Changes in plankton size structure and composition, during the generation of a phytoplankton bloom, in the central Cantabrian sea

Six stations situated on a transect perpendicular to the coast were sampled eight consecutive times in the central Cantabrian Sea, during February-March of 2005. A contrast was observed in the timing, magnitude and size structure of a phytoplankton bloom between coastal and oceanic stations, probably due to differences in the depth and mixing of the water column. The increase of biomass during the bloom occurred through the addition of "new" size classes of larger cells. An increase of small particle biomass was also observed at oceanic stations. In view of the results, it is proposed that at coastal stations, improved environmental conditions (nutrients and light) enhanced productivity and increased the number of both small and large cells. Because predation susceptibility is related to size, only large diatoms were able to escape from the predatory pressure exerted by microzooplankton. The results obtained indicate that imaging technology, combined with automatic recognition techniques, constitutes a powerful approach to describe plankton distributions at a fine temporal scale.