Spatial Patterns of Plankton Diversity and Abundance in the Calabar River, Okomita Axis, Cross River State, Nigeria

Calabar River is an important water body in south-eastern Nigeria that supports a thriving fishery to the surrounding communities. Effluents, run-offs and solid wastes from industries, farmlands, markets, slaughter houses, dumpsites and human settlements around Okomita area are discharged into the river causing pollutants that could adversely affect water quality and resident biota. This study evaluated the spatial dynamics in the diversity and abundance of plankton in connection to some physico-chemical factors in Calabar River at Okomita. Surface water and plankton sampling were carried out monthly from September, 2014 to August, 2016 at six sampling stations along the river. Physico-chemical parameters were measured following standard methods. Plankton net with a mesh size of 55µm was used to gather plankton samples. The biota were identified using standard identification guides. Descriptive statistics, ANOVA, student’s t-test, PCA and Shannon-Wiener’s species diversity and evenness were used to analyze the data. Significant differences (P<0.05) were observed in all physico-chemical parameters due to spatial variability, except biochemical oxygen demand. Station one had the highest overall percentage abundance of all the phytoplankton (33.83%) and zooplankton (35.97%) while Station five had the lowest (phytoplankton, 10.46%; zooplankton, 10.17%). Pollution-indicators were, phytoplankton: Oscillatoria tenuis (2.37%), Surirella oblonga (2.35%) and Melosira granulata (2.16%); zooplankton: Philodina species. (6.98 %), Brachionus forticula (6.53%) and Lecane lunaris (5.56 %). The range of Shannon-Wiener’s diversity indices (0.4 – 2.89) of plankton indicates that Calabar River at Okomita was moderately to highly polluted in all the stations. The principal component analysis revealed that parameters such as biochemical oxygen demand, dissolved oxygen, chemical oxygen demand, pH, total dissolved solids, and turbidity were the most important environmental factors influencing the plankton abundance in the river. Calabar River at Okomita is under pollution stress and may not be suitable for aquatic life.