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Linked river basin and coastal water models were
applied to analyse the effects of an optimal nitrogen
management scenario in the Oder/Odra river basin on water
quality in the Oder (Szczecin) Lagoon and the Pomeranian
Bay (Baltic Sea). This scenario would reduce nitrogen loads
into the coastal waters by about 35%, a level which is
similar to the load of the late 1960’s. During summer the
primary production and algae biomass in the Oder estuary
is limited by nitrogen, which makes a nitrogen management
reasonable. The comparison of the late 1960’s and the mid
1990’s shows that an optimal nitrogen management has
positive effects on coastal water quality and algae biomass.
However, this realistic nitrogen reduction scenario would
not ensure a good coastal water quality according to the
European Water Framework Directive. A good water
quality in the river will not be sufficient to ensure a good
water quality in the lagoon. Nitrogen load reductions bear
the risk of increased potentially toxic, blue-green algae
blooms, especially in the Baltic coastal sea. However, to
reach water quality improvements in lagoons and inner
coastal waters, nitrogen cuts are necessary. A mere focus on
phosphorus is not sufficient.
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