Sete Cidades and Furnas lake eutrophication (São Miguel, Azores): Analysis of long-term monitoring data and remediation measures

•Eutrophication of lakes in the Azores (Portugal) imply water quality deterioration•Long-term monitoring data from Sete Cidades and Furnas lake is discussed•Water chemistry influenced by anoxic environment in the bottom during summer•Ratio TN/TP suggests an improvement in water quality due to remediation measures•Internal P-loading influences trophic status

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 × 106 m3 and a depth in the range between 24 and 28.5 m. Furnas lake (1.87 km2) has a volume equal to 14 × 106 m3 and a maximum depth of 15 m.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.