Nutrient Concentrations in Different Land Use Types along River Riana, a Tributary of the Kuja Migori River Kisii – Kenya

Abstract

River systems draining catchments with diverse land use activities are increasingly vulnerable to nutrient enrichment, which poses significant risks to aquatic ecosystem health and downstream water quality. This study assessed spatial and temporal variation in nutrient concentrations across five land use types along River Riana, Kisii County, Kenya. Five sampling sites were established based on prevailing land use activities, namely a reference site, urban, forested, wastewater treatment effluent discharge, and agricultural sites. Water samples were collected monthly from September 2022 to August 2023, with triplicate samples analysed for total nitrogen, total phosphorus, nitrates, nitrites, soluble reactive phosphorus, and ammonium using standard analytical methods. Two-way analysis of variance was used to test for spatio-temporal differences at p<0.05. The urban site recorded the highest total nitrogen and total phosphorus in March 2023 (3138.74±147.45 µg/L and 841.29±16.54 µg/L, respectively), while the reference site consistently recorded the lowest concentrations. Nitrate levels peaked in the urban site in November 2022 (375.52±12.35 µg/L), nitrite and soluble reactive phosphorus were highest in the agricultural site in March 2023 (97.21±3.43 µg/L and 204.44±2.11 µg/L, respectively), and ammonium was high in the urban site in February 2023 (780.56±9.87 µg/L). Significant spatio-temporal differences (p<0.05) were recorded in all nutrients, with agricultural and urban sites recording consistently elevated levels. These findings indicate that agricultural runoff and urban activities are the primary drivers of nutrient loading in River Riana, underscoring the need for integrated land use planning and watershed management interventions.