Title:

Spatial and Temporal Occurrence of Preformed Nitrate Anomalies in the Subtropical Ocean

In the ocean, changes in dissolved nutrients and gases are linked by the stoichiometry of biochemical reactions such as photosynthesis and respiration. Preformed nitrate is a theoretically conservative tracer, derived to account for the stoichiometry of marine biological reactions involving oxygen and nitrate. However, significant preformed nitrate anomalies have been identified within the subtropical ocean, which describes biological consumption or production of oxygen without stoichiometric nitrate production or consumption, respectively. These anomalies highlight gaps in the current understanding of marine biogeochemical cycles of nitrogen, oxygen, and carbon in the ocean which oceanographers have been trying to explain for decades. Observations from the subtropical Pacific and Atlantic oceans taken by 20 Biogeochemical Argo profiling floats equipped with oxygen and nitrate sensors that collectively span from 2007 to 2019 have been used to answer two questions related to preformed nitrate anomalies. Are subsurface negative preformed nitrate anomalies and euphotic zone positive preformed nitrate anomalies recurrent in the global subtropical ocean? What are the spatial extents and seasonality of preformed nitrate anomaly formation? These analyses will improve understanding of how preformed nitrate anomalies vary geographically, vertically, and temporally, providing insight into the not yet fully explained mechanisms of preformed nitrate anomaly formation. The subtropical ocean ecosystem will expand as a result of changes in climate, which may result in the mechanisms driving the formation of preformed nitrate anomalies to play a greater role in ocean biogeochemistry into the future.

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