Question

The figure below shows the change in the concentration of various substances in a river following the discharge of untreated sewage into it.

1. Account for the changes in concentration of:
   • Organic matter. 
   • Nutrient ions.
   • Dissolve oxygen.
2. Describe the changes expected in:
   • Fish population between the point of sewage discharge and the point where the organic matter returns to normal levels.
   • Water plants and photosynthetic algae about one and half kilometres downstream from the point of sewage discharge.

Answer 1

1.   
   • Saprophytic bacteria breakdown the organic matter (into soluble simple materials) leading to decrease (in amount of) organic matter downstream;
   • Breakdown of organic matter by saprophytic bacteria releases nutrients/nitrates/ammonium/phosphates/sulphates increasing concentration of ions in the water;
   •    
     • Saprophytic bacteria use up dissolved oxygen for respiration decreasing the amount of oxygen in the river upstream;
     • Downstream the level of organic matter decreases decreasing activity of saprophytic bacteria increasing the level of oxygen to normal;
2.   
   • Fish population decreases(drastically) due to insufficient oxygen/decrease in oxygen concentration;
   • water plant and algae proliferate/increase due to increase in nutrients/phosphates/nitrates due to breakdown of organic matter by saprophytic bacteria;