A gulf-wide isoscape of zooplankton ?15N values reflects regional importance of NO3 and N2 fixation as sources of new N in the Gulf of Mexico

HERNÁNDEZ-SÁNCHEZ, OSCAR G.; HERZKA, SHARON Z.; CAMACHO-IBAR, VICTOR; COMPAIRE, JESUS C.; LE-ALVARADO, MELIZA; BARBERO, LETICIA

San Diego, California

Congreso; Ocean Sciences Meeting ? 2020; 2020

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. ?15N 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 ?15N 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 ?15N values of zooplankton (335-1000 ?m) 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 o/oo), and likely reflect denitrification. Values depleted in 15N were typical of the deepwater region (1.8 +-0.5 o/oo), and were associated with Loop Current eddies. Campeche Bay showed a heavier isotopic composition (5.1 +- 0.3 o/oo) 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 ?15N 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.