Biochemical Effects of Sewage Pollution on the Benthic Organism Nerita polita

Abstract

Rapid urbanization has resulted into substantial increase in quantities of sewage, which is traditionally discharged into the ocean, causing a number of environmental problems including threats to oceanic lives as well as ground water contamination. So far, there is limited data in Kenya regarding the toxic effects of sewage on benthic organisms. The aim of this study was to investigate the biogeochemical effects of sewage pollution on the mollusc Nerita polita by determining its impacts on the condition factor and energy reserves. A total of 135 molluscs were exposed to varying sewage concentrations of between 5% - 50% over a three week period. The physicochemical parameters of raw sewage were simultaneously determined with those of the nine treatment media and they included pH, temperature, biological oxygen demand, chemical oxygen demand, total suspended solids, total organic carbon, dissolved inorganic nutrients and heavy metals. Condition factor of Nerita polita was determined weekly. After an exposure period of three weeks the molluscs were sacrificed and energy reserves determined. Raw sewage had phosphates and total organic carbon levels above permissible limits while dissolved oxygen was below limits. Iron, zinc and copper were within permissible limits while cadmium, lead and mercury were absent. Increase in sewage pollution resulted in elevation of inorganic nutrient contents (0.009-0.077 mg/L; 0.045-155.92 mg/L; and 0.017-1.99 mg/L) for ammonia, phosphates and nitrates respectively while at the same time lowering pH from 7.75 to 7.29 and dissolved oxygen from 5.62-2.38 mg/L. Tukey’s post hoc analysis of pH at different sewerage concentrations indicated a statistically significant difference at 40% (7.44± 0.17; p < 0.05) and 50% (7.29 ± 0.10; p < 0.001) when compared to the control 0% (7.75 ± 0.03). pH levels for all the other treatments were insignificant (p > 0.05). Dissolved oxygen levels were statistically significant at p < 0.05 for concentrations of 15% (4.50±0.27 mg/L) and 20% (4.38±0.28 mg/L) while concentrations of 30% (4.10±0.39 mg/L), 40% (3.48±0.42 mg/L) and 50% (2.38±0.26 mg/L) were statistically significant at p < 0.001. Phosphates were significantly (p < 0.001) higher at concentrations of 15% (77.82±3.66 mg/L), 20% (88.69±2.67 mg/L), 30% (109.43±2.91 mg/L), 40% (144.43±2.94 mg/L) and 50% (155.92±2.74 mg/L) when compared to the control (0%; 0.05 ± 0.01mg/L). There was no statistical significant (p > 0.05) difference in temperature, ammonia, nitrates for all the nine treatment tanks. The relationship between the condition factor of Nerita polita and increasing sewage pollution was inconsistent and statistically insignificant (r = 0.234, p > 0.05, r = 0.011, p > 0.05 and r = -0.453, p > 0.05) for week one, two and three, respectively. Analysis of whole body tissues of Nerita polita indicated that lipid reserves (26.8 KJ) were highest in the mollusc tissues, followed by proteins (10.7 KJ) and glucose (1.1KJ) was least at the end of the experiment. Levels of the three biomolecules decreased with the increase in the pollution gradient. Glucose, lipid and protein concentrations in the mollusc tissues ranged from 29.6 -71.3 mg/L, 171- 677mg/L and 338-445mg/L, respectively, along the increasing sewage gradient. Findings of this work suggest that energy reserves are sensitive bio indicators but that conditional factor is an unreliable bio indicator 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.