Análise temporal do índice de estado trófico em reservatório artificial na região semiárida do nordeste brasileiro

Abstract

The semi-arid region of Ceará is marked by a climate of high temperatures throughout the year, in addition to irregular rains concentrated in the first semester, due to these peculiar characteristics the semi-arid is considered a fragile climate. The eutrophication process is a recurring problem in reservoirs in this region, which influences the quality of the water that supplies the population and agricultural activities. Due to the climatic conditions of the region, the tendency is to intensify this process due to the loading of loads. Land use and occupation are key factors in understanding the main impacts in a given region. Therefore, the objective was to evaluate and classify the degree of trophy of an artificial surface reservoir in the semiarid region using the Trophic State Index (TSI). The study was carried out in the General Sampaio reservoir, belonging to the Curu Basin, Ceará. Transparency data were measured in situ, chlorophyll-a (Cl-a) and total phosphorus (PT) were determined in the laboratory. The data refer to those available on the website of the Companhia da Gestão de Recursos Hídricos do Estado do Ceará – COGERH, together with data collected in the field. The in situ measurements and water samples were collected at one point (P01 – Water intake) of the reservoir, at which point COGERH data is used. To evaluate the different behaviors of the limnological variables during the dry and rainy season, the data were organized according to the period and statistical descriptions were made, also relating them to the water volume of the reservoir. Then, the average trophic state index was calculated. A correlation was made between the presence of the studied attributes and the precipitation for the day of collection and for the previous 10 and 30 days. To carry out the temporal study, images from the MAPBIOMAS collection were used for the years 2009, 2014 and 2019. In general, the limnological attributes chlorophyll-a, phosphorus and nitrogen in most of the study period presented concentrations above the limit established by CONAMA for human supply. The concentrations of the evaluated nutrients suffered little variation when compared to the seasonal, rainy and dry periods. It was observed that the reduced volume contributed to the intensification of the trophic index over the years, showing a good correlation. The rainfall of 10 days before the collection date did not show significance in terms of the influence on the presence of the analyzed attributes. The use and occupation of the basin is very homogeneous, requiring only the control of human actions to avoid the aggravation of polluting sources of water bodies.