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

Abstract:

Preformed nitrate (PreNO₃) was formulated to act as a conservative tracer of ocean circulation after accounting for the stoichiometry of marine biochemical reactions. However, PreNO₃ anomalies have been identified within the subtropical ocean, that describe the biological consumption or production of oxygen without stoichiometric changes in nitrate. Observations from the subtropical Pacific and Atlantic oceans taken by twenty Biogeochemical Argo (Bio-Argo) profiling floats which collectively span from 2007 to 2019 have been used to answer two questions related to PreNO₃ anomalies. What is the seasonality and geographic extents of subsurface negative PreNO₃ anomalies and euphotic zone positive PreNO₃ anomalies in the global subtropical ocean? What biogeochemical processes capable of generating PreNO₃ anomalies are consistent with the seasonality and spatial extents found in the Bio-Argo float records? Subsurface negative PreNO₃ anomalies appear in the subtropical North Pacific and Atlantic beginning at about 100m in January and decrease to roughly 140m depth by July, followed by shoaling to approximately 115m by December. Euphotic zone positive PreNO₃ anomalies are observed throughout the subtropical ocean whereas subsurface negative PreNO₃ anomalies are absent in parts of the Western Equatorial Pacific Ocean. The geographic extents of PreNO₃ anomalies are consistent with previous observations of vertically migrating phytoplankton (VMP) taxa throughout the global subtropical ocean. The timing of euphotic zone positive PreNO₃ anomalies in relation to subsurface negative PreNO₃ anomalies indicates that both VMP and the export and remineralization of N-deficient Transparent Exopolymer Particles (TEP) contribute to PreNO₃ anomalies. Since VMP include large diatoms that produce ballasted organic matter while TEP may only sink slowly, further investigation into mechanisms generating PreNO₃ anomalies is needed to assess their roles in the future biological carbon pump in an expanding subtropical ocean ecosystem.