Analysis of the Community Structure of Diatoms (Bacillariophyta) in Roraima Fluvial Systems, Amazon – Brazil

The State of Roraima (Amazon - Brazil) is considered well-preserved from an environmental point of view, sheltering a great variety of rivers and forest and “cerrado” streams, with white (muddy), black and clear waters. From the identification of the planktonic community of diatoms, ecological indices and statistical analyzes were applied in order to determine the degree of abundance and species richness among the different ecosystems in the region. Qualitative and quantitative samples of phytoplankton were obtained during periods of low water (December to March), as well as measurements and analyzes of limnological parameters: water temperature (ºC), pH, dissolved oxygen (mg/L), oxygen saturation (%), transparency (m), total dissolved ions Na+, K+, Ca2+, Mg2+, Fe2+, Fe3+ and Cl-, total P and N (mg/L) and Chlorophyll-a (µg/L). A total of 172 plankton samples from 43 sampling points (20 in rivers and 23 in streams) were carried out. The low water period was characterized by slightly acidic to neutral pH, with low transparency and variations in conductivity and concentration of the ionic composition. The results indicated similarity in diatom composition between rivers and streams, with little heterogeneity in “cerrado” streams. Considering climate oscillation and spatial distribution, the PCA highlighted the parameters conductivity, dissolved ions, total P and chlorophyll-a as preponderant to explain the behavior observed in the distribution of communities. For three components, the cumulative explanation was 68.6%. The ecological indices of richness and diversity indicated high similarity between rivers and streams, with good stability, homogeneity and ecological diversity for bacillariophytic diatoms. There were, however, some areas that, depending on the type of water and ionic concentration, showed some divergence in the composition of the diatom community. As environmental indicators, the diatoms showed that the water quality in the region is well-preserved, without eutrophication signals.