Stoichiometry of marine gels cycling in the North Pacific Subtropical Gyre: Insights on subsurface preformed nitrate anomalies

Abstract. Transparent exopolymer particles (TEP) and Coomassie-stainable particles (CSP) are gel-like exopolymers produced by phytoplankton and bacteria through the release of dissolved organic matter (DOM) into the water column. Despite their abundance (up to 10 % of dissolved organic carbon in the ocean is in the form of gels) their behavior and role in the ocean’s carbon cycle is not fully understood. TEP and CSP cycling may influence upper ocean carbon (C), oxygen (O₂), and nitrogen (N) budgets, but data on their biogeochemistry is lacking. Within the North Pacific Subtropical Gyre (NPSG), cycling of TEP and CSP has been hypothesized to help explain linkages between subsurface nitrate and oxygen fields and mixed-layer summertime dissolved inorganic carbon (DIC) drawdown. This study provides empirical estimates of TEP and CSP formation rates, remineralization rates, and C:N:O₂ stoichiometry during a June 2021 cruise from Station ALOHA (22°45’) to 31°N in the NPSG. Incubation experiments at 5 m and 125 m revealed TEP accumulation at Station ALOHA and TEP net remineralization at 31°N. CSP showed net remineralization across most depths and stations. Divergent cycling patterns between TEP and CSP suggest distinct roles within the exopolymer pool. The calculated exopolymer remineralization stoichiometry—C: N of 19.4:1, O₂:C of 2.7:1, and O₂:N of 52:1—indicates elevated oxygen demand relative to carbon and nitrogen, helping to explain ~15–20 % of the region’s subsurface preformed nitrate anomalies. By constraining the elemental stoichiometry of TEP and CSP and their patterns of change across depths and locations, this study highlights the importance of exopolymer cycling within upper ocean carbon, oxygen, and nutrient budgets within the NPSG.