Lagoa das Sete Cidades (Lagoon of the Seven Cities) is a twin lake situated in the crater of a dormant volcano on the Portuguese archipelago of the Azores. It consists of two small, ecologically different lakes connected by a narrow strait, which is crossed by a bridge. The volcano is located on the western third of the island of São Miguel. The Lagoa das Sete Cidades part of a natural landscape of communitarian interest: it is the largest body of water in the region and one of the most important freshwater resources in the archipelago. The lake is situated within the caldera of the Sete Cidades Massif, an ancient volcano built on various layers of ash, pyroclasts and trachyte and basaltic lavas. It is a stratovolcano constructed from alternating phases of explosive and effusive ejecta, from dominantly basaltic pre-caldera eruptions, a trachytic caldera-forming stage and a post-caldera stage consisting of alternating trachytic and basaltic eruptions. The caldera-forming eruption was triggered by a basaltic injection into a shallow trachytic magma chamber.

(Lagoa das Sete Cidades)

(Lagoa das Sete Cidades)

(Lagoa das Sete Cidades)

These geomorphological structures allowed varying hydrochemical properties and produced many types of springs and water circulation networks. Generally, formations with good hydrodynamic properties include areas with lava flows, basaltic pyroclasts and/or pumice, while solidified Ignimbrites, conditioned by high temperatures and paleosols, such as ash, have produced impermeable conditions. The aquifers are dependent on reduced permeability, or secondary volcanoes with hydrogeologically significant volumes. Consequently, the accumulation of ash at the bottom of the craters and caldera has conditioned the formation of lakes.

(Lagoa das Sete Cidades)

(Lagoa das Sete Cidades)

(Lagoa das Sete Cidades)

(Video from Lagoa das Sete Cidades)

As the existence of a eutrophication process in Sete Cidades lake was already known since the 1980s, the rise of cyanobacteria blooms was not unexpected by 2000. Although average total phosphorus concentration inside the lake was not very high, concentrations found in winter samples proved that there was an important runoff of fertilised water from the watershed. High levels of cynotoxins were found in 2002 in the Blue Lake. Lake water within the watershed is not potable, but recreational use, like swimming and windsurfing, were common practices, although advisories were placed along the lakes to warn of bloom toxicity and associated health dangers.

                 
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