Methane dynamics in the subsaline ponds of the Chihuahuan Desert: A first assessment

Highlights: • The ponds of the Chihuahuan Desert are subsaline ecosystems with low productivity. • These ecosystems present a sulfate concentration comparable to marine environments. • The ponds are net methane and carbon dioxide sources to the atmosphere. • Low rates of oxic methane production in the water column were also found. -- Abstract: The Cuatro Cienegas Basin (CCB) in the Chihuahuan desert is characterized by the presence of over 500 ponds located in an endorheic basin. These ponds are subsaline ecosystems characterized by a low productivity and a particularly high sulfate concentration, comparable to marine environments. This study focused on assessing the main physicochemical parameters in these ponds along with the characterization of the CH₄ dynamics through the determination of fluxes, dissolved CH₄ concentrations, and net methanotrophic and methanogenic activity. Despite a sulfate concentration ranging from 1.06 to 4.73 g L⁻¹, the studied ponds showed moderate but clear CH₄ production and emission, which suggests that methanogenesis is not completely outcompeted by sulfate reduction. CH₄ fluxes ranged from 0.12 to 0.98 mg m⁻² d⁻¹, which falls within the higher range of marine emissions and within the lower range reported for coastal saline lagoons and saline ponds. During summer, significant CH₄ production in the oxic water column was observed. In addition to CH₄, CO₂ fluxes were determined at levels from 0.2 to 53 g m⁻² d⁻¹, which is within the range recorded for saline lakes in other parts of the world. Our results provide additional evidence that subsaline/saline aquatic ecosystems play an important role in the emission of greenhouse gases to the atmosphere.