C-based gas (CH4 and CO2) fluxes from eutrophicated volcanic shallow lakes in Corvo Island (Azores, Portugal)

Abstract

A study to investigate spatial and temporal variations on the diffusive CO₂ and CH₄ fluxes in two volcanic shallow lakes on Corvo Island (Azores) was made, coupling geochemical and microbial tools. Water in both lakes is from the Na–Cl type, depicting alkaline pH values and low EC (63–90 μS cm⁻¹). The total CO₂ flux (calculated using sequential Gaussian simulation) range between 2.5 t km⁻² d⁻¹ and 6.5 t km⁻² d⁻¹, values closer to the mean value for lakes in the Azores with depth lower than 5 m. Values of total methane flux values are much lower, ranging from 31 to 38 kg km⁻² d⁻¹ falling in the lower range of worldwide datasets. Those emissions depict a seasonal effect, as total GHG emissions (CO₂ plus CH₄) range from 0.66 to 1.1 t CO_2-eq d⁻¹ in warmer conditions, about twice the values during colder periods.

The microbial characterization also depicts seasonal variations, showing a close relation with the diffusive CO₂ and CH₄ fluxes. During winter, anaerobic and sulfate-reducing bacteria are abundant, when microbial respiration intensifies, while in summer an enrichment in fermentative bacteria, including Clostridiaceae and Enterobacteriaceae is observed. High CH₄ production in summer results is expected from the presence of methanogenic archaea, instead methane oxidation is more relevant in winter, as shown by the higher abundance of methanotrophs, such as Methylomonadaceae and Methylococcaceae. Results show that small volcanic lakes are significative natural sources of C-based greenhouse gases, being CO₂ and CH₄ emissions enhanced by the trophic state of lake ecosystems.