Abstract

We investigated seasonal variations and influences of some physicochemical attributes of the Southeastern-Nigeria Imo River on the primary productivity at its middle course between March 2007 and February 2008. In situ determinations of water temperature, pH, turbidity, and dissolved oxygen (DO) were made with HORIBA U-10 water quality checker at 7 sampling locations and water samples collected in 500 mL sterile containers and transported to the laboratory in iced-coolers. The light and dark bottle technique for the measurement of primary production was utilized and gross and net primary production (GPP and NPP) as well as community respiration (CR) computed. In the laboratory, nitrate, sulphate and phosphate concentrations were determined according to standard methods by the cadmium reduction, barium chloride (turbidometric), and ascorbic acid methods, respectively. Correlation coefficient (r) was used to determine the influence of the physicochemical variables on primary production while studentized t-test was used to ascertain significant differences between seasons. DO varied from 4.50-8.81 (6.96 ±0.14) mgL^-1, turbidity from 11.0-279.00 (96.7 ±9.3) NTU, nitrate from 0.10-1.35 (0.53 ±0.04) mgL^-1, and sulphate from 0.90-8.10 (4.35 ±0.25) mgL^-1. Sulphate varied significantly at P<0.05, while turbidity showed high numerical variation only. Mean GPP, NPP, and CR were higher in the dry than wet season, even as there was no significant seasonal variation [tcal_(0.30)<t_crit(2.13)] at P<0.05. Annual yields were 10.9, 5.4, and 2.7 mgO₂L^-1d^-1 for GPP, NPP and CR, respectively.  The dry season months (January and February) yielded highest mean GPPs of 2.5 and 2.6 mgO₂L^-1d^-1, respectively. pH exerted significant positive influences on GPP, NPP, and CR at P<0.01. The observed oligotrophic production was attributed to low water nutrient level (most probably resulting from depletions through benthal excavations during sand mining) and high turbidity (due to re-suspension of benthal sediments), which limits solar radiation necessary for autotrophic production. The higher productivity recorded in the dry season was due to more concentration of nutrients (nitrate) and lowered turbidity.

Key words: Etche, Imo River, Niger Delta, sand mining, autotrophs.