Biological nitrogen cycling in the nitracline

Physical transport of nitrate across the nitracline is the main nitrogen source for new production by phytoplankton in the surface ocean. The intersection of the nitracline with the bottom of the photic zone makes this depth interval a region of rapid nitrogen cycling, involving assimilatory and regenerative transformations, as well as physical transport. We investigated the pathways of nitrogen transformations in surface, nitracline, and subnitracline samples by following changes in concentration and 15 N : 14 N ratio in dissolved and particulate pools in the same bottles for 24 h. Fluxes of nitrite and nitrate among pools were detected from tracer distributions but were not always reflected in changes in nutrient concentrations. For example, the production of nitrate by nitrification could be detected by tracer methods even when nitrate concentration decreased. Nitrification was detected within and below the nitracline by both oxidation of 15 NO 2 − and dilution of initially labeled nitrite (ammonium oxidation) or nitrate (nitrite oxidation) pools. Significant nitrate production by nitrification, relative to nitrate assimilation, implies that some of the nitrate assimilated by phytoplankton is functionally regenerated rather than new nitrogen. Our observations also suggest an important role for a labile organic nitrogen pool of unknown identity, possibly involving bacterial mediation, in the nitrogen cycling of surface waters.