April, at temperatures between 7.5 and 9.0 °C, was too warm with much rain and, in the first third of the month, much sun.
The above-average temperatures recorded since the beginning of the year continued also in May (mean temperatures: 12 - 14 °C). The month was too dry in many areas (25 - 55 mm), and had 170 - 223 hours of sunshine.
Temperatures of 15.3 - 17.3 °C were reached in June. Precipitation, at 70 - 110 mm, exceeded the mean values by up to 50 %. The duration of sunshine was 180 - 240 h.
The mean water temperatures remained above the climatological values (April: 7.8 °C; May: 9.7 °C; June: 12.9 °C).
In early April the usual Phaeocystis bloom was not observed this year near Norderney. From mid-April, concentrations of the bioluminescent Noctiluca increased sharply. The first red tide occurred in early May.
In early April, diatoms (Thalassiosira rotula), copepods, nauplia, and copepodites were abundant. Diatom concentrations continued to increase during the month (Chaetoceros socialis, Ch. didymus, Pseudonitzschia seriata, Guinardia delicatula, Ditylum brightwellii). Dinophysis acuminata occurred sporadically off Helgoland.
In early May, a decrease of phytoplankton stocks was observed. There were many diatoms (Chaetoceros debilis) and a very large number of small dinoflagellates. Among flagellates, raphidophyceae prevailed slightly. Myrionecta rubra (Mesodinium rubrum) was present with many small cells.
Phytoplankton growth increased in June, with flagellates the dominant species. Marine phosphorescence caused by Noctiluca scintillans could be observed.
In the North Sylt Wadden Sea, the spring bloom developed about 2 - 3 weeks later than last year and was dominated by diatoms (Odontella aurita and Skeletonema costatum). Species diversity increased steadily until the end of May.
In the coastal waters of Schleswig-Holstein, the summer phytoplankton in early June consisted of few species, with wide differences in distribution. Compared to the end of May, the species diversity and, in some areas, cell concentrations had clearly decreased. The first large colonies of Phaeocystis globosa developed near Süderpiep and south of Eiderstedt. In the middle of the month, rod-shaped diatoms (including Rhizosolenia imbricata) were transported from the North Sea into the coastal and Wadden Sea waters.
At the beginning of spring, a stable high pressure situation developed which lasted into the first decade of April. The second half of April was cold (mean temperature 6.4 - 7.5 °C), with little sunshine and much rain (40 - 50 mm rainfall).
The above-average temperature level (11.4 - 14.0 °C) continued in May, with 190 to 231 hours of shunshine and heavy rainfall (55 - 80 mm).
June was warm (15.6 - 17.3 °C) and, like the preceding months, too wet (60 - 105 mm precipitation). The duration of sunshine, at 240 - 280 hours, was average.
While water temperatures in April corresponded more or less to the reference series in the coastal waters of Mecklenburg-Vorpommern, a positive deviation was again recorded in May. Temperatures increased further in June (1 - 3 K above average).
Because of the mild winter, water temperatures during the first months exceeded the long-term mean values (1971 - 1990) both at the surface and in the deep basins of the Baltic Proper.
In the first half of the year, salinities in the coastal waters of Mecklenburg-Vorpommern were below average in the surface layer and bottom water.
An inflow of highly saline water in November 2001 had caused a salinity increase to 16.09 in the Bornholm Deep. By May, however, salinity had fallen again to 15.46. In February and March, the effects of the inflow were also measured in the Gotland Deep.
The monthly means in April in some areas were up to 11 cm above the reference series, in Koserow 3 cm below it. Water levels in May were balanced. The monthly mean values in some cases were up to 11 cm above the reference series. Above-average values were also measured in June.
At the Baltic coast of Schleswig-Holstein, the species spectrum of microalgae at the end of May was narrow, which is typical of the transition phase before the development of the summer population. Diatoms and small flagellates prevailed.
At the monitoring station "Seebrücke Heiligendamm", the typical spring development ended in mid-May, with Cryptophyceae (Teleaulax sp., Hemiselmis sp.) becoming dominant.
In April, chlorophyll-a concentrations in the outer coastal waters of Mecklenburg-Vorpommern increased to 1.2 to 7.5 times their long-term monthly mean (LMM). The abundance of Skeletonema costatum was still high. In the Pomeranian Bight, growth of Asterionella formosa/Diatoma elongatum led to this year's maximum spring bloom so far. The LMM was exceeded also in the inner coastal waters, with diatoms (Diatoma elongatum, Asterionella, Fragilaria, and Stephanodiscus) the dominant species. In the Lower Warnow, the first blue-green algae were observed, consisting of filamentous, potentially toxic species.
In May, phosphate and nitrate in the surface layer of the western Baltic were nearly depleted due to the spring algal bloom. In the western and eastern Gotland Basin, phosphate showed the usual seasonal level, and nitrate was depleted. In the Bornholm Basin, phosphate concentrations were still high, while nitrate was depleted. In the bottom layers, high phosphate and ammonium levels (exception: Bornholm Basin) were found, with nitrate concentrations close to zero.
Oxygen concentrations in the first few months of the year were normal in the coastal waters of Mecklenburg-Vorpommern.
Several minor inflows of saline water in October and November 2001 transported oxygen-rich, high-salinity water into the Baltic, causing complete oxidation of hydrogen sulphide in the Bornholm Basin. In the eastern Gotland Basin, H2S levels at the beginning of the year were low. However, this situation was of short duration. Measurements in May showed that hydrogen sulphide levels have increased steadily since then, marking a continuation of the stagnation period since 1995.