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.