Limnetica 31

This study investigated the changes in the cyanobacterial population and quantified the occurrence of microcystins in Lake Furnas during the first decade of the 21st century.

The eutrophication of Lake Furnas has been recognized since the 1980s. The lake’s phytoplankton population increased over the years in relation to this process of nutrient enrichment. Cyanobacteria began to dominate the phytoplankton and blooms of greater than 20 · 103 cells/ml occurred. After 2004, cyanobacterial blooms occurred regularly in the lake throughout the year. From 2000 through 2009, 30 blooms were detected. Of these blooms, 13 were dominated by Microcystis aeruginosa and 11 by Woronichinia naegeliana. In the other 6 blooms, the dominant cyanobacteria were Microcystis spp., Anabaena spp., Aphanocapsa spp. and Coelosphaerium kuetzingianum. A number of blooms involved more than 2 species simultaneously. The highest cell density (12.3 · 106 cells/ml) occurred during winter 2007. The predominant species in this bloom were M. aeruginosa (11.9 · 106 cells/ml) and W. naegeliana (83.8 · 103 cells/ml).

Because almost all of the cyanobacteria cited were considered toxin producers, a search for microcystins, the hepatotoxins most often found in freshwaters, was initiated in 2001. Samples were collected at four depths: surface, 2.5 m, 5.0 m and 0.5 m above the sediments.

From 2001 through 2009, soluble microcystins were detected six times during the summer, four times during the winter and autumn and three times during the spring (25 % of 129 samples). The average concentrations of soluble microcystins in the water column ranged from 0.1 µg/l to 0.5 µg/l. Intracellular microcystins were detected in 84 % of the samples. All samples collected after 2004 contained these cyanotoxins. The average concentrations of intracellular microcystins in the water column ranged from 0.1 µg/l to 11.2 µg/l. The highest value (154.5 µg/l) was found in a water sample collected from the lake surface during the winter of 2009, during a bloom dominated by W. naegeliana. The amounts of microcystins produced by cyanobacteria, expressed on a seston dry weight basis, varied between 24 mg/kg and 9737 mg/kg and showed an increase in 2008 and 2009. The concentrations of microcystins in samples from M. aeruginosa blooms ranged from 86 mg/kg to 1171 mg/kg and the highest values were recorded during the spring and summer of 2008.