An exploratory multi-indicator assessment of freshwater ecosystem health in two South African reservoirs

Abstract

Abstract This study assessed freshwater ecosystem health by integrating chemical, biological, and microbial indicators. We conducted an exploratory comparison of Loskop Dam and Nwanedi-Luphephe Dam, South Africa, using water quality parameters, fish Health Assessment Index (HAI) scores, parasite assemblages, and 16S rRNA gut microbiota profiling. Sampling occurred between March and May 2025. Selected water parameters were measured in situ, with nutrient and metal analyses conducted from 1 L water samples. Nineteen fish ( Oreochromis mossambicus , n = 10; Clarias gariepinus , n = 9) were examined for parasites, and gut microbial communities and the HAI scores calculated. The microbial functional prediction potential was inferred using PICRUSt2. Loskop Dam showed elevated total dissolved solids (272.75 mg/L vs. 40 mg/L), conductivity (41.07 vs. 6.0 mS/m), and pH (7.69 vs. 7.14) values compared with Nwanedi-Luphephe Dam. Fish from Loskop Dam exhibited a higher mean HAI score (45.0 ± 25.50) than Nwanedi-Luphephe Dam (34.4 ± 14.24) and greater endoparasite loads, particularly Contracaecum spp. , whereas fish from Nwanedi-Luphephe Dam harboured more ectoparasitic monogeneans. Gut microbiota was dominated by the phyla Fusobacteriota, Bacillota, Pseudomonadota and Planctomycetota, while classified genera were primarily represented by Cetobacterium, Romboutsia, Clostridium, Bacteroides and Sediminibacterium . In the final multivariable model, fish weight was the only significant predictor of HAI score (p = 0.03), whereas dam and fish species were not significant. PICRUSt2 identified ten metabolic pathways differing between dams, with nine core pathways enriched in Nwanedi-Luphephe Dam; PWY01479, PWY042, PWY-5747, PWY-5856, PWY-5857, PWY-5855, PWY-6708, UBISYN-PWY, and PWY-5920, while the nucleotide salvage pathway (PWY-6609) was enriched in Loskop Dam. Results reveal that Loskop Dam exhibits moderate anthropogenic impact, whereas Nwanedi-Luphephe Dam represents a relatively undisturbed ecosystem. Integrating water chemistry, fish health, parasite assemblages, and gut microbiota may offer a sensitive framework for monitoring freshwater ecosystem integrity and detecting early ecological disturbance.