This study presents the variation of cyanobacteria population and microcystins quantification in Lake Furnas during the first decade of the 21th 21st century. The eutrophication of Lake Furnas has been noticed since the eighties of last century. Related to this process of nutrient enrichment, phytoplankton population increased throughout the years, cyanobacteria began to be dominant and blooms (more than 20.103 cells/ml) arose. After 2004, cyanobacterial blooms became permanent in the lake throughout the year. From 2000 to 2009, 30 blooms were detected, 13 dominated by Microcystis aeruginosa and 11 by Woronichinia naegeliana. In the others 6 blooms, dominant cyanobacteria were Microcystis spp., Anabaena spp., Aphanocapsa spp. and Coelosphaerium kuetzingianum. Sometimes there were more than two species blooming at the same time. The highest cell density (12.3.106 cells/ml) took place in 2007's winter season, being the predominant species Microcystis aeruginosa (11.9.106 cells/ml) and Woronichinia naegeliana (83.8.103cells/ml). Since almost all of the mentioned cyanobacteria were reported as to be toxin producers, the search for the presence of microcystins, the hepatotoxins more often found in freshwaters began in 2001. Samples were collected at four depths:, surface, 2.5m, 5.0m and 0.5m above sediments. From 2001 until 2009, soluble microcystins were detected six times in summer, four times in winter and autumn and three times in spring (25% of 129 samples). Water column average concentrations ranged from 0.1mg/l to 0.5 mg/l. Intracellular microcystins were detected in 84% of the samples and all of those collected after 2004 presented these cyanotoxins. Water column average concentrations ranged from 0.1 mg/l to 11.2 mg/l. The highest value (154.5 mg/l) was found in a sample taken from water surface in 2009 winter, during a bloom dominated by Woronichinia naegeliana. Concentrations of 1.03 mg/l to 1.71 mg/l were found in the autumn of 2004 when Woronichinia naegeliana was blooming alone in the lake. The amounts of microcystins produced by cyanobacteria, expressed in seston dry weight basis, varied between 24 mg/kg and 9737 mg/kg and showed an increase in 2008 and 2009. The highest concentration was recorded in a surface sample (May 2009) during a bloom of Woronichinia naegeliana. Concentrations in samples from Microcystis aeruginosa blooms ranged from 86 mg/kg to 1171 mg/kg and the highest values were recorded in the spring and summer of 2008.
|Publication status||Published - 1 Jan 2012|