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)
(Summary of Baltic Sea Monitoring)
The winter 2005/2006 was a medium cold winter, the second coldest of the last decade after the winter 2002/2003,
stronger than the winter 2004/2005. The maximum ice cover of the Baltic Sea with
210 000 km² at March 16th amounted to 98 % compared with the long-term mean of
215 000 km² since 1720. The winter 2002/2003, the coldest of the last decade, had an ice
coverage of 232 000 km². The reduced ice sum at the German Baltic Sea coast amounted to
16 days (long term arithmetic mean: 22 days). Thus, the ice winter 2005/2006 at the German coast
belonged to the moderate ones. Caused mainly by the extreme cold January and the cold and snow rich March,
the "cold" sum 98 of the winter air temperatures in Warnemünde was only slightly below the
long term mean of 106 for the period 1946 - 2006. On January 26th, the lowest night temperatures
since February 1956 were measured in the German lowlands: -23 °C in West Pomerania and -14.5 °C
at the island Hiddensee. On the global scale, 2006 is considered even warmer than the very warm year 2005. The
annual "heat sum" of 355 in Warnemünde as well as the "heat sum" of July (184) and
September (58) are the highest on record since 1948 and are Iying around 20 % above the previous record values.
Never before in Germany a month was so sunny as July 2006. With more than 407 hours sunshine duration at the
lighthouse "Dornbusch" an the island Hiddensee the hitherto existing record of 403 hours at Cape Arkona
from July 1994 was broken. July 2006 was the hottest month since the beginning of weather observations 105 years ago.
For July 2006, the German Meteorological Service DWD registered
monthly temperature average of 21.9 °C at Warnemünde.
The meteorological conditions were reflected in the sea surface temperatures. The year 2006 was
characterized by comparatively warm months July, October and September. On an annual average it was the warmest
year within the observation period 1990 - 2006. The months February to May were relatively cold. March
belonged to the coldest months since 1990 for the whole Baltic Sea, only March 1996 was colder. July and October
were the warmest months in the last 17 years in the central and southern Baltic Sea. Maximum temperatures
were reached with 23 - 25 °C on July 8th in the central Baltic Sea and between July 20th
and 30th in the western Baltic Sea. Thus, huge areas of the Arkona Sea, the Bornholm Sea and the southern Gotland
Sea exhibited positive temperature anornalies of +4 K to +5 K.
In 2006, no prominent barotropic inflow events with more than 200 km³ occurred in the
Baltic Sea because the westerly storms were always of short duration only and not strong enough, and the Baltic Sea
filling factor was permanently high at the end of the year. Thus, the conditions in the deeper basins were still
coined by the after-effects of the warm and cold inflow events in 2002 and 2003 and by the stagnation period which
started afterwards. However, there are some hints that the deep basins were influenced in 2006 by two baroclinic
inflow events, one from September to December 2005 and the other from June to August 2006. In the Bornholm Basin a
certain improvement of the oxygen conditions was observed as a result. Having had in 2005 an
extreme annual mean of -0.67 ml/L at 80 m water depth, the baroclinic inflows of different densities
increased the annual average to 0.85 ml/L in 2006. In the eastern Gotland Basin the stagnation period was
enforced. The whole water column between 140 m depth and the bottom was free of oxygen. The annual mean for
the 200 m level decreased from 0.80 ml/L (2004) over -0.23 ml/L (2005) to -1.58 ml/L in 2006.
The baroclinic inflow from June to August 2006 could ventilate the near-bottom Iayer only shortly at the beginning
of November. Also in the northern and western Gotland Basin, the stagnation was intensified. Summing up, the new
stagnation period resulted in 2006 in a further deterioration of oxygen conditions in large areas of the deep basins
of the central Baltic Sea. Hydrogen sulphide concentrations as well as their spatial extension increased.
Normally the winter period is used for the description of nutrient trends in the
surface Iayer. Having had extremely high phosphate winter concentrations in 2005 in all sea areas
of the central Baltic Sea, in 2006 a notable decrease was observed. The winter nitrate values were comparable to the
previous years. Winter concentrations are also used for the calculation of the N/P ratios. These ranged in 2006 between
3.0 in the Bornholm Basin and 5.9 in the Farö Deep area and are Iying clearly below the Iong term mean for the
period 1990 - 2000. For example, the 11-year average was 7.3 for the Bornholm Basin and 7.9 for the eastern
Gotland Basin. lt has to be pointed out that the nutrient concentrations in the surface Iayer and thus logically also
the N/P ratios are influenced by the nutrient reservoir below the halocline which in turn is regulated by the interplay
of inflow events and stagnation periods in the deep basins. Deep reaching vertical mixing can transport parts of these
nutrients to the surface. The ratios clearly deviate from the classical Redfield ratio and ideal conditions for the
development of cyanobacteria in summer are given. Additionally, the high water temperatures caused
an early start of blooming. However, development of cyanobacteria was quite different in 2006 compared to 2005.
Already at the beginning of July, the mass occurrence started in the Bornholm and Arkona Basin and in the western
Baltic and continued until the end of August/beginning of September. The Gotland Basin was clearly less affected.
Weather conditions in both areas were similar. It has to be concluded that the triggering and regulating factors
of the cyanobacteria development are by far not well understood.
The intensification of the stagnation period in the deep basins of the central Baltic Sea is
accompanied by changes in the nutrient reservoirs. The extension of the anoxic layer is in accordance with an
enrichment of phosphate. The H2S zone was free of nitrate and ammonium did accumulate. The annual
mean of phosphate at 22 m water depth in the eastern Gotland Basin increased from 3.12 µmol/L (2005)
to 4.20 µmol/L (2006), ammonium increased from 1.7 µmol/L (2005) to 9.2 µmol/L in
2006. The extrme values from the end of the last stagnation period were not reached yet.