Primary productivity of Ifewara Reservoir, southwestern Nigeria

Abstract

The primary productivity of Ifewara reservoir, a potable water supply to Atakumosa area, Southwestern Nigeria, was investigated with a view to assessing its productivity status and hence its suitability for drinking. Sampling was done from February to December 2015, to cover an annual cycle. Four sampling stations were established on the reservoir, reflecting a comprehensive representation of its various zones. Parameters such as turbidity, transparency, alkalinity, temperature, dissolved oxygen (DO), biochemical oxygen demand (BOD₅), pH, conductivity, and total dissolved solids (TDS) were determined along with the primary productivity (PP) using standard methods. Productivity parameters—respiration (R), net productivity (NP), and gross productivity (GP)—were determined based on the evolution of oxygen method. Results showed that the reservoir was turbid with low light penetration, moderately alkaline, and poorly buffered. Respiration was about 66% of the gross primary productivity and generally higher than the net productivity. Net primary productivity (NPP) had significantly higher mean surface values than those of the bottom (P < 0.05). Mean water temperature and transparency values were significantly higher (p < 0.05) in rainy season than in dry season while reverse was the case for BOD5, NP and GPP which had significantly higher (p < 0.05) mean dry season values than those of rainy season. The significant (p < 0.05) transparency-NPP and transparency-GPP inverse relationship reveals the importance of light on the rate PP of the reservoir and generally, in aquatic environment. Physicochemical parameters studied (with the exception of turbidity) were within the recommended range set by both local and international standards. Season and depth dictates productivity of the reservoir characterized by being moderately alkaline, poorly buffered, and turbid during the period of study. The reservoir is productive, suitable for domestic water supply and irrigation, and can support aquaculture.

Keywords: productivity, aquaculture, correlation, phytoplankton, respiration, sampling, seasons