Diffuse CO2 emissions from Sete Cidades volcanic lake (São Miguel Island, Azores): Influence of eutrophication processes

Abstract

Sete Cidades Lake (São Miguel Island, Portugal) is subdivided into two interconnected branches: the Green Lake and Blue Lake. The lake has an area and maximum depth of 4.39 km2 and 29.5 m (Blue Lake), respectively, with evidence of eutrophication, particularly in the northern area of the Green Lake. In this study, we conducted a sampling survey during January 2017 to measure CO2 fluxes from the lake using a floating accumulation chamber. We also produced two hydrogeochemical profiles for each of the lake’s branches. A total of 1760 CO2 flux measurements were taken along the lake’s surface. The lake water was relatively cold (14.0 °C on average) and weakly mineralised (average electrical conductivity of 116 μS cm−1) with a neutral pH (7.7 on average). The relative composition of major ions occurred in the following decreasing order: Na+ > Mg2+ > Ca2+ > K+ for cations and Cl− > HCO3− > SO42− for anions. The lake water was mainly the Na–Cl type due to sea salt input from seawater spraying. CO2 fluxes ranged from 0.3 to 17.2 g m−2 d−1 and from 2.1 to 17.9 g m−2 d−1 for the Blue and Green Lakes, respectively. Highest CO2 degassing occurred in areas dominated by macrophytes and algal blooms. The measured values suggest that the CO2 was predominantly biogenically sourced, which was further supported by the δ13C isotopic data. The estimated total CO2 emissions varied between 5.8 t d−1 (Green Lake; area = 0.81 km2) and 24.9 t d−1 (Blue Lake; area = 3.58 km2). This study further elucidates the lake’s trophic and chemical pollution status and has major implications for lacustrine CO2 emissions to the atmosphere. Our study also provides a reference for understanding potential future variations in volcanic activity.