Impact of a kraft pulp and paper mill effluent on phytoplankton and macro invertebrates in river Nzoia, Kenya

Abstract

Phytoplankton and macroinvertebrate assemblages were used to assess the impact of a kraft pulp and paper mill effluent in Kenya, on River Nzoia downstream of the discharge point in relation to changes in water quality during May to June and November 2008 (rainy and dry seasons, respectively). Total phosphorus concentration increased from 0.027 mg.L-1 upstream to 0.04 mg.L-1 downstream. Ammonia nitrogen (NH3-N) concentration was 0.51 mg.L-1 upstream and 0.86 mg.L- 1 downstream. Nitrate concentration stood at 1.18 mg.L-1 upstream compared with the 2.23 mg.L-1 downstream. The pH changed from 4.5 to 5.0 upstream to 5.5 to 6.0 downstream, while DO increased from 6.57 to 7.03 mg.L-1 downstream. The BOD5 (biochemical oxygen demand after five days) values remained almost unchanged from 4.63 mg.L-1 upstream to 4.67 mg.L-1 downstream. Taxon composition of phytoplankton and macroinvertebrates correlated with adverse environmental gradients resulting from the mill's effluent discharge. Overall, there was a shift in composition and abundance of both phytoplankton and macroinvertebrates, with the downstream site recording high numbers of tolerant taxa (i.e., Microcystis sp. and Chironomus sp.). It was recommended that water quality monitoring with effluents of this nature be done using a combination of chemical analysis and biological indicators such as phytoplankton and macroinvertebrates.