fluorescence were measured every 2 minutes using autonomous sensors, while discrete samples were taken daily during 12 days to study the dynamic of chemical parameters and plankton abundances, diversities and productions. The phytoplankton bloom picked two days after the nutrient addition with chlorophyll a concentrations attaining up to 4.7 µg L-1. Two independent measurements of the chlorophyll a (HPLC and by continuous in situ fluorescence monitoring) showed that acidification tends to enhance the phytoplankton biomass. In particular, pigment biomarkers highlighted that mainly diatoms tend to increase under acidification. Shifts in the plankton community structure, as well as interactions between acidification and temperature increases will be further discussed.