Shelf Sea Oxygen Dynamics: A year of Glider Measurements

Charlotte Anne June Williams, National Oceanography Centre, Marine Physics and Ocean Climate, LIVERPOOL, United Kingdom, Matthew Palmer, National Oceanography Centre, Liverpool, United Kingdom, Claire Mahaffey, University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom and Jenny Jardine, University of Liverpool, Liverpool, United Kingdom
Oxygen (O) is involved in most biogeochemical processes in the ocean, and dissolved oxygen (DO) is a well-established indicator for biological activity via the estimate of apparent oxygen utilisation (AOU). In the deep waters of the open ocean, the AOU provides a valuable insight into the ocean’s biological carbon pump. However, in the physically dynamic and highly productive shallow shelf seas, interpretation of the O distribution and the magnitude of AOU is complex. Physical processes, such as diapycnal mixing, entrainment and horizontal advection act to ventilate waters below the thermocline and thus increase O and decrease AOU. In contrast, biological remineralisation of organic material below the thermocline will consume O and increase AOU.
Here, we use 1 year of high-resolution data from >20 glider deployments in the seasonally stratified NW European Shelf Sea to identify and quantify the physical and biological processes that control the DO distribution and magnitude of AOU in shelf seas. A 200km transect between the shelf edge and the central Celtic Sea (CCS) was repeated between November 2014 and August 2015, thus capturing key periods in the seasonal cycling in shelf seas, specifically the onset of stratification, the spring bloom, stratified summer period and breakdown of stratification. The gliders collected data for DO, temperature, salinity, chlorophyll fluorescence, CDOM, backscatter and turbulence. In addition, direct measurements of turbulent dissipation from the Ocean Microstructure Glider deployed during the campaign provided estimates of mixing at CCS and the shelf break, allowing accurate quantification of the vertical fluxes of O.
At the end of the stratified period the DO concentration was higher and AOU was lower at the shelf break (80 µM) compared to at CCS on shelf (>95 µM) (Fig 1). Estimates of vertical DO fluxes indicate that this horizontal variation in DO and AOU was partly attributed to enhanced mixing via internal waves at the shelf break ventilating waters below the thermocline, rather than decreased biological O consumption at the shelf break. Taking into consideration physical mixing processes, we provide a robust estimate of the biological O consumption over a seasonal cycle and highlight the need to consider the impact of physical processes on O dynamics in shallow shelf sea systems.