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Nitrification and nitrous oxide dynamics in the Southern California Bight

The amount of primary production fueled by upwelled “new” nitrate can be used to estimate the amount of organic carbon available for export to the deep ocean. Nitrate production in the euphotic zone from the microbial process of nitrification affects these estimates, yet the controls on nitrification in the upper ocean are debated. This study examines how seasonal cycles in primary production influence rates of nitrification fueled by both ammonia and urea‐derived nitrogen (N), and how these processes relate to the distribution of the greenhouse gas nitrous oxide (N2O) using monthly rate measurements from the San Pedro Ocean Time‐series (SPOT) station. Nitrification rates were highest at the onset of upwelling and were correlated with depth‐integrated primary production in the lower euphotic zone. Similar ammonia and urea‐derived N oxidation rates suggest urea is a significant N source fueling nitrification, particularly below the euphotic zone. Nitrification supplied a large proportion of phytoplankton N demand in the lower euphotic zone, implying significant regenerated production. The Southern California Bight was always a source of N2O to the atmosphere, which likely was advected into the system from the eastern tropical North Pacific, rather than produced locally from nitrification, and ventilated to the atmosphere during upwelling. Together, the results suggest the coupling of N remineralization and primary production in the upper ocean have important implications for the amount of organic carbon available for export out of the surface ocean, but that transport may dominate over local production in explaining local N2O dynamics.