CO2 and CH4 fluxes from monomictic volcanic lakes in a remote oceanic island: Flores island case study (Azores)

Abstract

Flores Island, situated in the Azores archipelago, is a volcanic island formed by overlapping stratovolcanoes. The island hosts several volcanic freshwater bodies, some of them present poor ecological state. Flores Island maar craters lakes were studied to assess greenhouse gas emissions, through investigation of temporal and spatial variations of the diffuse CO2 (ΦCO2) and CH4 (ΦCH4) flux. Comprida, Negra, and Funda lakes were the selected study cases, with four field surveys combining geochemical and microbial analyses. Water composition varies between Na-HCO3, Na-HCO3-Cl, and Na-Cl-HCO3, showing low variation across surveys. The total CO2 flux (calculated by Graphical Statistical Approach) shows higher emissions at Funda Lake (1.06 ± 0.26 and 1.02 ± 0.36 t d− 1), as the other lakes present values as high as 0.39 ± 0.08 t d− 1. All of them showed lower ΦCO2 during the second survey (August), as high temperatures enhance photosynthetic activity. ΦCH4 values range between 0.64 and 6.28 kg d− 1, with higher emissions at Funda Lake, although bellow worldwide lake averages. ΦCO2 values were 2 to 3 orders of magnitude higher than ΦCH4. Microbial communities and their diversity varied between lakes, with Negra and Funda lakes depicting stronger seasonal shifts. During winter mixing of the water column appears to favor anaerobic processes, enhancing microbial respiration and potential CO2 release from sediments, and enrichment in syntrophic taxa and potential CH4 oxidizers (e.g. Syntrophaceae, Methylomonadaceae). Results indicate that ΦCO2 and ΦCH4 emissions are mainly controlled by biogeochemical processes, and show that the trophic state of a lake ecosystem can enhance the greenhouse gases emissions.