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The summer of 1997 was extraordinarily warm, at mean air
temperatures between 8.5 and 9.5 °C. With a yearly precipitation
from 460 mm (Kap Arkona) to 660 mm (Elbe estuary), 1997 was the third
year in sequence which was too dry. Sea surface temperatures up to
19.4 °C (2.5 K higher than the year before) were measured in the North
Sea. Oxygen levels did not fall below critical values at any time.
North Sea:
In the North Sea, phytoplankton showed a typical seasonal
development. As in the year before, Coscinodiscus concinnus was dominant.
However, the abundance was lower than in 1996. In the North Sylt Wadden
Sea, rod-shaped diatoms Rhizosolenia imbricata were very abundant
from late June to mid-November. "Black spots"
were scarcely observed in the Wadden Sea of Lower Saxony in 1997.
Cod, smelt, mackerel as well as haddock have recovered in the
North Sea.
Baltic Sea:
In the Baltic Sea significantly raised sea temperatures (about
4.5 k higher than the long-term mean) were measured. In 1997, there
was no inflow of salt water. As in 1995 and 1996 the
nutrient concentrations were in line with the season. In the deep
water of the eastern Gotland Basin hydrogen sulphide was found.
During the past summer, the surface accumulations of blue-green algae
were the most extensive and prolonged ones ever recorded in the whole Baltic
Sea. A dinoflagellate bloom (Gyrodinium aureolum) with toxic effects
on the bottom fauna and fish was observed in September-October in
the northern Belt Sea and western Kattegat.
An unusual meteorological situation caused a dramatic flooding event in
the Baltic drainage area in summer 1997. Heavy rainfalls led to extreme
water levels of the rivers Oder and Vistula. Because of a high
load of nutrients local impacts in the Pommeranian Bight occured.
These caused oxygen depletion (between Ahlbek and Greifswalder Oie)
which influenced the bottom fauna. Long-term effects on the overall Baltic
ecosystem are not to be expected.
The stocks of fully developed cod are extremely reduced. However,
the cod year-class 1997 is found to be the strongest one in the
1990s. The 1995 flounder year-class hardly reached average
size, like the 1993 and 1994 year classes. Dab stocks
continued to decline.
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