Nitrogen sources (NO3− vs N2 fixation) inferred from bulk δ15N values of zooplankton from the deep water region of the Gulf of Mexico

Abstract

Abstract Dinitrogen (N2) fixation is an important source of new N in oceanic oligotrophic waters, but its contribution has not been quantified in the central and southern Gulf of Mexico (GM), where mesoscale anticyclonic (ACE) and cyclonic eddies (CE) influence NO3− availability for phytoplankton. We investigated the temporal and spatial variation in zooplankton δ15N values as a proxy for baseline nitrogen sources and estimated the contribution of N2 fixation and subsurface nitrate to secondary production. Copepods, euphausiids and two size classes of zooplankton were collected from 0 to 200 m during five cruises. Zooplankton δ15N values were higher during winter due to wind-driven deepening of the mixed layer, reflecting NO3− transport toward the surface. There was a higher estimated contribution of N2 fixation during the highly stratified summer months. Zooplankton δ15N values from ACEs were significantly lower than in CE, associated with a deepening of the 25.5 isopycnal and lower depth-integrated [NO3−]. N2 fixation contributed 60–80% of the N in ACEs. Some stations within the Bay of Campeche showed higher δ15N values due to NO3− transport to the euphotic layer due to a semipermanent CE, regional upwelling and river discharge. Our results highlight the importance of fixed nitrogen over an extensive region of theGM.