While June 1998 was still warmer on average than in the preceding years (mean temperature: 16.0 °C), the mean temperature in July was 1 K below average
(15.0 - 16.0 °C). Accordingly, while the sea surface temperature of 12.3 °C in June was 0.3 K above the long-term monthly mean, it was 0.5 K lower in July, at 13.8 °C. Oxygen conditions in the German Bight were generally good. Low levels were found in some deep areas near the western Baltic coasts, with low nutrient concentrations in the surface layer. Hydrogen sulfide formation was observed in deepwater of the Bornholm Basin, the eastern Gotland Basin, and the Farö Deep.
In the North Sea, phytoplankton showed its normal seasonal development from mid-June to the end of July. Noctiluca scintillans was frequent, with mass developments, while the foam-producing Phaeocystis globosa occurred only sporadically. Diatoms(Rhizosolenia imbricata, Guinardia delicatula) prevailed at the west coast of Schleswig-Holstein.
Sprat stocks are at a low level presently. The seal population in the Schleswig-Holstein Wadden Sea has continued to grow.
At the Baltic coast of Schleswig-Holstein, cell numbers of the dinoflagellates Gymnodinium, Scrippsiella, Prorocentrum minimum and Heterocapsa triquetra increased steadily until mid-July. In the second half of July, diatom stocks (Skeletonema costatum, Chaetoceros curvisetus, Proboscia alata) increased and algal blooms formed in the inner Flensburg Bight, Kiel Bight, and in the inner Bay of Lübeck. In the Bay of Lübeck and Mecklenburg Bight, blue-green algae (Nodularia spumigena, Aphanizomenon sp., Anabaena sp.) followed the diatoms at the end of July. Large stocks of small centric diatoms were observed in the outer Pomeranian Bay in June. During the same period, mass developments of blue-green algae occurred in the inner Pomeranian Bay.
In July, potentially toxic blue-green algae were observed in the entire Baltic Sea.
In the Odra bank area, flounder showed a good recruitment, with a strong year-class 1996. Turbot and plaice catches were not satisfactory. The cod year-class 1997 was not comparable to the results in 1996 and 1997.