The Oder Estuary - IOW

40
µmol/l
1
0
-1
-2
-3
-4
-5
-6
-7
J F M A M J J A S O N D
Kl. Haff
Gr. Haff
Bucht
Figure 6.3: Differences in median phosphate-P concentrations (µmol/l) between 1992-1999
compared to the 1980-1991 in the Kleines Haff, the Wielki Zalew (Grosses Haff) and
the Pomeranian Bay (Bucht). Data source: State Agency of Environment, Protection
of Nature and Geology Mecklenburg-Vorpommern (LUNG) and West Pomeranian
Voivodeship Inspectorate in Szczecin (WIOS).
In the Pomeranian Bay these long-term trends are visible, too, but less pronounced.
The annual dynamic of the phosphate concentrations in all three systems reflects the biological
activity in these systems in an unusual manner. During winter we find the high concentrations and
a steady decrease towards May. At that time phosphate is taken up by phytoplankton and largely
depleted in the water column. Phosphate is another major nutrient for the flora and during spring
and summer stored in form of organic compounds (Fig. 6.2). Unusual is the steady increase in
phosphorus concentrations already in early summer. Phosphorus becomes abundant in the water
during the entire summer and autumn. This is caused by a fast mineralization of organic material, a
release from the sediments and ongoing supply by the Oder river.
The comparison between the annual phosphorus dynamics in the 1980’s and the 1990’s shows that
the concentrations in all three systems decreased during the 1990’s. In the Wielki Zalew phosphate
is still available during late spring in the 1980’s, but becomes a scarce resource for phytoplankton
in the 1990’s. In the Kleines Haff and the Pomeranian Bay this effect is even more pronounced and
the concentration drops temporarily below analytical detection limit.
The spatial pattern of phosphate concentrations in the Oder Lagoon is often dominated by the
discharge of the Oder river into the south-eastern part of the Wielki Zalew (Fig. 6.4, 6.5). The
concentrations show a reduction with increasing distance from the river mouth. Exceptions are the
summer months, where high concentrations occur in large parts of the lagoon. The discharge of
lagoon water into the Baltic Sea via the Swina channel causes gradients in the entire year.