As in the preceding months, above-average temperatures were also recorded from October through December 1999.
1999 generally was very warm and sunny. At the coast and on the islands of Schleswig-Holstein, it was too dry, while at the coast of Mecklenburg-Vorpommern it was too wet.
Both in September and October the water temperatures reached record levels. In early December, a severe storm surge occurred causing water levels to rise up to 4 m above normal in the Elbe area and in northern Friesland.
As early as mid-September, the abundance of plankton decreased markedly. In the Wadden Sea north of Sylt, diatoms prevailed in October. A conspicuous dominance of the large centric diatom Coscinodiscus radiatus was observed. Another frequently found species was Odontella sinensis, which formed a bloom at the end of October.
On the whole, no extraordinary algal blooms occurred in the coastal waters in 1999. The plankton development was normal and rather moderate, as in 1998. The development of the foam-forming Phaeocystis globosa was at a lower level than the year before. Light-emitting Noctiluca scintillans appeared later than in 1998 and did not reach the levels of that year. Potentially toxic algae occurred only sporadically.
In the tidal flats of Lower-Saxony, a stronger development of green algae was observed during the summer of 1999 than in the last four years.
The cod year-classes 1997 to 1999 appear to be weak. Also the stocks of young plaice and dab were smaller than in the preceding years.
The stagnation in the Baltic Proper continued. This led to oxygen depletion and formation of hydrogen sulphide in the largest areal extent observed in the past 15 years. While in the mid-90s a decrease in nitrogen and phosphate concentrations had been observed, measurements in 1999 showed that this trend has not continued.
In mid-October, the dinoflagellates Ceratium tripos and C. fusus developed in the Mecklenburg Bight, and a bloom of the potentially toxic diatom Pseudonitzschia pungens was observed, which had not shown comparable mass developments in the past years. In the southern Arkona Sea and in the bodden waters, the phytoplankton composition in the period October through December corresponded to long-term observations, with a dominance of Dinophyceae and centric diatoms.
Phytoplankton development in the Baltic coastal waters in 1999 generally was moderate, continuing the trend toward decreasing eutrophication which has been observed since 1990. As in the summer periods of the past 10 years, both toxic and non-toxic blue-green algal blooms occurred in the Baltic Proper. However, they were less pronounced than in 1997.
In the Arkona Sea, an average strength of the 1999 cod year-class was observed. Flounder stocks appear to be stable.
In the summer of 1999, a new species in northern waters was observed in the Estonian coastal waters. It is a freshwater medusa (Maeotias inexpectata) from the Black Sea.
A crustacean species from the Caspian Sea has increased strongly, with mass developments in the coastal waters of Lithuania. This species has been observed earlier in the Gulf of Finland and Bay of Riga.