The structure of the plankton community of the Öregrundsgrepen (southwest Bothnian Sea)

Abstract

Taxonomic composition and variations in density and biomass of the plankton community in the Öregrundsgrepen, a shallow coastal area, were investigated from June 1972 to November 1973. The phytoplankton biomass was large in spring but small during the rest of the year. The spring bloom was dominated by diatoms and dinoflagellates, especially byThalassiosira spp. which were also important during other seasons. Small forms, such asCryptomonas spp.,Rhodomonas spp. and monads, dominated during summer. Blue-green algae were never of any major importance. During the summer, the trophogenic layer exceeded 10 m in thickness. The metazoan fauna was of lower diversity than the plankton flora. The dominating species, the copepodsAcartia bifilosa andEurytemora affinis, constituted on the average 83% of the standing crop. The low salinities, 5–6‰ S, were regarded as the principal pertinent limiting factor. The metazoan fauna reached large biomass values from July to October. The protozoan fauna (in the case of ciliates), obtained biomass maxima during the spring bloom. It is suggested that the Öregrundsgrepen represents an area of elevated productivity within a region of low overall production, presumably due to local upwelling. From June 1972 to May 1973, the average biomasses were: phytoplankton 0.464 g C m⁻², ciliates 0.040 g C m⁻², copepod nauplii 0.010 g C m⁻², micro-rotifers 0.004 g C m⁻², and mesozooplankton (larger than 0.2 mm) 0.312 g C m⁻². It is estimated that about than 60% of the phytoplankton production is consumed by the microzooplankton (<0.2 mm).