Organic matter subducted to the seasonal thermocline supports summertime heterotrophy in the lower euphotic zone of the North Atlantic subtropical gyre

Abstract:

In the oligotrophic subtropical gyre of the North Atlantic, summertime surface drawdown of dissolved inorganic carbon (DIC) and build-up of oxygen (O₂) in the absence of nutrients to support primary productivity has been an unresolved puzzle for decades. We use 4 years of biogeochemical data from profiling floats in the northwestern subtropical North Atlantic and 24 years of bottle samples from the BATS timeseries to address this puzzle. We show that oxygen-rich, low-nitrate surface waters are subducted into the seasonal thermocline after the spring bloom. These waters are trapped on isopyncals that are progressively displaced downward over the season. Because of this downward displacement, a 50-100m depth-integrated O₂ anomaly appears at an average rate of 1.64 ± 0.60 mol/m² from May to October. Similarly and apparent nitrate loss occurs at an average rate of -0.028 ± 0.022 mol/m². Organic matter is also subducted to these isopycnals and its remineralization counters the tendencies towards oxygen build-up and nitrate loss in the lower euphotic zone. This compensation, however, is incomplete (70-80%). The result is a positive rate of change in the O₂ anomaly from May to October (0.32 ± 0.15 mol/m²) despite net heterotrophy. For Bermuda, 30-50% of positive net community production (NCP) within the mixed layer could be supported by vertical supply of the nutrients remineralized at 50-100m.