Late summer phytoplankton species composition and biomasses in the Eastern Gulf of Finland

Abstract

The nutrient load of the River Neva and the St. Petersburg region strongly affects the trophic status of the eastern Gulf of Finland. The extent of eutrophication and its effect on the phytoplankton community were studied in the eastern Gulf of Finland in August 1990–1992. The phytoplankton of the easternmost part was dominated by an assemblage of fresh-water and brackish-water species, including numerous indicators of eutrophication. Inside the St. Petersburg flood-protection barrier, phytoplankton growth was limited, probably mainly by turbidity; and it consisted essentially of fresh-water cryptophytes. In 1990 and 1992, marine cryptophytes dominated in the open Gulf. The highest phytoplankton biomasses occurred in the estuary outside the barrier, where polluted river water with a high N/P-ratio mixes with brackish deep water with a low N/P-ratio. The phytoplankton community in the Neva estuary was dominated by the blue-green alga Planktothrix agardhii. In 1991, strong outflow of river water pushed the bloom of Planktothrix agardhii exceptionally far westwards. In addition to the heavy loading, meteorological and hydrographical factors strongly influence the distribution of blue-green algal blooms in the eastern Gulf of Finland.