Lagoa Das Sete Cidades - Human Impacts

Human Impacts

In the mid-20th Century, the destruction of forested areas and their transformation into grassland and pasture in the natural catchment area of the lake, increased the run-off of nutrients, particularly phosphorus and nitrogen, from the steep land. Lacking a domestic sewage system until recently, the community of Sete Cidades contributed to gradual pressures on the ecosystem, resulting in an over-abundant development of rooted aquatic macrophytes and algae in the water bodies. The eutrophication of the Lagoa das Sete Cidades lakes, due to nutrient enrichment was enhanced by human agricultural and domestic activities, began to be noticed in 1987 and led to the implementation of a monitoring program. Measurements taken at the time, that included the examination of Carlson's Trophic state index, the measure of Chlorophyll, inorganic nitrogen and phosphorus in the water column were made, showing increases in all measures. While phytoplankton populations varied throughout the year in the Lagoa Azul, the presence of phytoplankton in the Lagoa Verde persisted annually, dominated by Cyanobacteria. When temperature, light and nutrient availability are adequate to phytoplankton growth, surface waters may host algae or cyanobacteria blooms. In eutrophic waters, cyanobacteria often dominate the summer and early autumn phytoplankton, while during winter and spring they are replaced by diatoms.

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 run-off 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 warm of bloom toxicity and associated health dangers.

Although the state of the lake's water quality has improved since 2000, Cyanobacteria blooms have negatively affected the situation in the lake. Consequently, the Regional Government began preparation of a new territorial examination of the situation within the crater. The Plano de Ordenamento da Bacia Hidrográfica da Lagoa das Sete Cidades (POBHLSC) (English: Sete Cidades Watershed Land Management Plan) was implemented to evaluate the conditions of the lake, establish methods of interdiction and monitorization of landuse initiatives.