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Dynamics of Ecosystems

Chapter 57

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Biogeochemical Cycles

• Biogeochemical cycles: chemicals moving through ecosystems; biotic and abiotic– Can cross the boundaries of ecosystems (one

ecosystem might import or export chemicals to another)

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Biogeochemical Cycles• Water Cycle– All life depends on the presence of water– Amount of water available determines the nature

and abundance of organisms present– It can be synthesized and broken down• Synthesized during cellular respiration• Broken down during photosynthesis

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Biogeochemical Cycles• Basic water cycle– Evaporation– Transpiration: 90% of evaporation is through plants– Condensation– Precipitation– Groundwater• Aquifers : provide 95% fresh water used in US

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Biogeochemical Cycles• Water cycle

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Biogeochemical Cycles

• Changes in the supply of water to an ecosystem can radically alter the nature of the ecosystem

• Deforestation disrupts the local water cycle

• Water that falls as rain drains away

• Tropical rain forest semiarid desert

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Biogeochemical Cycles• Nitrogen Cycle– component of proteins and nucleic acids– Usually the element in shortest supply– Availability• Most cannot use N2 (gas)

• Use NH3, and NO3-

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Biogeochemical Cycles• Nitrogen fixation: synthesis of nitrogen

containing compounds from N2

• Nitrification: N2 --> NH3 --> NO3-

• Denitrification: NO3- --> N2

– carried out by microbes: free or living on plant roots – Nitrogenous wastes and fertilizer use radically alter

the global nitrogen cycle

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Biogeochemical Cycles• Nitrogen Cycle

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Biogeochemical Cycles• Phosphorus cycle– Occurs in nucleic acids, membranes, ATP– Exists as PO4

3- in ecosystems– Plants and algae use free inorganic phosphorus,

animals eat plants to obtain their phosphorus

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Biogeochemical Cycles• Phosphorus cycle

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Biogeochemical Cycles• carbon cycle (important component of bodies

~20% of human body weight)

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Biogeochemical Cycles• Respiration/photosynthesis

• Methane producers: anaerobic cellular respiration releases CH4

• Decomposition: releases CO2

• Combustion: burning fossil fuels produces CO2

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Biogeochemical Cycles• Over time, globally, the carbon cycle may

proceed faster in one direction– Human burning of fossil fuels is creating large

imbalances in the carbon cycle– The concentration of CO2 in the atmosphere is going

up year by year

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Biogeochemical Cycles

• Limiting nutrient: weak link in an ecosystem; shortest supply relative to the needs of organisms

• Fe for algal populations• N and P for both terrestrial and aquatic

ecosystems

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Biogeochemical Cycles• Biogeochemical cycling in a forest

ecosystem-- Hubbard Brook Experiment• Undisturbed forests are efficient at

retaining nutrients• Disturbed (cut trees down) amount of

water runoff increased by 40%– Loss of Ca; increased nine fold– Loss of Phosphorus did not increase – Loss of NO3

-; 53kg/hectare/yr

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Biogeochemical Cycles

The Hubbard Brook Experiment38-acre watershed. Orange curve shows nitrate concentration in the runoff water from the deforested watershed. Green curve shows the nitrate concentration in runoff from an undisturbed watershed