Nitrogen fixation in mesoscale eddies of the North Pacific Subtropical Gyre: patterns and mechanisms

Abstract Recent studies suggest that mesoscale eddies may support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), yet the strength and mechanistic underpinnings of this trend are not fully understood. We explored the relationships among NFRs, cyanobacterial diazotroph abundances, and environmental conditions of mesoscale eddies by sampling three pairs of eddies of opposite polarity in the North Pacific Subtropical Gyre. Using the Hawaii Ocean Time-series for historical context, we found that NFRs were anomalously high (up to 18.6 nmol N L−1 d−1) in the centers of two anticyclones. The highest NFR was linked to a positive anomaly in Crocosphaera abundance. To unravel the main processes and players, we used high-resolution abundance data, metatranscriptomes, deck-board incubations, and population models to evaluate three mechanisms for each diazotroph population: bottom-up control, top-down control, and physical control. Based on cell division and grazing rate estimates, Crocosphaera anomalies appeared to be partially driven by both bottom-up control (due to elevated surface phosphate concentration promoting high growth rates) and top-down control (reduced grazing pressure from putative predators). In contrast, although large, buoyant Trichodesmium and symbionts of diatoms (Richelia) appeared to have accumulated at the fronts between cyclonic and anticyclonic eddies, they had lower growth rates and contributed less to bulk NFRs. Together, the interplay of these three complex mechanisms may explain reports of elevated diazotroph abundances and NFRs associated with anticyclones and eddy fronts.