Spatial and seasonal dynamics of the methane cycle in a tropical coastal lagoon and its tributary river.

Coastal aquatic ecosystems such as estuaries and coastal lagoons are important atmospheric methane sources that must be better constrained. This work presents a detailed characterization of the methane cycle in a tropical coastal lagoon (La Mancha, Veracruz, Mexico) and its tributary river over three distinct seasons, along a transect from the river to the sea connection. In addition to several physicochemical parameters, the dissolved methane, carbon dioxide, and oxygen concentrations were measured with high resolution in the sediments and the water column, combined with production/uptake rates. Methane and carbon dioxide cycles were further constrained by determining atmospheric flux over the entire river and lagoon sections. The results indicate that La Mancha is a highly contrasted ecosystem. The river section is characterized by a strong pycnocline, relatively high methane concentration, and active methanogenesis and methanotrophy, discharging into a relatively homogeneous lagoon section where the methane and carbon cycles are less active. Overall, both the river and the lagoon were a net source of methane and carbon dioxide, with an annual emission of 2.9 metric tons of methane and 2757 metric tons of carbon dioxide. The spatial structure of the main components of the methane, carbon dioxide, and oxygen cycles was established, and it was observed that depthwise heterogeneities predominated in the river section. In contrast, lengthwise heterogeneities dominated in the lagoon section.

Overall spatiotemporal dynamics of greenhouse gasses and oxygen in two subtropical reservoirs with contrasting trophic states.

The impact of cultural eutrophication on carbon cycling in subtropical reservoirs was assessed using high-resolution measurements of dissolved gas concentration, atmospheric exchange, and uptake/production rates of methane, carbon dioxide, and oxygen. Seasonal measurements were performed in two reservoirs that pertain to the same hydrological basin but are drastically different in terms of allochthonous carbon input. These results were used to feed a mass balance model, from which a large number of overall parameters were determined to explicitly describe the dynamics and spatial attributes of the carbon cycle in the reservoirs. A single graphical representation of each reservoir was created to facilitate an overall appraisal of the carbon cycle. The impact of cultural eutrophication was profound and resulted in a complete redistribution of how the various bioprocesses participated in the methane, carbon dioxide, and oxygen cycles. Among several identified impacts of eutrophication, it was observed that while eutrophication triggered increased methane production, this effect was followed by a similar increase in methane emissions and methanotrophic rates, while gross primary production was depleted.

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

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.