Transcript Chapter 6: Energy in the Ecosystem

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Chapter 11: flux of energy and matter
through ecosystems
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“Like all biological entities, ecological communities require
matter for their construction and energy for their activities.
We need to understand the routes for which matter and energy
enter and leave ecosystems, how they are transformed into
plant biomass, and how this fuels the rest of the community –
bacteria and fungi, herbivores, detritivores and their
consumers.”
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Background: Organizing Concepts
 In
1920s, English ecologist Charles Elton and others
promoted a revolutionary concept:


organisms living in the same place not only have similar
tolerances of physical factors, but
feeding relationships link these organisms into a single functional
entity
 This
system of feeding relationships is called a food
web.
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The Ecosystem Concept
 The
English ecologist A.G. Tansley took Elton’s ideas one
step further:

in 1935 Tansley coined the term ecosystem, the fundamental unit
of ecological organization
 the ecosystem concept: “the biological and physical parts of
nature together, unified by the dependence of animals and plants
on their physical surroundings and by their contributions to
maintaining the conditions and composition of the physical world.”
-R.E. Ricklefs
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Some key terms
Standing crop


Biomass


Mass of organisms per unit area of ground (or water); usually expressed in units
of energy or dry organic matter
Primary productivity


Bodies of the living organisms within a unit area
Rate at which biomass is produced per unit area by plants
Gross primary productivity

Total fixation of energy by photosynthesis

Net primary productivity

Secondary productivity

=
GPP - Respiration
Rate of production of biomass by heterotrophs
4/12/2017
GPP can be
partitioned
into
respiration
and NPP
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More key terms

Live consumer system


Proportion of primary production consumed by herbivores – who
are then consumed by carnivores
Decomposer system

Fraction of NPP not eaten by herbivores reaches decomposer
system

Two groups responsible for decomposition of detritus

Bacteria and fungi: decomposers

Animals that consume dead matter: detritivores
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Geographic patterns in PP
Productivity of
forests,
grasslands,
crops and
lakes follows a
latitudinal
pattern
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+ NPP among ecosystems
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What limits PP?

Terrestrial communities:


Solar radiation, carbon dioxide, water and soil nutrients: resources
required for PP
Temperature, a condition, strong influence

IF other resources were in abundant supply, radiation would be
used more efficiently [eg: conifer communities only uses between
1 to 3 % of available radiation]

Rainfall strongly correlated with productivity

Of the minerals, the one with strongest influence on community
productivity: fixed nitrogen [not atmospheric N]

May be limited by a succession of factors
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What limits PP in aquatic
environment?
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
Availability of nutrients (nitrate and phosphate)

Intensity of solar radiation that penetrates water column
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Relationship between PP and
SP
Positive relationship

Secondary productivity by zooplankton, eat phytoplankton cells,
positively related to phytoplankton productivity

Productivity of heterotrophic bacteria – also +ive with phyotplankton

Caterpillars abundance linked to primary productivity (which is linked
to annual rainfall)

Seed-eating finch – raises more broods
In wet years (increased plant production)
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Where does the energy go?

In aquatic and terrestrial communities: SP is 1/10 of PP
(1) not all of plant biomass is consumed alive by herbivores
(2) not all plant biomass eaten by herbivores is assimilated and
available for incorporation into consumer biomass. [what
happens to the rest?]
(3) not all energy assimilated is converted to biomass [what
happens to the rest?]
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Alfred J. Lotka, the Thermodynamic
Concept, and Lindeman’s concept
 Alfred
J. Lotka introduced the concept of the ecosystem
as an energy-transforming machine:



described by a set of equations representing exchanges of
matter and energy among components, and
obeying thermodynamic principles that govern all energy
transformations
In 1942, Raymond Lindeman brought Lotka’s ideas of the ecosystem
as an energy-transforming machine to the attention of ecologists.
He incorporated:



Lotka’s thermodynamic concepts
Elton’s concept of the food web as expression of the ecosystem’s
structure
Tansley’s concept of the ecosystem as the fundamental unit in ecology
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Thermodynamics and Ecology

1st law of thermodynamics - Energy can be neither created
nor destroyed. It can only change forms.

