Increased summer storms will reduce bottom water oxygen concentrations in a temperate shelf sea.

Oxygen is a vital resource in the ocean, particularly for the high oxygen demand consumers such as fish. In temperature shelf seas, the bottom water oxygen is frequently seen to decrease during the stratified period as a natural consequence of organic matter being remineralised and the seasonal thermocline preventing the replenishment of oxygen from the atmosphere. However, the subsurface chlorophyll maximum (SCM) is a generator of oxygen in the base of the thermocline. Mixing across the thermocline by episodic strong wind events could supply oxygen from the SCM into the bottom water and so offset some of the oxygen reduction arising from organic matter degradation. To explore this possibility, we set up a simple 1-D numerical model to simulate the seasonal cycle of stratification, phytoplankton, nutrients and oxygen in a temperate shelf sea. By adding strong wind mixing, the oxygen concentration in the bottom water becomes lower by the end of autumn than in the case with no wind events. This paradoxical result occurs because the wind mixing also brings organic matter from the SCM into the bottom water, which increases respiration and degradation. A warmer climate will lead to lower oxygen concentrations simply as a result of the reduction in oxygen solubility in seawater; our results also suggest that any climate-driven increases in wind mixing could further worsen bottom water oxygen conditions in temperate shelf seas.