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

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.