Dynamics of the zooplankton assemblage of Ojirami Reservoir, Edo State, Nigeria

Abstract

Zooplankton form an important trophic link between phytoplankton and fish in the aquatic food chain. Investigation was carried out on the zooplankton community of Ojirami, Reservoir, Nigeria, to document the composition, abundance and distribution and their interactions with the physico-chemical conditions. Surface water and zooplankton samples were collected from six sampling stations (A, B, C, D, E, and F) monthly from January 2009 to December 2010 and analyzed following standard methods. Twelve out of the 17 physico-chemical conditions showed higher wet season mean levels. The zooplankton community comprised 28 species of Rotifera, 12 Cyclopoida, 17 Cladocera, 6 Calanoida, 5 Diptera (larvae), 1 Ephemeroptera and 2 Arachnida. Rotifera was the most dominant group and Arachnida the least. No species was dominant; Brachionus falcatus, Asplanchna brightwelli and Tropodiaptomus species were subdominant. Rotifera was also the most abundant group in all stations, followed by Calanoida and Cladocera. Abundance of major groups was higher in wet than dry season, but not significant (P>0.05), except for Diptera that was higher in dry season (P<0.05). Overall community structure exhibited high species diversity and high taxa richness. Total zooplankton, Cyclopoida, Cladocera and Calanoida were negatively correlated with water temperature and water level; and positive with alkalinity, free CO₂ and
phosphate. The study revealed low pollution indicator species in the reservoir suggestive of moderate water quality.

Keywords: Brachionus, Correlation, physico-chemical conditions, Ojirami Reservoir