Influence of anthropogenic activities on water quality of a tropical stream ecosystem

Water samples were collected from three sites located in the middle reach of the Njoro River, Kenya, and analysed for total phosphorus (TP), orthophosphate, ammonia‐nitrogen, and nitrate‐nitrogen to evaluate stressor sources (e.g. factories and wastewater ponds) and the general stream water quality. The stream surface water was also analysed for biochemical oxygen demand (BOD₅) to provide an overview of organic matter loading. Mugo, Egerton Bridge and the canning factory sites of the Njoro River had low water quality which is likely to be due to poor farming, partially treated effluents and poor provision of sanitation facilities to the riparian communities. The concentrations of the selected nutrients did not differ significantly among the three sites, presumably due to pollution of the whole stream reach by the catchment nutrient sources. High phosphate concentrations (i.e. ～0.76 mgPO₄ l^?1 and ～0.87 mgTP l^?1) at Canning Factory were recorded during the low flow dry season. Nitrate‐nitrogen concentrations varied significantly with water discharge which explained between 63 and 87% of the nutrient variability in the three sites. BOD₅ differed significantly among the three sites, with historical effects of wastewater and factory effluent discharge being reflected in the results of Egerton Bridge and Canning Factory. The concentrations of ammonia‐nitrogen, TP and orthophosphate were higher in the wastewater than in the river water whereas nitrate‐nitrogen was lower. This study indicates that the Njoro River is stressed by nutrients from the activities within its catchment. With the increasing population, the nutrient load to the river will continue to increase and the water quality will continue to deteriorate. Reductions of nutrient loads into the river as well as provision of sanitation facilities to the riparian communities are needed to control further water degradation.