The sustainable management of water supply reservoirs requires analysis of spatiotemporal variations in nutrient levels, phytoplankton composition, and trophic status. The Katse Dam (KD) is a strategic raw water supply source that generates hydropower and sustains aquaculture. However, it is exposed to nutrient enrichment from mining and aquaculture, whose impact on its trophic status necessitates monitoring. This study applies the organic pollution index (OPI), the modified pollution index (MPI), and Carlson’s trophic state index (CTSI) to assess the trophic status of KD. The results from the first decade (FD) (2003–2013), when the intensity of mining and aquaculture activities was minimal, were compared with the results from the second decade (SD) (2014–2024) when there was higher activity. The MPI revealed that KD transitioned from a contaminated status during the FD to a greatly contaminated status during the SD. KD shifted from mesotrophic to eutrophic in the transitional zone and from eutrophic to hypereutrophic in the lacustrine zone. The cyanobacteria Radiocystis sp. replaced Asterionella sp. and became the most abundant algae in the SD, followed by the diatom Flagilaria sp. Principal component analysis (PCA) indicated stronger correlations between NH4, PO4, NO3, and NO2, while canonical correspondence analysis (CCA) indicated a strong correlation between PO4 and Fragilaria sp. in the SD. The OPI classified KD water quality as excellent, with the exception of the lacustrine zone, where the water quality was polluted in 2016 and 2021. The data analysis revealed how long-term variations in KD water chemistry and phytoplankton influenced trophic status. This study thus provides water managers with a template for assessing water quality to secure the strategic value of the KD.
Moahloli et al. (Sat,) studied this question.