The lake that turned green
308 words over 29 numbered lines, then 6 parts worth 13 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
The lake that turned green
Every summer for the last few years, a small lake surrounded by
farmland has turned bright green. Visitors complain about the smell of
rotting algae, and in some years dead fish have been found floating
near the shore. The water looks cloudy, and the plants that once grew
on the bottom of the lake have disappeared. Local people asked a team
of scientists to find out what was happening.
The scientists found two sources of mineral ions entering the lake. A
stream carried nitrate and phosphate ions washed from fertiliser
spread on nearby fields. In addition, an old sewage pipe from a
village sometimes overflowed after heavy rain, sending untreated
sewage into the stream.
The extra mineral ions allow algae in the water to grow very quickly,
especially in the warm, bright days of summer. This is called an algal
bloom. The algae form a green layer near the surface that blocks light
from reaching plants growing on the bottom of the lake. These plants
cannot photosynthesise, so they die. When the algae die too, bacteria
decompose the dead plants and algae. The bacteria respire aerobically
and use up oxygen dissolved in the water.
Untreated sewage makes the problem worse. It contains organic matter
that bacteria also feed on, and it may contain pathogens that cause
disease in people who swim in the water.
Each month for a year, the scientists measured the concentration of
dissolved oxygen in water taken from one metre below the surface. They
always took the sample at the same time of day. Their results are
shown in Figure 1. Most fish need at least 4 mg of oxygen per dm³ of
water to survive.
The village has now replaced the sewage pipe, and farmers have agreed
to leave a strip of grass at least ten metres wide between their crops
and the stream.
Figure 1 Concentration of dissolved oxygen in the lake water over one year
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term eutrophication.
(1)Use Figure 1 to calculate the percentage decrease in dissolved oxygen between month 4 and month 8. Give your answer to one decimal place.
(2)Explain why the concentration of dissolved oxygen fell between month 5 and month 8.
(4)Describe two ways in which untreated sewage harms the organisms in a lake or the people who use it.
(2)Explain why the scientists always took the samples at the same time of day.
(2)Suggest how a strip of grass between the crops and the stream could reduce algal blooms in the lake.
(2)Total for this question: 13 marks
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