This study identified the phytoplankton assemblage of the increasingly in-stream sand-mined Imo River in Etche, southeastern Nigeria, as bioindicators of water quality. Plankton samples were collected once monthly for 24 months (March 2007-February 2009) with net of 55 μm mesh size that was hauled horizontally along the river course for 5 minutes at 7 sampling locations. Samples were later transferred to plastic containers and fixed/preserved in a 4 % formalin solution. In the laboratory, a wide-mouthed pipette was used to withdraw 1ml of the plankton subsample and to place it on a Sedge-wick rafter-counting chamber for direct microscopy observation. Standard keys were used for species identifications. The ANOVA, means plots, hierarchical cluster analysis (HCA), and the Margalef’s species diversity index were used to analyze data. Phytoplankton comprised 43 genera and a mean density of 1859 cells/ml. The dominant phytoplankton was the Bacillariophyceae (53.25%), followed in order by Cyanophyceae (21.25%), Chlorophyceae (10.33%), Chrysophyceae (4.84%), Pyrrophyceae (4.57%), Xanthophyceae (3.39%) and Euglenophyceae (2.42%). Highest abundance was recorded in the reference sampling location 1 (527 cells/ml; 28.23%) and least abundance in location 6 (139 cells/ml; 7.45%). There was significant spatial heterogeneity in the plankton taxa [F(20.94)>Fcrit(3.94)] at P<0.05, and the diatoms and blue-green algae were most responsible for the observed inequality. The Chrysophyceans, Euglenophyceans, Cyanophyceans and Chlorophyceans formed the first and richest phytoplankton cluster. Comparatively low phytoplankton biotic diversity in the study could mostly be attributed to sand-mining-induced perturbations in water column, which exerted selective effects on the biological assemblage.
