Proposal for a trophic status index for brazilian semi-arid reservoirs

Abstract

ABSTRACT The Brazilian semi-arid region, which is characterized by high climate vulnerability, intense and frequent droughts, and irregular rainfall, has a restricted quantity of water in its reservoirs and low quality at several points. Regarding quality, the literature presents indexes that evaluate eutrophication in different conditions, using specific expressions that may not be applied universally. To assist in decision-making and minimize this problem in the semi-arid region, the present work proposes a new trophic state index (TSI) taking into account data and conditions of reservoirs in the semi-arid region of Ceará. In total, 18 variables and the correlations between them were evaluated. In the years 2014-2022, data was made available by COGERH for 25 reservoirs managed by the company. Descriptive statistics were performed for each variable data set, removing outlier values to analyze trends. An exploratory investigation was also carried out, studying correlations between variables and transparency. Phosphorus, nitrogen, chlorophyll a, and turbidity presented the most significant correlations for the composition of the proposed TSI. An adaptation was made in the base expression of the TSI, which is associated with the values obtained in the semi-arid region of Ceará. Trophic classes were suggested for this new TSI along with their respective expressions. The proposed index yielded comparable results in certain classifications when compared with other established indexes. High concentrations of cyanobacteria occurred in reservoirs classified as eutrophic. To enhance the robustness of this novel index, it is recommended to extend its application to additional reservoirs within the semi-arid region.