Impact of dredging on the water quality and macrobenthic invertebrate fauna of the Ikpoba River in southern Nigeria
The effect of dredging on the water quality and macrobenthic fauna of the erosional and bank-root biotopes of the Ikpoba River was investigated. Among the physical parameters, transparency, turbidity, flow rate, total suspended solids, total dissolved solids and total solids differed significantly (P<0.01, 0.001) among the stations. The a posteriori Duncan Multiple Range test revealed that transparency was significantly higher (P < 0.01) at station 1, the upstream control station than at the other stations, while turbidity and total solids (suspended and dissolved) were significantly lower (P < 0.01) at station 1 than at the other stations which did not differ significantly. Among the chemical parameters, only pH, conductivity and iron were significantly different (P < 0.05) among the stations; pH and iron were lower in station 1, while conductivity was significantly higher in station 1 than in the other stations. All the parameters in the dredged stations 2, 3 and 4 were not significantly different. With the exception of transparency, flow rate and conductivity, all significant parameters were higher at the dredged stations than at the control station. The overall abundance of macrobenthic invertebrates was not significantly different (P > 0.05) among the erosional biotope station. In the bank-root biotope, the overall abundance was significantly different (P < 0.05) among the stations; stations 1 and 4 were significantly higher than stations 2 and 3. Dredging altered the community structure in both biotopes but the impact was more pronounced in the bank-root biotope. The erosional biotope witnessed the elimination sensitive organisms and the dominance of stress-resistant chironomids and tubificids in the dredged stations. In the bank-root biotope, the decapod crustacean and ephemeropterans decreased progressively in importance from station 1 through 4. Variations in faunal diversity (H’) at both biotopes are reported. The impact of dredging on macroinvertebrate fauna was more pronounced in the bank-root biotope. Future dredging operations should consider pre-existing densities of benthic macroinvertebrate taxa as part of baseline data for reasonable impact assessment.
Key words: Dredging, macrobenthic invertebrates, water quality, biotopes, tropics