Estudo dos índices de estado trófico em reservatórios do semiárido

Abstract

Eutrophication has become a global problem that can increase water quality, due to the excess of nutrients, mainly phosphorus, stimulating the increase in primary productivity. One way of classifying and presenting the trophic state of a body is the water index (IETs) that allows to follow an evolution of the eutrophication process in using the environments. The research aimed to analyze and compare two IETs widely used in Brazil, Carlson (1977) and Lamparelli (2004), and apply them to reservoirs in the semiarid region. The hypothesis is that Lamparelli's (2004) model increases the sensitivity in the trophic classification system by increasing its classification range, with the purpose of differentiating the stages of eutrophication. Sampling in the three reservoirs of the semi-arid region (Dourado, Cruzeta and Passagem das Traíras) was carried out between January 2012 and December 2019. The annual averages of the variables (chlorophyll-a, total phosphorus and Secchi) and the global indices were calculated for each model. In addition, linear regression was performed at a 95% confidence level (p < 0.05) and a Spearman correlation was performed to indicate whether there was a positive or negative relationship between all variables and to examine the degree of correlation (weak, moderate, strong or very strong) between them. The results showed that these reservoirs were eutrophic and worsened over time, given the high average annual concentrations of chlorophyll-a and total phosphorus. The results obtained from the global average of each reservoir for the values of the Trophic State Indexes (IETs) showed an increase and decrease trend over the years in which the study was carried out, and the Carlson model (1977) practically there was no variation in the trophic classification, indicating that, in general, the environments are considered hypereutrophic. Meanwhile, Lamparelli (2004) presented three distinct classifications (eutrophic, supereutrophic and hypereutrophic) expanding the trophic categorization and confirming the working hypothesis. Regressions and Spearman's correlation showed that both the Carlson and Lamparelli models were statistically significant (p < 0.05). In the Spearman correlation, the IET Chla x IET PT was moderate and the IET Chla x IET Secchi resulted in a weak correlation for the two models. The results of this study can help future research and managers in future decision-making regarding the model that best adapts to EIT studies in the semiarid region and other regions of the country.