THE RESPONSE OF PHYTOPLANKTON COMMUNITIES TO PHOSPHORUS INPUT REDUCTION IN MESOCOSM EXPERIMENTS

This study examines the effects of a reduction in the phosphorus input (in comparison to field levels) on natural coastal phytoplankton comm unities. Experiments were conducted in four land-based mesocosms consi sting of 3 m(3) black polyethylene tanks. Light climate and mixing reg ime reproduced conditions in Dutch coastal waters, temperature was mai ntained within 2 degrees C of the ambient seawater temperature. Phytop lankton species succession and primary production were studied in two mesocosm experiments of 5 and 4 weeks in April and July 1993. In both experiments, inorganic nutrients were continuously added to four mesoc osms. N (nitrogen) and Si (silicon) inputs were identical for the four mesocosms and resembled the inputs to the Dutch coastal zone. The pho sphorus (P) input established a N/P ratio gradient among the mesocosms : 16, 32, 64 and 128. The first experiment was carried out during the spring diatom bloom. The initial diatom dominance was maintained in th e mesocosms with reduced phosphorus loads. In the mesocosm with the hi ghest P input, flagellates developed to the detriment of the diatoms a t the end of the experiment in parallel with the establishment of a si licate limitation. The second experiment coincided with the spring/sum mer phytoplankton bloom dominated by the colony of Phaeocystis sp. Dia toms outcompeted Phaeocystis sp. in the mesocosms with reduced phospho rus inputs (N/P = 32, 64, 128). In both experiments, maximum primary p roduction rates were observed in the mesocosm with the highest P input and decreased with the phosphorus gradient. It was concluded that a r eduction of the phosphorus loads to eutrophicated coastal waters deter mined a proportional reduction of the primary production and stimulate d the dominance of diatoms over flagellates.