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The hydrographic-hydrochemical state of the Baltic Sea in 2014

The winter of 2013/14 in the southern Baltic Sea area was a mild one, with a cold sum at Warnemünde of 65.8 Kd, a value that falls well short of the long-term average of 104.5 Kd. The winter of 2013/14 is thus the twenty-seventh warmest winter since the comparative record began in 1948. This cold sum was due exclusively to the month of February; all other months had a cold sum of 0 Kd. The coldest winter in the observation series was that of 1962/63 with a cold sum of 395.2 Kd.

With a warm sum of 236.9 Kd, recorded at Warnemünde, 2014 comes in tenth place in the time series that began in 1948, forcing 2013’s value of 230.4 Kd into eleventh place. The 2014 value far exceeds the long-term average of 150 K d, and exceeds the standard deviation. 2014 can therefore again be classed as a particularly warm year, even if average monthly temperatures in June and August were slightly below the long-term average, and April and October were balanced. In comparison, May, July and September present warm sums that are twice as high; they are exceptionally warm months that come eighth, fourth, and sixth respectively in the time series since 1948.

In terms of Sea Surface Temperature, 2014 was the warmest year since 1990, and was about 1.2 K above average for 1990-2014; it was 0.4 K higher than 2008, formerly the warmest year. Except for February and June, all other months, and especially July and August in the north of the Baltic Sea, contributed to that value. The western Baltic Sea exceeded long-term mean values by +1 to +3 K in every month except February. After a mild start to the year, a cold spell starting around 20 January produced strong cooling until the beginning of February. Nevertheless, the monthly average for January exceeded the long-term averages for 1990-2014 by +2 K; January was the second-warmest month in the western Baltic since 2007. February lay in the range of long-term averages, and in the Arkona Sea and Gulf of Bothnia was the coldest month of the year. As usual, March developed into the coldest month of the year in the Gotland Sea. From March to May throughout the Baltic Sea, and even from March to December in its western section, anomalies of +1 to +3 K were recorded.

Inflow events with estimated volumes between 100 and 400 km³ occurred in the Baltic Sea on four occasions in 2014. In February, an inflow volume of 141 km³ was calculated based on changes in sea level at Landsort Norra. After a brief outflow phase, another inflow occurred in March with an estimated volume of 203 km³. In combination with the earlier effects of hurricane ‘Xaver’ in December 2013 (inflow volume of 147 km³) and the long phase of westerly winds from the end of October to the beginning of November 2013, both these events produced a complex interaction that resulted in water spilling over Stolpe Channel in late April / early May. These water masses reached the Gotland Deep in late May and oxygenated its deep water for the first time since 2003. Although none of these three events fulfilled the typical characteristics of a Major Baltic Inflow, combined they imported comparably large amounts of water, salt, and oxygen into the deep water of the Baltic Sea (a novel form of deep-water ventilation that was described for the first time). In August, another inflow phase with a volume of some 164 km³ was detected. In terms of ventilation of the deep basins, its effects were more restricted to the Arkona Basin and the Bornholm Basin.

A very large inflow began on 13 December; it had a volume of 358 km³, of which 198 km³ was saline water bearing around 4 Gt salt. According to latest estimates, this event represents the third-largest salt-water inflow into the Baltic Sea since the beginning of the record in 1880.

The annual cycle of oxygen saturation in the surface water was again typical in 2014. As a result of the dominance of oxygen-consuming processes and low productivity, the surface water in February in all sea areas was slightly undersaturated at 95 % to 96 %. In March, too, undersaturation continued to be observed in all sea areas except the Arkona Basin where saturation levels were 104.3 %. The peak of the 2014 spring bloom occurred in all sea areas in May: in the Bornholm Basin and in the eastern Gotland Basin, saturation values between 120 % and 125 % were determined. Summer presented a familiar picture with saturation values around 105 %. Only in the eastern Gotland Basin did saturation values around 115 % indicate an intensive cyanobacteria bloom. In the autumn, intensified degradation processes again led to undersaturation. Overall it can be concluded that the annual range of variation for saturation was relatively small, as in previous years. This indicates a healthy oxygen balance in the surface water.

Oxygen conditions in the deep water of the basins of the central Baltic Sea are primarily influenced by the occurrence or absence of strong barotropic and/or baroclinic inflows. The Bornholm Basin is the westernmost of the deep basins, and inflows are often able to ventilate its deep water. As described above, the inflow events of autumn 2013, and February and March 2014 led in the spring to a lasting improvement in oxygen conditions in the deep water of the Bornholm Basin, where at the end of April 5.65 ml/l oxygen was measured. Once these water masses had spilled over Stolpe Sill, they advanced into the Gotland Basin whose deep water they repeatedly - if briefly - ventilated for the first time since 2003.

At the same time, this meant that towards the end of the year relatively low concentrations of hydrogen sulphide were measured in the eastern Gotland Basin. Usually at the end of a long period of stagnation, values between− 5 ml/l and −7 ml/l are found, as was the case at the beginning of the year. By the end of 2014, therefore, favourable conditions had developed for the Major Baltic Inflow in December.

Oxygen conditions in the deep water were also reflected in nutrient concentrations. The good oxygen supply in the deep water of the Bornholm Basin resulted in phosphate concentrations <2 μmol/l because dissolved phosphate is precipitated under oxic conditions. In contrast, ammonium concentrations of 0.1 ± 0.2 μmol/l were in the range of the detection limit, while nitrate values, with two exceptions, were between 7 μmol/l and 9 μmol/l.

The water masses that advanced into the eastern Gotland Basin were characterised by low concentrations of phosphate and high concentrations of nitrate, and led to a reduction in concentrations of phosphate there. The brief ventilation of its deep water in July led to the equally brief formation of nitrate, and ammonium values were also lower than in previous years. Also in terms of nutrients, favourable conditions existed for the Major Baltic Inflow of December 2014.

Complete report in:
Marine Science Reports 96 (2015)
Nausch, Günther; Naumann, Michael; Umlauf, Lars; Mohrholz, Volker; Siegel, Herbert:

Hydrographic-hydrochemical assessment of the Baltic Sea 2014

Annual hydrographic-hydrochemical Assessments

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