title: Sete Cidades and Furnas lake eutrophication (São Miguel, Azores): analysis of long-term monitoring data and remediation measures.
abstract: Eutrophication of lakes in the Azores archipelago (Portugal) has been described since the 80s, and due to water quality deterioration several remediation methodologies were applied. Long-term monitoring data on water quality from Sete Cidades and Furnas lakes (São Miguel island) were analyzed in order to address that the success of the ongoing policy reverses their eutrophic status. Sete Cidades lake (4.46 km2), made by two branches (so-called Green and Blue lakes), has a water volume equal to 58×10(6) m3 and a depth in the range between 24 and 28.5m. Furnas lake (1.87 km2) has a volume equal to 14×10(6) m3 and a maximum depth of 15m. In Sete Cidades lake mean water temperature and pH range respectively between 13.4°C to 17.7°C and 7.04 to 8.06. Instead, mean temperature in Lake Furnas ranges between 16.6°C and 17.2°C, with a pH between 7.36 and 7.84. Mean water conductivity shows that both lakes correspond to very diluted waters, with values between 98.1 and 136.5 μS/cm. Water chemistry is influenced by the monomictic character of the lakes, as anoxic environment in the hypolimnion due to O2 consumption during the summer, when stratification prevents mixing along the water column, explains sharp increase in TP, TN, N-ammoniacal, Mn and Fe. Major-ion content is rather constant along time, with some seasonal fluctuations, despite a decrease in SO4 observed in Sete Cidades lake which is suggested to be a result of runoff diversion, a result also shown by the decrease of the molal ratio TN/TP in both studied lakes. Nevertheless, the application of the trophic state index (TSITP) does not suggest an improvement in what concerns trophic status, which is explained by the internal P-loading, a source that have to be managed by environmental authorities in the near future in order to revert eutrophication.
authors: CVARG - Centre of Volcanology and Geological Hazards Assessment, Department of Geosciences, University of the Azores, Apartado 1422, 9501-801 Ponta Delgada, Portugal. Electronic address: jvc@uac.pt.
authors: J Virgílio
authors: JV
authors: Cruz
authors: Directorate-Regional for the Environment, Av Antero de Quental, 9C, 2° Piso, 9500-160 Ponta Delgada, Portugal.
authors: Dina
authors: D
authors: Pacheco
authors: CIBIO - Azores, Research Center in Biodiversity and Genetic Resources, Department of Biology, University of the Azores, Apartado 1422, 9501-801 Ponta Delgada, Portugal.
authors: João
authors: J
authors: Porteiro
authors: Directorate-Regional for the Environment, Av Antero de Quental, 9C, 2° Piso, 9500-160 Ponta Delgada, Portugal.
authors: Raquel
authors: R
authors: Cymbron
authors: Directorate-Regional for the Environment, Av Antero de Quental, 9C, 2° Piso, 9500-160 Ponta Delgada, Portugal.
authors: Sandra
authors: S
authors: Mendes
authors: Directorate-Regional for the Environment, Av Antero de Quental, 9C, 2° Piso, 9500-160 Ponta Delgada, Portugal.
authors: Andrea
authors: A
authors: Malcata
authors: CVARG - Centre of Volcanology and Geological Hazards Assessment, Department of Geosciences, University of the Azores, Apartado 1422, 9501-801 Ponta Delgada, Portugal.
authors: César
authors: C
authors: Andrade
mesh_terms: D001394:Azores; D004784:Environmental Monitoring; D052918:Environmental Restoration and Remediation; D005068:Eutrophication; D060106:Lakes / Q000737:chemistry*; D009584:Nitrogen / Q000032:analysis; D010758:Phosphorus / Q000032:analysis; D012621:Seasons; D014876:Water Pollution / Q000032:analysis* / Q000517:prevention & control
chemical_list: D010758:Phosphorus; D009584:Nitrogen
keywords: Azores; Eutrophication; Island hydrology; Volcanic lakes; Water quality