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Volume 13, issue 6 | Copyright
Ocean Sci., 13, 1061-1075, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Dec 2017

Research article | 18 Dec 2017

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

Morten Holtegaard Nielsen1, Torben Vang2, and Lars Chresten Lund-Hansen2 Morten Holtegaard Nielsen et al.
  • 1Marine Science & Consulting, Peder Lykkes Vej 8, 4. th., 2300 Copenhagen S, Denmark
  • 2Marine Ecology, Biological Sciences, Aarhus University, Ole Worms Allé 1, Building 1135, 8000 Aarhus C, Denmark

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 two-layered 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.

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Short summary
Using field observations we studied the effects that the narrow contraction in the Little Belt, one of the three straits connecting the Baltic Sea and the North Sea, has on currents and water masses. We found the existence of internal hydraulic control, a phenomenon well-known from theoretical and laboratory studies, but surprisingly rarely observed in nature. This phenomenon plays an enormous role due to intense mixing between the water masses, which may drive a highly productive ecosystem.
Using field observations we studied the effects that the narrow contraction in the Little Belt,...