Application of Different Indices to Assess the Trophic Status of a Warm Monomictic Reservoir in the Lesotho Highlands, Southern Africa

Katse Dam(KD), a strategic raw water source to South Africa, is exposed to pollution from mining and aquaculture production. The organic pollution index (OPI), the modified pollution index (MPI), and Carlson's trophic state index (CTSI) have not been previously applied to KD. The current study applies these indices to assess the trophic status of KD in the first decade (FD) (2003-2013), when the intensity of mining and aquaculture activities was minimal, and compares with the second decade (SD) (2014-2024) when production was higher. The Pollution Index of KD revealed that it transitioned from contaminated during the FD to greatly contaminated during SD. KD shifted from eutrophic status to hypereutrophic status in the lacustrine zone during the SD. The cyanobacteria Radiocystis sp. replaced Asterionella sp. and became the most pollution-tolerant algae in the SD, followed by the diatom Flagilaria sp. The pollution index (PI) values of physico-chemical parameters increased from 65 in the FD to 160 in the SD. OPI classifies KD as extremely polluted, with values above the threshold of 5 OPI in the SD. Application of the different indices, attribute mining, and aquaculture as influential to the transition of KD from mesotrophic to eutrophic in the transitional zone. The findings provide environmental managers with a basis to mitigate pollution at source to secure good water quality.