The physico-chemical characteristics, trace metal levels and plankton community structure of Ogunpa and Ona rivers, Ibadan, Nigeria were studied to determine the impact of pollution in both rivers.

Physico-chemical parameters monitored differed significantly with river and season at P less than 0.05.
Conductivity, pH, biochemical oxygen demand, ammonia-nitrogen, total dissolved solids were highly elevated during the dry-low flow season due to non-point source organic pollution.

Iron (0.017 - 0.877 milligramme per litre)was the most abundant trace metal followed by zinc. The relatively high levels of these metals was associated with the geochemical nature of the catchment area which was composed of rock types such as pegmatites, amphibolites and quartz. Other trace metals (chromium, lead, cadmium, manganese, copper, cobalt) recorded lower levels than the WHO Standards.

A total of 34 phytoplankton and 47 zooplankton species were identified with the Chlorophyceae and Rotifera being dominant. Seasonal succession in both rivers involved the abundance of Microcystis aeruginosa and Oscillatoria / Phormidium species in Ogunpa and Ona rivers respectively.

Hierarchical clustering of plankton and correlation with environmental parameters showed two major species associations in both rivers - a high density species group exhibiting strong correlations with environmental factors and a moderate to low density group which were weakly correlated with environmental factors.