FY 2018

Seaglider surveys at Ocean Station Papa: Oxygen kinematics and upper-ocean metabolism

Pelland, N.A., C.C. Eriksen, S.R. Emerson, and M.F. Cronin

J. Geophys. Res., 123(9), 6408–6427, doi: 10.1029/2018JC014091, View online (2018)

Abstract. Understanding variability in annual net community production (NCP) and factors affecting its estimation are important priorities for the study of ocean biogeochemistry. The time evolution of dissolved oxygen is useful proxy for NCP if the physical tendency can be isolated and removed; a key challenge is the resolution of the terms contributing to this tendency. Here oxygen balances are examined in the upper 200 m at Ocean Station Papa (50°N, 145°W) using data from Seaglider surveys June 2008 to January 2010. Sampling of horizontal gradients of oxygen, temperature, and salinity during these surveys allows the inference of monthly three‐dimensional advection and turbulent diffusion of oxygen. The resulting monthly oxygen balances show strong variability with depth, with similarities to the temperature balances shallower than 100 m and salinity balances deeper. The estimated annual NCP of 2.2 ± 1.2 mol C·m⁻²·year⁻¹ in the top 120 m is in agreement with contemporary studies in the subarctic North Pacific Ocean. Horizontal advection in the surface layer is found to be small as previously assumed but important at greater depths: Assuming zero horizontal advection in the top 120 m would have resulted in an overestimate of annual NCP by 50%. Horizontal and vertical advection together dominated the oxygen balance in the permanent pycnocline. Results emphasize the three‐dimensional nature of the oxygen balance, even in this relatively quiescent location, and further demonstrate the viability of autonomous spatial surveys for resolving physical oxygen transport terms.

Plain Language Summary. In the same manner as plants on land, small organisms in the surface of the ocean produce oxygen and organic carbon through photosynthesis. Some of this carbon sinks to the deep ocean in a process known as the ocean's biological pump. Over long periods of time, the biological pump is important for regulating the concentration of carbon dioxide, an important greenhouse gas, in the Earth's atmosphere. One way to measure the biological pump is by monitoring changes in ocean oxygen to determine how much is produced biologically. This study employs this method by deploying small robot submarines, known as ocean gliders, to monitor oxygen and ocean flows at a well‐studied location in the Gulf of Alaska. The most important finding is that in certain conditions, ocean flows can produce changes in oxygen that are a large fraction of biological production; if they are unaccounted for, they could result in large errors or uncertainties. Encouragingly, results also suggest that gliders—which are a relatively cost effective but still emerging observational technology—are a viable method for estimating such flows. Pairing gliders with other new technologies holds promise for more widespread measurements that could greatly increase our understanding of the biological pump.