The monthly mean temperatures in September, at 14 to 15 °C, were mostly 0.4 to 0.5 K above the long-term reference values. Precipitation, at 50 to 70 mm was below average in most areas. The average sea surface temperature of 14.6 °C was 0.9 K above the mean value.
The weather in October, with average rainfalls of 55 - 100 mm, was about 1 K too warm (monthly mean temperatures: 11 - 12.9 °C). The duration of sunshine, at 80 - 100 hours, differed only slightly from normal. The mean sea surface temperature of 12.7 °C, therefore, exceeded the climatological value by 0.9 K.
In late August/early September, nutrient concentrations had their seasonal low. Nitrate and phosphate were largely depleted. Oxygen supply in the deeper layers of the German Bight during the summer was good.
In September, the plankton composition was characterized by a high species diversity and low abundance. The overall development of phytoplankton was normal. Potentially toxic algae occurred only sporadically.
Phytoplankton stocks in October generally showed the normal seasonal development. Off Helgoland, the diatom species Odontella sinensis and Chaetoceros were observed. The dominant flagellate species was Prorocentrum triestinum. Decreasing water temperatures and windy weather continued to reduce phytoplankton stocks in the second half of October. In the Wadden Sea area north of Sylt, only a large diversity of diatom species were found, which is the typical seasonal pattern. Rhizosolenia prevailed, and the diversity of Thalassiosira species (typical winter species) increased strongly.
September, at temperatures between 13.4 and 14.1 °C, was slightly too warm, but no more summer days were recorded. Due to some strong rainfalls, monthly totals exceeded the mean values by up to 154 %. Above-average water temperatures were measured along the coasts of Mecklenburg-Vorpommern, in the nearshore shallow water zone.
October was noticeably too warm (monthly mean temperatures: 11 - 12 °C Oct.) and much too dry.
No extreme water levels occurred at the German Baltic Sea coast in September. In October, as in September, water levels were below the reference series.
In September, salinity in the surface layer (5.7 - 10.6) of the outer coastal waters was clearly reduced due to the above-average precipitation. Salinity in the bottom water (6.5 - 18.0) was also below normal. In almost all inner coastal waters, salinity in the surface layer was close to normal in spite of the strong rainfalls (1.2 - 13.7).
At the end of October, salinity in the Fehmarnbelt area rose due to the prevailing westerly winds (10 - 15 m/s).
In the coastal waters of Mecklenburg-Vorpommern, nutrient levels in August were normal for the season. Hardly any phosphate was measured in the entire sea area. Nitrate occurred only off Boltenhagen and in the Pomeranian Bight at very low concentrations. Silicate levels were normal at most measuring points. Also in September, the situation hardly changed at the coasts of Mecklenburg-Vorpommern.
Measurements in the Baltic Sea showed depletion of nitrate and traces of phosphate in September. In the bottom water of the central basins, phosphate levels increased while nitrite and nitrate values were close to zero because of the poor oxygen situation.
In areas with low water exchange (coastal inlets and bights), only about 10 % O2-saturation was measured in the bottom water in August. Critical values were found locally. In September, the oxygen situation normalized in almost all of these areas, which led to improved living conditions.
Measurements in late October/early November confirmed the trend toward increasing oxygen deficiency in the deepwater of the Baltic Proper.
Compared to 1999, hydrogen sulphide in August increased in the deep areas of the northern Baltic, but decreased in the Bornholm deep. Late October/early November Landsort Deep showed unusual high hydrogen sulfide concentrations.
In September, the population of microalgae in the coastal waters of Schleswig-Holstein was characterized by different Ceratium species, Dactyliosolen fragilissima, a small algal bloom of Prorocentrum micans, and Emiliana huxleyi.
At the measuring station "Seebrücke Heiligendamm", a typical autumn bloom of the dinoflagellate Ceratium tripos occurred. A cyanophyceae bloom of the genus Planktothrix (1.5 million filaments/L) was observed in the coastal waters of Mecklenburg-Vorpommern, north of Ahlbeck. High abundances of microalgae were recorded in the areas of Strelasund and Greifswalder Bodden, at 70.5 and 138 million cells per liter, respectively. In the Stettiner Haff (Firth of Stettin), algae of the genera Planktothrix and Aphanizomenon continued to be dominant.
In October, the phytoplankton biomass at the measuring station "Seebrücke Heiligendamm" decreased sharply.
Because of oxygen deficiency in the Arkona Sea and Bornholm Sea in late October/early November, hardly any macrozoobenthos was found. At the Fehmarnbelt station, Ophiura albida occurred in large quantities. Saduria entomon was abundant in the southern Arkona Sea.
The poor condition of cod stocks in the western Baltic Sea was confirmed. The strong 1997 year-class has been largely harvested, and older cod hardly exist any more. The largest number of cod was recorded for the 1999 year-class. Also remarkable were the clearly smaller catches of herring and sprat in the entire area of the Arkona Sea.