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

Abstract

AbstractIn 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.