Original paper

Cyanobacteria blooms in Sete-Cidades lake (S. Miguel Island - Azores)

Santos, Maria da Conceição Raimundo; Pacheco, Dina Maria Medeiros; Santana, Fernando; Muelle, Helena

Abstract

Sete-Cidades is a beautiful softwater lake located in a volcanic crater at S. Miguel island (Azores's archipelago). The lake has an area of 445.5 ha and in it's narrowest zone is crossed by a hand made bridge dividing it in two interconnected sub-units, Lagoa Azul (the Blue Lake - 357.3 ha, depth 24 m) and Lagoa Verde (the Green Lake - 88.2 ha, depth 21 m). In the mid 20 th century, some forested areas of their steep watersheds were transformed in grasslands and pastures. This human environmental disturbance and a rainy climate increased soil erosion and led to a progressive enrichment in nutrients of the water bodies. Sete-Cidades lake can still be classified as mesotrophic, having presented an average chlorophyll a concentration of 3-8 mg.m-3 and a total phosphorus average concentration of 15-23 mg.m-3 for the last three years. As a result of eutrophication processes phytoplankton population increased along the years and cyanobacteria blooms began to develop, especially after the year 2000. Since the beginning of monitoring programs, in 1988, until 1998, phytoplankton population varied from 1.0 106 cells.l-1 to 8.0 106 cells.l-1. In the autumn and winter seasons of 1997/98, severe raining events gave rise to intense runoff from watersheds and edges leading to an increase of phytoplankton population during the next few years, reaching the maximum cell density of 4842.7 106 cells.l-1 inside Lagoa Azul, in July 2003. Cyanobacteria began to dominate and they represent now the major phytoplankton group all over the year. Cyanobacteria blooms in this lake normally begin in April-May and continue through October. Bloom conditions were also detected in winter samples (446.7 106 cells.l-1 in a water sample collected from Lagoa Azul in February 2003). The dominant cyanobacteria species blooming in the lake was Microcystis sp., mainly Microcystis aeruginosa, which formed yellowish-green dense “scums” floating on the surface, when calm weather conditions prevailed. Water and biomass samples analysis enabled to detect the presence of soluble and intracellular microcystins. Maximum levels were detected in Lagoa Azul samples collected in July 2002. The concentration of soluble toxins was only 0.03-0.05 mg.m-3 MC-LR eq., but in biomass samples from surface to 5 m deep, toxin concentrations reached 3.78-7.83 mg.m-3 MC-LR eq., corresponding to 154-3780 mg.Kg-1 dry weight.

Keywords

water qualitynutrientseutrophicationcyanobacteria blooms