Marine biological production from in situ oxygen measurements on a profiling float in the subarctic Pacific Ocean

Abstract

Evaluating the organic carbon flux from the surface ocean to the interior (the marine biological pump) is essential for predictions of ocean carbon cycle feedback to climate change. One approach for determining these fluxes is to measure the concentration of oxygen in the upper ocean over a seasonal cycle, calculate the net O2 flux using an upper ocean model, and then use a stoichiometric relationship between oxygen evolved and organic carbon produced. Applying this tracer in a variety of ocean areas over seasonal cycles requires accurate O2 measurements on autonomous vehicles. Here we demonstrate this approach using an O2 sensor on a profiling float that is periodically calibrated against atmospheric pO2. Using accurate data and a model that includes all physical and biological processes influencing oxygen, we determine an annual net community production of 0.7 ± 0.5 mol C m−2 yr−1 in the northeast Pacific Ocean (50°N, 145°W) from June 2012 to June 2013. There is a strong seasonal cycle in net biological oxygen production with wintertime fluxes caused by bubble processes critical to determining the annual flux. Approximately 50% of net autotrophic production during summer months is consumed by net respiration during the winter. The result is a biological pump in the subarctic Pacific Ocean that is less than that determined by similar methods in the subtropics to the south. This estimate is significantly lower than that predicted by satellite remote sensing and global circulation models.

Interannual variability in net community production at the Western Antarctic Peninsula region (1997–2014)

“…For example, the assumption of steady state may be violated during vigorous blooms in the early growing season (Figure ). Previous studies also observed strong seasonality in the NCP [ Bushinsky and Emerson , ; Emerson , ; Fassbender et al ., ; Ostle et al ., ; Palevsky et al ., ]. Net heterotrophy and entrainment of newly exported production back into the mixed layer, more commonly observed in the fall and winter [ Bushinsky and Emerson , ; Emerson , ; Fassbender et al ., ; Kortzinger et al ., ; Ostle et al ., ; Palevsky et al ., ], are not captured by our current algorithms.…”

“…Net heterotrophy and entrainment of newly exported production back into the mixed layer, more commonly observed in the fall and winter [ Bushinsky and Emerson , ; Emerson , ; Fassbender et al ., ; Kortzinger et al ., ; Ostle et al ., ; Palevsky et al ., ], are not captured by our current algorithms. Therefore, our ANCP is different from the one presented in previous studies [ Bushinsky and Emerson , ; Emerson , ; Palevsky et al ., ] and may overestimate NCP in the late part of growing season (i.e., austral autumn).…”

Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production

“…Thus, it is important to have both long‐term and spatially resolved measurements of ANCP throughout the Southern Ocean. Long‐term deployments of chemical sensors on profiling floats now provide the ability to estimate ANCP in remote regions of the ocean [ Riser and Johnson , ; Martz et al ., ; Bushinsky and Emerson , ; Plant et al ., ; Hennon et al ., ]. Measurements of the annual production of oxygen [ Riser and Johnson , ; Bushinsky and Emerson , ] or depletion of nitrate [ Plant et al ., ] using sensors mounted on profiling floats can provide direct estimates of ANCP after the Redfield ratio is used to convert the measured quantity to carbon uptake.…”

“…Long‐term deployments of chemical sensors on profiling floats now provide the ability to estimate ANCP in remote regions of the ocean [ Riser and Johnson , ; Martz et al ., ; Bushinsky and Emerson , ; Plant et al ., ; Hennon et al ., ]. Measurements of the annual production of oxygen [ Riser and Johnson , ; Bushinsky and Emerson , ] or depletion of nitrate [ Plant et al ., ] using sensors mounted on profiling floats can provide direct estimates of ANCP after the Redfield ratio is used to convert the measured quantity to carbon uptake. While these sensors can be deployed on a variety of other platforms, such as moorings [ Shadwick et al ., ] or gliders [ Nicholson et al ., ], the high operational costs of these platforms [ Rudnick et al ., ] generally preclude large arrays from being deployed [ Rudnick , ].…”

Towards a transformative understanding of the ocean�s biological pump: Priorities for future research - Report on the NSF Biology of the Biological Pump Workshop

“…This has historically not been well observed in the ocean, but a number of studies (e.g. Bushinsky et al, 2015;Fassbender et al, 2016;Kortzinger et al, 2008;Palevsky et al, 2016;Quay et al, 2012) have shown that much of export from the stratified summertime mixed layer is subsequently respired and ventilated back to the atmosphere, either by remineralization below the mixed layer that is ventilated during winter mixing or by wintertime net heterotrophy.…”