September 1997 was extraordinarily warm, at air temperaturesbetween 14.0 and 15.5 °C. Compared to long-term mean values, October and November were up to 1.5 K below the long-term mean temperatures. All three months covered by the report were too dry.
Sea surface temperatures during this period were above the climatological mean values (1971 - 93). In the eastern Gotland Basin, hydrogen sulfide was found below 130 m. Anoxic areas had spread in this basin since August.
On the coasts of Lower-Saxony, plankton samples in September
showed a mode-rate concentration of diatoms. Flagellates in the plankton
were dominated by dinoflagellates. Colonies of the foam-forming
Phaeocystis and light-emitting Noctiluca algae occurred only
sporadically. With falling water temperatures in October and
November, the abundance of plankton decreased as well. Potentially
toxic species of the genus Dinophysis were found at the stations Osterems,
Jade, and Harle. In the North Sylt Wadden Sea, rod-shaped plankton dominated
by the species Rhizosolenia imbricata was frequent, with mass development
in some places. On the west coast of Schleswig-Holstein, a marked increase
in the range of species was observed in the two large groups of diatoms and
dinoflagellates in September. In the following months, falling water
temperatures caused a decrease in the algal population.
In the September plankton in the near-shore area of the Arkona Sea,
the blue-green algae Nodularia spumigena and Anabaena spp. continued to be present. Dinophyceae were present from late September to early October. From mid-October, the overall
plankton concentration was conspicuously low.
In September large-scale blooms of blue-green algae did not occur
any more in the northern Baltic Sea.