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The winter 2003/04 was an average-cold winter, milder than 1995/96 and 2002/03,
comparable with 1999/2000, and colder than the other winters during the past
decade. The winter 'cold sum' in Warnemünde amounted to 49,5 and was lying
clearly below the long-term mean of 113. 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. Whereas the year 2004 was the fourth warmest on a
global scale since recordings started in 1861 the summer in the Baltic Sea region
was on average. Thus, the summer 'heat sum' in Warnemünde was 143 (mean
142). However, the months June and July where clearly too cold. This was compensated
by an very warm August.
The meteorological conditions were reflected in the sea surface temperatures.
The winter temperatures in the central Baltic Sea can assessed as normal. The
temperatures in the central Baltic Sea showed negative anomalies of - 1,5 K
in summer, esp. in July, whereas the Bothnian Sea was too warm by + 1,5 K during
that time. In August the whole Baltic Sea reached positive anomalies/deviations.
The whole year 2004 was characterized by only low inflow activities. Only in
February, May and at the end of September relatively faint inflows of highly
saline water occurred. Thus, the main focus was directed to the lingering effects
of the major Baltic inflow of January 2003 in the deep water of the different
basins. With that inflow about 200 km3 salt- and oxygen-rich water
were transported into the Baltic Sea resulting in a remarkable improvement of
the oxygen conditions. 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 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. It is noteworthy
that a continuous salinity increase took place in the 200 m level there. 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 1976/77. 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. 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.
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 pre-requisite 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.
Günther Nausch
The
hydrographic-hydrochemical state of the Baltic Sea in 2004
