Hydrographisch-hydrochemische Zustandseinschätzung der ... - IOW

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µmol/l in the Gotland Deep, 4.45 µmol/l in the Farö Deep) and ammonium (26.2 µmol/l in the
Gotland Deep, 12.2 ml/l in the Farö Deep) highest concentrations for the present stagnation
period were reached.
In the surface layer, phosphate and nitrate show the typical behavior found in temperate
latitudes. Comparing the years 2011 and 2012, considerable differences were obvious in the
Arkona and Bornholm Basin. As a result of mineralization processes and only low activity,
highest nutrient concentrations were measured during winter. This winter plateau phase in the
central Baltic Sea usually lasts over 2 – 3 months, but can be observed in the western Baltic
often only in rudimentary form because the spring bloom of the phytoplankton starts earlier
there. Nitrate winter concentrations were on a comparable level in both years. The spring bloom
resulted in a rapid uptake of nitrate. The nitrate reservoir was exhausted in 2011 already at the
end of March, in 2012 most probably somewhat later. Nitrogen limitation caused the
breakdown of the spring bloom. During the further course of the two years, nitrate
concentrations were near the detection limit in both cases. Only in late autumn they start to
increase slowly due to the proceeding mineralization and reach their maximum again in
February of the following year. In contrast, the annual cycles of phosphate were quite different.
Whereas in winter 2011 relatively low phosphate concentrations were measured, they were
considerably higher in winter 2012. In the year 2011, the expected scenario was in fact
observed. Phosphate concentration decreased during the spring bloom. By the end of April
around 0.20 µmol/l phosphate remained. Until summer, phosphate concentrations decrease
further and can reach the detection limit if strong cyanobacteria blooms occur. In 2012,
however, winter values were conserved in the Arkona Basin until the end of April, and in the
first May decade still 0.5 µmol/l were measured. Also in summer, phosphate values were
clearly higher than normal. In the Bornholm Basin still 0.32 µmol/l were registered in the mid of
July, the ‛minimum“ was reached at the beginning of August with 0.15 µmol/l phosphate, a
value clearly above the detection limit. In the eastern Gotland Basin, in contrast, a ‛typical“
annual cycle was found: In May there remained around 0.25 µmol/l, in the first decade of
August 0.05 µmol/l were measured. Due to the unsettled weather and low water temperatures
no intensive development of cyanobacteria took place in 2012. A similar annual cycle in both
last mentioned sea areas was described already in 2004 and 2005. In the following years, the
annual cycle returned back to ‛normal“.