Intrinsic and extrinsic sources of phosphorus loading into the Nyando River, Kenya

Abstract

The Nyando River supports a large human population and an enriched biodiversity. The basin has undergone extreme modification over the last century through changes in hydrological changes and land‐use activities. The impacts of the changes have severely shifted the ecology of the river from desirable to less desirable. Of all the major rivers draining into Lake Victoria‐Kenya, the Nyando River is consistently the most turbid and exhibits the greatest phosphorus load. The present study investigated the present limnological status and sources of phosphorus loads between 2015 and 2016 through water and sediment sampling during high and low flow regimes. The samples were analysed for various phosphorus fractions, including soluble reactive phosphorus (SRP), total phosphorus in water (TPw), non‐apatite inorganic phosphorus (NAIP), apatite phosphorus (AP), inorganic phosphorus (IP), organic phosphorus (OP), total phosphorus in sediments (TPs) and elemental compositions. The measured SRP and TPw concentrations indicated hypertrophic conditions, with means for both high and low flows ranging from 79.3 to 165.7 μg P/L, and 200.0 to 243.9 μg P/L, respectively. The AP concentrations were high, with mean measurements for high and low flows being 766.9 and 790.5 mg/kg, respectively. The sources of high phosphorus loads were established to be both natural and anthropogenic, with major drivers being the industrial establishments along the river basin. Erosion of the catchment phosphorus‐enriched carbonatite rocks and river banks also contributed significant phosphorus loads. Enhanced effluent treatment, quarry dust containment, improved forest cover and regeneration of riverine buffer strips are recommended to restore the river's ecosystems.