Flows of Energy & Matter (SL IB Environmental Systems & Societies (ESS))

Discuss how human activities affect the nitrogen cycle and the impacts of this on ecosystems.

Describe the role of bacteria in the nitrogen cycle.

The figure below shows a diagram of a stream that runs past a farm. A group of students took water samples at various locations along the stream and measured the nitrate concentration in parts per million (ppm) in those water samples. The students have marked these nitrate concentrations on their diagram.

Explain how the farm may be causing the increase in nitrate concentration in the water.

Define the term gross primary productivity.

Only around 1% of the light energy that reaches the Earth’s atmosphere is captured by plants and made available to other organisms in the food chain.

Suggest reasons for this low value.

Woodlice are detritivorous insects (they feed on the dead remains of plants). A biologist estimated the numbers of woodlice at two different sites in a woodland ecosystem. They also estimated the net primary production of the sites to see if this influenced the numbers of woodlice present. Their results are shown in the table below.

Although net primary production is a measure of plant biomass formed per year, it does not represent the total amount of plant biomass formed per year by photosynthesis.

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This woodland ecosystem has a total average gross primary productivity of 20 000 kJ m^-2 y^-1 but the net primary productivity is calculated to be 12 000 kJ m^-2 y^-1.

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The figure below shows the energy transfers taking place in a simple food chain. The energy values are all given in kJ m^-3 y^-1.

The efficiency of energy transfer can be calculated using the following equation:

Ecological efficiency = (energy used for new biomass ÷ energy supplied) × 100

Use information in the figure and the equation provided to calculate the efficiency of energy transfer for zooplankton.