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)
(This summary has been communicated to HELCOM as National Comment, Germany 2008)
The report summarises the hydrographic and hydrochemical conditions in the western Baltic Sea and Arkona Basin in 2008. Based on
the meteorological situation, the horizontal and vertical distribution of temperature, salinity, oxygen and nutrients is described
on a seasonal scale.
The winter of 2007/2008 was mild and short. With air temperatures in Warnemünde reaching a "cold sum"
of 20.0 K d, this winter ranked eighth among the mildest winters since the beginning of records in 1948. The 60-year mean
is 103 K d. Ice formation in Germany's territorial waters was limited to the coastal waters and harbours, whereas the
open sea remained free of ice. Only eight winters since 1896/1897 have been weaker than the ice winter of 2007/2008. The summer
"heat sum" of 196 K d computed for Warnemünde was higher than the 60-year mean (146 K d), but
well below the record value of 355 K d in 2006.
Sea surface temperatures in 2008, as in 2007, were characterised by a warm first half of the year and a mild
late autumn, which led to the highest mean annual surface temperature measured in the entire Baltic Sea since 1990:
8.5 °C. Except in August and September, monthly averages exceeded the long-term means.
Especially the months of January - May were among the warmest months in the observation period from 1990 to 2008. The highest
temperatures of the year were measured on 31 July . From 1 August, temperatures gradually decreased.
Five barotropic inflow events carrying an estimated 200 km³ each
occurred in 2008: in January, February/March, June, September/October, and November. The path of these small barotropic inflow
events was mainly through the Sound into the Arkona Basin, but owing to their small size and the fact that they spread mainly along
the border of the basin they were not registered by the MARNET station
"Arkonaboje" . They were identified by changes in the Landsort gauge data and in the
data from representative stations east of Bornholm. Only part of these barotropic inflows and of four baroclinic inflows in January,
May, August, and September reached the Baltic Proper in the form of single pulses having different characteristics.
In 2008, oxygen saturation in the surface layer showed the typical seasonal pattern. In February, slight
undersaturation was observed due to low production and the dominance of oxygen-consuming processes. The spring bloom led to
increasing oxygen concentrations. At the end of the summer, oxygen saturation reached, or slightly exceeded, 100 %. In autumn,
increased degradation of organic matter in both areas studied caused saturation values of around 95 %.
A pronounced seasonal cycle was also observed in the near-bottom layer. During winter, frequent deep convection normally prevents
stable stratification. Consequently, oxygen saturation in the near-bottom layer is hardly lower than in the surface layer. The lowest
oxygen values are normally measured in September. Therefore, since 2001,Landesamt für Natur und Umwelt Schleswig-Holstein
(LANU) has carried out its spatial
measurements of oxygen levels in the western Baltic Sea in autumn. In 2008, 62 % of all measured oxygen values in the near
bottom layer were <2 mg/L = 1.40 ml/L.
In 2007, this share was 43 %, i.e. clearly lower. However, oxygen deficiency was observed in a smaller
area than in the extreme year of 2002.
In 2008, the nutrients phosphate and nitrate in the surface layer of the western Baltic Sea and the Arkona Basin
showed the typical seasonal pattern. As usual, the characteristic stable winter plateau observed in the Baltic Proper, which is
preferably used for trend analysis, was not found in the western Baltic Sea and Arkona Basin because the spring bloom of phytoplankton
in the western part of the Baltic started too early for observation. At the end of March, the nitrate pool was already exhausted
whereas sufficient phosphate still remained. Nitrate depletion ended the spring bloom due to nitrogen limitation. Phosphate
concentrations decreased only slowly and reached the detection limit in summer. In autumn, nutrient levels increased again due to
intensified mineralisation of organic matter and reached their winter levels in February 2009.