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Ecosystems

•  All organisms living in a given area with the abiotic factors with which they interact

•  Not discrete areas •  The most inclusive of biological

organization •  At the ecosystem level we often study the

flow of energy and nutrients in the system

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Generalized energy flow

•  Introduced to the base •  Complete picture with

the recycling of nutrients

•  Detritus and decomposers

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Can look at energy transfer in terms of Biomass as well: 3

Food Webs and Trophic Levels

•  Productivity refers to the amount of biomass produced in a given area during a given time. –  Primary Producers - Photosynthesize. –  Consumers - Eat other organisms.

•  Food Webs are series of interconnected food chains in an ecosystem. –  Trophic Level refers to an individual’s feeding position

in an ecosystem.

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Primary production •  Each day 100 million atomic bombs worth of solar

radiation strikes the earth (~1000 Watts/m2; 1.740×1017 Watts total)

•  Only a small fraction used for photosynthesis •  Gross Primary Productivity

–  Total production per unit time; not all stored, some used for immediate respiration

•  Net Primary Productivity –  Gross P.P. - respiration = Net P.P.

•  Light/Dark Bottles –  What is the ecological unit of interest…why?

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Primary production- biomass 6

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So, what controls Primary production in the ocean?

•  Light? Or something else?

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Nutrient limitation-Aquatic

•  Limiting nutrients –  Nitrogen; Phosphorus; Iron, Silica; others

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Limiting nutrients

•  Book examples: nitrogen and phosphorus •  Iron and Silica are also very important:

IronEx

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IronEx •  1993 and 1995 •  In two weeks, 1,000 pounds of iron produced two million pounds

of additional phytoplankton, sucking 2,500 tons of CO2 from the sky.

•  “Give me half a tanker of iron and I’ll give you the next ice age.” J. Martin, MLML (maybe more like 16)

•  use for Climate Change? www.planktos.com/

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Limiting nutrients in freshwater

•  University of Manitoba’s Experimental Lakes Area

If you fertilize it, they will come…

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Terrestrial limitation

•  Light, temperature and moisture •  Primary production removes nutrients from

soil, sometimes faster than they are replaced

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Amazon Rain Forest

•  Amazonian rainforest depends on an interesting source for many of its nutrients, including iron and phosphorus

From the Bodélé depression, a now-dry basin on the southern edge of the Sahara that in wetter times held a body of water the size of Lake Erie. • 700,000 tons of dust each day! • Other effect-

• Coral Death in the Caribbean • Toxic Algae blooms • Ice Age Triggers?!

13 Cycling of chemicals in ecosystem: Biogeochemical cycles

•  Limited elements are continuously recycled •  Two general categories of biogeochemical

cycles – Gaseous forms: carbon and oxygen from CO2 – Nongaseous forms: phosphorous, potassium,

calcium, others

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General model of cycling 15

The water cycle

•  More of a physical than chemical process

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The Carbon Cycle •  Carbon is the basic constituent of all organic

compounds and life •  “Fixed” into living matter via photosynthesis •  Following the carbon through an ecosystem is

very close to following energy •  Just as energy flows through food web, so

does carbon and other compounds •  Carbon is released back into the system via

respiration and decomposition (and combustion)

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The carbon cycle 18

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Carbon Pools 19

Carbon Dioxide in water •  CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3

- ⇔ H+ + CO32-

Carbon Water Carbonic bicarbonate carbonate

Dioxide Acid

•  Bicarbonate ion is the major reservoir of CO2 in the sea. (7th most common ion)

•  Equilibrium: bicarbonate ~90% •  About 50x more CO2 in water than in air! Why?

CO2 in hydration sphere

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Nitrogen Cycling

•  Nitrogen is essential component of protein •  80% of atmosphere is nitrogen gas (N2), but

not directly available to plants for uptake •  Usually available to plants in two forms

–  Ammonium (NH4+) –  Nitrate (NO3-)

•  90% of N uptake is via nitrogen fixation by bacteria –  10 grams of glucose to fix 1 gram of N!

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Nitrogen cycle 22

Phosphate Cycle •  Phosphorous: important organic form:

PO43- (phosphate)

•  Important for nucleic acids, phospholipids, ATP, bone…etc.

•  Somewhat simpler that previous cycles because there is no gas phase.

•  Linked to hydrologic cycle…at least part of the way! •  Localized recycling on both short and long-term

scales

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Phosphate cycle 24

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Other cycles

•  Just about all nutrients have a “cycle” •  Other examples:

–  Sulfur: complex and not well understood; important human influence-Acid rain and pH changes

–  Oxygen: water vapor and photosynthesis

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Regulation of nutrient cycling •  Example: Long-term ecological

research –  1963: started studying nutrient cycling in a

forest ecosystem in New Hampshire –  Carefully measured nutrient “budget” of

system –  Found that in natural state most things were

in good internal balance –  1966: large area logged and all plants killed –  Monitored changed in cycles in logged area

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Hubbard Brook experiment 27

Many cycles are linked

•  Introduced as independent, but all are linked in various ways

•  Nutrients are all part of living organisms and organic matter- travel together in cycling

•  These cycles can also easily be disturbed by human activity

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Human Impact

•  Human activities can have serious impacts on chemical cycles

•  Human activity has intruded on nutrient cycles to such an extent that we can’t understand any cycle without taking humans into account

•  We artificially transport nutrients from one place to another and add new materials to ecosystems

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Agricultural example •  Natural vegetation is cleared and crops are grown

without supplementation •  But biomass and nutrients are exported •  “Free” period from 1 year to decade •  Stores are exhausted so fertilizer must be added •  Tilling increases rate of decomposition

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Nitrogen cycle 31

Human altered nitrogen cycle

Fertiliz

er

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