Monitoramento de alta frequência no reservatório de Ribeirão das Lajes: análise da estabilidade térmica e química da coluna d’água entre os anos de 2018 a 2020

Abstract

Measurements of temperature, dissolved oxygen, pH and fDOM through continuous high frequency logging of the water column were carried out with the objective of studying the variations in the limnological conditions of the water column of the Ribeirão das Lajes reservoir located in Rio de Janeiro. to characterize the physical and chemical conditions in the different stability scenarios of the water column during the period from October 2018 to February 2020. The influence of cold fronts on the short-term stratification pattern of the reservoir was also evaluated. The association relationships between the physical and chemical variables of the water in Lajes were verified by applying tests such as Shapiro-Wilk, Mann-Whitney, Kuskal-Wallis and Spearman Correlation. Thermal stability was evaluated using the indicators: temperature, RWCS, RTR and Schmidt stability. Dissolved oxygen, pH and fDOM were used to study the chemical characteristics of the reservoir waters. The results obtained using the physical parameters showed that the Lajes reservoir has periods of strong stability and instability of the water column, varying over time, showing a seasonality coupled with the climatological regime, thermal stratification of the water column quite pronounced in the summer months lasting 3 months (December to March) in 2018/2019 and 2 months (January and February) of 2019/2020. The RWCS, RTR and Schmidt Stability showed the same results, with greater stability during the summer and periods of instability in the winter months. The climatic conditions of the region are the main factors responsible for the stratification pattern of the water column of the reservoir area and the entrances of cold fronts were important for changes in short periods in the stability of the water column of the reservoir. Passages of cold fronts were observed, which influenced the weakening of the strong thermal stratifications existing during the summer, which made the water column unstable for a few days and for a longer period during the winter. These periods of instability were associated as a result of the low temperatures existing during the passage of these fronts. The concentration of fDOM showed a negative correlation with dissolved oxygen and with pH, demonstrating great power of decomposition in the first meters of the water column. Thus, it was possible to conclude that the Ribeirão das Lajes reservoir remains stratified throughout the year with periods of strong thermal stability that are interrupted by the entrance of cold fronts that weaken the existing layers in the water column, but not to the point of occurrence. a complete water mixture from this column. The stratification of the water column was also verified by the oxygen and pH parameters, which are stratified even during periods of thermal destratification, unlike fDOM, which is stratified inversely with oxygen and pH.