A time series of urea degradation, nitrification, and nitrous oxide concentrations off the coast of Southern California

Microbial nitrogen (N) transformations regulate N availability for phytoplankton and nitrous oxide (N₂O) production in the ocean. Coastal waters often experience dynamic N fluxes and periods of high productivity, remineralization, and N₂O production. Here we present a time series of urea degradation, nitrification, and N₂O concentrations from monthly cruises at the San Pedro Ocean Time-series (SPOT) station off the coast of Southern California. Nitrification rates were measured throughout the euphotic zone directly using ¹⁵NH₄Cl and ¹⁵N-urea additions at in situ temperature and light. Nitrification rates were highest at night, suggesting nitrifying organisms were released from light inhibition and competition. Profiles of urea concentration show a seasonal maximum in the euphotic zone during periods of high production. Our results show that incubations amended with urea had comparable rates of nitrification to bottles amended with ammonium (NH₄⁺). Kinetic experiments were conducted in the upper water column to compare ammonia- and urea-based nitrification. These parameters were used to estimate the contribution of nitrification in the euphotic zone to the production of N available to fuel phytoplankton production.