A NEW TREND IN THE DEVELOPMENT OF THE PHYTOPLANKTON IN THE OOSTERSCHELDE (SW NETHERLANDS) DURING AND AFTER THE CONSTRUCTION OF A STORM-SURGE BARRIER

During the pre-barrier period (1982-83), the Oosterschelde phytoplankt on were a diatom-dominated community, comprising a species-rich assemb lage throughout the year. Assemblages of spring, early summer and summ er, developed in response to a gradually evolving turbidity-light grad ient during the course of the year. During the barrier-construction pe riod (1984-87), characterized by decreasing current velocities, increa sing sedimentation of suspended matter, increasing water transparencie s and unchanged nutrient conditions, the growth season for the phytopl ankton started earlier and lasted longer. Some flagellate species resp onded by much higher biomass than before. The impact of short-term cli matic factors during this period, notably severe winters, could be ill ustrated with examples of clear responses of some species (e.g. Biddul phia aurita). In the post-barrier years (1987-90) a changed light-nutr ient-salinity regime (i.e. much light, limitation of nitrate, high sal inity) was demonstrated and an extended summerseason developed, withou t the original gradual transitions. This was reflected in an a-seasona l trend of the phytoplankton assemblage, where summer species were alr eady observed in spring and spring species decreased in abundance. In summer small flagellates increased and some weakly silicified diatom s pecies made their appearance. In the eastern compartment no colony for mation of Phaeocystis occurred in summer and this was thought to be du e to nitrate limitation. Changes in abundance of some species (Phaeocy stis, Ditylum brightwellii, Skeletonema costatum), occurring during th e entire period of investigation (I 982-90), could be explained using field observations compared with experimental evidence from the litera ture. The relationship between species composition and biomass on the one hand and environmental variables on the other hand, was analysed i n a Canonical Correspondence Analysis, for both compartments separatel y.