Absence of hypoxia events in the adjacent coastal waters of Grijalva-Usumacinta river, Southern Gulf of Mexico

Abstract

Globally, oxygen concentration in many coastal areas is depleting. River nutrient discharges may produce hypoxia events. The Southern Gulf of Mexico receives the discharges of the Grijalva-Usumacinta River System, the second largest in the Gulf of Mexico. To evaluate the influence of river discharges on dissolved oxygen concentrations in the receiving coastal ecosystem, we studied the variation of physicochemical variables in the water column. During the dry season, the influence of the river waters to the coastal area is scarce, but during the rainy season the river plume reached ~9 km offshore. The lowest concentration of dissolved oxygen (3.6 mg L^-1) was observed within the river plume. We concluded that, in the studied area, hypoxia events (oxygen concentrations ≤ 2 mg L^-1) would occur during the rainy season, low winds and in deeper waters (>80 m depth).

Keywords: Coastal ecosystem; Dead zones; Grijalva-Usumacinta river system; Oxygen depletion.