2nd law of thermodynamics - spontaneous natural processes
increase entropy overall

the total biomass ALWAYS decreases with increasing trophic
levels, as energy is constantly being lost to the atmosphere

So?
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Lindeman’s Foundations of
Ecosystem Ecology
 The
ecosystem is the fundamental unit of ecology.
 Within
the ecosystem, energy passes through many steps
or links in a food chain.
 Each
link in the food chain is a trophic level (or feeding
level).
 Inefficiencies
in energy transformation lead to a pyramid
of energy in the ecosystem.
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Odum’s Energy Flux Model

Eugene P. Odum popularized ecology to a generation of
ecologists.

Odum further developed the emerging framework of
ecosystem ecology:
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
he recognized the utility of energy and masses of elements as
common “currencies” in comparative analysis of ecosystem
structure and function
Odum extended his models to incorporate nutrient cycling.


Fluxes of energy and materials are closely linked in ecosystem
function. However, they are fundamentally different:
 energy enters ecosystems as light and is degraded into heat
 nutrients cycle indefinitely, converted from inorganic to organic
forms and back again
Studies of nutrient cycling provide an index to fluxes of energy.
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Simple
Ecosystem
Model
energy
input from
sun
PHOTOAUTOTROPHS
(plants, other producers)
nutrient
cycling
HETEROTROPHS
(consumers, decomposers)
energy output (mainly heat)
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Models of ecological energy flow
A single trophic level
A food chain
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An ecological pyramid of energy
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Only 5% to 20% of energy passes
between trophic levels.
 Energy
on:
reaching each trophic level depends
net primary production (base of food chain)
efficiencies of transfers between trophic levels
- More on this later 

 Plant
use between 15% and 70% of light
energy assimilated for maintenance – thus that
portion is unavailable to consumers
 Herbivores
and carnivores expend more
energy on maintenance than do plants:
production of each trophic level is only 5%
to 20% that of the level below it.
Energy:
how
many
lbs
of
grass
to
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support one hawk
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Ocean food pyramid – roughly
2500 lbs/1136 kg of phytoplankton
to support 0.5lb/0.23 kg of tuna
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Only 5% to 20% of energy passes
between trophic levels.
 Energy
on:


reaching each trophic level depends
net primary production (base of food chain)
efficiencies of transfers between trophic levels
 Plant
use between 15% and 70% of light
energy assimilated for maintenance – thus that
portion is unavailable to consumers
 Herbivores
and carnivores expend more
energy on maintenance than do plants:
production of each trophic level is only 5%
to 20% that of the level below it.
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Ecological Efficiency
 Ecological
efficiency (food
chain efficiency) is
the percentage of
energy transferred
from one trophic
level to the next:



el
range of 5% to 20% is
typical, as we’ve seen
to understand this more
fully, we must study the
use of energy within a
trophic level
Undigested plant fibers in elephant dung
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Intratrophic Energy Transfers
 Intratrophic
transfers involve several
components:
ingestion (energy content of food ingested)
 egestion (energy content of indigestible materials
regurgitated or defecated) (the elephant dung)
 assimilation (energy content of food digested and
absorbed)
 excretion (energy content of organic wastes)
 respiration (energy consumed for maintenance)
 production (residual energy content for growth and
reproduction)
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Fundamental Energy Relationships
 Components
of an animal’s energy budget are
related by:
 ingested
energy - egested energy = assimilated
energy
 assimilated energy - respiration - excretion =
production
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Assimilation Efficiency
 Assimilation
efficiency =
assimilation/ingestion
 primarily
a function of food quality:
 seeds: 80%
 young
vegetation: 60-70%
 plant foods of grazers, browsers: 30-40%
 decaying wood: 15%
 animal foods: 60-90%
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Net Production Efficiency
 Net
production efficiency =
production/assimilation
 depends
largely on metabolic activity:
birds: <1%
 small mammals: <6%
 sedentary, cold-blooded animals: as much as 75%

