Nitrogen fixation in the Gulf of California and the Eastern Tropical North Pacific

Di-nitrogen (N₂) fixation plays a well-recognized role in the enhancement of primary production and arguably particle export in oligotrophic regions of the subtropical and tropical oceans. However, recent evidence suggests that N₂ fixation may also be significant in regions of the surface ocean proximate to or overlying zones of intense subsurface denitrification. In this study, we present results from a series of research cruises in the Gulf of California (GoCal) and adjacent waters of the Eastern Tropical North Pacific (ETNP). Measurements include microscopy, genomic analyses, incubations, stable isotopic measurements, and sediment traps coupled with ²³⁸U:²³⁴Th disequilibria. Combined, these results suggest that N₂ fixing microorganisms are present and active throughout the region, with larger sized Richelia and Trichodesmium spp. recorded within the warmer waters at the entrance to and within the GoCal, and smaller, unicellular diazotrophs observed in the cooler waters of the northern ETNP. N₂ fixation rates in the summer varied from 15–70 μmol N m⁻² d⁻¹, with episodic blooms contributing as much as 795 μmol N m⁻² d⁻¹. While the estimated contribution of N₂ fixation to particle export was highly variable, blooms of diatom-Richelia symbioses accounted for as much as ∼44% of the measured summer carbon flux at 100 m. Alternately, evaluation of the N isotopic composition of sinking material and the magnitude of measured N₂ fixation rates indicate negligible to small enhancements of new production when blooms of either colonial Trichodesmium spp. or unicellular diazotrophs were encountered. Consistent with previous research, we also found that while fluxes of C to sediment traps are similar in winter and summer months, the efficiency of C export (export/surface productivity) in the GoCal region is elevated during summer relative to the more productive diatom-dominated winter phase of the seasonal cycle. The episodic and variable nature of N₂ fixation recorded in this region make it unlikely that new production via diazotrophic activity is solely responsible for the observed patterns of C transport efficiency; rather, we hypothesize that eolian inputs and/or efficient transport of picocyanobacterial biomass via grazing or aggregation may further explain the enhanced export efficiency observed in the GoCal summer. In sum, diazotrophy typically supports <10%, but as much as 44% of export production. The high variability of direct measurements of N₂ fixation implies that other mechanisms contribute to the seasonal invariance of C flux in this region. If this region is indicative of other oxygen minima zones with active diazotrophs, our results indicate that export-mediated feedback mechanisms between N₂ fixation and denitrification are not as strong as previously hypothesized.