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5e. Lessons learnt from TIDE: towards ecosystem services and measures , recommendations for further estuarine management
Water quality management is a measure that seems to pay off in all estuaries (read also Saathoff et al. 2012 for other type of measures). Although highest nitrogen and phosphorus concentrations are observed in the Scheldt estuary, water quality improvement has been observed most recently in this estuary for the time period studied in TIDE. It seems organic matter input is the largest driver in general, but that freshwater discharge, residence time, light climate and bathymetry can be important constraints for further improvement of ecological functioning and ecosystem services following from it (read also Sanders et al. 2012 ). Within the Humber estuary, pelagic primary productivity most likely will always be very limited or insignificant because of high suspended matter concentrations, which implies the estuary is more susceptible to organic matter pollution. Hence, monitoring of biological oxygen demand in the Humber estuary would be interesting. However, the Humber comprises most shallow subtidal area compared with the other TIDE estuaries in which microphytobenthos can contribute up to 50 % of primary production (Underwood & Kromkamp 1999 and refs herein). Nevertheless, more managed-realignment sites could increase sedimentation and decrease turbidity again. In the Elbe it will be very difficult to restore ecological functioning because of local deepening in the upper reaches of the estuary. In this instance the best measure is prevention. Installation of shallow water zones will not contribute sufficiently to restore oxygen deficiencies. Further reduction of chlorophyll a input in the Elbe is questionable, since biological oxygen demand is already very low compared to the Scheldt estuary. The Scheldt and Weser are more similar to the Elbe estuary, while the Humber is really type specific. As the Scheldt water quality is recently improving, it is important to keep in mind the effect of deepening as observed within the Elbe in future estuarine management. Otherwise recent improvements of water management might be reset. For the Weser it is suggested to increase monitoring effort, since data in only the upper reaches is too limited to see any pattern or trend.