 Gross
production efficiency = assimilation
efficiency x net production efficiency =
production/ingestion, ranges from below 1%
(birds and mammals) to >30% (aquatic
animals).
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Active, warm-blooded animals – low net
production efficiencies; hummingbird: <1%
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Production Efficiency in Plants
 The
concept of production efficiency is
somewhat different for plants because
plants do not digest and assimilate food:
 net
production efficiency = net
production/gross production; varies between
30% and 85%
 rapidly growing plants in temperate zone have
net production efficiencies of 75-85%; their
counterparts in the tropics are 40-60% efficient
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Detritus Food Chains
 Ecosystems
support two parallel food
chains:
 herbivore-based
(relatively large animals feed
on leaves, fruits, seeds)
 detritus-based (microorganisms and small
animals consume dead remains of plants and
indigestible excreta of herbivores)
 herbivores consume:



1.5-2.5% of net primary production in temperate forests
12% in old-field habitats
60-99% in plankton communities
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Exploitation Efficiency
 When
production and consumption are not
balanced, energy may accumulate in the
ecosystem (as organic sediments).
 Exploitation
efficiency / trophic transfer
efficiency = ingestion by one trophic
level/production of the trophic level below it.
 To
the extent that exploitation efficiency is
<100%, ecological efficiency = exploitation
efficiency x gross production efficiency.
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Reminder of key terms

Consumption efficiency (CE)

Assimilation efficiency (AE)

Production efficiency (PE)
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Some General Rules
 Assimilation
efficiency increases at higher
trophic levels.
 Net
and gross production efficiencies decrease
at higher trophic levels.
 Ecological
efficiency averages about 10%.
 About
1% of net production of plants ends up as
production on the third trophic level: the
pyramid of energy narrows quickly.
 To
increase human food supplies means eating
lower on food chain!
 [virtual
water]
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Virtual water
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Virtual water
http://environment.nationalgeographic.com/
environment/freshwater/embedded-water/
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Virtual water
http://environment.nationalgeographic.com/enviro
nment/freshwater/embedded-water/
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Virtual water
http://environment.nationalgeographic.com/
environment/freshwater/embedded-water/
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Virtual water
http://environment.nationalgeographic.com/
environment/freshwater/embedded-water/
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Virtual water
http://environment.nationalgeographic.com/
environment/freshwater/embedded-water/
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Admin notes..

Need to get your syllabi signed by the end of today, or status
quo applies to you. Need hypothesis etc

Quizzes. Set times.

Chapters 5 and 7: December 8
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Chapters 11 and 8: December 15
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Chapters 9 and 10 : January 5
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Chapters 12 to 14: January 19
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Extra slides’ [climate change, for example] will go into final exam
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Food…Homework: reminder
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For one week (7 days), write
down everything that you
consume, including
cigarettes, gum, etc.
Include as much locationalinformation about the
product that you know

By December 7th

Give me your weekly diet

A one to two paragraph
reflection on your eating
habits
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A one to two paragraph
reflection on the relationship
between what you eat and
ecology

[Please be sure that your
writing is properly edited.]
Be detailed. A “salad” is not
enough.
Think about what you are
eating
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Documentaries – must see

Thursday December 2nd, 12.30 to 2.00 pm: Food, Inc.
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Thursday December 9th, 12.30 to 2.00 pm: Mircocosmos.
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Location for both: Hariri Auditorium
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systems: general patterns of
energy flow
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+ Live consumer and decomposer systems:
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general patterns of energy flow
DOM: dead
organic
matter
LCS: live
consumer
system
Relative sizes of
boxes and
arrows are
proportional
to the relative
magnitude of
compartment
s and flows
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Process of decomposition

Immobilization – when an inorganic nutrient element is
incorporated into organic form, primarily during the growth
of green plants [carbon dioxide becoming carbohydrates, eg]

Mineralization – conversion of elements from organic back to
an inorganic form

Decomposition – the gradual disintegration of dead organic
matter by both physical and biological agents
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Who decomposes?

