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5c. How can we avoid oxygen deficiency situations in the TIDE estuaries?
Main problems of oxygen deficiency are observed in the Elbe and Scheldt estuary. Although, overall dissolved oxygen concentrations are lowest in the Scheldt estuary, since 2009 oxygen drops below 5 mg/l are no longer observed along the estuarine gradient. In the Elbe a zone of oxygen deficiency nearby the freshwater deeper part of the estuary appears to persist. When oxygen dynamics are compared, it can be concluded that in the Scheldt estuary the oxygen deficiencies are mainly related to water quality, while in the Elbe oxygen sags are related to the particular morphometric characteristics of the estuary. With water quality improvements, oxygen in the nearby future is not likely to drop below 5 mg/l again. In the Elbe the effect of the installation of shallow water zones and reduction of algal input upon estuarine oxygen concentrations has been examined (Schöl et al. 2012 ). None of these measures seem to be sufficient to restore good ecological functioning.