Estrutura e funcionamento da comunidade fitoplanctônica em ambientes lacustres do estado de Roraima – Brasil

Abstract

The aim of the present study was to analyze the structure of the phytoplankton community on the nyctemeral and seasonal scales and the influence of limnological variables in two Amazon lakes: Caracaranã Lake (03º50’41” N and 59º46'52.1 W) and Lago dos Reis (01º30’59.5” N and 61º15'50.4" W). Sampling was performed with nictemeral frequency in 4-hour intervals, completing two 24-hour cycles in two distinct climate periods: rainy season (August and September 2005 and June 2006) and dry season (December 2005 and November 2006). Collections were made at a central station in the lakes at the subsurface (0.20 m), middle and approximately 30 cm above the bottom of the water column, using a van Dorn bottle. The following abiotic variables were analyzed: temperature, dissolved oxygen, pH, electrical conductivity, turbidity, total nitrogen, nitrate, total dissolved phosphorus and orthophosphate. The phytoplankton community was assessed with regard to composition and density. The relationship between the phytoplankton community and abiotic variables was assessed using canonical correspondence analysis. Caracaranã Lake proved oxygenated, witha predominance of an ortograde profile, slightly acidic, polymythic waters with diurnal stratification and nocturnal circulation during the rainy season and homogeneous layers during the dry season, with low concentrations of nutrients, characterizing an oligotrophic environment. A total of 60 taxa were identified, with a predominance of desmids (50% of the taxa). The species Botryococcus terribilis J. Komárek & P. Marvan e Sphaerocystis shroeteri Chodat had a 92.86% frequency of occurrence. The nictemeral variations in population densities tended toward the formation of vertical gradients, with slightly higher values at the subsurface. Regarding seasonality, the greatest densities occurred in the rainy season and the lowest densities occurred in the dry season. Lago dos Reis had low concentrations of oxygen, a clinograde profile, waters ranging from slightly acidic to alkaline, stratified during the day and homogenous at night, with low concentrations of nutrients. The phytoplankton community was represented by 54 taxa, with a predominance of Chlorococcales (22 species). Small densities of phytoplankton occurred in both nictemeral cycles, with an accentuatedvertical gradient. The highest densities were recorded in the dry season and lowest densities were recorded in the wet season. Lago dos Reis exhibit characteristics that classify it as apolymythic, oligotrophic environment. The canonical correspondence analysis demonstrated that the variability in the data was more important seasonally than on the nictemeral scale. It was concluded that differences found in the structure and dynamics of the phytoplankton communities are related to the particular characteristics of each lake, such as low concentrations of dissolved oxygen, turbid waters, pH ranging from slightly acidic to alkaline at Lago dos Reis, contrasting the well-oxygenated waster, high transparency and slightly acid pH in Caracaranã Lake.