Question

gcse biology ecology & ecosystems
high demand questions questionsheet 15
the graph shows the effect of sewage entering a river on the oxygen level and the relative numbers of bacteria and fish.
(a) how are the bacteria affected by the sewage entering the river?
1
*(b)(i) how are the fish affected by the sewage entering the river?
1
(ii) what is the reason for this change?
2
(c) as the distance downstream increases the concentration of sewage falls.
suggest two reasons for this.
2

Explanation:

• (a) Analyze the bacteria - related data in the graph.
• (b)(i) Examine the fish - related data in the graph.
• (b)(ii) Relate the fish change to oxygen levels.
• (c) Consider physical and biological processes that reduce sewage concentration downstream.

Answer:

• (a) The number of bacteria increases initially.
• (b)(i) The number of fish decreases.
• (b)(ii) Sewage causes a decrease in oxygen levels (from the graph, as bacteria consume oxygen for decomposition, fish need oxygen to survive).
• (c) Dilution (mixing with more river water) and decomposition (by bacteria and other decomposers).