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5a. What are the important factors controlling ecosystem functioning within the TIDE estuaries?

Chlorinity is the most structuring element in all estuaries, next temperature (winter-summer opposition). Dissolved constituents decrease towards the sea while pH and dissolved oxygen increase. Chlorinity (zonality) is more important in the Scheldt estuary explaining 66.5 % of the variation than in the Elbe explaining only 32.5 % of the variation. In the Elbe estuary seasonality plays a much greater role compared to the Scheldt and Humber estuaries. Patterns are least conspicuous in the Elbe, because of the very high freshwater discharge continuously disrupting the salinity gradient. Nitrate constitutes the major part of total dissolved inorganic nitrogen in all estuaries, indicating nitrification and mineralization’s relevance over denitrification and primary production. Physical correlations, like between temperature and dissolved oxygen are to be found in all estuaries. However, the latter least pronounced in the Scheldt together with highest dissolved inorganic nitrogen, phosphate and biological oxygen demand concentrations, reflect the importance of other biological oxygen influencing processes within this estuary. Within the Elbe the correlation found between phaeopigments, chlorophyll a and biological oxygen demand might well indicate the importance of autochthonous organic matter input of the algae in the freshwater zone, as also found by the contributions of BfG in TIDE (Schöl et al. 2012 ). Concentrations of nutrients are much lower in the Elbe. This is likely the combined effect of the largest river drainage basin, highest freshwater discharge and earlier water treatment since the 80ies. Primary production peaks earlier in spring in the Elbe, while in summer in the Scheldt. In the Humber primary production can be considered insignificant and estuarine functioning is mostly regulated by the suspended matter dynamics. For the Weser no typical patterns could be found, since too few data was available for a more detailed analysis. However, likely, as seen from gross primary production estimates and high dissolved oxygen concentrations, the Weser might give some interesting estuarine patterns different from those observed in the Scheldt, Elbe and Humber.

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