PS 22-49
Temporal variation in abundance of zooplankton along a horizontal salinity gradient in cross river estuary, Nigeria
The Cross river estuary is highly endowed with abundant fisheries resources. Being the widest in Nigeria, the estuary has been known to provide enormous breeding and nursery grounds for several ichthyofaunal species. Zooplanktons constitute an important diet for fish larvae and many other fauna in the estuary. The need to evaluate the dynamics of this important component of the aquatic food chain in the estuary necessitated this study. Samples were collected from six stations, delineated into three sampling zones over a horizontal salinity gradient from freshwater in the upper reaches, through brackish water to marine water in the lower reaches (mouth) of the estuary. Samples were collected over a period of twelve months covering wet and dry seasons.
Results/Conclusions
Seasonal variation showed higher wet season peaks in total zooplankton density in the months of April, September and October. The wet season was characterized by higher abundance of copepods, contributing 68% and 67% of the total population in the months of April and October, respectively with Calanoid copepods alone contributing 49.33% of all individuals. Fish larvae, Cnidarians and Chaetognths also exhibited increased wet season abundance contributing 36%, 14% and 6.12%, respectively.The Cnidarians and Chaetognaths were completely absent from the samples in the upper reach stations and also during the dry season, while mollusk larvae showed no seasonal bias.The ciliate population increased remarkably during the dry season, especially in the upper reach stations, contributing 56% of total zooplankton population, followed by copepods with 30%. Analysis of variance showed significant seasonal variation (P< 0.05) in zooplankton abundance. Generally, copepods were more abundant in the lower reach stations while cladocerans were more important in the upper reach stations. A significant positive correlation (r= 0.879; P< 0.05) was observed between zooplankton densities and salinity during the study,