Variability of currents and vertical stratification in the Marsdiep

Velocity and salinity data is presented from 22 13-hour anchor stations (AS) collected between 2004 and 2011 at different locations in the Marsdiep. For 13 of the 22 AS, the largest velocities occur close to the surface during the entire tidal cycle. For the other 9 AS, a mid-depth velocity maximum is observed during the late flood phase in combination with a vertically stratified water column and strong cross-stream velocities. A mid-depth maximum has only been observed towards springtide conditions. Cross-stream circulation cells with velocities up to 0.4 m/s are present during different phases of a tidal cycle together with a vertically stratified water column. Vertical stratification is highly variable within a tidal cycle and seems to be driven by cross-stream circulation patterns. The onset of vertical stratification is during the late ebb (flood) phase and it can persist until the consecutive peak flood (ebb) phase. Stratification is observed just as frequently during springtide as during neaptide conditions. It suggests that changes in fresh water influx are more important for vertical stratification than changes in tidal stirring driven by the spring/neaptide cycle. These observations show that vertical stratification is more frequent and important in the Marsdiep than previously thought.