April was very warm, with temperatures of 8 - 10 °C which were about 2.5 K above the reference values. The sea surface temperatures accordingly reached above-average values (April: 7.2 °C).
The weather in the first half of May was exceptionally dry, warm and sunny. Although the second half of May was changeable and cool, the mean temperatures reached 15.2 °C. The average sea surface temperature, at 10.3 °C (1.2 K above the climatological value), was the second highest temperature since records began in 1971.
In May, the oxygen saturation measured in the German Bight was approx. 100 %.
Due to the continuing warm weather since mid-April, many typical summer species (increased abundance of dinoflagellates and Chaetoceros species) were found in plankton as early as the beginning of May. Off Helgoland, a very large number of flagellates from the Rhaphidophyceae class were observed. In mid-may, diatoms were quite abundant (Guinardia delicatula, Pseudonitzschia seriata, Eucampia zoodiacus).
Northwest of Sylt, water discolouration was observed in early May which was related to the occurrence of an algal bloom of the fish toxic flagellate Chattonella off the Danish coast. However, mass mortality of fish occurred only off the Danish west coast.
A rather moderate development of the foam-forming alga Phaeocystis globosa was observed, as in 1999. Off the coasts of Lower Saxony, it occurred in the second half of April, as expected. From mid to late May, Phaeocystis colonies off the western beaches of Sylt and Amrum caused dark brown water discolouration. In early June, westerly winds led to minor foam formation.
By contrast, the light-emitting zooplankton species Noctiluca scintillans developed rapidly. On windless days, small and medium-sized accumulations at the sea surface occurred as early as the second half of May. Marine phosphorescence was observed for the first time this year.
April was warm, dry, and sunny. The temperatures (7.6 - 10.8 °C) exceeded the long-term reference values by 2 - 4 K. The high air temperatures led to above-average water temperatures (Wismar Bight, Unterwarnow: deviation <3 K). No major fluctuations of the water level occurred in April.
The weather in the first half of May was dry, warm, and sunny. It was followed by a period of changeable, sometimes cool weather in the second half of May. At a mean temperature of 13.5 °C, the month was generally too warm by 2 K. Also the water temperatures were above the long-term
In May, salinity in the surface layer of the coastal waters of Mecklenburg-Vorpommern was about normal.
In May, nutrient levels from the Kiel Bight to the northern Gotland Basin were low; they were on the same order as in 1999. On the coasts of Mecklenburg-Vorpommern, the phosphate concentrations in June were near the limit of detection, nitrate was not detectable, silicate was slightly below normal.
At the end of April, the Gulf of Finland was ice-free. The skerries in the Bothnian Bay were still covered by thick fast ice. Local remnants of decaying ice were still found in the inner skerries and bays of the northern Sea of Bothnia.
In May, the water levels at the Baltic coasts of Schleswig-Holstein were mostly below the normal mean sea level.
In the coastal waters of Mecklenburg-Vorpommern, crypto- and dinoflagellates (Teleaulax acuta, Heterocapsa rotundatum) prevailed in April. Also the diatoms Skeletonema costatum and Diatoma
elongatum) as well as centric diatoms were found. Northwest of Hiddensee, first occurrences of Aphanizomenon baltica were observed.
The beginning of the spring bloom in the area between the western Baltic Sea and the Arkona Basin was recorded in late March/early April.
In May, high cell concentrations of the diatom Skeletonema costatum occurred in the inner Flensburg Bight. The ciliate Myrionecta rubra and the dinoflagellate Heterocapsa triquetra were observed in the Bay of Gelting/Sonderburg. In the Eckernförde Bight, increased cell concentrations of dinoflagellates of the genus Dinophysis were observed, which are toxic to mussels. As no mussel fisheries exist in this area, this was without relevance.
In the Pomeranian Bight, diatoms of the genera Diatoma and Asterionella prevailed in May. Also the blue-green alga Aphanizomenon "baltica" was observed. In the Firth of Stettin, filamentous, potentially toxic Cyanophyceae (blue-green algae) were increasingly observed. Also cryptoflagellates were present.