In recent decades, the central North Sea has been experiencing a general trend of decreasing dissolved oxygen (O_2) levels during summer. To understand the potential causes driving lower O_2, we investigated summertime turbulence and O_2 dynamics in the thermocline and bottom boundary layer (BBL). The study focuses on coupling biogeochemical processes with physical transport processes to identify key drivers of the O_2 and organic carbon turnover within the BBL. Combining our flux observations with an analytical process-oriented approach, we resolve the key drivers that ultimately determine the BBL O_2 levels. We report substantial tidally-driven turbulent O_2 fluxes from the thermocline into the otherwise isolated bottom water. This con tribution to the local bottom water O_2 and carbon budgets has been largely overlooked and might be a central factor maintaining relatively high O_2 levels in the bottom water throughout the stratification period. With the current climate warming projections, we propose that higher water temperature and reduced turbulence could favour migrating algal species that could out-compete other species for light and nutrients, and shift the oxygen production zone higher up within the thermocline while maintaining similar organic carbon export to the bottom water. Due to the substantially lower turbulence levels in the central region of the thermocline as compared to the higher turbulence observed at the thermocline-BBL interface, such a shift in the production layer could lead to further isolation of the bottom water and promote the seasonal occurrence of lower O_2 concentrations.
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