The warm weather of the preceding months continued in May. Therefore, sea surface temperatures in the North Sea in May, at 11 - 13.5 °C, were up to 1 K above average. At the monitoring network station LT Kiel, the sea surface temperature rose to values around 15.4 °C (2 K above average) in early June.
Nutrient concentrations at the monitoring network station UFS Elbe during the first half of the year showed the normal seasonal development.
Typical summer microalgae species occurred in the entire sampling area. At the coasts of Schleswig-Holstein, a wide species spectrum of rod-shaped diatoms and dinoflagellates was observed. At the end of May, an increase in the diatom Cerataulina pelagica was reported at Helgoland Roads. In the northern Sylt Wadden Sea, the bloom of the foam-forming alga Phaeocystis globosa reached its peak at the end of May and gradually decreased in early June. The typical seasonal occurrence of Noctiluca scintillans was observed everywhere. Potentially toxic algae occurred only sporadically. The development of green algae in the coastal waters of Lower-Saxony was found to be low.
In May, the typical seasonal exhaustion of phosphate levels was observed. Also the nitrate values in many areas were below the limit of detection. Stagnation in the eastern Gotland Basin persisted, and hydrogen sulphide was measured below 140 m depth.
In the coastal area between Hiddensee and the Pommeranian Bight, a strong increase in the phytoplankton biomass was observed in May. The typical spring algae species (Dinophyceae, Cryptomonads, small flagellates) prevailed. Also in the inner coastal waters of Mecklenburg-Vorpommern and in the coastal waters of the southern Arkona Sea, a marked increase in phytoplankton biomass was observed as compared to the preceding years. In the inner bodden lagoons of northern Rügen, plankton production was dominated by a species-poor biocoenosis of blue-green algae (Microcystis botrys, M. flos-aquae, Woronichinia compacta). In the outer coastal waters of the islands of Rügen, Hiddensee, Darss-Zingst and Usedom, the mass development of filamentous algae (Pilayella litoralis) continued.
In mid-June, the species spectrum in the Baltic coastal waters of Schleswig-Holstein still was much too small for the summer season.