4.1 Water systemsIB Environmental Systems and Societies HL: Subtopic test

1. 1
   The Aral Sea in Central Asia was once one of the largest lakes in the world. From the 1960s, water from its two feeder rivers, the Amu Darya and the Syr Darya, was diverted along canals to irrigate cotton fields in a dry region. The lake shrank dramatically and much of the former lake bed is now desert.

   (a)

   Which change in the water budget of the Aral Sea best explains why it shrank?

   [1 mark]

   • AInputs of streamflow rose above the outputs by evaporation
   • BStreamflow input fell far below the output by evaporation
   • CPrecipitation on the lake increased and evaporation stopped
   • DGroundwater flow out of the lake stopped

   (b)

   According to the global distribution of water, approximately what percentage of the Earth's water is held in surface freshwater stores such as lakes and rivers?

   [1 mark]

   • A0.02%
   • B1.7%
   • C96.5%
   • D0.0001%

   (c)

   Explain how irrigation can alter the flows and stores of the hydrological cycle in a drainage basin such as that of the Aral Sea.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Since the start of the industrial revolution the surface waters of the oceans have absorbed roughly a quarter of the carbon dioxide released by human activities, and their average pH has fallen by about 0.1 units. Phytoplankton in the surface layer fix carbon dioxide by photosynthesis. When they and the animals that feed on them die, some of their remains sink and are buried in sediments on the seabed.

   (a)

   Which explains why absorption of carbon dioxide by the oceans causes ocean acidification?

   [1 mark]

   • ACarbon dioxide reacts with sodium chloride to form an acid
   • BCarbon dioxide raises the temperature of sea water, which lowers pH
   • CCarbon dioxide dissolves and reacts with water to form carbonic acid, which releases hydrogen ions
   • DCarbon dioxide is converted to oxygen in the water, which makes it acid

   (b)

   Which process is responsible for the longer-term sequestration of carbon in the oceans?

   [1 mark]

   • ACarbon dioxide remaining dissolved in the surface layer
   • BUptake of carbon into living organisms as biomass that accumulates on the seabed
   • CEvaporation of carbon dioxide from warm surface water
   • DRelease of carbon dioxide by respiration of fish

   (c)

   Explain why carbon stored as dissolved carbon dioxide is only a short-term store, whereas carbon in seabed biomass is longer term.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In midsummer, the temperature of a deep temperate lake is measured at different depths. From the surface to 8 m the water is 22 °C. Between 8 m and 14 m the temperature falls steadily from 22 °C to 8 °C. Below 14 m, down to the lake bed at 40 m, the water is a constant 6 °C. Dissolved oxygen is high in the top layer but close to zero below 14 m. In winter the whole lake is about 4 °C and the water mixes completely.

   (a)

   Identify the depth range of the thermocline and explain why there is little mixing between the layers in summer.

   [3 marks]

   (b)

   Explain how global warming could affect stratification in this lake and the supply of oxygen to the deeper water.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the North Atlantic, warm salty surface water flows north, cools, and sinks near Greenland and Norway before flowing south as deep water, as part of the global ocean conveyor belt. Off the coast of Peru, winds push surface water away from the shore and cold, nutrient-rich water rises to replace it. This supports one of the world's largest fisheries, for anchoveta, but in El Niño years the upwelling weakens and catches collapse.

   (a)

   Explain how thermohaline circulation is driven and how it affects global climate.

   [6 marks]

   (b)

   Discuss the importance of upwellings for aquatic productivity and food production, and how stratification can affect them.

   [6 marks]

   Total for question 4: 12 marks

End of questions