Can oxygen utilization rate be used to track the long-term changes of aerobic respiration in the mesopelagic ocean?

Quantifying possible changes in oceanic aerobic respiration will contribute to the understanding of marine deoxygenation. Here we use a high-resolution Earth system model to investigate the ability of oxygen utilization rate (OUR) to track the temporal change of marine respiration. Results show that in intermediate waters of the North Atlantic Subtropical Gyre (200m-1000m), vertically integrated OUR shows the same trend as the model’s vertically integrated true respiration (decrease by around 0.2 $\mathrm{mol O_{2}/m^{2}/y}$) for the time period 1850 to 2100. However, in the mesopelagic Tropical South Atlantic, integrated OUR increases by 0.2 $\mathrm{mol O_{2}/m^{2}/y}$, while the local true respiration decreases by 0.3 $\mathrm{mol O_{2}/m^{2}/y}$. We identify changes in water mass mixing over time, affecting apparent oxygen utilization (AOU) and age in different ways, as one explanation for this divergence. Quantitatively assessing changes in aerobic respiration from OUR in a changing ocean will require accurate knowledge of changes in mixing processes.