Observational evidence for estuarine circulation in the German Wadden Sea

Observational evidence is presented, which corroborates the hypothesis of the general presence of estuarine circulation in the Wadden Sea as put forward in a previous study (Burchard et al., 2008). Current velocity data from moored ADCPs (in the Hörnum Deep south of Sylt Island, 2002–2009) and ship cruises (in several locations in the German Wadden Sea, 2000–2008) were analysed. As a general result, the vertical current profiles above the benthic boundary layer are usually more homogeneous during flood than during ebb, with a pronounced dependence on the cross-shore horizontal density difference. This tidal asymmetry consequently must lead to a residual outflow of Wadden Sea waters in the upper part and a residual inflow of water in the lower part of the water column, thus giving a generic explanation for the obvious net import of suspended sediments from the German Bight into the Wadden Sea.

► We present observational evidence for the existence of estuarine circulation in the German Wadden Sea.
► The evidence is derived from long-term continuous ADCP measurements and from ship campaigns.
► Our findings virtually should apply to all shallow coastal seas in humid areas.