DYNAMICS IN TROPHIC STATUS, WATER AND NUTRIENT MASS BALANCE AND COMMUNITIES OF LAKE KANYABOLI, KENYA

Abstract

Fishing pressure and nutrient enrichment from the catchment area threaten Lake Kanyaboli's water quality and communities. Understanding these impacts and their effect on the lake's water quality and communities is crucial for lake management and conservation. This study assessed the spatio-temporal variations in the physical, chemical, and biological variables, nutrient budget, plankton and fish communities of Lake Kanyaboli in western Kenya from February 2020 to February 2021. Primary and secondary data on water quality variables, including Secchi depth (SD), electrical conductivity (EC), temperature, pH, dissolved oxygen (DO) concentration, nitrates (NO3-), nitrite (NO2-), ammonium (NH4+), soluble reactive phosphorus (SRP), total phosphorus (TP) and total nitrogen (TN), and chlorophyll-a (Chl-a) were collected. Plankton and fish community data were also collected. Water quality variables and plankton communities were compared using two-way ANOVA, PCA, Pearson correlation, and descriptive statistics. The lake’s trophic status was calculated using SD, TP and Chl-a Carlson trophic status index (TSI) equations. Furthermore, PERMANOVA, Two-way nested ANOSIM, species diversity indices, CCA, NMDS, Kruskal Wallis test and SIMPER were also used to compare plankton and fish community data. Mass balance models were used to calculate water, nitrogen, and phosphorus mass balances. Two-way ANOVA on water quality variables showed no site × season interactions, while only Chl-a showed spatial variability. Significant seasonal differences were observed in SD, DO and Chl-a. Based on the Chl-a and SD TSI values, Lake Kanyaboli is eutrophic, whereas the TP value indicates hypereutrophic conditions. The nutrient mass balance showed that the lake receives more nutrient loads of N (90.78 t N year-1) and P (24.06 t N year-1) than it loses for both N (73.42 t N year-1) and P (15.76 t P year-1). Phytoplankton consisted of 30 genera dominated by Chlorophyceae (42.30%), with Bacillariophyceae, Cyanophyceae, Cryptophyceae, Euglenoidae, Trebouxiophyceae, and Zygnematophyceae also present. Fifteen zooplankton genera were also identified, including Cladocera (16.73%), Rotifera (27.86%) and Copepoda (55.41%). Although plankton abundance differed significantly between seasons, Two-way ANOVA showed no significant site × season interaction. The 2020 fish catch survey showed that Tilapia (50%), Clarias sp. (23%), Protopterus aethiopicus (20%), and haplochromines (7%) were the main fish species in the catch. But Kruskal Wallis showed no monthly variations in fish catches. The inter-annual fish catch and catch per unit effort (2003 to 2020) showed a decreasing trend. Despite infrequent historical data, physical, chemical, and biological variables captured anthropogenic effects over time, and the lake was eutrophic due to nutrient load. The phytoplankton community also showed that the lake was in a non-equilibrium state due to short water residence time and the uptake of nutrients by macrophytes bordering the lake. The major management issues identified are nutrient loading, fishing effort, and lake connectivity with the Yala River. Managing the catchment area holistically and monitoring the lake's water quality and fishing effort will help sustain goods and services, and livelihoods of Lake Kanyaboli riparian communities