August 1999, at average temperatures of 17 - 18 °C, was warmer than in the preceding years but cooler and more changeable than July 1999. Frequent and heavy rainfall, sometimes in connection with thunderstorms, ended the dry period.
At the monitoring station UFS Elbe, water temperatures up to 18.7 °C were measured. While nutrient concentrations in the German Bight were found to be very low, the Elbe monitoring station reported
elevated phosphate levels as early as the second half of July and, from mid-August, increased ammonium concentrations. Oxygen depletion in the near-bottom area did not reach critical levels despite the warm weather.
Plankton found along the East Frisian coast contained a wide variety of diatom species (Chaetoceros socialis, Bacteriastrium hyalinum, Rhizosolenia imbricata, Guinardia delicatula), dinoflagellates, small flagellates (Gyrodinium aurolum) and ciliates (Mesodinium rubrum). High cell concentrations were not reached. Colonies of the foam-forming alga Phaeocystis globosa were not very abundant. Potentially toxic algae like Dinophysis accuminata, Chrysochromulina sp., Chattonella sp., and Fibrocapsa sp. were found only sporadically. In North Sea fisheries, a strong 1999 year-class of haddock was observed.
Sea surface temperatures in August fluctuated between 19 and 21 °C. Phosphate was still detectable in the surface water. Nitrate levels were low. The oxygen concentration in the surface water was about 100%. Near the bottom, oxygen levels in the Kiel and Mecklenburg Bights ranged between 20 and 27 %. Hydrogen sulphide was found in all deep basins of the Baltic Sea.
Plankton in the Baltic Sea waters off the coast of Schleswig-Holstein showed a clear decrease in the population densities. Dinoflagellates such as Prorocentrum micans and Ceratium fusus were found regularly. The diatoms Cerataulina pelagica and Rhizosolenia pungens occurred as well. Blue-green algae were hardly observed any more. In the coastal waters of the southern Arkona Sea, the abundance of the usually predominant N2-fixing cyanophyceae was low. Coccoid blue-green algae
(Aphanothece, Lemmermanniella, Snowella, Woronichinia) prevailed.
In some parts of the eastern and southern Baltic Sea, blue-green algae were still abundant in early August, but there were hardly any major surface accumulations in the open sea. In the second half of August, cell concentrations of blue-green algae in open Baltic Sea waters continued to decrease. The potentially toxic blue-green alga Nodularia spumigena was found at very low concentrations, while near the coast an increase in the non-toxic Aphanizomenon sp. was observed.