Seasonal stages of phytoplankton community structure and sinking loss in the Gulf of Riga

In this paper, we present the biomass, species composition and sinking losses of phytoplankton and heterotrophic flagellates obtained during three seasonal stages (May 1995; June–July 1994; August 1993) from the Gulf of Riga — a eutrophied, semi-enclosed area in the Baltic Sea. The Gulf was characterised by intensive dinoflagellate (mainly Peridiniella catenata) dominated spring bloom (2700–7600 μg l−1 wet weight) while the diatom Thalassiosira baltica contributed most (80–90%) to the settling phytoplankton biomass (up to 6.5 g m−2 day−1). The mineral nutrients were abundant during the bloom and it is suggested that the differential sedimentation of species was caused by physical factors (thermal stratification of the water column). The phytoplankton biomass in summer (780–2600 μg l−1) was dominated by high abundance of filamentous cyanobacterium Aphanizomenon flos-aquae, autotrophic nanoflagellates and picoplankton. The primary vertical flux of phytoplankton (<1 g m−2 day−1) was mainly due to non-motile species and aggregate-bound picoplankton and A. flos-aquae. Below the pycnocline, resuspended dormant diatom populations caused a significant (up to 1 g m−2 day−1) secondary flux. Large heterotrophic dinoflagellates (Gyrodinium/Gymnodinium) were abundant (up to 500 μg l−1) in the middle layers and associated to ammonium regeneration. The early autumn stage was a post cyanobacterial bloom situation with relatively low phytoplankton abundance (330–860 μg l−1) dominated by A. flos-aquae and nanoflagellates. Also, the sedimentation of phytoplankton was lowest (up to 340 mg m−2 day−1). Our results indicate high seasonal differences in the phytoplankton community structure and sedimentation. However, the high within-season temporal variability overrules the variability between different areas of the southern part of the Gulf.