Leibniz-Institut für Ostseeforschung, Warnemünde
(IOW) (Baltic Sea Research Institute, Warnemünde)
Bundesamtes für Seeschifffahrt und Hydrographie, Hamburg, Rostock
(Federal Maritime and Hydrographic Agency of Germany, Hamburg, Rostock)
The report summarizes the hydrographic- hydrochemical conditions in the western Baltic Sea and the Arkona Basin in 2009. Based on the meteorological situation, the horizontal and vertical distribution of temperature, salinity, oxygen and nutrients is described on a seasonal scale.
The winter 2008/2009 was the third mild and short winter in series. With the "cold sum" of the air temperature in Warnemünde of 32.8 K d it ranked on place 14 amoung the mildest winters since the beginning of the record in 1948. The year was warm above the average. The summer "heat sum" 2009 in Warnemünde of 208 K d was only slightly higher than in the year before (196 K d),
well above the 60-years mean of 146 K d, but well below the record value of 355 K d in 2006.
Sea surface temperature in 2009 was, as already in the two years before, characterized by comparatively warm months January to May in wide areas of the Baltic Sea. However, temperatures were somewhat below those of 2007 and 2008. August was clearly the warmest month of the year, with August 8th as the warmest day. Due to only slow cooling, September 2009 was the fourth warmest since 1990. Westerly winds during September and October coined the horizontal temperature distribution with negative
anomalies along the Swedish coast. All in all, 2009 was - after the last four years - the fifth warmest year since the beginning of investigations in 1990.
In 2009 two barotropic inflow events were estimated to carry each about 200 km³ in September/October and in November. From the four smaller barotropic inflows which were recorded in 2009 at the Landsort gauge not any could be identified in the deep water of the Bornholm or Gotland Basin. The relatively strong inflow from November 2009 was only detected in January 2010 in the Bornholm Basin. Due to present knowledge, storm driven small barotropic inflows
take place mainly on the direct way through the Sound into the Arkona Basin In contrast, baroclinic inflow processes occur preferentially over the Darss Sill due to the shallow depth of the Drogden Sill in the Sound. The relatively strong baroclinic inflow in April/May 2009 also showed no remarkable signature in the Bornholm Basin.
The oxygen saturation in the surface layer in 2009 also showed the typical seasonal cycle. As a result of the dominance of oxygen consuming processes and low productivity, the winter surface layer was slightly under saturated (97 %). With the onset of the spring bloom oxygen saturation increased. During the summer period a slight over saturation prevailed due to the activity of algae. In autumn, increased degradation of organic matter caused again under saturation. Generally, the yearly
range of fluctuation in oxygen saturation is relatively low indicating a healthy oxygen balance of the surface water.
In the near-bottom layer a pronounced seasonal cycle could also be observed. In the shallow western Baltic Sea and in the Arkona Basin frequent vertical mixing or deep convection normally prevents a stable stratification in winter. Additionally, inflow processes cause repeated water renewing. Consequently, a good oxygen supply down to the bottom can be observed in winter. The formation of thermal stratification and the increased mineralization of organic matter in summer result in decreasing oxygen saturation
in the near bottom layer of both sea areas. The comparison between 2008 and 2009 shows a widely identical behaviour.
Lowest oxygen values are normally detected in late summer/early autumn. Therefore, the Landesamt für Landwirtschaft, Umwelt und ländliche Räume des Landes Schleswig-Holstein (LLUR) performs since several years intensive spatial observations of the oxygen situation in the near bottom area during that period. Compared to 2008, in 2009 less stations with bad or worse oxygen content (oxygen content <2 mg/L = 1.40 ml/L).were observed (2009: 50 %; 2008: 62 %).
The nutrients phosphate and nitrate showed the typical annual cycle in the surface layer. However, the stable winter plateau which is characteristic for the central Baltic Sea and preferably used for trend analysis can be seen in the western Baltic Sea and the Arkona Basin more as a peak because phytoplankton spring bloom starts early. In 2008 and 2009 winter phosphate (0.60 µmol/L) and nitrate concentrations (2.7 - 2.8 µmol/L) were at a similar level. In both years it was obvious that the nitrate pool was completely exhausted already in mid March. Nitrogen limitation caused the breakdown of the spring bloom. In contrast, still 0.20 µmol/L phosphate were available at the end of April. During the further course of the year, also phosphate concentrations decrease and can reach the detection limit if heavy cyanobacteria blooms develop. In autumn, phosphate and later
on nitrate concentrations start to increase due to intensified mineralization of organic matter and reach typical winter values during January/February of the following year.