Variação dos parâmetros físicos e químicos das águas superficiais da bacia do Rio das Velhas - MG e sua associação com as florações de cianobactérias

Abstract

Velhas rivers basin is located in the Minas Gerais central state, drains an area of 29,173 km ² and is part of the metropolitan area, covering a total of 51 municipal seats. The occurrence of cyanobacteria blooms in the Velhas river began to be observed from the rainy period of 2007, affecting water use and representing a public health risk. The objective of this research was to investigate the influence of the physical, chemical and biotic factors in the occurrence of these blooms in sixteen sampling stations in the Velhas river, using secondary data monitoring provided by IGAM (state agency that manages water) during January 1998 to October 2010. A cluster analysis was performed in an attempt to highlight the groups of similar points of monitoring, considering the limnological characteristics of sampling stations. As a support tool for the interpretation of results obtained in cluster analysis were presented graphs "boxplot" made after the application of Kruskal-Wallis ANOVA and median with an level of significance of 5%. In this analysis it was evident the interference of the uses and occupation of the basin, which resulted in the definition of three groups. For monitoring stations that showed a number of cells of cyanobacteria over 20.000 (considered a threshold value for cyanotoxins analysis) a principal component analysis was made in an attempt to explain the main trends in the variation of water quality. Subsequently we calculated the pearman correlation matrix using all the parameters analyzed for each of these sampling stations. The results that were significantly correlated were expressed by means of graphical analysis of time series representing the evolution of these parameters, linking them with density of cyanobacteria. In general, the parameters that best correlated with the density of cyanobacteria were: dissolved oxygen, pH and chlorophyll-a and the only parameters with a negative correlation with the density of cyanobacteria were turbidity and total suspended solids, that are indicative of the physical characteristics of water. After calculating the correlation matrix the nonparametric Mann-Whitney test was used for the verification of significant differences between the concentrations measured during the dry and rainy seasons considering the significance level of 5%,. The results of this study demonstrate that the parameters affecting the density of cyanobacteria suffer interference of seasonality and also indicate the need to revise what are the relevant parameters for assessing water quality.