The nutrient load of the River Neva and the St. Petersburg region stro
ngly affects the trophic status of the eastern Gulf of Finland. The ex
tent of eutrophication and its effect on the phytoplankton community w
ere studied in the eastern Gulf of Finland in August 1990-1992. The ph
ytoplankton 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 i
t consisted essentially of fresh-water cryptophytes. In 1990 and 1992,
marine cryptophytes dominated in the open Gulf. The highest phytoplan
kton biomasses occurred in the estuary outside the barrier, where poll
uted 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, s
trong outflow of river water pushed the bloom of Planktothrix agardhii
exceptionally far westwards. In addition to the heavy loading, meteor
ological and hydrographical factors strongly influence the distributio
n of blue-green algal blooms in the eastern Gulf of Finland.