Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea

摘要: Abstract The fraction of phytoplankton growth that leads to the rain organic carbon out sunlit surface ocean (“export production”) is central ocean׳s sequestration atmospheric dioxide. Nitrate assimilation has long been taken as a measure export production; however, this recently questioned by suggestions much nitrate in layer (the “euphotic zone”) originates from biological nitrogen (N) already waters. This view supported recent observations euphotic zone elevated δ 18 O relative its 15 N, indicative nitrification oxidation recycled ammonium nitrite and then nitrate). To evaluate potential importance Sargasso Sea zone, we measured N seawater for samples with ≥0.2 µM collected on cruises Sea, here present first large data set correct low but often measurable concentrations nitrite. Regardless season, (i.e., nitrate-only rather than nitrate+nitrite commonly reported) increase unison below base toward surface. pattern derives phytoplankton, implying slower upward transport into lower zone. In twilight observe rise (relative deeper waters) not accompanied suggesting co-occurring assimilation. Nitrification, therefore, does appear occur waters overlapping only ~150 m-thick it. net, argue simpler cycle suggested, rate approximating deep ocean.