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 (O 2 ) levels during summer. To understand potential causes driving lower O 2 , we investigated a 3-day period of summertime turbulence and O 2 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 O 2 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 O 2 levels. We report substantial turbulent O 2 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 O 2 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 O 2 concentrations. This process may become suppressed with warming climate and stronger stratification, conditions which could promote migrating algal species that potentially shift the O 2 production zone higher up within the thermocline.

Physical dynamics structures and oxygen budget of summer hypoxia in the Pearl River Estuary

“…Based on the local TKE budget between shear/buoyancy production and dissipation, Osborn () derived vertical diffusivity for density K z , as follows where Γ is mixing efficiency, and N 2 = − ( ∂ρ / ∂z ) g / ρ is buoyancy frequency, in which ρ is seawater density, g is gravity acceleration, and z is vertical coordinate with positive upwards. Similar to the Fickian diffusion, the vertical diffusion for DO concentration ( C DO ) is defined as (Rovelli et al ) where ∂C DO / ∂z is the vertical gradient of mean DO concentration.…”

“…where Γ is mixing efficiency, and N 2 = − (∂ρ/∂z)g/ρ is buoyancy frequency, in which ρ is seawater density, g is gravity acceleration, and z is vertical coordinate with positive upwards. Similar to the Fickian diffusion, the vertical diffusion for DO concentration (C DO ) is defined as (Rovelli et al 2016)…”

Determining Near‐Bottom Fluxes of Passive Tracers in Aquatic Environments

“…For this study, bottom landers were deployed in 74 m of water for about three tidal cycles (40 h During the 40 h deployment, the semidiurnal tide dominated the measured velocities at the site (McGinnis et al, 2014). From principal component analysis on the total velocities, water generally flowed at 45 ∘ from due east, essentially cross isobath-consistent with the major axis of the semidiurnal barotropic tidal ellipses at the site (Rovelli et al, 2016). The major axis of the total velocity ellipses were about 5 cm s −1 at the lowest EC instrument and increased to about 10 cm s −1 at 0.75 m above the seabed (not shown).…”

“…We applied the theory above to observations collected in the North Sea during a study on benthic oxygen exchanges (McGinnis et al, ; Rovelli et al, ). For this study, bottom landers were deployed in 74 m of water for about three tidal cycles (40 h) in August 2009 (56.502133°N, 3.002233°E) to characterize the mean and turbulence properties near the SWI.…”

“…We applied the theory above to observations collected in the North Sea during a study on benthic oxygen exchanges (McGinnis et al, 2014;Rovelli et al, 2016). For this study, bottom landers were deployed in 74 m of water for about three tidal cycles (40 h During the 40 h deployment, the semidiurnal tide dominated the measured velocities at the site (McGinnis et al, 2014).…”