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Hydrographic-chemical Conditions in the Baltic Sea in 2007 (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)

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

by Günther Nausch, Rainer Feistel, Lars Umlauf, Klaus Nagel, Herbert Siegel

(Summary of Baltic Sea Monitoring)

The winter 2006/2007 was mild and short. According to the "cold sum" of 10.3 K d of the winter air temperatures in Warnemünde, it was the mildest one since the beginning of the record in 1948. This "cold sum" amounted to only 10 % of the 60-years long-term mean, ranging before the second mildest winter 1999/2000 (10.7 K d) and 1989/1990 (15.0 K d). The maximum ice cover of the Baltic Sea of 139 000 km² at February 23rd amounted to 65% compared with the long-term mean of 214 000 km² since 1720. This value is not exceptionally low as compared to the extreme low "cold sum" in Warnemünde. Already in the last 10 years, 5 winters had a lower ice coverage; during the last 20 years there were 11 such winters. The coldest winter during the last decade was in 2002/2003 with an ice coverage of 232 000 km². The reduced ice sum at the German Baltic Sea coast, as a measure to assess the intensity of the ice winter, was zero in 2007 (long-term arithmetic average: 22 days). Only in the inner part of the Schlei river and some protected areas near to the coast of the Bodden waters in Pomerania, some ice with minor extent was observed for a few days. The months of January to April were extremely warm, only the October was somewhat too cold. Precipitation varied extremely between the extraordinary dry April with only 20 % of the normal rain and the wet months of June and July with around 300 % of the average. The annual "heat sum" of 153.1 K d in Warnemünde was only slightly greater than the 60-years mean. However, due to the mild winter and warm spring, 2007 was the second warmest year in Germany since the beginning of the 20th century, after 2000 and before 1994.

Sea surface temperature in 2007 was characterized by a comparatively warm first half of the year and was, on the annual average, the warmest year of the observation period since 1990. Especially the months of January, February and June were responsible for this extremum. In the southern and central Baltic Sea, they belonged to the warmest years of the observation period 1990 - 2007. After the sea surface temperature in June was very high, its values of July, August and September were below the long-term mean. Maximum temperature was reached with 18 - 20 °C on August 14th. Due to the water temperature in summer and windy weather, the development of cyanobacteria was only minor.

In 2007, barotropic inflow events carrying more than 200 km³ water volume occured three times: end of January, at the beginning of March, and at the beginning of November. Two additional events with less than 150 km³ took place in June/July and August/September. Pulses with warm water >6 °C did flow into the Gotland Basin at 180 m water depth in the second and third week of January. A similar pulse was observed at the end of March, covering however the whole water column between 180 and 220 m. The conditions in the deep 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 were some hints of baroclinic inflow events already in 2006 which, followed by the five small barotropic inflow events in 2007, influenced the central Baltic Sea. Especially the near-bottom layer region from the Bornholm Basin to the Gdansk Deep was ventilated repeatedly, a process otherwise typical for baroclinic inflows. Despite these ventilations, the annual oxygen mean of 0.46 ml/L in 80 m water depth in the Bornholm Basin was lower than in the year before (0.85 ml/L), and by far off the extreme average of -0.67 ml/L in 2005. The different inflow events could only shortly improve the oxygen situation in the eastern Gotland Basin. Thus, traces of oxygen could be detected in 225 m water depth (0.02 - 0.04 ml/L) in May. But already in autumn, high hydrogen sulphide concentrations were found again. At the end of 2007, the water column of the Gotland Deep between 137 m and the bottom (239 m) was completely anoxic indicating the continuation of the stagnation period. The annual mean 2007 for 200 m water depth of -1.46 ml/L was only marginally lower than in 2006 with -1.58 ml/L. In the western Gotland Basin, throughout the whole year of 2006, a stagnation period prevailed. This situation continued in 2007.

In the surface layer of the Baltic Sea, the nutrients phosphate and nitrate showed an annual cycle typical for the temperate climate zone, although with some regional particularities during the last years with respect to phosphate. In the eastern Gotland Basin, phosphate concentrations decreased after the spring bloom and were near to the detection limit during the summer. This is in conformity to the typical course. In the Bornholm Basin, already in summer 2003 the phosphate reservoir of the surface layer was not consumed completely. During the two following years this "summer phosphate pool" increased further. In 2005, no concentrations <0.20 µmol/L were measured. Whereas in the summer of 2006 an intensive cyanobacteria bloom consumed the phosphate reservoir completely, the "summer phosphate pool" was formed in 2007 again. The reasons for this different behaviour as well as the triggering and regulating factors of the cyanobacteria development are by far not well understood. Noteworthy are also the high phosphate winter concentrations in all sea areas. Together with relatively low winter nitrate concentrations a shift took place in the N:P-ratio. In general, N:P-ratios in the mixed winter surface layer are far away from the classical Redfield ratio of 16:1. Over a longer period of time, a stable N:P- ratio of 7:1 - 9:1 had established. In the last five years, a remarkable decrease was observed. In 2007, the N:P ratio ranged between 4.3:1 in the Bornholm Basin and 6.6:1 in the Landsort Deep area. In the coming years, we shall have to observe whether the N:P- ratios remain on this new low level or if they return to the "normal” values which were found until the end of the 1990s.
The nutrient situation in the deep basins of the central Baltic Sea is mainly coined by the occurrence or absence of inflow events. The ventilation of the Bornholm Basin nearly throughout the whole year of 2007 resulted in low phosphate concentrations. The annual mean of 2.73 µmol/L in 80 m water depth was below that of 2006. The ventilation allowed nitrification processes. Thus, high nitrate concentrations were found down to the bottom. The annual mean of 6.6 µmol/L was only slightly lower than in the year before. In the eastern Gotland Basin, the ventilation of the deep water was irrelevant and may just have served for some temporal lowering of the H2S concentrations. The annual mean for phosphate in 200 m water depth of 4.03 µmol/L was in the same range as in 2006; nitrate was not existent under anoxic conditions and ammonium concentrations increased further from 1.7 µmol/L (2005) over 9.2 µmol/L (2006) to 11.1 µmol/L in 2007. The prevailing stagnation period in the deep water of the western Gotland Basin caused no significant changes in the nutrient concentrations compared to the year before.

The complete report "Hydrographisch-chemische Zustandseinschätzung der Ostsee im Jahre 2007" together with the report &quot;Die Schwermetallsituation in der Ostsee im Jahre 2007" is available in German as no 72 of the series "Meereswissenschaftliche Berichte (Marine Science Reports) of the IOW " on the internet at:
(http://www.io-warnemuende.de/publikationen.html)

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