Non-equilibrium Interactions of a Zooplankton Community on Olero Creek, Benin River, Nigeria

The Olero creek Zooplankton was sampled in wet season from 13 stations across a conductivity range of 150 µmhos to 8000 µmhos. The abundance of Copepods, some Cladocerans (Daphnia sp. and Ceriodaphna sp.) and Rotatoria did not vary together with conductivity in a correlation analysis. Correlation was high (p<0.05) only among the Cilliata where abundance of Opecularia sp. and Vorticella sp. were closely related to variations in conductivity values while Opisthostyla sp. exhibited marginal correlation with values of conductivity. However, among Rotatoria abundance variable for Brachionus sp., Euchlanis sp., Notholca sp. and Asplanchna sp. was not explained by variations of single variable of conductivity. The occurrence of only three genera of Cladocera, three genera of Cilliata and four genera of Rotatoria with relatively lower abundances than Copepod group recorded, suggest in this tidal creek the non- significance of freshwater Zooplankton. The diversity values were low ranging from 0.7 to 1.3 in addition to low evenness ranging from 0.1 to 0.2 across all stations. Copepods dominated by 92% followed by Rotatoria 3%; Cladocerans 1.2% and Cilliata 0.5%. The observed non-equilibrium in abundance of Copepods with significant low numbers of Rotatoria, Cladocerans and Cilliata suggest biological interactions that support ecological principles of a trophic cascade. The low numerical abundance of Ciliates was in sharp contrast to their ubiquity, rapid reproductive rate and capacity to survive in extreme environments, including adaption to anaerobic conditions. This fact provided surrogate information of a cascade effect and intraguild competition. The observed numerical relationship within trophic levels shows that top-level consumers (copepods) were operating a top-down system driven by predation and competition. The top-down cascading of this inverse correlation in numerical density between copepods and other zooplankton should be included in directions of future research. A more appropriate basis of cross system explanation of top down trophic cascades in many natural systems is required for marine, estuaries and freshwater systems.