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

Physical-chemical variables, phytoplankton biomass, species composition and photosynthesis-irradiance (P-I) parameters were analysed during 1 year in the Santo Andre Lagoon, Southwest Portugal - a land-locked coastal ecosyste m with temporary connections with the sea. When the lagoon stayed closed th e observed phytoplankton blooms were mainly caused by Prorocentrum minimum, a potentially toxic dinoflagellate. It was dominant during most of the yea r but the seawater inflow to the lagoon triggered a decrease in phytoplankt on biomass and an abrupt shift in species composition. ph I The maximum pho tosynthetic 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 muE m (2) s(-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, sal inity and wind velocity are high. A mathematical model to describe photosyn thetic rate as a function of irradiance and temperature [P (I, t)] was appl ied to the samples in which P minimum was the dominant species.