Munyasya JN, Juma KK, Burugu MW, Mburu DN and Okuku EO
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.
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