Subsurface Frontal Dynamics Induce Nitrate Regeneration and Phytoplankton Accumulation

Abstract Oceanic fronts significantly affect primary production. While surface fronts are well‐studied, subsurface fronts have received relatively little attention. The impacts and underlying mechanisms of subsurface fronts on phytoplankton distribution and nitrogen cycle remain unclear, limiting our understanding of primary production. Based on data from in situ sampling, satellite, and reanalysis, pronounced thermal fronts occurred in the subsurface layers but weakened and disappeared toward the surface and deeper layers of the northern South China Sea. Despite differing formation mechanisms (i.e., dipole eddies and warm offshore water intrusion), both frontal zones exhibited substantially higher chlorophyll a (Chl a ) levels than non‐frontal zones (on average, Chl a concentrations increased by 77.78% and inventories rose by 88.56%). Positive correlations between frontal intensities and Chl a concentrations, along with enhanced convergence‐divergence and vertical processes, suggested that Chl a aggregate relates to physical accumulation. Additionally, evident nitrate (NO 3 − ) loss and isotope enrichment factors ( 15 = 3.4‰ and 18 = 4.5‰) supported that Chl a increase were also associated with NO 3 − assimilation. However, 22.7% of the total NO 3 − pool in frontal zones was from nitrification, representing an increase of 15.2% compared to non‐frontal zones. Elevated regeneration was attributed to enhanced oxygen exchange related to frontal dynamics, as suggested by comparable dissolved oxygen and ammonium levels in both zones, yet with elevated apparent oxygen utilization and NO 3 − in frontal zones. This study highlights that subsurface fronts not only facilitate phytoplankton aggregation but also drive active NO 3 − regeneration, thereby leading to an overestimation of new production in oligotrophic oceans.