Fases de equilíbrio de cianobactérias em reservatório eutrófico do semiárido durante período de estiagem prolongada

Abstract

The work aim to detect equilibrium phase (s) or non-equilibrium in a eutrophic reservoir of tropical semi-arid northeast, during a period of prolonged drought. Phytoplankton community structure with dominance of cyanobacteria was studied in association with the endogenous (physical and chemical water) and exogenous factors (climatic and hydraulic) that may influence the ecosystem dynamics. The study, conducted in the reservoir Argemiro de Figueiredo (Acauã), in the watershed of the Rio Paraíba - Paraíba state, was sampling weekly during September to December 2012, in the subsurface of two points: P1 (side arm) and P2 (near the dam). The variables analyzed were hydrometeorological (precipitation; share / volume), water temperature, underwater light climate (transparency; reason Zeuf: Zmis), pH, electrical conductivity, dissolved oxygen, soluble reactive phosphorus - RSP, dissolved inorganic nitrogen - DIN ( nitrite, nitrate and ammonia), DIN reason RSP, trophic state index - TEI, chlorophyll-and a quantitative characterization of phytoplakton (density and biomass) and qualitative (taxonomic identification, diversity, life strategy, functional group) . The prolonged drought (since January 2012) caused high evapotranspiration, so the reservoir had a water volume around 50% of its maximum capacity at the beginning of the research, with environmental stability (spatial and temporal homogeneity) and high levels of eutrophication (54 < EIT &#8804; 74), high water temperature (average> 27 ° C), basic pH (mean > 8.5), low availability of underwater light (Zeuf reason: Zmis <1) and high nutrients concentration. These conditions contributed to the establishment of a community dominated by filamentous cyanobacteria, low richness and high biomass represented by Planktothrix isothrix, Planktothrix agardhii and Cylindrospermopsis raciborskii (R strategist, functional groups S1 and Sn), and they were responsible for formation and maintenance of a steady-state during. In the absence of disturbances (rain events and flooding, disturbance factors that influence the dynamics of the ecosystem and its algal community) theb reservoir supported the establishment of functional phytoplanktonic groups which are specific biomarkers of eutrophic hypereutróphic levels with dominance of few taxa that formed a community ranging between phases steady-state and not steady-state , or equilibrium phases and non-equilibrium. The establishment of equilibrium phases of the periodical may be constant because of the presence of cyanobacteria in this steady-state reservoir The scenario described is an alert for environmental and public health managers because the public risk since the treatment methodologies adopted in the two potable work plants, which supply about 40,000 inhabitants, does not us technologies compatible with the removal of cyanotoxins.