Understanding the Spatiotemporal Variation of Water Quality and Phytoplankton Biomass in Subtropical Reservoir Using the Blue–Sky Multispectral Data

Abstract

Chlorophyll-a (chl-a) is an optically active compound used as a proxy for phytoplankton biomass to determine the trophic states of aquatic ecosystems. Blue–sky remote sensing technologies present low-cost and effective monitoring techniques for water quality on a large scale. This study was aimed at using readily available Landsat multispectral images to assess the spatial and temporal variation of phytoplankton biomass in Nandoni reservoir, Limpopo Province, and at examining the relationships that exist between the physicochemical variables and chl-a concentration. Landsat 7 ETM+ and Landsat 8 OLI images for June (dry) and December (wet), for the years 2008–2020, were used to derive the distribution of chl-a concentration. Using regression techniques, in situ measured chl-a showed a strong and perfectly linear relationship to the predicted Landsat chl-a in the Nandoni reservoir. There was a negative significant correlation between land use and land cover and water quality variables. Using permutational multivariate analysis of variance (PERMANOVA) analysis, we uncovered significant differences for chl-a concentration in sites, seasons and zones. A significant positive correlation was observed between water temperature and chl-a concentration. In contrast, a strong negative significant correlation was observed for chl-a with salinity and total dissolved solids. chl-a concentration in the Nandoni reservoir was derived using Landsat remote sensing images, suggesting that the Landsat data is suitable for monitoring small reservoirs in a short timescale. The results of this study suggest that remote sensing techniques can be used to control the development of an early warning system for this study and other reservoirs. Furthermore, the results highlight the role of using analysis ready Landsat series data in monitoring phytoplankton biomass and chl-a abundance in freshwater systems.