The Baltic Sea and Its Transition Zones
143
annual salinity and wind velocity variations, suggesting variations in the
degree of mixing between surface water and the phosphorus-rich bottom
water.
More indicative than the observed trends in nutrient concentrations
are the symptoms of eutrophication. During the past 30 years, in large
areas of the Baltic a rapid decrease in oxygen in the deep water and
prevailing anoxic conditions in the deep basins have been observed
(Andersin and Sandler 1983). This phenomenon has been attributed to
an increased inflow of saltier water into the deep basins in the Baltic
since the beginning of this century, resulting in a higher stability of the
halo cline and thus decreased ventilation of the deep water. More recently, it appears more likely that the observed oxygen depletion can be
interpreted as a sign of eutrophication, i.e. increasing production and
thus sedimentation of organic matter results in increased oxygen consumptionbelow the halocline (Larsson et al. 1985; HELCOM 1990; Nehring and Matth1ius 1991).
Since the beginning of the 1980s; several extreme oxygen depletion
events with hydrogen sulfide development and killing of fish and macrozoobenthos have been reported for the Belt Sea and the Western Baltic
(e.g. Ehrhardt and Wenck 1984; Weigelt and Rumohr 1986).
The increased nutrient levels in the photic layer of the Baltic should
ultimately have caused an increased planktonic primary production.
While the results of earlier investigations are contradictory, probably
because the observation periods were not long enough and were partly
overlapping with natural cyclic variations (Kayser et al. 1981), more
recent studies seem to prove effects of eutrophication on plankton production in most areas of the Baltic. Phytoplankton biomass (chlorophyll
a) as well as pelagic primary productivity increased in the western and
southern Baltic and showed positive trends in the other parts, except for
the Arkona Sea and the Gulf of Finland. Also, zooplankton biomass
showed generally positive trends, which, however, were significant only
in the Mecklenburg Bight (Schulz and Kaiser 1986; HELCOM 1990).
Toxic blooms, in particular red tides, have not occurred so far in the
Baltic, although phytoplankton species otherwise known to produce
toxins have eventually been observed, sometimes even mass development
(Graneli et aI. 1990). However, harmful blooms of dinoflagellates, chrysophyceans and prymnesiophyceans seem to become more frequent in
the Kattegat-Belt-Arkona Sea region. In the inner Baltic, blooms were
mostly caused by cyanobacteria. Several potentially toxic species of cyanobacteria, dinoflagellates, chrysophyceans and diatoms occur, which may
increase the risk of development of noxious blooms.
143
annual salinity and wind velocity variations, suggesting variations in the
degree of mixing between surface water and the phosphorus-rich bottom
water.
More indicative than the observed trends in nutrient concentrations
are the symptoms of eutrophication. During the past 30 years, in large
areas of the Baltic a rapid decrease in oxygen in the deep water and
prevailing anoxic conditions in the deep basins have been observed
(Andersin and Sandler 1983). This phenomenon has been attributed to
an increased inflow of saltier water into the deep basins in the Baltic
since the beginning of this century, resulting in a higher stability of the
halo cline and thus decreased ventilation of the deep water. More recently, it appears more likely that the observed oxygen depletion can be
interpreted as a sign of eutrophication, i.e. increasing production and
thus sedimentation of organic matter results in increased oxygen consumptionbelow the halocline (Larsson et al. 1985; HELCOM 1990; Nehring and Matth1ius 1991).
Since the beginning of the 1980s; several extreme oxygen depletion
events with hydrogen sulfide development and killing of fish and macrozoobenthos have been reported for the Belt Sea and the Western Baltic
(e.g. Ehrhardt and Wenck 1984; Weigelt and Rumohr 1986).
The increased nutrient levels in the photic layer of the Baltic should
ultimately have caused an increased planktonic primary production.
While the results of earlier investigations are contradictory, probably
because the observation periods were not long enough and were partly
overlapping with natural cyclic variations (Kayser et al. 1981), more
recent studies seem to prove effects of eutrophication on plankton production in most areas of the Baltic. Phytoplankton biomass (chlorophyll
a) as well as pelagic primary productivity increased in the western and
southern Baltic and showed positive trends in the other parts, except for
the Arkona Sea and the Gulf of Finland. Also, zooplankton biomass
showed generally positive trends, which, however, were significant only
in the Mecklenburg Bight (Schulz and Kaiser 1986; HELCOM 1990).
Toxic blooms, in particular red tides, have not occurred so far in the
Baltic, although phytoplankton species otherwise known to produce
toxins have eventually been observed, sometimes even mass development
(Graneli et aI. 1990). However, harmful blooms of dinoflagellates, chrysophyceans and prymnesiophyceans seem to become more frequent in
the Kattegat-Belt-Arkona Sea region. In the inner Baltic, blooms were
mostly caused by cyanobacteria. Several potentially toxic species of cyanobacteria, dinoflagellates, chrysophyceans and diatoms occur, which may
increase the risk of development of noxious blooms.