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Productivity

Productivity. Think back to the work on food webs/chains It is often useful to know how much energy is passing through a trophic level over a period of time. This is called productivity

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Productivity

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  1. Productivity • Think back to the work on food webs/chains • It is often useful to know how much energy is passing through a trophic level over a period of time. • This is called productivity • Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in an area, over a certain period of time.

  2. Specifications • 2.2.4 – Define the terms gross productivity, net productivity, primary productivity, secondary productivity, gross primary productivity and net primary productivity. • 2.2.5 – Calculate the values of gross and net productivity from given data

  3. Productivity • The area is normally one square metre and the time is usually one year. • It is therefore measured in units of kilojoules per square metre per year (kJm-2year-1) • The rate at which producers convert light energy into chemical energy is called primary productivity.

  4. Gross Productivity • Gross Productivity (GP) – is the total gain in energy or biomass per unit time. • This is sometimes shown as GPP – Gross Primary Productivity • It is related to the total amount of chemical energy incorporated into the producers. • The producers use some of this energy during respiration and energy needs which is eventually lost to the environment as heat. • The remaining energy is available to the herbivores and is known as net primary productivity (NPP)

  5. Recap of Definitions! • Productivity = production per unit time • Primary Productivity = The rate at which energy/biomass is formed through photosynthesis • Secondary Productivity = The rate at which primary material is synthesised into animal tissue per unit area in a given time. • Gross Productivity (GP) = the total gain in energy/biomass per unit time. • Gross Primary Productivity (GPP) = the total gain in energy of the producers. • Net Productivity (NP) = the gain in energy/biomass per unit time remaining after allowing for respiration (R) loses. • Net Primary Productivity (NPP) = the gain in energy/biomass per unit time remaining after allowing for respiration loses which is passed onto the herbivores.

  6. Environmental Productivity • Primary productivity varies greatly in different environments. • The rate at which plants can convert light energy into chemical energy is affected by many factors: • Sunlight • Water • Temperature • Amount of nutrients

  7. Environmental Productivity • In natural ecosystems primary productivity tends to be highest in tropical regions. • This is due to good light levels and high temperatures in the tropics. • In the oceans however, the most productive areas are in cold regions due to the up-welling of water bringing plant nutrients with it.

  8. Important Calculations • We can calculate GPP as follows: GPP = NPP + R • We can calculate NPP for both producers and consumers as: NPP = GPP – energy used in respiration • In addition, the equation for consumers only is: GP = food eaten – faecal losses

  9. Calculating Productivity Values • Some easy ones to start you off! • What is the % energy from sunlight that is fixed as GPP if the total energy from the sun in 3 x 106 and the gross primary productivity = 2.8 x 104? • What is the GPP of an ecosystem if the NPP is 1660 kJm-2yr-1 and the energy lost during respiration is 573 kJm-2yr-1 ? • What is the NPP if the GPP is 2700 kJm-2yr-1 and the energy used in respiration is 1850kJm-2yr-1?

  10. Calculating Productivity Values Now for some slightly harder ones!

  11. Energy Flow Diagrams Energy flow diagrams illustrate energy flow through communities and include values for respiratory losses and energy flow through the decomposers Information from energy flow diagrams can be used to calculate ecological efficiencies Ecological Efficiency is the net production of new biomass at each trophic level as a percentage of the total energy flowing through that trophic level Therefore, for photoautotrophs, photosynthetic efficiency is determined as: Photosynthetic Efficiency = Net production ÷ Light Energy Absorbed

  12. Use information from the energy flow diagram to: • Explain the meaning of the term Gross Primary Production • Explain the meaning of the term Net Primary Production • Calculate the Photosynthetic Efficiency of the phytoplankton

  13. Gross Primary Production is the total energy fixed by photoautotrophs during photosynthesis Photosynthetic Efficiency = 3.7 x 104 ------------ x 100 172 x 104 = 2.15% Net Primary Production is the energy stored as biomass (gross production – energy lost as heat in respiration)

  14. NPP = 800 kJ m-2year-1 Photosynthetic efficiency = 1.3% GPP = 24 x106 kJ m-2year-1 NPP = 200 kJ m-2year-1 NPP = 69.7 x 103 kJ m-2year-1 114 x 103 kJ m-2year-1

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