AbstractThe availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. In coastal waters, high dissolved inorganic N concentrations were historically thought to inhibit dinitrogen (N₂) fixation, however, recent N₂ fixation measurements and the presence of the N₂-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N₂ fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N₂ fixation rates, as well as diazotroph community composition and abundance. UCYN-A1 and UCYN-A2 symbioses dominated diazotroph communities throughout the region during upwelling and oceanic seasons. Bulk N₂ fixation was detected in most surface samples, with rates up to 23.0 ± 3.8 nmol N l⁻¹ d⁻¹, and was often detected at the deep chlorophyll maximum in the presence of nitrate (>1 µM). UCYN-A2 symbiosis N₂ fixation rates were higher (151.1 ± 112.7 fmol N cell⁻¹ d⁻¹) than the UCYN-A1 symbiosis (6.6 ± 8.8 fmol N cell⁻¹ d⁻¹). N₂ fixation by the UCYN-A1 symbiosis accounted for a majority of the measured bulk rates at two offshore stations, while the UCYN-A2 symbiosis was an important contributor in three nearshore stations. This report of active UCYN-A symbioses and broad mesoscale distribution patterns establishes UCYN-A symbioses as the dominant diazotrophs in the SCCS, where heterocyst-forming and unicellular cyanobacteria are less prevalent, and provides evidence that the two dominant UCYN-A sublineages are separate ecotypes.