Summer oxygen dynamics in the bottom waters of a wide, temperate shelf sea

Abstract. Seasonal depletion of dissolved oxygen (DO) in stratified shelf seas has important consequences for benthic and pelagic ecosystems and biogeochemical cycling. There is a need to understand the importance of different processes contributing to the bottom water oxygen budget, and how those processes might change in a warmer ocean. Using CTD observations from the UK Shelf Sea Biogeochemistry programme, we construct a summer DO budget for the bottom waters of the Celtic Sea and use the budget to assess how bottom water DO might change in a warmer climate. Across the shelf, bottom water DO concentration declined during summer stratification, with greater losses in shallow northern waters and slower depletion in deeper southern regions. At a well-sampled site in the central Celtic Sea, the bottom water shows a consistent net DO loss of –44 ± 4 mmol m⁻² d⁻¹. Respiration and remineralization dominate this decline (–54 ± 19 mmol m⁻² d⁻¹), while vertical turbulent fluxes from the subsurface chlorophyll maximum (SCM) form an important DO source (30 ± 17 mmol m^-2 d^-1). Episodic wind events enhance DO supply from the SCM, helping to offset some of the DO consumption in the bottom water. Benthic oxygen demand and horizontal transports make minor contributions to the DO budget. A +2 °C “business as usual” climate warming will reduce oxygen solubility and lead to a 12 mmol m^-3 drop in DO concentration in the bottom water over the summer stratified period. However, we find that increased microbial metabolic rates in the warmer bottom water are more important for changes in bottom water DO concentrations, potentially driving a decrease in DO concentration of about 35 – 66 mmol m^-3. Combined, these effects will lead to increased oxygen deficiency in the central and northern Celtic Sea. Our results demonstrate the importance of metabolic responses to a warmer ocean, but also the need to better understand changes in winds and wind-driven mixing across the seasonal thermocline.