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

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.

Chemical monitoring of river water bodies in an EU outermost region: examples from the Azores archipelago (Portugal).

Chemical monitoring of water quality on a total of 16 rivers in the Azores archipelago (Portugal), since 2003, made it possible to identify the major pressures and spatial geochemical variations along main course of the rivers. River water pollution is to a large extent associated to point sources, namely domestic wastewater discharges, especially in urban areas, and diffuse sources, associated with pasture land, and explain the high values on BOD(5) and nutrients (P and N). Heavy metals and metalloids, as well as hydrocarbons and pesticides, are generally under the detection limits of the analytical methods. Generally, river water reflects pollution loads according to a simple model, derived from land use in the watershed: in the upper part conditions are pristine, in the intermediate portion of the basin pasture land dominates and near the coast urban discharges are increasingly important. Results stress the role that an approach based on the watershed scale, coupled with land use management measures, are crucial to water management procedures and a successful WFD implementation in small river basin districts like the Azores. The paper also shows the need for full compliance regarding EU directives on urban wastewater and nitrate pollution due to agriculture.