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

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.