Comunidade fitoplanctônica de dois reservatórios eutrofizados do semiárido Paraiba.

Abstract

The spatial and temporal variation and adaptive strategies of phytoplankton were analyzed as a function of chemical and physical settings and the volume of water accumulated at the moment of the samplings in two eutrophic reservoirs in the semiarid region of northeastern Paraiba. Samples of water were collected monthly from January 2008 to July 2009, in the Cacimba da Várzea (Cacimba de Dentro municipality, Curimataú) and Várzea Grande (Picuí municipality, Eastern Seridó sub basin) reservoirs at three collection stations: E1- entrada de tributários, E2 - Next to fishponds and E3-dam zone, and at three depths (sub-surface, 50% and 0% incidence of light). The variables studied were: Sechhi transparency, attenuation coefficient of light (K), temperature, pH, dissolved oxygen, electrical conductivity, total inorganic nitrogen, reactive soluble phosphorus, total phosphorous, compared NID / PSR, Trophic State Index (ETI). The concentration of chlorophyll a was evaluated, as an indirect indicator of phytoplankton biomass and the phytoplankton community was characterized (qualitatively and quantitatively), its density was determined and converted into biomass. The adaptive strategies were verified from the functional traits of each species. The presence of microcystin LR was determined by high performance liquid chromatography (HPLC). The two reservoirs are located in close basins although with different precipitation values and with similar limnological characteristics: mean temperatures of the water from 26.3 to 27.3°C, high pH (near 8), low transparency (extremes of 0.2 meters) and high concentrations of nutrients, which favor the development of cyanobacteria that dominated reservoirs throughout the study period. In Cacimba da Várzea biomass fitoplactônica had a maximum concentration of biovolume at 10.15 mm3.L which were 10 times smaller than in Várzea Grande (128.56 mm3.L) associated with overflow events during the rains (the winter period), while peak concentrations of total phosphorus were 1269.17 μg.L in Cacimba da Várzea and 1170.83 μg.L in Várzea Grande. 19 functional groups were identified: 10 in Várzea Grande (C, H1, J, Lo, Mp, P, S1, Sn, W2, X1) and 9 in Cacimba da Várzea (C, J, Lo, P, S1, Sn, W2, X1), with predominance of S1 and Sn in the two reservoirs, standing out in the dry season or summer when there were smaller volumes of water accumulated in the reservoirs. The morpho-functional traits of phytoplankton revealed the predominance of species with maximum linear dimension - MDL (greater than 50μm) and cell volume between 103 and 104 μm3 R-strategists, attributes that translate eutrophic conditions in the ecosystem. The results show that the functional groups are efficient tools in monitoring aquatic ecosystems in semiarid to express physical, chemical, biological and hydraulic conditions. The Microcystin LR concentrations detected in the reservoirs were 11.54 μg.L in Cacimba da Várzea and an average of 5.03 μg.L in Várzea Grande. High values are what require applying the guidelines of Ordinance 2914/2011, the weekly control of the cyanotoxins levels in the spring catchment area as well as in the treated water at the ETA outlet before distribution. The maximum value of microcystin allowed in drinking water is 1 μg.L. The results show that monitoring bodies of water intended for multiple uses and especially for human consumption shows functional groups identification as an excellent tool for assessing levels of eutrophication and guides to the possible presence of cyanotoxins in these waters. Given the social and economic importance of both reservoirs studied its characterization as eutrophic environments (the composition of the dominant phytoplankton groups confirmed by the contents of N and P and the EIT) is useful tostate managers of water resources for the adoption of prevention methods or the delay of eutrophication.