Índice de estado trófico adaptado aos reservatórios do semiárido cearense

Abstract

Reservoirs in the Northeastern semi-arid are naturally susceptible to eutrophication phenomena, due to environmental peculiarities. The increase in the availability of nutrients in these reservoirs, mainly phosphorus and nitrogen associated with local conditions, is the ideal environment for the occurrence of excessive blooms of cyanobacteria impacting their trophic state. The Trophic State Index - EIT, is a methodology adopted to quantify the state of eutrophication of water bodies, allows condensing various information about water quality variables, facilitating the interpretation of the trophic conditions of the dams and in making decisions for adequate management of reservoirs in the semiarid region. The objective of this work was to propose and evaluate a Trophy Index for the Semi-Arid - ITS, for four Ceará reservoirs: Gavião, Pacoti, Riachão and Pentecoste. Multivariate statistics were used, mainly, the analysis of the main components - PCA, with the aid of the PAST software. The index was consistent, coherent, precise and more sensitive to environmental variations present in the reservoirs under study: Gavião, Pacoti, Riachão and Pentecoste. The reservoirs showed a high state of eutrophication, being classified between eutrophic and hypereutrophic in the majority, with worsening in the trophic state over the years of the study (2014 to 2017). The percentage of volume available in the reservoirs, despite having a low correlation with the calculated ITS, there was a worsening in the trophic state with the reduction in the volume of water. However, the proposed ITS showed excellent correlations with the concentrations of total nitrogen (0.9165 ≤ r ≤ 0.9415) for all reservoirs, except Pentecost, which had a low negative correlation (r = -0.49). Although the analysis of the limiting nutrient indicated the collimation of primary productivity by total phosphorus and total nitrogen, it was found that phosphorus behaved as the main limiting factor. The proposed trophic status index proved to be efficient in assessing the degree of trophy for the Ceará reservoirs.