40. The effects of a release of sewage or nitrate fertilizer into a river include...........

a) rise in the number of saprotrophic bacteria that decompose the sewage.

b) As the organic matter decays, it is changed into first ammonia (poisonous) then into nitrate. The nitrate, in conjunction with release of fertilizer in the river causes eutrophication.

c) Increased eutrophication increases the population of algae which absorb the nutrients and thus increase their numbers exponentially.

d) Oxygen levels decrease due to the increased bacteria, or during long nights, as the algae do aerobic respiration. The algae and animal life may die due to lack of oxygen.

e) Far downstream, elevated nutrient levels cause a small increase in algae, which produce more oxygen and reduce the amounts of the pollution nutrients.

f) The primary conumers increase, reducing the numbers of algae.

g) The river returns to pre-sewage release levels.

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