Ship-borne contour integration for flux determination

The consumption of nutrients by mussel beds can be monitored by measuring the net nutrient flux across a circumscribing vertical surface. Measuring this nutrient flux not only requires resolving the spatial (and temporal) distribution of nutrients at the bounding contour, but also an ability to unambiguously measure net water motion across it. As a prerequisite for future application to nutrient flux determination, we here describe results of a field campaign in the inland Wadden Sea in which we aim to measure just the latter water balance. This is done by comparing the water transport across a vertical surface, spanned by the closed contour and the bottom, to the net tidal displacement of the free surface enclosed by this contour. The former is obtained by measuring currents while traversing the contour repeatedly within one tidal cycle. The latter is estimated using sea-level observations at nearby tidal stations as well as by means of direct sea-level observations at the location of measurements. The current measurements are used to reveal the spatial structure of the tidal and tidally rectified fields, which are divided into cross and along contour currents. These measurements show how the presence of a steep slope in the bathymetry has an impact on the presence of circulation cells that are relevant for the mixing and flushing of water. A neat closure of the water budget is obstructed since it is dominated by the difference between a strong inflow and a concurrent, nearly equally strong outflow. For future application we recommend using observed tidal elevations within the closed contour as a constraint in determining the best estimates for cross-contour water fluxes, thus opening the route to genuine nutrient flux measurements by ‘contour integration’.