Geoscience Reference
In-Depth Information
of the early 1960s were significantly below 0.1 mmol/m
3
DIP. In 2000 and 2001, the
concentrations were higher during these months but remained below 0.1 mmol/m
3
as well. Altogether, the period with very low DIP concentrations was not longer
in the 1960s and significant limiting effects on algae biomass were not obvious.
However, more detailed studies are necessary to answer this question finally.
18.7 Discussion and Conclusion
The application of the 3D flow and ecosystem model ERGOM in the Oder estuary is
a clear step forward compared to the box model approach in the lagoon (Wielgat and
tool to understand anoxic processes and the exchange between sediment and water
body. The model is well able to simulate the long-term behaviour of the estuary
and the impact of changing loads and can be regarded as a reliable tool. However,
the model in its present state is not able to simulate short-term anoxic sediment
processes. The process formulation is not the major shortcoming. Problems result
from the model's horizontal and vertical resolution. A horizontal grid significantly
below 500 km seems necessary to calculate the exchange between lagoon and Baltic
Sea. Four to five vertical layers instead of two will be needed to simulate short-term
oxygen depletion above the sediment and simultaneous inflow of Baltic water and
outflow of lagoon water into the lagoon. Further, the accuracy as well as the spatial
and temporal resolution of input data now becomes a limiting factor for the quality
of the model performance. Wind data with hourly resolution, provided in a 5-km
grid, will be necessary to simulate short-term stratifications in the water column.
The model allowed the calculation of a coarse phosphorus budget for July 2000
and the quantification of internal eutrophication. Due to a strong riverine phospho-
rus load reduction during the last decade, the process of internal eutrophication
gained relative importance. The model clearly shows that riverine loads and inter-
nal processes in the lagoon influence the coastal Baltic Sea. The lagoon serves as a
converter, sink and sometimes as a source of nutrients for the Baltic Sea. However,
in the present state, the model is not able to simulate the consequences of internal
eutrophication in the lagoon during summer on the ecology of the coastal sea.
The Oder example shows that nutrient management between land and sea
requires a comprehensive approach, has to link external and internal management
measures and has to follow guiding principles. First, the application of nutrients on
terrestrial systems and their loss to the sea has to be minimized. Second, nutrient
cycles have to be established and/or strengthened.
Nutrients are used as fertilizer in agriculture and are partly lost to ground and
surface waters and end up in the river and finally in the sea. The application of
fertilizer and agricultural practice has to be optimized to reduce the loss. Measures in
the river basin can increase the retention of nutrients. Denitrification in wetlands and
tile drainage systems are examples. Vegetated strips along watercourses to reduce
run-off and sediment input are another example.
