Spatio-Temporal Variations of Zooplankton Fauna of Oluwa River, Ilaje LocalGovernment Area, Ondo State, Nigeria

The study identified zooplankton composition of Oluwa River, Ilaje Local Government Area, ondo State, Nigeria, as indicators of the health of the aquatic ecosystem. The major objective of the study was to provide baseline information on aspects of the biology (taxonomic composition, occurrence, distribution and abundance) of the zooplankton. Zooplankton samples were collected with 55μm mesh size plankton net, samples were identified and counted microscopically for eighteen months (June, 2014 – November, 2015) covering both the rainy and dry seasons. A total of six sampling stations were established to represent the various sections and regime of the river. Altogether a total of thirty-eight species belonging to three groups; Rotifera, Copepoda and Cladocera. The Rotifera were represented by thirty species, four species of Copepoda and four species of Cladocera. Rotifera dominated the zooplankton groups, accounting for 87%. Asplanchna brightwelli (3.65%) was the most abundant, followed by A. girodideguerne (3.47%). The dry season abundance (3373 Org/L) was higher than the rainy season (3025 Org/L) Copepoda accounted for 2% of the percentage abundance of zooplankton. Eucyclops macrurus (3.64%) dominated; followed by Pseudocalanus elongatus with 3.57% in the relative abundance the rainy season abundance (475 Org/L) was higher than dry season (311 Org/L). Cladocera accounted for 11% of the percentage abundance of zooplankton. Podon leucarti (3.64%) dominated, followed by Ceriodaphinia cornuta (3.57%) in the relative abundance. The dry season abundance (489 Org/L) was higher than rainy season (476 Org/L). In conclusion, Oluwa River can be inferred to be rich in zooplankton fauna composition and therefore fairly clean and unpolluted.