Photosynthetic Characteristics of Phytoplankton During the Development of a Summer Bloom in the Urdaibai Estuary, Bay of Biscay

Day-to-day variations in the photosynthetic characteristics of phytoplankton were studied over a fortnight-long period in summer in the Urdaibai estuary (Bay of Biscay) and related to changing environmental conditions. Both physical and chemical properties of the estuary were profoundly influenced by changes in river flow and tidal stirring. Phytoplankton biomass and productivity increased during neap tides, coinciding with the improvement of weather conditions. A highly productive bloom, composed mainly of cryptophyceans,Euglenasp., and the dinoflagellateGlenodinium foliaceumwas a major feature in the upper estuarine section. There was a link between the physical changes in the environment and the time-scale of phytoplankton responses. Photosynthetic parameters were determined for phytoplankton collected at two stations characterized by different light regimes. While photoadaptation processes were distinguished both in time and space, daily variability appeared to be more important than diel periodicity in the photosynthetic capacity of phytoplankton. Much of the spatio-temporal variability (81%) in primary production was explained by light availability and phytoplankton biomass. Finally, two approaches for predicting productivity were compared: the ‘ light×biomass model ’ and the photosynthesis-irradiance (P-I) model. Results indicated that primary production can be estimated directly from environmental variables on a broad scale, but for finer resolution it is more appropriate to use the P-I model.