Planktonic communities as indicators of water quality in a tropical reservoir

Surface water bodies have been significantly altered by various human activities in the watersheds. Assessing these ecosystems is crucial for maintaining an ecological balance and protecting public health. This study aimed to analyse the water quality of a tropical public supply reservoir by evaluating the dynamics of planktonic communities. A total of 69 water quality parameters, in addition to phytoplankton and zooplankton biomonitoring data, were analyzed between 1989 and 2020. Preliminary and descriptive statistical analysis, trophic state index, Shannon-Wiener diversity index, statistical tests, and multivariate statistics analysis were applied. The results showed a change in the composition of the planktonic communities over time, with a greater abundance of cyanobacteria and an increase in the density of the Crustacea class in the final years of monitoring, indicating a more degraded environment. There was a reduction in the diversity of phytoplankton species. However, the concentrations of physical and chemical parameters remained low over the years, and oligotrophic conditions were prevalent in the reservoir. Therefore, the analysis of bioindicators provided information that was not observed when the physical and chemical parameters of water quality were analyzed. Possibly due to environmental variables not considered in this study, or due to the cumulative or synergistic effects caused by changes in the environment. It is evident that integrating bioassessment into the monitoring of water bodies is essential for a comprehensive ecological classification and for maintaining the integrity of the ecosystem.