Effect of anthropogenicity on the temporal diversity and abundance of macro-invertebrates in Calabar River, Okomita, Cross River State, Nigeria

Abstract

The influence of anthropogenicity on the abundance and diversity of macro-invertebrates was investigated in Calabar River, Okomita, Cross River State for a period of 24 months (August, 2014-September, 2016) in six sampling stations. Bank root macro-invertebrates were collected by kick sampling while substrata macro-invertebrate samples were collected with a van-Veen grab. Seasonal variations in the values of the macro-invertebrates abundance were determined by pooling together the values for the wet and dry seasons. The difference in values in the six stations in each season was tested for significant difference (p < 0.05) while the pooled differences in the two seasons were tested using unpaired (independent) student’s t-test. Spcies diversity was estimated using the Shannon-Wiener index (H’), species richness (d) and evenness (J) of individual distribution among species. All macro-invertebrate groups had highest percentage abundance in the dry season months. Seasonal percentage abundance of insects was significantly different (p˂0.05). Higher seasonal abundance of the total macro-invertebrates was recorded in the dry season and the seasonal variation was significant (p˂0.05). Lower number of pollution intolerant species of macro-invertebrates (Sudanonautes africanus, Orectochilus orbisonorum, Dineutus discolour, Phaon iridipennis, Pila ovata, Natica flammulata and Eulima fischeri) recorded during the rainy season was an indication that the river was more perturbed during the rainy season as a result of influx of pollutants from refuse dumps, farmlands, Okomita Market, abattoir and mechanic workshop. The results of the PCA revealed that DO, hardness, alkalinity, magnesium and cadmium were the most important parameters influencing the abundance of macro-invertebrates in Calabar River, Okomita. Intermediate values and values less than one of Shannon-Weiner diversity index were observed, indicating that Calabar River, Okomita is moving from moderately polluted state to severely polluted condition.