September 1999 was the warmest September month of the century (mean temperatures up to 18.5 °C). Also the water temperatures reached record levels (mean SST of the North Sea in September: 15.7 °C).
Measurements showed low nutrient levels, which is the typical seasonal pattern. Oxygen conditions in the German Bight
On the coasts of Lower-Saxony, a wide species spectrum of plankton was found consisting of diatoms, dinoflagellates, small flagellates, and ciliates, all characterized by low abundance. Potentially toxic Dinophysis acuminata were rarely observed. Green algae still occurred in almost all Wadden Sea areas in September.
Phytoplankton stocks showed a declining tendency near Helgoland. A wide variety of diatom species was found (Coscinodiscus wailesii); among the dinoflagellates, Prorocentrum triestinum prevailed. On the coasts of Schleswig-Holstein and in the northern tidal flats of Sylt, diatoms were the predominant species (Chaetoceros socialis, Coscinodiscus radiatus, Odontella sinensis, Rhizosolenia imbricata, Leptocylindrus danicus). Dinoflagellates (Gymnodinium species) were less abundant.
A mass development of the subtropical Cladocera Penilia avirostris (planktonic crustacean) was observed in the North Sea.
Nutrient levels were low. While the surface water in all parts of the Baltic was saturated with oxygen, critical values were reached in the bottom water of inlets and bays. Hydrogen sulphide was found in the Hanö Bay and Bornholm Basin (below 70 and 80 - 85 m, respectively), Karlsö Deep (below 100 m), the eastern Gotland Basin, and in the western and northern Baltic Sea (below 150 m).
The species spectrum and abundance of plankton was normal for the summer/autumn season. Dactyliosolen fragilissimus, a typical summer diatom, was ubiquitous in September. Also the dinoflagellates Ceratium tripos and C. fusus were found. In the Wismar Bay, cell concentrations of the potentially toxic Prorocentrum minimum reached 1.1 million cells per liter. Coccal blue-green algae of the species Woronichinia and Snowella predominated in the Pomeranian Bight. Along the outer coasts of the island of Hiddensee, in the Strelasund area and in the bodden waters between Rügen and Hiddensee, an intensive bloom of blue-green algae was observed ( Nodularia spumigena, Aphanizomenon 'baltica', Anabaena lemmermannii). In October, a bloom of the potentially toxic diatom Pseudonitzschia pungens of unprecedented intensity was observed at the measuring station "Seebrücke Heiligendamm". Filamentous brown algae and sea grass were found increasingly on the beaches along the outer coasts.