Abstract
Abstract. Continental shelves are the most productive areas in the seas with the strongest implications for global nitrogen cycling. The Yucatán shelf (YS) is the largest shelf in the Gulf of Mexico (GoM); however, its nitrogen budget has not been quantified. This is largely due to the lack of significant spatio-temporal in situ measurements and the complexity of the shelf dynamics, including coastal upwelling, coastal-trapped waves (CTWs), and influence of the Yucatán Current (YC) via bottom Ekman transport and dynamic uplift. In this paper, we investigate and quantify the nitrogen budget of dissolved inorganic nitrogen (DIN) and particulate organic nitrogen (PON) in the YS using a 9-year output from a coupled physical–biogeochemical model of the GoM. The sum of DIN and PON is here referred to as total nitrogen (TN). Results indicate that the main entrance of DIN is through its southern (continental) and eastern margins. The TN is then advected to the deep oligotrophic Bay of Campeche and central GoM. It is also shown that the inner shelf (bounded by the 50 m isobath) is “efficient” in terms of TN, since all DIN imported into this shelf is consumed by the phytoplankton. Submarine groundwater discharges (SGDs) contribute 20 % of the TN, while denitrification removes up to 53 % of TN that enters into the inner shelf. The high-frequency variability of the TN fluxes in the southern margin is modulated by fluxes from the YC due to enhanced bottom Ekman transport when the YC leans against the shelf break (250 m isobath) on the eastern margin. This current–topography interaction can help to maintain the upwelling of Cape Catoche, uplifting nutrient-rich water into the euphotic layer. The export of TN at both western and northwestern margins is modulated by CTWs with a mean period of about 10 d in agreement with recent observational and modelling studies.
Highlights
Climatological values were calculated for the Mexican rivers and Submarine groundwater discharges (SGDs) systems, using temporally scattered information found in the literature (e.g. Rojas-Galaviz et al, 1992; Milliman and Syvitski, 1992; Poot-Delgado et al, 2015; Conan et al, 2016) and a data collection effort within Mexican institutions led by Jorge Zavala-Hidalgo and from the GOMEX IV cruise of CINVESTAV (Centro de Investigación y Estudios Avanzados in Merida Yucatan) within the Consorcio de Investigación del Golfo de México” (CIGoM) project
The results indicate that total nitrogen (TN), especially dissolved inorganic nitrogen (DIN), enters the Yucatán outer shelf through the southeastern and eastern margins
The biogeochemical nitrogen-based cycle seems to be very efficient for NO3 remineralization and consumption by the phytoplankton, converting most of the DIN to particulate organic nitrogen (PON)
Summary
Continental shelves are the most productive areas in the ocean, widely recognized to play a critical role in the global cycling of nitrogen and carbon (e.g. Fennel, 2010; Liu et al, 2010) with direct implications for human activities, such as fisheries, tourism, and marine resources (Zhang et al, 2019). The easterly winds drive a westward circulation over the inner shelf (Enriquez et al, 2010; Ruiz-Castillo et al, 2016) They are responsible for the upwelling along the zonal Yucatán coast due to divergent Ekman transport (Fig. 2). Observational studies suggest high intrusions of upwelled waters during spring and summer which are suppressed during autumn–winter (Merino, 1997; Enriquez et al, 2013) This seasonal variability is not easy to explain since the YC near the YS does not show such a clear seasonal signal and is dominated by higher-frequency mesoscale variations (Sheinbaum et al, 2016), so several mechanisms have been proposed to understand it. The main objectives of this study include (i) quantification of the TN budget within the inner and outer YS, (ii) investigation of the sources and sinks of nitrogen in the continental shelf, and (iii) analysis of the physical mechanisms that modulate the cross-shelf TN transport
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