Bacteria and fungi: begin the process of decomposition. Use
soluble materials (amino acids and sugars)

Microbial specialists: break down residual resources
(structural carbohydrates and complex proteins)

Some specialist microbivores feed on bacteria and fungi

Microbivores: group of animals that operate alongside the
detritivores; minute animals that specialize at feeding on bacteria
or fungi but are able to exclude detritus from their guts
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What do they eat?

Plant detritus

Two of the major components of dead leaves and wood: cellulose
and lignin

Lacking cellulase enzymes, majority of detritivores depend on
production of cellulases by associated bacteria or fungi or
protozoa
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What do they eat?
Feces and carrion [decaying flesh of dead animals]


Carnivorous vertebrates: poor quality dung [feces, manure].
Why?

Elephant dung  within minutes eaten by adult dung beetles
feed on the the dung, bury large quantities along with their
eggs to provide food for their larvae

Without those beetles, though…
Cattle dung. Cow pop increased from 7 in 1788 to 30 million
in 1988 – producing 300 million cowpats/day – in Australia

Lack of native dung beetles  loss of 2.5 million ha/year/ under
dung. So introduced 20 species of beetles
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Elephant dung? Into paper?
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Energy moves through ecosystems at
different rates.
 Other
indices address how rapidly energy cycles through
an ecosystem:


residence time measures the average time a packet of energy
resides in storage:
 residence time (yr) = energy stored in biomass/net productivity
biomass accumulation ratio is a similar index based on biomass
rather than energy:
 biomass accumulation ratio (yr) = biomass/rate of biomass
production
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Residence Time for Litter
 Decomposition
of litter is dependent on conditions
of temperature and moisture.
 Index
is residence time = mass of litter
accumulation/rate of litter fall:




3 months in humid tropics
1-2 yr in dry and montane tropics
4-16 yr in southeastern US
>100 yr in boreal ecosystems
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Biomass Accumulation Ratios
Biomass accumulation ratios
become larger as amount of
stored energy increases:
 humid tropical forests have
net production of 1.8
kg/m2/yr and biomass of 43
kg/m2, yielding biomass
accumulation ratio of 23yr
 ratios for forested terrestrial
communities are typically >20
yr
 ratios for planktonic aquatic
ecosystems are <20 days
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Flux of matter through
ecosystems
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+ Activities of organisms strongly influence the
patterns of flux of chemical matter
Cycling of elements and flux of energy in ecosystems are
fundamentally different:

Energy: lost as heat [once carbon is transformed into heat, it can
no longer be used by living organisms; heat is lost]

Chemical elements: remain within the biosphere – where they
cycle continually between organisms and the physical
environment [carbon in carbon dioxide can be used again in
photosynthesis, can be incorporated into biomass]

Inorganic compounds: used by organisms to synthesize organic
compounds, then recycled over and over before being lost in
sediments, streams, and groundwater or escaping to the
atmosphere as gases
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Energy flows through biochemical
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pathways
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Ecosystems may be modeled as linked
compartments.

An ecosystem may be viewed as a set of compartments
among which elements are cycled at various rates:

photosynthesis moves carbon from an inorganic compartment (air
or water) to an organic compartment (plant)

respiration moves carbon from an organic compartment
(organism) to an inorganic compartment (air or water)
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Nutrient budgets in terrestrial
ecosystems
Weathering of parent bedrock and soil [physical and chemical
processes] main source of nutrients

