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Data of Dissertation

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Author: Andreia Maria da Anunciação Gomes.

Title: Ecophysiological study of cyanobacteria forming blooms in Jacarepaguá lagoon..

Year: 2011.    Full text (in Portuguese)

Abstract

Jacarepaguá Lagoon is located in the South coast of Rio de Janeiro State, in the metropolitan zone of Rio de Janeiro City, Brazil. It is an oligohaline, shallow lagoon that has undergone intense eutrophication in the past decades, with cyanobacterial blooms reported since the 90'. Sporadically, a break of cyanobacteria dominance is followed by an increase of phytoplankton diversity. The aim of this study was to evaluate, through field and laboratory studies, which factors influence phytoplankton succession in this environment. During a seasonal cycle (2007-2008) we analyzed phytoplankton community and water chemical and physical parameters. During cyanobaterial blooms, the most prominent species were Microcystis aeruginosa, Aphanizomenon sp. and Planktothrix sp. Although still dominant, a significant reduction in biomass was observed during winter, when water temperature decreased from about 30oC(summer) to 18oC. Bloom collapse resulted in the release of a large quantity of DIN, allowing other species to occupy the niche released by cyanobacteria: Cyclotella sp. among diatoms and Desmodesmus quadricauda. among green algae. Considering these field observations, we tested, in laboratory, the effect of nutrient reduction on phytoplankton, as well as the effect of temperature on direct and indirect competition among the main species of Jacarepaguá lagoon. The reduction in nutrient concentration decreased cyanobacterial biomass but did not reduce their dominance. Even so, the low nutrient condition favored other phytoplanktonic groups, especially diatoms. The reduction of phosphate input in Jacarepaguá lagoon could recover this system. Temperature can influence growth and phytoplanktonic species replacement. We established, under laboratory conditions, optimum growth temperatures for P. agardhii (27oC), C. meneghiniana (18 and 21oC), D. quadricauda (18 and 30oC) and M. aeruginosa, (all those tested between 18 and 30oC). In competition tests with mixed cultivation M. aeruginosa was more competitive than others in both temperatures (18 and 30oC). In cultivation with exudates, different allelopathic responses (growth inhibition or stimulation) were attained in different temperatures. Observing the community dynamics in the lagoon, M. aeruginosa is favored during periods of higher temperatures. Exudates of its competitors stimulated its growth at 30oC, but not at 18oC. Adaptation of one strain to a specific temperature will determine not only its vulnerability to allelopathic compounds, but also its ability to express the allelochemical potential. Finally, it is difficult to point one factor that dictates the occurrence of cyanobacterial blooms in the Jacarepaguá lagoon. A synergy of environmental and biological factors combined with the physiology of the species will determine the dynamics of phytoplankton in this system.

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