Abstract:
Degradation of coastal ecosystems is an issue of growing concern. Discharges of sewage effluents to sea water
is a major contributor to marine pollution. This study examines the biogeochemical effects of sewage pollution on
the condition factor and energy reserves of Nerita polita. A total of 135 molluscs were exposed to varying sewage
concentrations of between 5%-50%. Physicochemical parameters of the exposure media were characterized using
standard techniques. Condition factor of Nerita polita was determined weekly using the Fulton’s index. After the exposure
period, the molluscs were sacrificed and energy reserves determined. Increase in sewage pollution resulted in elevation
of ammonia (0.01-0.08 mg/L), phosphate (0.05-156 mg/L), nitrates (0.02-1.99 mg/L) and temperature (24.34-25.12oC),
while pH (7.75 to 7.29) and dissolved oxygen (5.62-2.38 mg/L) were lowered. There was no correlation between the
condition factor of Nerita polita and the sewage pollution. Glucose, lipid and protein concentrations in the mollusc tissues
ranged from 29.6-71.3 mg/L, 171-677 mg/L and 338-445 mg/L, respectively, and they decreased along the increasing
sewage gradient. Energy reserves in Nerita polita were highest in lipids, followed by proteins and glucose the least
and they were affected by the pollution gradient. Findings of this work suggest that energy reserves are sensitive bio
indicators but that conditional factor is an unreliable marker to assess acute sewage toxicity. In addition, increase in
sewage pollution also leads to a decrease in the water quality and that sewage concentrations above 30% can have
profound effects on Nerita polita.
