Mechanistic explanation of the imbalance between the net community production and nutrient supply in the North Atlantic subtropical gyre

In the oligotrophic subtropical gyre, seasonal drawdown of summertime dissolved inorganic carbon (DIC) and oxygen (O₂) build-up in the surface layer (~0-50m) without a source of nutrients to support primary productivity has been an unresolved puzzle for decades (Michaels et al., 1994). Here to resolve this puzzle, we analyzed biogeochemical data from recently-available profiling floats deployed in the northwestern subtropical North Atlantic (Johnson et al., 2013) together with bottle samples from a timeseries near Bermuda (Lomas et al., 2013).

Our results show that subduction of oxygen-rich, nitrate-low surface waters occurring after the spring bloom can increase depth-integrated (50-100m) O₂ anomaly by 1.64 ± 0.60 mol/m² from May to October, and decrease nitrate by 0.028 ± 0.022 mol/m². Due to simultaneous injection of very high non-sinking organic matter with subduction, the tendency towards large oxygen build-up is suppressed by organic matter remineralization as the watermass travels through the shallow subsurface. However, this compensation is incomplete (70-80%), such that a small net increase of O₂ anomaly from May to October (0.32 ± 0.15 mol/m²) in the seasonal thermocline is observed, despite negative net community production (NCP). Positive NCP within the mixed layer is potentially supported by vertical supply of remineralized nutrients into the mixed layer, and we estimate these processes can contribute ~30%-50% of the annual NCP near Bermuda. However, the net impact of horizontal advection is to decrease thermocline nitrate at seasonal time scale, thus leads to the previous conundrum of “DIC drawdown in the mixed layer but no visible nitrate”. The mechanistic understanding of biogeochemical cycles in the NASG would improve our understanding how future climate change may impact the ocean biological carbon pump via perturbations to the oceanic upper layer circulation.