Isopycnal Processes Allow for Summertime Heterotrophy Despite Net Oxygen Accumulation in the Lower Euphotic Zone of the Western North Atlantic Subtropical Gyre

Abstract

In the oligotrophic subtropical gyre of the North Atlantic, the processes that allow for an imbalance between annual biological productivity and organic carbon export have been sought for decades. We use biogeochemical data from profiling floats and 26‐year bottle samples off Bermuda to provide the first evidence for a mechanism that allows for heterotrophy in the presence of oxygen accumulation in the lower euphotic zone (50–100 m) during the stratified season. After the spring bloom, surface waters that are enriched in oxygen and organic matter, but low in nitrate, are subducted and transported along the seasonal isopycnals that progressively displace downward. Due exclusively to this downward displacement, a positive 50‐ to 100‐m depth‐integrated O2 anomaly appears (1,688 ± 545 mmol O2/m2) from mid‐May to mid‐October. Neglecting this effect of isopycnal displacement would suggest an excess of biological productivity over remineralization at 50–100 m (344 ± 330 mmol O2/m2). Yet, when these changes are differenced, significant along‐isopycnal oxygen consumption (−1,344 ± 537 mmol O2/m2) is identified. After accounting for mixing, net biological‐driven oxygen consumption is still found (−827 ± 509 mmol O2/m2), which indicates heterotrophy. Remineralization of sinking and suspended organic matters at 50–100 m could support 90 ± 67% of the heterotrophic demand. Our analysis also shows that the spread in the biological‐driven oxygen sink is linked to the strength of isopycnal displacement that modulates the supply of nutrients and organic matters. This along‐isopycnal transport and heterotrophy in the lower euphotic zone reduces carbon export at 100 m and helps to resolve previously noted imbalances between surface biological productivity and total organic carbon export.

Subsurface oxygen maximum in oligotrophic marine ecosystems: mapping the interaction between physical and biogeochemical processes

“…In this case, an isopycnal downward displacement driven by the temperature from late spring to half summer occurs (top of Fig. S3), similar to the findings by Chen and McKinley (2019). Then, in July and August, low-salinity waters appear at depths of 40-90 m (bottom of Fig.…”

“…4. In areas of subduction (eastern Levantine, case E), the SOM is deep and not connected to local summer production but to downward transport, as in Chen and McKinley (2019).…”

“…Martz et al (2008) observed an SOM at 40 • S in the South Pacific Ocean, which was formed because of the loss of oxygen by heating and solubility-related processes in the above mixed layer and respiration at depths below the SOM. Chen and McKinley (2019) found that a downward isopycnal displacement of oxygen produced during the spring bloom was responsible for oxygen accumulation in May-October in the 50-100 m layer of the oligotrophic subtropical gyre of the North Atlantic. Finally, Smyth et al (2017) observed an SOM 13-87 m above the DCM in permanently stratified tropical and subtropical areas of the Atlantic Ocean and interpreted this mismatch as a signal of productivity enhanced by high light levels rather than by high values of chlorophyll, with the DCM indicating a community of high chlorophyll : carbon ratio phytoplankton adapted to low light conditions.…”

Vertical fluxes in the upper ocean

“…The traditional view of carbon export is a vertical flux due to sinking organic material (Siegel et al, 2016;Palevsky and Doney, 2018). However, current estimates of sinking fluxes of organic matter are not sufficient to account for the observed levels of heterotrophic production in the mesopelagic or the observed levels of annual net community production (Palter et al, 2005;Alonso-González et al, 2009;Baltar et al, 2012;Emerson, 2014;Giering et al, 2014;Roshan and DeVries, 2017;Chen and McKinley, 2019).…”