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Hydrographic-chemical Conditions in the Baltic Sea in 2005 (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)

(This summary has been communicated to HELCOM as National Report 2005)

The winter 2004/2005 was an average-cold winter for the whole Baltic Sea area, however, the third coldest winter of the last decade after 1995/1996 and 2002/2003. The maximum ice cover in the Baltic Sea with 177 000 km² amounted to 82 % compared with the long-term mean of 215 000 km² since 1720. The coldest winter of the last decade 2002/2003 had an ice coverage of 232 000 km². At the German Baltic Sea coast, the ice winter was only weak. The "cold sum" 32 of the air temperature in Warnemünde, which was lying clearly below the long-term mean of 112, showed this also. Only March was too cold. On a global scale, the year 2005 was the second warmest since registration started in 1861 and followed directly after 1998. Separating both hemispheres, 2005 was the warmest year on the northern one and the fourth warmest on the southern one. Also with 188 the summer "heat sum" in Warnemünde was well above the long-term mean of 143. It ranks 13 in the record since 1947, whereby the years 1997 and 2002 were on top, both with a "heat sum" of 294. Only the August 2005 was clearly too cold.
The meteorological conditions were reflected in the sea surface temperatures. The water temperature of the year 2005 was characterised by the relatively warm months January and July as well as October, November and December. As annual mean, 2005 was to the warmest one during the investigation period. January was the second warmest month in the whole Baltic Sea since 1990. Only January 2001 was warmer. Also February was lying over the average of the last 15 years. Already in July the maximum surface temperature with around 20 °C was reached. This observation happened only once before, in 2001. Normally the August is the warmest month of the year.
The salt content in the deeper basins of the Baltic Sea increased, with some interruptions like in 2005, continuously already since 1993. However, the similar trend change in the surface layer took place not before 2002/2003 and is still continuing since then.

In 2005, no important inflow events occurred because the winter storms at the beginning of the year were always only of short duration. The conditions in the deeper basins of the central Baltic Sea were still coined by the after-effects of the warm and cold inflow events in 2002 and 2003.
Especially the salt water inflow in January 2003 resulted in a sustainable improvement of the oxygen situation. Already in 2004, a clear decrease of the oxygen concentrations was observed and the onset of a new stagnation period was stated. This development continued in 2005. Thus, at the end of August, with -3.12 ml/L the highest hydrogen sulphide concentration ever observed in the Bornholm Basin was detected near the bottom. In the eastern Gotland Basin, the layer free of oxygen increased continuously. At the end of 2005 the whole column between 150 m depth and the bottom was anoxic. Near the bottom a negative oxygen concentration of –3.75 ml/L was measured. In the western Gotland Basin the influence of the salt water inflow could be detected only delayed and with reduced intensity. Although in the deep water of the Landsort and Karlsö Deeps traces of oxygen were found occasionally, a definite improvement of the situation could not be stated. At the end of the year both deeps displayed again anoxic conditions.
Normally the winter period is used for the description of nutrient trends in the surface layer. The winter phosphate concentrations in the surface layer were high in all areas of the central Baltic Sea. They are comparable with values from 1993. The years in between showed lower phosphate concentrations. The winter nitrate concentrations were similar to the years before after having the lowest values for the last 15 years in 2004.
Extremely high concentrations were measured in the Bornholm Basin, but also in the Karlsö Deep area. Thus, in February a maximum of 1.14 µmol/L was detected. Such high concentrations were found in the mid 1980s for the last time. The reason can be seen, in contrast to the normal development, in that the phosphate was not used up in summer 2004. This resulted, together with the autumnal mineralization, in extremely high phosphate values in the winter surface layer 2005. After the spring bloom still 0.5 - 0.6 µmol/L phosphate were measured in May 2005. Until summer, a further decrease could be detected. But, the concentrations did not fall below 0.3 µmol/L. Therefore, it can be expected that the phosphate surface winter concentration in the Bornholm Basin will be also high in 2006.

The measured winter nutrient concentrations affected also the molar N/P-ratios. These were lying at 5.1 in the winter surface layer of the Landsort and Farö Deep areas. The lowest value with 3.0 was found in the Bornholm Basin. They are well below the long-term mean and far away from the classical Redfield ratio. Consequently, ideal conditions for the development of cyanobacteria were given. Whereas in the whole eastern, northern and western Gotland Basin, the Gulf of Finland and the southern Bothnian Sea very intense cyanobacteria blooms were observed in 2005, no development of cyanobacteria worth to note occurred in the Bornholm Basin despite meteorological conditions were comparable. The reasons for this development have to be clarified still.

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