Seasonal succession and growth in the plankton communities of the Gulf of Riga in relation to long-term nutrient dynamics

Abstract

To study temporal changes in phyto- and zooplankton in the Gulf of Riga monitoring data covering the period of 1972–1995 were analyzed. Changes in external nutrient supply have turned P limitation, during the 1980s, into the present deficiency of Si for the spring bloom. Contrary to depletion of the N and Si pools, the internal sources and sinks were responsible for input of P into the pelagic ecosystem in the 1990s. Restructuring of the nutrient pool reshaped the pelagic planktonic communities. Changes of the species composition and food-web relationships occurred. Till the 1990s, an increase of diatoms in the summer phytoplankton and an increase of herbivores in zooplankton took place. Later, the role of dinoflagellates, greens, blue-greens and heterotrophic algae increased and more intense competition between herbivorous species of zooplankton developed. The nutrient accumulations in winter and the riverine input defined the phytoplankton spring growth, whereas the decline phase was determined by sedimentation and grazing of the bloom by crustacean zooplankton. Heterotrophic dinoflagellates formed only a negligible share of the vernal phytoplankton. The nutrient requirements of the summer algae were covered by vertical transport and regeneration, where zooplankton-mediated fluxes could be important. There were environmental indications on long-term modification of nutrient summer cycling. The vigorous blue-green algae blooms may have contributed to the sharp increase of the P surface values in summer of the 1990s.