Sources, Dynamics and Management of Phosphorus in a Southern Baltic Estuary - The Baltic Sea Basin - page 379

1996 or 1998. In autumn, winter, spring and early summer of most years, the model
results are well in agreement with the data. However, between July and September,
many years show much higher concentrations (up to 20 mmol/m 3 ) than is predicted
by the model. Nearly all very high concentrations in the lagoon (station KHM)
are observed during this period. In the Pomeranian Bay, a few kilometres off the
Oder Lagoon outlet (OB4), DIP concentrations are in general much lower, because
the bay is part of the Baltic Sea. Extremely high concentrations are rare and the
model is well able to reflect the annual pattern. Unusual high concentrations above
3 mmol/m 3 are, in most cases, observed during winter and cannot be easily related
to high concentrations in the lagoon. The same is true for the station OB14 in the
Pomeranian Bay.
According to the model simulation, the three stations show a strong gradient
with declining concentrations from the lagoon towards the outlet and the open bay.
However, similarities in the long-term concentration pattern are obvious. The model
clearly suggests that high riverine loads, like during the late 1980s, are causing
increased DIP concentrations at all three stations in the lagoon as well as in the bay.
In the data, this is not visible, but this is very likely an effect of insufficient data
sampling frequencies. Further, concentrations simulated with the ERGOM model
are aggregated to monthly averages, while the data represent single samplings near
the water surface.
18.4 Annual Dynamics and the Role of Sediments
In the following, we focus on the years 2000 and 2001 when outstandingly high
phosphorus concentrations in the lagoon have been observed. Figure 18.3 shows the
weather conditions during these years and the dates with extreme total phosphorus
concentrations. The green bars indicate periods with low average wind speeds of
about 3 m/s. It becomes obvious that high phosphorus concentrations occur only
between July and September and seem to be linked to calm periods with low wind
speeds. Wind direction, temperature or cloudiness, as a measure for global radiation,
seems to play only a minor role.
Detailed model results are shown in Fig. 18.4 . The model separates the
lagoon's water body into two horizontal layers, a bottom layer and a surface layer.
Figure 18.4b shows the oxygen concentration in the bottom layer, iron-phosphates
in the sediment and the dissolved inorganic phosphorus concentrations in the water
body. In early spring of both years, the oxygen concentration shows a decline to val-
ues below 6 ml/l which are maintained until autumn. Anoxia in the bottom layer is
not observed in the model results. Bottom oxygen concentrations are not included in
the German monitoring but data from Polish stations confirm concentrations below
7 ml/l near the sediment in summer. Anoxia was found neither in the Polish mon-
itoring data nor in the model results but has been reported for the central lagoon
by scientists for the mid-1990s. As mentioned before, the model ERGON is very