Estrutura e dinâmica da comunidade fitoplanctônica em um reservatório eutrófico do trópico semiárido brasileiro.

Abstract

This work studied the structure and dynamics of the phytoplankton community of a eutrophic reservoir of semiarid region, with intensive fish farming in cages, focusing on the patterns of temporal and spatial distribution of life strategies and functional groups characteristic. Sampling occurred between August 2007 and July/2009, with bimonthly frequency in the first year and at monthly intervals thereafter, in the euphotic and aphotic zone of the points of confluence of tributary rivers (PC), near the fish net cages (PT) and near the dam itself (PB). The variables analyzed were: rainfall, water temperature, transparency, pH, electrical conductivity, alkalinity, dissolved oxygen, soluble reactive phosphorous (SRP), dissolved inorganic nitrogen (DIN), phytoplankton biomass and the presence of cyanotoxins (microcystin-LR) in water. The approach of phytoplankton functional groups and ecological and morphological attributes was used to describe the trajectory of species between periods. Seasonality promoted the establishment of distinct periods, drought and rainy. During the drought favorable environmental conditions in the reservoir such as high temperature (>24°C), alkaline pH (minimum 7.0 and maximum 10), limited availability of underwater light (Zeu:Zmis≤1), eutrophic conditions (IET=61.2±5.1-PC, 62.5±5.4-PT, 60.7±6.6-PB) and environmental stability associated with high water residence time (146 days), contributed to the establishment of high phytoplankton biomass consisting mainly of filamentous cyanobacteria (R strategists), belonging to the functional groups S1, Sn, H1 and MP, typical of eutrophic environments with low light availability. Coccoid cyanobacteria of M and K were also recorded. During this period, the dominant species were: Planktothix agardhii, Pseudanabaena limnetica, Cylindrospermopsis raciborskii, Dolicospermum circinalis, Oscillatoria lacustre, Microcystis aeruginosa, Microcystis sp. and Aphanocapsa incerta. Reduction in biomass and in the dominance of cyanobacteria occurred in the rains, especially in times of overflow of the reservoir, when invasive species (C strategists) and stress tolerant (S strategists), not colonial flagellate (Botryococcus braunii), single-celled flagellate (Chlamydomonas sp. and 13 Peridinium umbonatum) and non-flagellated unicellular (Closterium sp.) dominated or co-existed in the environment, favoring the increase in biomass of the groups: F, P, Lo and X2, typical of meso-eutrophic environments. These taxa were predominant especially at the confluence of tributary rivers and near the dam itself, during overflow events. Next to the cages cyanobacteria dominated and the biomass values were higher, indicating that the activity of intensive fish farming was an important factor of anthropogenic disturbance in the reservoir. Of the eight species of cyanobacteria dominant (≥ 50% of the total biomass), Planktothrix agardhii, Pseudanabaena limnetica, Cylindrospermopsis raciborskii, Dolicospermum circinalis, Oscillatoria lacustre, Microcystis aeruginosa, Microcystis sp. are capable of producing toxins. During the study the formation of toxic blooms and production of microcystin-LR was recorded, with the highest values observed in the dry season (minimum=0.93μg.L-1- dezembro/2008 and maximum=27.29μg.L-1-december/2007). The occurrence of microcystin alert to possible risks to public health since this reservoir is used to supply human activity and intensive fish farming. The use of functional groups, ecological and morphological attributes was effective tools for understanding the links between environment and species, and assessing the ecological health of the reservoir.