Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

Abstract

Abstract. Internal hydraulic control, which occurs when stratified water masses are forced through an abrupt constriction, plays an enormous role in nature on both large and regional scales with respect to dynamics, circulation, and water mass formation. Despite a growing literature on this subject surprisingly few direct observations have been made that conclusively show the existence of and the circumstances related to internal hydraulic control in nature. In this study we present observations from the Little Belt, Denmark, one of three narrow straits connecting the Baltic Sea and the North Sea. The observations (comprised primarily of along-strait, detailed transects of salinity and temperature; continuous observations of flow velocity, salinity, and temperature at a permanent station; and numerous vertical profiles of salinity, temperature, fluorescence, and flow velocity in various locations) show that internal hydraulic control is a frequently occurring phenomenon in the Little Belt. The observations, which are limited to south-going flows of approximately twolayered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one. This is connected to the depth of the pycnocline on the upstream side and the topography, which is both deepening and contracting toward the narrow part of the Little Belt. The existence of two possible flow configurations is known from theoretical and laboratory studies, but we believe that this has never been observed in nature and reported before. The water masses formed by the intense mixing, which is tightly connected with the presence of control, may be found far downstream of the point of control. The observations show that these particular water masses are associated with chlorophyll concentrations that are considerably higher than in adjacent water masses, showing that control has a considerable influence on the primary production and hence the ecosystem in the area.

The impact of urea on toxic diatoms – Potential effects of fertilizer silo breakdown on a Pseudo-nitzschia bloom

“…The Lillebaelt area is characterized by strong currents and rapid water flow, which therefore potentially distributed the added nitrogen, primarily to the north (Kattegat). The area is characterized by estuarine circulation (Nielsen et al, 2017). This phenomenon occurs when surface and bottom currents flow in opposite directions, meaning that even if the nitrogen load was transported northwards with the surface waters, the uptake and sedimentation in the bottom waters may have retained the nitrogen in the area.…”