In June, warm inflowing air from the Mediterranean led to the highest June temperatures (up to 33 °C) ever recorded in the past 100 years. However, the last week of June was cool, with little sun. The mean water temperature reached 11.7 °C (- 0.3 K).
In June, diatoms (Guinardia flaccida, Guinardia delicatula, Guinardia striata) prevailed in the Northern Sylt Wadden Sea as well as off Helgoland and in the coastal waters of Schleswig-Holstein. Throughout the month of June, the dinoflagellate Noctiluca scintillans, which causes marine phosphorescence, was abundant and showed mass developments. Another commonly found species was the autotrophic ciliate Myrionecta rubra. Toxic algal blooms were not observed.
In the period from July 1998 to April 1999, oil polluted dead sea birds washed ashore on the beaches of the German North Sea coast were systematically recorded. Feather analyses allowed conclusions as to the origin of the oil. 91% of the samples analysed contained pollutant substances originating from ship’s engines.
In June, high insolation and inflows of Mediterranean air led to unusually hot weather (record temperatures up to 36 °C). Cooler periods occurred in the last week of June.
The oxygen values were still normal. A decrease to 5.1 - 6.9 mg O2/L was only observed in the western part of the Mecklenburg Bight. The stagnation continued in the deep water of the Baltic Proper.
The mean water level in June was 2 to 7 cm above the long-term reference values.
The ice winter of 1999/2000 in the northern area of the Baltic Sea has been classified as weak, based on the maximum extent of the ice cover. It ranks among the lower third in the severity ranking covering the past 100 years. Nevertheless, navigation in many areas was temporarily difficult due to the ice conditions.
In June, the species spectrum of microalgae observed on the Baltic coast of Schleswig-Holstein was too low. Low cell concentrations of dinoflagellates (Heterocapsa triquetra, Scripsiella sp.) and of the ciliate Myrionecta rubra (red discoloration of water) were observed. At the measuring station "Seebrücke Heiligendamm", small dinoflagellates and Haptophyceae prevailed in mid-June. On 21 June, the typical summer diatom Dactyliosolen fragilissimus appeared.