Estrutura das assembléias de bacterioplâncton, arqueoplâncton e fitoplâncton em estuários sob estresses antropogênicos (NE-Brasil)

Abstract

This study evaluated the influence of nutrients and salinity on the structure of bacterioplankton, archaeoplankton and phytoplankton in four low-inflow estuaries from Ceará, northeastern Brazil (Ceará, Cocó, Pacoti e Pirangi). Sampling occurred during the dry (Nov/10) and rainy (May/11) periods in the surface layer of the water column. The richness and composition of the prokaryotes assemblages were assessed by denaturing gradient gel electrophoresis of 16S rRNA gene. Composition, species richness and density of the phytoplankton assemblage were analyzed by the traditional Utermöhl method. The estuaries were classified as eutrophic or hypereutrophic, according to the trophic state index modified for tropical environments, which take into account the contributions of total phosphorus and chlorophyll a. In the dry season, the salinity values were higher in all estuaries due to the decrease of freshwater inflow, increase of saltwater intrusion and higher evaporation rates. In the rainy season estuaries were characterized by increased river and inorganic suspended solids discharge. Regardless of environmental condition, the richness of bacteria was higher than that of archaea. Redundancy analysis (RDA) showed that the variation in the composition of particle-attached bacteria was significantly (p < 0.05) influenced by phosphorous and nitrogen concentrations, while the particle-attached archaea was driven by these nutrients and also by pH. The RDA for free-living bacteria showed that this assemblage was significantly (p < 0.05) influenced by TN:TP, while the variation in the composition of free-living archaea was not explained by any of the measured environmental variables. The highest phytoplankton densities occurred in Ceará (2.4 × 107 organisms.L-1) and Cocó (5.3 × 107 organisms.L-1) in the dry period. For these samples, the predominant taxa were the nanoplanktonic phytoflagellate Cryptomonas/Rodhomonas 1 and the cyanobacteria Synechocystis cf. aquatilis, respectively. In the dry period the Pacoti and Pirangi estuaries, where the hypersalinity condition occurred, the phytoplankton densities were low (1.0 × 105 organisms.L-1 and1.3 × 105 organisms.L-1, respectively), with the predominance of a small unidentified diatom (length = 11-15 μm) and the marine diatoms Cylindrotheca closterium and Thalassionema nitzschioides in both samples. The four estuaries in the rainy season presented the predominance of freshwater phytoplankton species, which were mostly typical of eutrophic environments (e.g. Planktothrix isothrix and Trachelomonas cf. volvocina var. volvolcina). The phytoplankton assemblage was mainly influenced by the concentrations of nitrogen, silica and salinity, according to the CCA. Taken together, our findings indicated the influence of nutrients to select populations of bacterioplankton, archaeoplankton and phytoplankton, showing the importance of pollution control in low-inflow estuaries. Regarding the salinity levels, prokaryotic assemblages seem more resistant to seasonal changes in this variable than the phytoplankton.