Species composition and distribution of zooplankton in the lower cross river estuary

Abstract

Species composition, spatial distribution, abundance and diversity of zooplankton in the Cross River estuary were investigated over a period of 24 months. A total of 66 taxa belonging to eleven phyla were identified. Copepoda was the most abundant, with 17 taxa followed by the Cladocera with 11 taxa. Overall contribution of crustaceans to the total zooplankton population was 74.16%, while Chaetognaths and Cnidarians contributed 6.3% and 6.1% respectively. Densities ranged from 40 organisms/l to 1,660 organisms/l. Copepod presence was high in all the sampling zones but more in the lower reaches of the estuary. Cladocerans, Ciliates and Rotifers were more important in the upper reaches of the estuary, whereas the Cnidarians and Chaetognaths were absent upstream but highly abundant in the downstream reaches. Zooplankton composition showed significant spatial variation (p < 0.05) in taxa occurrence and density across the sampling zones. Copepods had the highest dominance value of 0.73, followed by Cladocera with 0.51. Jaccard's coefficient of similarity of species revealed dissimilarity between the upper reach stations and the lower reaches. Taxa richness was highest in the lower reach station 6 with a value of 6.79. A general trend of increase in species diversity and richness from upstream to downstream was observed. Inter-and intra-specific relationship revealed highly significant positive correlation (p < 0.01) between Cnidarians and Copepods (r = 0.896), Cladocerans (r – 0.841) and between chaetognaths and Copepods (r = 0.725, P < 0.05) and Cladocerans (r = 0.451, P < 0.05). Paucity of zooplankton in terms of occurrence and abundance at certain sampling locations of higher proximity to household and industrial effluents sources is indicative of anthropogenic perturbations. Keywords: Zooplankton, abundance, distribution, diversity, estuary AJAZEB Vol. 7 2005: pp. 5-10