Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 13, 2971-2979, 2016
https://doi.org/10.5194/bg-13-2971-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
20 May 2016
Biogeochemical characteristics of a long-lived anticyclonic eddy in the eastern South Pacific Ocean
Marcela Cornejo D'Ottone1, Luis Bravo2, Marcel Ramos3, Oscar Pizarro4, Johannes Karstensen5, Mauricio Gallegos6, Marco Correa-Ramirez7, Nelson Silva8, Laura Farias9, and Lee Karp-Boss10 1Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, P.O. Box 1020, Valparaíso Chile and Millennium Institute of Oceanography, Valparaíso, Chile
2Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Facultad de Ciencias del Mar, Universidad Católica del Norte & Millennium Nucleus for Ecology and Sustainable Management of Oceanic Islands (ESMOI), Coquimbo, Chile
3Departamento de Biología Marina, Facultad de Ciencias del Mar, Universidad Católica del Norte & Millennium Nucleus for Ecology and Sustainable Management of Oceanic Islands (ESMOI) &Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Coquimbo, Chile
4Department of Geophysics, University of Concepcion, Chile and Millennium Institute of Oceanography, Concepción, Chile
5Physical Oceanography, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
6Departamento de Ecología, Pontificia Universidad Católica de Chile, Santiago, Chile
7Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, P.O. Box 1020, Chile and Instituto Milenio de Oceanografía (IMO), Valparaíso, Chile
8Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, P.O. Box 1020, Valparaíso, Chile
9Departamento de Oceanografía, Centro ciencia de Clima y la Resielcia (CR2) and Instituto Milenio de Oceanografia (IMO), Concepción, Chile
10School of Marine Science, University of Maine, Orono, ME 04469, USA
Abstract. Mesoscale eddies are important, frequent, and persistent features of the circulation in the eastern South Pacific (ESP) Ocean, transporting physical, chemical and biological properties from the productive shelves to the open ocean. Some of these eddies exhibit subsurface hypoxic or suboxic conditions and may serve as important hotspots for nitrogen loss, but little is known about oxygen consumption rates and nitrogen transformation processes associated with these eddies. In the austral fall of 2011, during the Tara Oceans expedition, an intrathermocline, anticyclonic, mesoscale eddy with a suboxic (< 2 µmol kg−1 of O2), subsurface layer (200–400 m) was detected  ∼  900 km off the Chilean shore (30° S, 81° W). The core of the eddy's suboxic layer had a temperature-salinity signature characteristic of Equatorial Subsurface Water (ESSW) that at this latitude is normally restricted to an area near the coast. Measurements of nitrogen species within the eddy revealed undersaturation (below 44 %) of nitrous oxide (N2O) and nitrite accumulation (> 0.5 µM), suggesting that active denitrification occurred in this water mass. Using satellite altimetry, we were able to track the eddy back to its region of formation on the coast of central Chile (36.1° S, 74.6° W). Field studies conducted in Chilean shelf waters close to the time of eddy formation provided estimates of initial O2 and N2O concentrations of the ESSW source water in the eddy. By the time of its offshore sighting, concentrations of both O2 and N2O in the subsurface oxygen minimum zone (OMZ) of the eddy were lower than concentrations in surrounding water and “source water” on the shelf, indicating that these chemical species were consumed as the eddy moved offshore. Estimates of apparent oxygen utilization rates at the OMZ of the eddy ranged from 0.29 to 44 nmol L−1 d−1 and the rate of N2O consumption was 3.92 nmol L−1 d−1. These results show that mesoscale eddies affect open-ocean biogeochemistry in the ESP not only by transporting physical and chemical properties from the coast to the ocean interior but also during advection, local biological consumption of oxygen within an eddy further generates conditions favorable to denitrification and loss of fixed nitrogen from the system.

Citation: Cornejo D'Ottone, M., Bravo, L., Ramos, M., Pizarro, O., Karstensen, J., Gallegos, M., Correa-Ramirez, M., Silva, N., Farias, L., and Karp-Boss, L.: Biogeochemical characteristics of a long-lived anticyclonic eddy in the eastern South Pacific Ocean, Biogeosciences, 13, 2971-2979, https://doi.org/10.5194/bg-13-2971-2016, 2016.
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