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Hydrographic-chemical Conditions in the Baltic Sea in 2004 (Summary)

  Leibniz-Institut für Ostseeforschung, Warnemünde
(IOW) (Baltic Sea Research Institute, Warnemünde)
commissioned by
Bundesamtes für Seeschifffahrt und Hydrographie, Hamburg, Rostock
(Federal Maritime and Hydrographic Agency of Germany, Hamburg, Rostock)

The winter 2003/2004 was an average-cold winter, milder than 1995/1996 and 2002/2003, comparable with 1999/2000, and colder than the other winters during the past decade. The maximum ice cover in the Baltic Sea of 152 000 km² was found in the middle of March and accounted for 2/3 of the long-year average. At the German Baltic Sea coast only a light ice winter was registered. The summer in the Baltic Sea region was on average. The months June and July where clearly too cold. This was compensated by a very warm August.
The meteorological conditions were reflected in the sea surface temperatures. The winter temperatures in the central Baltic Sea can be assessed as normal. The temperatures in the central Baltic Sea showed negative anomalies of –1,5 K in summer, especially in July. In August the whole Baltic Sea reached positive anomalies/deviations.

The whole year 2004 was characterised by only low inflow activities. Only in February, May and at the end of September relatively faint inflows of highly saline water occurred. However, the major Baltic inflow from January 2003 continued to have effects.
During the course of 2004 the oxygen content in the Bornholm and eastern Gotland Basin decreased continuously due to mineralisation processes. In December only 0,25 ml/L oxygen remained in the Bornholm, Gotland and Farö Deeps. In the Gotland Deep, again anoxic conditions have restored below 200 m water depth indicating the beginning of a new stagnation period. In the western Gotland Basin the influence of the salt water inflow can be detected only delayed and with reduced intensity. In the Landsort Deep traces of oxygen were found only shortly and the Karlsö Deep remained anoxic throughout the whole year.

In the Gotland Deep a continuous salinity increase took place in the 200 m level. The annual mean reached 12,7 psu accompanied by an increase in temperature. The reason can be seen in the warm baroclinic inflow of summer 2003. Such high salinities were measured for the last time from April to August 1977 as a result of the huge major Baltic inflow of 1976/1977. In contrast to that event, however, the amount of transported oxygen was extremely low this time and caused no significant effects in the oxygen regime. The baroclinic warm water inflows and their effects in the central deep basins of the Baltic Sea were described for the first time in 2002 and 2003 and seem to be an indication of a new quality in the long-term behaviour of the Baltic Sea.

As a result of the water renewing, the oxygen-poor or even oxygen-free near-bottom layer with high phosphate and low nitrate concentrations was lifted up to a depth range directly below the halocline. Erosion during the vertical convection in winter enabled an immediate effect on the nutrient concentrations in the surface water. Thus, the winter nitrate values in the Arkona and Bornholm Sea as well as in the eastern and western Gotland Sea were the lowest during the last 15 years. In contrast, the phosphate concentration was comparably high in the eastern but esp. in the western Gotland Sea. On short time scales, internal processes are dominating over allochthonous inputs. The measured winter nutrient concentrations affect also the molar N/P-ratio. These are lying in the western Gotland Sea at around 3,5, in the eastern one at 4,3 and in the surface layer of the Bornholm Sea at 4,9 - well below the long-term mean. These conditions should be an excellent prerequisite for the development of cyanobacteria in summer. But in 2004, the corresponding expected intense blooms failed to appear and the causes have still to be clarified.

The complete report is available in German on the internet at:
http://www.bsh.de/de/Meeresdaten/Beobachtungen/MURSYS-Umweltreportsystem/PDF/osretro022005.pdf

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