Variação espaço-temporal da comunidade fitoplanctônica em um reservatório eutrófico do semi-árido do Nordeste (Pernambuco - Brasil)

Abstract

Although the study of phytoplankton is of fundamental importance to understanding the ecology of aquatic ecosystems, there are few studies on phytoplankton in tropical environments, especially in semi-arid regions. The aim of the present study was to identify the environmental variables that exercise the greatest influence over the seasonal and spatial dynamics of the phytoplankton community in a reservoir located in a semi-arid region of northeastern Brazil (Jucazinho Reservoir). Collections were carried out at three sampling stations and two depths during the rainy (Aug 2008, Feb and Mar 2009) and dry (Oct, Nov and Dec 2008) seasons. The phytoplankton was identified and its density was determined, which was subsequently converted to biomass values. Concomitantly, analyses were performed for the characterization of the system. The reservoir was very homogeneous with regard to the spatial-temporal variation in hydrological variables: water well oxygenated at the surface and anoxic at the bottom; pH ranging from neutral to alkaline; temperatures always above 25º C; high turbidity; and high electrical conductivity at all sampling stations and depths. In both seasons, there was limited nitrogen and high concentrations of phosphorus. Cyanophyta species were predominant in the environment, generally representing 80% of the phytoplankton biomass throughout practically the entire study at all sampling stations and depths. Co-dominance of cyanobacteria belonging to H1, MP, S1 and Sn associations was recorded in most of the months studied, except August 2008, where there was a substitution of the S1 association (Planktothrix agardhii (Gomont) Anagnostidis & Komárek) for the P association (Aulacoseira granulata (Ehrenberg) Simonsen). Water temperature, precipitation and pH were the parameters with the greatest influence over the temporal variation in phytoplankton, whereas the vertical distribution of the phytoplankton biomass was directly related to the availability of light in the water column.