A gulf-wide isoscape of zooplankton δ15N values reflects regional importance of NO3 and N2 fixation as sources of new N in the Gulf of Mexico

Oscar Hernández1, Sharon Z Herzka2, Victor F Camacho-Ibar3, Jesus C Compaire1, Meliza Le-Alvarado1 and Leticia Barbero4, (1)CICESE, Biological Oceanography, Ensenada, BJ, Mexico, (2)Center for Scientific Research and Higher Education at Ensenada, Biological Oceanography, Ensenada, BJ, Mexico, (3)Autonomous University of Baja California, Instituto de Investigaciones Oceanologicas, Ensenada, Mexico, (4)NOAA Atlantic Oceanographic and Meteorological Laboratory, Ocean Chemistry and Ecosystems Division, Miami, United States
In oligotrophic waters, N2 fixation can play an important role in supporting primary production. Oceanographic features such as anticyclonic and cyclonic eddies modify water column structure and change N availability. δ15N values of zooplankton reflect the distinct isotopic composition of NO3, N2 fixation, and denitrification (or a mix of these sources) because they feed at different depths and integrate short-term temporal variation at the base of the food web. We constructed an isoscape for the whole Gulf of Mexico (GM) based on a synoptic characterization of the δ15N values of zooplankton as tracers of nitrogen sources and cycling. Zooplankton samples were collected at 93 stations during two oceanographic cruises in summer 2017 (August-September; XIXIMI-06 and GOMECC-3) with a bongo net towed obliquely from the surface to 200m or ca. 20 m off the bottom over the continental shelf. The δ15N values of zooplankton (335-1000 µm) ranged from 0.9 to 11.6‰ with a clear latitudinal pattern: the highest values were observed throughout the northern continental shelf, including the region of influence of the Mississippi-Atchafalaya river system (8.7 ± 1.6 ‰), and likely reflect denitrification. Values depleted in 15N were typical of the deepwater region (1.8 ±0.5 ‰), and were associated with Loop Current eddies. Campeche Bay showed a heavier isotopic composition (5.1 ± 0.3 ‰) than the central zone but lighter than the northern zone, which is likely due to the presence of a semi-permanent cyclonic Campeche eddy that leads to NO3 pumping to the euphotic zone and to the discharge of the Grijalva-Usumasinta river system. Depleted δ15N values in the central zone indicate that N2 fixation can be a very important biogeochemical pathway. Mixing models will be used to estimate the contribution of various N sources.