The weather from January to April was warmer than usual (average air temperature in April: 8,2 °C). Therefore, the sea surfacetemperatures in the North Sea were up to 0.6 K above the climatological value (average water temperature in April: 7,2 °C). Also in the Baltic Sea, high air temperatures in March and April led to above-average water temperatures. Precipitation was regionally very different, exceeding the reference values by up to 90 % in some places. The winter of 1998/99 was characterized by a weak ice season.
The water was mostly well-mixed, which is typical of the season. The phosphate concentrations differed hardly from the reference value of 1936. In late March/early April, similar as in 1998, the spring plankton bloom set in. While in the preceding years mainly large diatoms were observed near Helgoland, smaller species (e.g. Thalassiosira species) were found in that area in 1999. In late April/early May, the typical foam-forming alga of that season - Phaeocystis globosa - began to develop. In mid-May, Rhizosolenia and Guinardia species were increasingly observed. At water temperatures around 11 °C, the abundance of Noctiluca scintillans increased.
The juvenile 1998 year-classes of sprat, whiting and Norway pout are good, while those of cod, herring, haddock and mackerel are weak.
In the Baltic Proper, nutrient levels in February and March were typical of the winter season. Near the coasts, the trend toward lower nutrient concentrations which has been observed since 1992 continued in the winter of 1998/99. Oxygen levels in the sea surface layer were high. Hydrogen sulfide has been measured in the eastern Gotland Basin since March. In late March/early April, dinophyceae (Heterocapsa rotundata, Prorocentrum balticum, Gymnodinium spp.), cryptophyceae (Teleaulax spp., Hemiselmis sp.) and diatoms (Chaetoceros species, Skeletonema costatum) prevailed, which is the typical seasonal pattern. As in the preceding years, this was followed by a transition phase characterized mainly by the group of small flagellates. A strong vegetation of filamentous brown algae was observed on the outer coasts of the isle of Rügen and in the Bodden lagoons close to the Baltic Sea.
Standard catches of cod in the Kattegat, Belt Sea, Arkona Sea, and Bornholm Sea were higher than last year.