Unmanned aerial vehicles and low-cost sensor as tools for monitoring freshwater chlorophyll-a in mesocosms with different trophic state

Eutrophication is one of the leading causes of compromising the quality of freshwater and marine ecosystems, where the concentration of chlorophyll-a is an essential variable to monitoring the water quality. Moreover, monitoring in situ chlorophyll-a require constants samplings, high laboratory, and logistics costs, and sometimes in regions not accessible. Therefore, new technology can be important to increase the monitoring the water quality. In this sense, this work aimed to evaluate remote sensing techniques, from unmanned aerial vehicles (UAV), onboard low-cost sensors (RGB), to determine chlorophyll-a in aquatic environments. The experiment consisted of 26 mesocosms, where phytoplankton samples were inserted, simulating small shallow lakes, with gradual additions of nitrogen and phosphorus, until a trophic gradient was obtained. Subsequently, in situ concentrations of chlorophyll-a and aerial images with the UAV were obtained. The images were processed to generate orthorectified mosaics and calculate eight vegetation indices (NGBDI, SI, NGRDI, SCI, VWRI, GLI, EXG, and VARI) by which simple linear regressions were adjusted as a function of chlorophyll-a concentrations. All indexes were able to detect the gradient of chlorophyll-a, and the best index was NGBDI (R-2 = 0.88). The vegetation indices already used in aquatic environments showed greater efficiency in detecting chlorophyll-a in situ. Therefore, our results indicated that monitoring water quality for the evaluated parameters could be carried out by remotely piloted aircraft, onboard with standard RGB cameras, with faster, simpler, and lower cost protocols.