Physico-Chemical Water Quality Parameters and Heavy Metal Concentrations in Fish from Lake Cyambwe, Rwanda

This study assessed the physico-chemical parameters of water and the heavy metal concentrations in fish and water from Lake Cyambwe, Rwanda, to evaluate the lake's suitability for fish production, irrigation, and drinking purposes. Water samples were collected from three sampling points around the lake, and physico-chemical parameters pH, salinity, dissolved oxygen, total nitrogen, total phosphorus, total alkalinity, and total suspended and dissolved solids were determined using standard titrimetric and spectrophotometric methods. Concentrations of Fe, Cu, Mn, Cd, Cr, and Pb in water and fish tissue were determined using Atomic Absorption Spectrophotometry (Shimadzu AAS-6800). Electrical conductivity (EC) and total dissolved solids (TDS) were measured with a HACH 440d multimeter, turbidity with a HACH 2100P turbidimeter, and nitrogen and phosphorus species with a DR 6000 UV-Vis spectrophotometer; potassium and sodium were measured by flame photometry. Mean results were: total alkalinity 146 ± 13 mg/L CaCO₃, EC 125 ± 3 µS/cm, turbidity 10.3 ± 0.8 NTU, TDS 59 ± 7 mg/L, pH 7.99 ± 0.09, salinity 0.061 ± 0.005%, total nitrogen 20.9 ± 0.3 mg/L, total phosphorus 0.107 ± 0.038 mg/L, potassium 120.07 ± 0.001 mg/L, and sodium 1.03 ± 0.04 mg/L. In fish tissue, Fe (105.847 mg/kg), Cu (2.981 mg/kg) and Mn (23.34 mg/kg) were detected, while Cd and Cr were below the detection limit; corresponding water concentrations were Fe (52.305 mg/kg-equivalent), Cu (0.249 mg/kg-equivalent) and Mn (27.6375 mg/kg-equivalent), with Cd and Cr again undetected. Turbidity and total nitrogen exceeded recommended surface-water thresholds, and Fe and Mn in fish tissue exceeded FAO guideline limits, indicating elevated metal bioaccumulation. These results are attributed to anthropogenic activities around the lake, including irrigation, domestic washing, and nearby mining operations. Continued monitoring and management of catchment activities are recommended to safeguard water quality and public health.