Avaliação do estado ecológico de sistemas lóticos subtropicais e temperados brasileiros para determinação de sítios de referência na bacia hidrográfica do Arroio Andréas, município de Vera Cruz, RS, Brasil.

Abstract

This research aimed at determining reference sites in southern Brazilian lotic systems, using the Water Quality Trophic Index (WQTI), based on epilithic diatom communities. Within the period of 2012 and 2016, 140 epilithic diatom samples from seven sampling sites within the spring area of the Andreas Stream Hydrographic Basin, Vera Cruz County, RS, Brazil, were analyzed quarterly. Similarly, from 2012 and 2014, water samples in theses sampling sites were collected monthly for determination of physicochemical and microbiological parameters, such as water temperature, pH, turbidity, dissolved oxygen, biochemical oxygen demand, nitrate, phosphate, total dissolved solids and fecal coliforms. The results indicated the occurrence of two diatom samples groups with significant differences (p<0.05). Group 1 (sampling sites P1, P4 and P5) was classified in the oligotrophic category with an average WQTI of 1.3 ± 0.2, while Group 2 (sampling sites P2, P3, P6 and P7) was classified in the β-mesotrophic category with an average value of 2.0 ± 0.4. The species representing oligotrophic waters throughout the time series, were Platessa hustedtii (Krasske) Lange-Bertalot and Achnanthidium minutissimum (Kützing) Czarnecki, the latter being common to both sampling groups, demonstrating to be representative of oligotrophic to mesotrophic waters. Thus, we proposed the upper limit of the 25% quartile for ITQA score scale, equal to 1.5, as a reference value to differentiate the limits between the “high” and “good” ecological status to determine “reference sites” for subtropical and temperate Brazilian aquatic systems. The physical, chemical and microbiological water quality evaluation gives support to this proposal, as far as the results indicated a significant improvement in the water quality, classifying sampling sites P1, P4 and P5 as having good quality appropriate for multiple uses.