Articles | Volume 13, issue 5
Biogeosciences, 13, 1609–1620, 2016

Special issue: Low oxygen environments in marine, fresh and estuarine...

Biogeosciences, 13, 1609–1620, 2016

Research article 15 Mar 2016

Research article | 15 Mar 2016

Thermocline mixing and vertical oxygen fluxes in the stratified central North Sea

Lorenzo Rovelli1,2,a, Marcus Dengler1, Mark Schmidt1, Stefan Sommer1, Peter Linke1, and Daniel F. McGinnis1,3 Lorenzo Rovelli et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Scottish Association for Marine Sciences (SAMS), Oban, UK
  • 3Institute F.-A. Forel, Faculty of Science, University of Geneva, Geneva, Switzerland
  • anow at: Nordic Center for Earth Evolution (NordCEE), Department of Biology, University of Southern Denmark, Odense, Denmark

Abstract. In recent decades, the central North Sea has been experiencing a general trend of decreasing dissolved oxygen (O2) levels during summer. To understand potential causes driving lower O2, we investigated a 3-day period of summertime turbulence and O2 dynamics in the thermocline and bottom boundary layer (BBL). The study focuses on coupling biogeochemical with physical transport processes to identify key drivers of the O2 and organic carbon turnover within the BBL. Combining our flux observations with an analytical process-oriented approach, we resolve drivers that ultimately contribute to determining the BBL O2 levels. We report substantial turbulent O2 fluxes from the thermocline into the otherwise isolated bottom water attributed to the presence of a baroclinic near-inertial wave. This contribution to the local bottom water O2 and carbon budgets has been largely overlooked and is shown to play a role in promoting high carbon turnover in the bottom water while simultaneously maintaining high O2 concentrations. This process may become suppressed with warming climate and stronger stratification, conditions which could promote migrating algal species that potentially shift the O2 production zone higher up within the thermocline.

Final-revised paper