Constraining the sources of nitrogen fueling export production in the Gulf of Mexico using nitrogen isotope budgets. (18th August 2021)

• Record Type: Journal Article
• Authors: Knapp, Angela N; Thomas, Rachel K; Stukel, Michael R; Kelly, Thomas B; Landry, Michael R; Selph, Karen E; Malca, Estrella; Gerard, Trika; Lamkin, John
• Editors: Moisander, Pia
• Is Part Of: Journal of plankton research. Volume 44:Number 5(2022)
• Page Start: 692
• Page End: 710
• Publication Date: 2021-08-18
• Subjects: Gulf of Mexico -- δ15N budget -- nitrate δ15N -- regenerated production; Plankton -- Periodicals; 578.776
• DOI: 10.1093/plankt/fbab049
• Languages: English
• ISSNs: 0142-7873

Abstract

Abstract: The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2 ) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition ("δ 15 N") of nitrate with the δ 15 N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradients in upper ocean dissolved organic N and suspended particulate N concentration and/or δ 15 N were found that would indicate significant N2 fixation fluxes accumulated in these pools, consistent with low Trichodesmium spp. abundance. Finally, comparing the δ 15 N of sinking particulate N captured within vs. below the euphotic zone indicates that during late spring regenerated N is low in δ 15 N compared to sinking N.