Calcium
Iron

Magnesium
Phosphorus
Potassium
Atmospheric carbon dioxide: source of carbon content of
terrestrial communities
Gaseous nitrogen: source of most of nitrogen content [with
enzyme nitrogenase to convert gaseous nitrogen into
ammonium ions]
Dryfall or wetfall: Rain: chemicals from: trace gases of sulfur and
nitrogen oxides; particles rich in sodium, magnesium, chloride,
sulfate; dust particles rich in calcium, potassium, and sulfate
Generalized compartment model of the cycling of elements within
ecosystems
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+ Nutrient budgets (nutrient circulation) of a
terrestrial and aquatic system
Inputs: blue
Outputs: black
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+ Nutrient budgets in aquatic communities
Main transformers of
dissolved inorganic
carbon: small
phytoplankton cells –
carbon recycled near
ocean surface
Larger phytoplankton:
majority of carbon flux
to deep ocean floor 
consumed by deepsea animals;
mineralized to
inorganic…
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Yes
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I do want you to know the boxes

11.2

11.3
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Global biogeochemical cycles

Nutrients are moved by winds in atmosphere and by moving
waters of streams and ocean currents

No boundaries

Hydrological cycle
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Phosphorus cycle

Nitrogen cycle
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Sulfur cycle

Carbon cycle
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A Physical Model for the Water
Cycle
 The
biosphere contains 1,400,000 teratons (TT,
1012 metric tons) of water, 97% of which
resides in the oceans.
 Other






water compartments include:
ice caps and glaciers (29,000 TT)
underground aquifers (8,000 TT)
lakes and rivers (100 TT)
soil moisture (100 TT)
water in atmosphere (13 TT)
water in living things (1 TT)
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Global water cycle; units in
billion billion grams (10^18)
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The water cycle is solar-powered.
The water cycle consumes one-fourth (1/4) of the total solar energy
striking the earth during a year:



precipitation over land exceeds evaporation by 40 teratons/yr;
surplus returns to the ocean in rivers
evaporation over the oceans exceeds precipitation by 40 teratons/yr;
surplus is delivered by winds to the land masses
Can calculate the energy that drives the global hydrologic cycle



Total weight of water evaporated (456 tt/year) * energy required to
evaporate 1 g of water (2.24 kJ)
= ~10 to the 21st power kJ/yr = 32 billion megwatts
¼ of the total energy of the sun’s radiation striking the earth
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The water cycle is solar-powered.
The residence time of water varies by compartment.

Residence time of water = average time a water molecule will spend in
that compartment; measure of the average age of the water in that
reservoir

The atmosphere contains 2.5 cm of moisture at any time; annual flux
into and out of the atmosphere is 65 cm/yr:
 residence time is compartment size/flux, or 2.5 cm / 65 cm/yr =
0.04 yr, about 2 weeks. (for water to condense and fall as rain)
 Only 0.08% of water in flux – in transit

Soils, rivers, lakes and oceans have same flux rates as atmosphere,
but they contain about 100,000 times as much water, yielding a
mean residence time of 2,800 yr.

Groundwater can spend 10,000 yrs beneath the Earth’s surface;
fossil water
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Hydrological cycle

Terrestrial vegetation can modify the fluxes that occur

Vegetation can intercept water at 2 points:

Catching some water in foliage from which it evaporates

Preventing some water from draining from soil water by taking it
up via roots into the plant’s transpiration ssytem
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Human activities
Human activities that alter the water cycle include:



agriculture
industry
alteration of the chemical composition of the atmosphere

construction of dams

deforestation and afforestation

removal of groundwater from wells

urbanization
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The Phosphorus Cycle

Phosphorous is an essential element, constituent of nucleic
acids, cell membranes, energy transfer systems, bones, and
teeth.

Phosphorus may limit productivity:


in aquatic systems, sediments act as a phosphorus sink unless
oxygen-depleted

in soils, phosphorus is only readily available between pH of 6 and 7
Phosphorus undergoes relatively few transformations:


plants assimilate P as phosphate (PO43-) and incorporate this into
organic compounds
animals and phosphatizing bacteria break down organic forms of
phosphorus and release the phosphorus as phosphate
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Phosphorus cycle

Released from the rock by chemical weathering

Enters and cycles within terrestrial community for years, decades,
or centuries

Carried via ground water into a stream

Weeks, months, or years later, atom is carried to the ocean

Makes, on average, ~ 100 round trips between surface and deep
waters

Each time: taken up by surface-dwelling organisms

After 10 million years, fails to be released as soluble phosphorus,
and enters bottom sediment in particulate form

