Dominant and co-existing species of Cyanobacteria from a Eutrophicated reservoir of Rio Grande do Norte State, Brazil

This paper reports on the cyanobacterial population, its ecology, dominance and coexistence characteristics in one annual cycle of a eutrophicated Gargalheiras reservoir (Marechal Dutra) of Acari, in the state of Rio Grande do Norte of Brazil. The study was conducted between November 1999 and May 2000, which included the dryand wet seasons. It is an old, man-made impounded water reservoir. The methodology included the collection of phytoplankton samples from the surface to the bottom in a fixed station to include four vertical profiles and to analyse them for physico-chemical and algological characteristics. Ecological indices, such as Shannon-Weaver for species diversity, Margalef's species richness and Pielou's evenness were calculated. The results reveal the characteristics of the eutrophic to the hyper trophic state for the Gargalheiras reservoir with the dominance of Cylindrospermopsis raciborskii, Raphidiopsis curvata, Microcystis aeroginosa and Oscillatoria sp during the dry period. Cages were suspended to culture Amazonian fish, Tambaqui, at the beginning of the wet period close to the sampling stations in the reservoir and treated for point source nutrient loading and disturbance factor. This localised disturbance stimulated the growth of Chlorophyceae species, which co-existed along with the species of cyanobacteria to finally dominate; coinciding with the stability of monsoon rain in May. The conclusion drawn from the study period revealed that the reservoir with high inorganic nutrients had reduced transparency, anoxic hypolimnion, elevated levels of electrical conductivity and the dominance of cyanobacteria during the dry period and the dominance of chlorophyceae in the wet period. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.