Dados da Tese de Doutorado
Author: Gina Luísa Boemer Deberdt.
Title: Cyanobacterial study in high trophic degree reservoir (Salto Grande Reservoir - Americana - SP).
Year: 2002. Full text (in Portuguese)
Abstract
The study of the bloom of potential toxic cyanobacteria is of paramount significance, mainly when a reservoir endowed with great social and economical values due to both its multiple uses and its nearness to big urban centres such as the Salto Grande reservoir, located near the city of Americana in the inland of the State of São Paulo, Brazil is concerned. Based on such actuality, this work was aimed at supplying resources to devise forecasting the occurrence of cyanobacteria as well as the production of toxins in aquatic environments in which high eutrophication levels are observed. Towards this target, the research was carried out in the three steps that follow. In macrocosmic level (reservoir), the variations of the occurrence of both the cyanobacteria species and the remaining phytoplankton classes were found out, and then the toxic potentiality of the samples collected during the rainy season (January, February and March 1998) and the dry season (June, July, August and September 1998) in two sampling stations at the Salto Grande reservoir was analysed. In mesocosmic scale (tanks), the variations in the occurrences of the classes of phytoplankton and of the cyanobacteria species as well as the variations in production of toxins as a function of the ratio N/P that was dealt with by means of dosing nitrogen and phosphorus in the water were observed during the dry season. In microcosmic level (glass bottles “in lab”), the effect of reducing phosphorus (and consequently increasing the ratio N/P) on the growth and the production of toxins in cultures of Microcystis aeruginosa (Kützing) Kützing strains isolated from bloom samples of this species at the Salto Grande reservoir, during the wet and the dry seasons was tested. At the environment, the presence of microcystin in the water of all the samples, with the exception of the 25th of February 1999, was detected. Apart from the station 2 during January 27th 1999 (39.53 μg.L-1) and March 22th 1999 (3.98 μg.L-1), the concentrations stayed below the limit of acceptability (1 μg.L-1). At the experiments in mesocosmic scale, in 3 different conditions, increasing in phytoplankton density was observed. At the control condition (without manipulation), both reasonable augmentation of the percentage of the cyanobacteria contribution and diminishment of the other groups during the 11 days were detected. Under the condition of low N/P ratio, a slight rise of the percentage of the cyanobacteria and chlorophycea contribution and a decrease of the cryptophycea were observed; the other groups did not present much change. Under the condition of high N/P ratio, it was seen that the cyanobacteria increased, the chlorophycea remained unaffected and the cryptophycea decreased in comparison to the initial percentages. The conditions of the tanks that had been maintained at low N/P ratio favoured more the chlorophycea. The cyanobacteria presented biomass augmentation under the conditions of the tanks that had been maintained with high N/P ratio. For every test at the microcosmic level, the exponential stage had begun at the 8th development day. After being experimented for nearly 18 days, yield decrease of the cultures at the environment ASM-1 with phosphorus reduction was observed. In every scale that had been studied, it was noted that the microcystin concentration is related to unfavourable factors as far as the development of the toxic species is concerned. However, the determining causes for the growth of toxic cyanobacteria played different roles in each scale studied. At the macrocosmic level, enrichment was the responsible for increasing the density of the toxic species. At the microcosmic level, phosphorus availability had been directly related to Microcystis aeruginosa growth rate and, therefore, to the increase of microcystin concentration.
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