Maybe, 100 million years later, ocean floor becomes dry land…
+ Phosphorus cycle
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Nitrogen - A Most Versatile
Element!
 Ultimate
source (largest reservoir) of this
essential element is molecular N2 gas in the
atmosphere, which can also dissolve in water
to some extent.
 Nitrogen
is absent from native rock.
 Nitrogen
enters biological pathways through
nitrogen fixation:

these pathways are more complicated than those of the carbon
cycle because nitrogen has more oxidized and reduced forms
than carbon
+ Biological pathways of the nitrogen
cycle
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Nitrogen Fixation
 Loss
of nitrogen to atmosphere by
denitrification is offset by nitrogen fixation:
 fixation is carried out by:
 free-living bacteria such as Azotobacter
 symbiotic bacteria such as Rhizobium, living in root nodules of
legumes and other plants
 cyanobacteria
 N-fixation
is an energy-requiring process, with
energy supplied by oxidation of organic detritus
(free-living bacteria), sugars supplied by plants
(bacterial symbionts), or photosynthesis
(cyanobacteria)
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Significance of Nitrogen
Fixation

Nitrogen fixation balances denitrification on a global basis:


these fluxes amount to about 2% of total cycling of nitrogen
through ecosystems
Nitrogen fixation is often very important on a local scale:

N-fixers dominate early colonizers on nitrogen-poor substrates,
such as lava flows or areas left bare by receding glaciers
+ Nodules on the roots of soybeans
harbor symbiotic nitrogen-fixing
bacteria
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Nitrogen and us

human beings have more than doubled the annual transfer of
nitrogen into biologically available forms

Chemical fertilizers

Pollution from vehicles and industrial plants

N2O has risen in the atmosphere as a result of agricultural
fertilization, biomass burning, cattle and feedlots, and other
industrial sources
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Human activities

The impacts of human domination of the nitrogen cycle
that we have identified with certainty include:




Increased global concentrations of nitrous oxide (N2O), a
potent greenhouse gas, in the atmosphere as well as
increased regional concentrations of other oxides of
nitrogen (including nitric oxide, NO) that drive the
formation of photochemical smog;
Losses of soil nutrients such as calcium and potassium that
are essential for long-term soil fertility;
Substantial acidification of soils and of the waters of
streams and lakes in several regions;
Greatly increased transport of nitrogen by rivers into
estuaries and coastal waters where it is a major pollutant.
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consequences
human alterations of the nitrogen cycle have:

* Accelerated losses of biological diversity, especially
among plants adapted to low-nitrogen soils, and
subsequently, the animals and microbes that depend on
these plants;

* Caused changes in the plant and animal life and
ecological processes of estuarine and nearshore ecosystems,
and contributed to long-term declines in coastal marine
fisheries.
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The Sulfur Cycle

Sulfur is an essential element and, like nitrogen, has many
oxidation states and follows complex chemical pathways.

Sulfur reduction reactions include:



assimilatory sulfate reduction to organic forms and dissimilatory
oxidation back to sulfate by many organisms
reduction of sulfate when used as an oxidizer for respiration by
heterotrophic bacteria in anaerobic environments
Sulfur oxidation reactions include:

oxidation of reduced sulfur when used as an electron donor (in
place of oxygen in water) by photosynthetic bacteria

oxidation of sulfur by chemoautotrophic bacteria that use the
energy thus obtained for assimilation of CO2
+
86
Sulfur in Coal and Oil Deposits

