Annual variation of environmental variables, phytoplankton species composition and photosynthetic parameters in a coastal lagoon

Physical-chemical variables, phytoplankton biomass, species composition and photosynthesisirradiance (P-I) parameters were analysed during 1 year in the Santo André Lagoon, Southwest Portugal - a land-locked coastal ecosystem with temporary connections with the sea. When the lagoon stayed closed the observed phytoplankton blooms were mainly caused by Prorocentrum minimum, a potentially toxic dinoflagellate. It was dominant during most of the year bur the seawater inflow to the lagoon triggered a decrease in phytoplankton biomass and an abrupt shift in species composition. The maximum photosynthetic rate (P_max) ranged from 2.0 to 22.5 mg C (mg chlorophyll a)^-1 h^-1 and the light saturation index (I_k), ranged from 5.2 to 335.0 μE m^-2s^-1, with winter minima and summer maxima. P_max and I_k were both positively correlated to temperature. Abundance of P. minimum was associated with high nitrate concentrations whereas diatoms appear when ammonium, salinity and wind velocity are high. A mathematical model to describe photosynthetic rate as a function of irradiance and temperature [P (I, t)] was applied to the samples in which P. minimum was the dominant species.