Spatial and temporal occurrence of preformed nitrate anomalies in the subtropical North Pacific and North Atlantic oceans

Abstract

Preformed nitrate (PreNO3) was formulated to act as a conservative tracer of ocean circulation after accounting for the stoichiometry of marine biochemical reactions involving oxygen and nitrate. However, PreNO3 anomalies (e.g. negative values) have been identified within the shallow subsurface of the subtropical ocean which still have yet to be fully explained. The mechanisms proposed to drive the formation of PreNO3 anomalies include: vertical export and remineralization of N-poor dissolved organic matter (DOM) or transparent exopolymer particles (TEP), and the actions of vertically migrating phytoplankton. In this study we use observations from the subtropical Pacific and Atlantic Oceans taken by twenty Biogeochemical Argo profiling floats to assess the spatiotemporal variability of negative subsurface and positive euphotic zone PreNO3 anomalies and their formation rates. After explicitly accounting for the magnitude of DOM cycling, residual euphotic zone positive PreNO3 anomalies and subsurface negative PreNO3 anomalies are consistently observed between 40°S and 40°N. The seasonal timing of euphotic zone positive residual PreNO3 anomaly formation in relation to subsurface negative residual PreNO3 anomalies suggests that both vertically migrating phytoplankton and the remineralization of N-deficient TEP likely contribute to the formation of residual PreNO3 anomalies, but with regional variance suggesting a potentially larger role for migrating phytoplankton in the North Pacific and TEP processes in the North Atlantic. Since migrators include large diatoms that produce ballasted organic matter while TEP may only sink shallowly before remineralization, further investigation into the mechanisms generating PreNO3 anomalies and their quantitative significance is needed to assess the future trajectory of the biological carbon pump in an expanding subtropical ocean ecosystem.