Use of benthic macroinvertebrates as indicators of water quality in River Njoro, Kenya

Abstract

Unsustainable anthropogenic activities, including intensive agriculture and grazing along River Njoro has resulted in the deterioration of river water quality. This is mainly through runoff and increased effluent discharge to the river. The river is the most important source of freshwater for the riparian communities and Lake Nakuru, a Ramsar site. In its middle reaches, the water quality is gradually declining. The study was done in order to determine the water quality in the middle reaches of River Njoro using benthic macroinvertibrates assemblage as an indicator. The assemblage of benthic macroinvertebrates was compared with selected physicochemical variables along the middle reaches of River Njoro during
both wet and dry seasons. Ten sites in River Njoro were established and sampled. Kenyatta and Ngata had significantly (p< 0.05) high temperatures than all the upstream sites. Nitrates, ammonia and total
nitrogen concentration were more than four times higher at Bora than at both Confluence and Njoro over the dry season. At Canners Up and Bora, total phosphorus concentration was more than double that at Confluence and Kenyatta. Higher richness of benthic macroinvertebrate (mean=28.6±5.96) was recorded at Njoro. At Egerton, Canners Up and Canners Down, the abundance of macroinvertebrates in the dry season was more than ten times less that in the wet season. Chironomidae, Oligochaete, Moina sp and Cyclopoidae were characteristic of the impacted
sites. Potamon sp, Orthotrichia sp, Ecdyonurus sp and Catenula sp were present at the upstream sites but absent from all sites receiving effluents. This study shows that only tolerant taxa inhabit the impacted sites especially over the dry season.

Key words: physicochemical parameters, anthropogenic activities, tolerant taxa, middle reaches, wet season, dry season, impacted sites, effluents.