Estrutura da comunidade fitoplanctônica, em cinco lagos marginais do Rio Turiaçu, (Maranhão, Brasil) e sua relação com o pulso de inundação.

Abstract

The phytoplankton communities of five lakes in the floodplain of Turiaçu River (MA) were analysed for taxonomic composition and evaluated regarding temporal dynamics. Changes on phytoplankton densities were analysed as well as the variations in physical and chemical water characteristics along a hydrological cycle, attempting to correlate them to the flood pulse and the consequent water level fluctuations. Samplings were carried out in lakes Cabeludo, Caboclo, Arrodiador, Bate Terra and Quebra Pote, five periods of representative phases: dry, water rising, flooding, water lowing and a second dry. Qualitative samples were taken by horizontal hauls with a 20 µm phytoplankton net, and quantitative samples were taken at the surface and at 20 cm above the bottom with a Van Dorn bottle. Samples were preserved with in 4% formalin and the phytoplankton was identified and enumerated in settling chambers under a Zeiss inverted microscope. Turbulence, rainfall and water level fluctuations were considered the main controlling factors of the successional dynamics. The lakes are shallow and nutrient rich, being eutrophic during the dry period and mesotrophic in the other hydrological phases. The phytoplankton communities of the five lakes are represented by 139 taxa belonging to 9 Classes: 22 Bacillariophyceae, 34 Chlorophyceae, 1 Cryptophyceae, 33 Cyanophyceae, 2 Dinophyceae, 20 Euglenophyceae, 1 Xanthophyceae and 25 Zignematophyceae. Most taxa are new records for Maranhão State. Cyanophyceae was the numerically dominant group in three periods: water rising (March), water lowing (August) and dry (December), with relative abundances varying from 23.13% in Lake Caboclo during the water raising period to 65.72% of the total, at the same lake, in the dry period. Bacillariophyceae were the most abundant organisms in the flooding period (May), with relative abundances varying from 38,14 % in Lake Bate-Terra to 49,70% in Lake Caboclo. The most abundant species of Cyanophyceae and Bacillariophyceae were Planktolyngbya circumcreta and Aulacoseira granulata, respectively. Total phytoplankton densities varied from 1725 ind.mL-1 in Lake Arrodiador, in the flood period, to 8272 ind.mL-1 in Lake Cabeludo, at the dry period. The highest densities occurred in dry periods, related to the greater nutrient availability and to the concentration factor resulting from the water level decrease. There was a tendency to higher species richness, lower dominance, higher equitativity and higher diversity of phytoplankton during water rising and water lowering.