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Hydrographic-chemical Conditions in the German Exclusive Economic Zone (EEZ) of the Baltic Sea in 2011

  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)

(http://www.io-warnemuende.de/)

(Summary of monitoring results from the Baltic Sea: Meereswiss. Ber. 86 (2012). Nausch, G.; Feistel, R.; Umlauf, L.; Mohrholz, V.; Nagel, K.; Siegel, H.:
Hydrographisch-hydrochemische Zustandseinschätzung der Ostsee 2011

The report summarizes the hydrographic-hydrochemical conditions in the western Baltic Sea and the Arkona Basin in 2011. Based on the meteorological situation, the horizontal and vertical distributions of temperature, salinity, oxygen and nutrients are described on a seasonal scale.

The moderate to strong, long and cold winter 2009/2010 was immediately followed by a similar strong winter 2010/2011. With the "cold sum" of the air temperature in Warnemünde of 177.7 K d it ranks on place 13 of the coldest winters since the beginning of the record in 1948, directly behind the winter before with 203.7 K d. The coldest winter was 1962/63 with 395.2 K d. The year was somewhat to warm. The summer "heat sum" 2011 in Warnemünde of 174.5 K d was lower than the years before and was lying only somewhat above the average of 148 K d of the observation period since 1948. The value is clearly within the statistical spread and only half of the record value of 355 K d in 2006. In the list of the last 64 "heat sums", the year 2011 ranks on place 20.

Sea surface temperature in 2011 was characterized in the western Baltic by relatively cold months January to March and a warm June. The strong cooling in December 2010 continued in the Mecklenburg Bight in January and February and until March in the Arkona Sea. As a result, February was the coldest month in the Mecklenburg Bight with temperatures shortly below 0 °C and an anomaly of -2 K. In the Arkona Sea, surface temperatures were lying around 1 °C in the first two months of the year and decreased to 0.5 °C in March, a period when temperatures already started to increase in the Mecklenburg Bight. Highest positive anomalies were observed in June with up to +1.5 K. In July, monthly mean was in the range of the long-term average whereas August was to cold (-1.6 K). Maximum temperatures at the beginning of August were only 18 - 19 °C. September and October were characterized by persistent westerly winds causing upwelling at the Swedish coast and resulting in negative anomalies in the northern Arkona Sea of -4 K. In November, surface temperatures were in the range of the long-term mean; December was around 1 K to warm.

In 2011, barotropic inflow events with estimated volumes between 100 and 300 km3 took place four times: in January/February, in July, in March/April, in May and in November/December. The relatively strong inflow signal of November/December 2011 transported about one billion tons (1 GT) of salt into the Baltic Sea; it remains clearly behind the Major Baltic Inflows in the winter 2003 (2.0 GT) and the winter 1993 (3.4 GT).The salt of the inflow could only be quantified in January 2012 in the Bornholm Basin. To the present knowledge, small barotropic inflows, driven by storm events, take mainly the short way via the Sound into the Arkona Basin. Baroclinic events in contrast, take place preferentially across the Darss Sill due to the shallow depth of the Drogden Sill in the Sound.

The annual cycle of the oxygen saturation in the surface layer showed in 2011 the typical behaviour. As a result of the dominance of oxygen-consuming processes and low productivity, the winter surface layer of the investigation area was with 97 - 98 % slightly subsaturated. In the western Baltic, the phytoplankton spring bloom caused a supersaturation especially of March. In the Arkona Basin, the bloom started with delay due to the long-lasting winter and continued until May. As also documented by the oxygen concentrations, the huge range of values reflected the unequal development stage of the bloom. In autumn, increased degradation of organic matter caused again subsaturation between 95 % and 96 %. Generally, the yearly range of fluctuations in oxygen saturation is relatively narrow, thus indicating a healthy oxygen balance of the surface water.

The nutrients phosphate and nitrate showed an annual cycle in the surface layer which is typical for the moderate latitudes. Due to the long winter 2010/2011, nutrients in the Arkona Basin suggestively showed a plateau phase which is common in the central Baltic Sea. Normally, the early onset of the phytoplankton blooms prevents this plateau in the Arkona Basin, but especially in the western Baltic. But already at the end of March, the nitrate pool in the surface layer was completely exhausted whereas sufficient phosphate remains. Thus, the spring bloom was terminated by nitrogen limitation. The phosphate concentrations decreased only slowly and reached the detection limit in summer, giving preconditions for intense cyanobacteria blooms. In autumn, an increase of nutrient concentrations starts due to the favoured mineralisation of organic matter, reaching winter concentration latest in February of the following year.

The report summarizes silicate measurements for the period 1993 - 2011. On average, sufficient silicate is available throughout the whole year so that diatom growth is normally not limited by silicate. However, monthly measurements in 2009 and 2010 in the western Baltic showed that in February/March a drastic decrease in silicate can take place. Values below 1 µmol/L indicated a short limitation of diatoms by silicate. The mean annual silicate cycles in the Mecklenburg Bight and in the Arkona Sea differ clearly. In the Mecklenburg Bight, normally an intensive diatom bloom is observed resulting in a silicate decrease of about 10 µmol/L between February and May. In the Arkona Sea, a diatom bloom can also be seen, but starting later. Between February and March values decrease only slightly. Also it seems to be that the intensity of the bloom is lower on average because the difference between February and May is only 6 µmol/L. Until the middle of the last century, remarkably higher silicate concentrations were measured (20 - 30 µmol/L). A significant decrease could be observed at the end of the 1950s/beginning of the 1960s. At the end of the 1980s silicate concentrations in the surface layer of about 10 µmol/L were reported. Since then, a new equilibrium state seems to have established. This conclusion is supported by the data series 1993 - 2011. This 19 years series gives an annual average of 9.6 µmol/L in the western Baltic and of 9.5 µmol/L in the Arkona Sea. Trend investigations document no further decrease of silicate concentrations. For both sea regions, the coefficient of determination, r2, is 0.01.

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