Iron sulfide (FeS) commonly associated with coal and oil
deposits can result in environmental problems:


oxidation of sulfides in mine wastes to sulfate, which combines
with water to form sulfuric acid, associated with acid mine
drainage
oxidation of sulfides in coal and oil releases sulfates into
atmosphere, which then form sulfuric acid, a component of acid
rain
+
Sulfur cycle
+ Acidic streams from refuse of coal mines
(Pennsylvania)
+
94
The carbon cycle is linked to
global energy flux.
 The
carbon cycle is the focal point of
biological energy transformations.
 Principal
classes of carbon-cycling
processes:
 assimilatory/dissimilatory
processes (mainly
photosynthesis and respiration)
 exchange of CO2 between atmosphere and
oceans
 sedimentation of carbonates
+
Global carbon cycle; units in
billions of metric tons or
gigatons (GT) and GT/yr
+
96
First class of carbon cycling:
Photosynthesis and Respiration

Approximately 85 GT of carbon enter into balanced
assimilatory/dissimilatory transformations each year.

Total global carbon in organic matter is about 2,650 GT
(living organisms plus organic detritus and sediments).

Residence time for carbon in biological molecules = 2,650
GT / 85 GT/yr = 31 years.
+
97
Second class of carbon cycling:
Ocean-Atmosphere Exchange

Exchange of carbon across the atmosphere-ocean interface links
carbon cycles of terrestrial and aquatic ecosystems.

Oceans contain 50 times as much CO2 as the atmosphere (oceans as
sink. Oceans as source?)

Dissolved carbon pool is 30,000 GT, nearly 50 X that of atmosphere
(640 GT).

Net atmospheric flux (assimilation/ dissimilation and exchange with
oceans) is 119 GT/yr for mean atmospheric residence time (640 GT /
119 GT/yr) of about 5 years.

By 1990: combustion of fossil fuels -> 6 GT / year = ~ 1% of total
atmospheric carbon dioxide

(Read up on climate change – again)
+
98
Third class of carbon cycling:
Precipitation of Carbonates

Precipitation (and dissolution) of carbonates occurs in
aquatic systems:

precipitation (as calcium and magnesium carbonates) leads to
formation of limestone and dolomite rock
 turnover of these sediments is far slower than those associated
with assimilation/dissimilation or ocean-atmosphere exchange
 carbonate sediments represent the single largest compartment
of carbon on planet (18,000,000 GT)
+
Most of the earth’s carbon is in sedimentary rocks (southern
Texas)
+
100
Precipitation of Calcium and Carbon
Through the Ages

CO2 dissolves in water to form carbonic acid, which
dissociates into hydrogen, bicarbonate, and carbonate ions:
CO2 + H2O  H2CO3
H2CO3  H+ + HCO3-  2H+ + CO32-

Calcium ions combine with carbonate ions to form slightly
insoluble calcium carbonate, which precipitates:
Ca2+ + CO32-  CaCO3
When precipitation > respiration (as in algal blooms) –
calcium tends to precipitate out of the system
+
101
Slow Release of Sedimentary Calcium
and Carbon

Calcium removed from the water column in the oceans is
replaced by calcium dissolved from limestone sediments on
land by slightly acidic water of rivers and streams.

Carbon is also slowly released from oceanic sediments as
limestone is subducted beneath continental plates, and CO2
is outgassed in volcanic eruptions.
+
102
Reef-Builders extract carbon
from water.

In neutral conditions of marine ecosystems, extraction of CO2
from water column drives precipitation of CaCO3:
CaCO3 + H2O + CO2  Ca2+ + 2HCO3-

Reef-building algae and coralline algae incorporate calcium
carbonate into their hard structures, forming reefs.
+
Skeleton of caralline algae made of calcium carbonate
+
104
Changes in the Carbon Cycle
Over Time
 Atmospheric
CO2 concentrations have varied
considerably over earth’s history:



during the early Paleozoic era (550-400 Mya), concentrations
were 15-20 X those at present
concentrations declined to ca. present level by 300 Mya (during
which saw development of forests on land), then increased again
to 5 X present level through the early Mesozoic era (250-150 Mya)
and have declined gradually since
early Paleozoic and early Mesozoic eras were extreme
greenhouse times (hot temperatures), unlikely to be equaled by
effects of current human enhancement of atmospheric CO2