Systems and Cycles of the
Biosphere
Chapter
20
Energy Flow in Ecosystems
The Food Web
Energy transformations in an ecosystem occur through a series of
trophic levels, or feeding levels, that are collectively referred to as a
food chain or food web.
The plants and algae in a food web are the primary producers and
make up the first trophic level.
These organisms use light energy to convert CO2 and water into
carbohydrates, and eventually into other biochemical molecules
needed to support life.
Energy Flow in Ecosystems
The Food Web
The primary producers support the consumers — organisms that ingest
other organisms as their food source.
The primary consumers or herbivores are the lowest level of
consumers.
Secondary consumers or carnivores feed on the primary consumers.
Some animals are omnivores and can feed on both plant and animal
materials - decomposers use detritus, or decaying organic matter,
derived from all feeding levels.
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Energy Flow in Ecosystems
Photosynthesis and Respiration
Photosynthesis is the production of carbohydrate — a
general term for a class of organic compounds consisting
off the
th elements
l
t carbon
b (C),
(C) h
hydrogen
d
(H)
(H), and
d oxygen (O)
with the general molecular formula CH2O (sugar or sucrose
(C12H22O11)).
H2O + CO2 + light energy → CH2O + O2
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Energy Flow in Ecosystems
Photosynthesis and Respiration
Three different photosynthesis processes have been identified:
About 95 percent of plants use the C3 pathway (Calvin cycle or CalvinBenson cycle), so designated because the first stable product
manufactured
f t d from
f
CO2 is
i th
the 3
3-carbon
b compound
d 3-phosphoglyceric
3 h
h l
i
acid (PGA) with chemical formula C3H7O7P.
About 1 percent of plants use the C4 photosynthetic pathway - in C4
plants, CO2 is converted into oxaloacetic acid, a 4-carbon compound
with the chemical formula C4H4O5.
Plants that use the CAM (Crassulacean acid metabolism) pathway
initially manufacture oxaloacetic acid in the same way as C4 plants.
Energy Flow in Ecosystems
Photosynthesis and Respiration
Respiration is the opposite of photosynthesis in that
carbohydrate is broken down and combined with oxygen to
yield CO2 and water.
water
The reaction can be simplified as follows:
CH2O + O2 → CO2 + H2O + chemical energy
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Energy Flow in Ecosystems
Net Photosynthesis
Gross photosynthesis is the total amount of carbohydrate produced by
photosynthesis.
Net photosynthesis is the amount of carbohydrate remaining after
respiration
i ti h
has b
broken
k d
down enough
h carbohydrate
b h d t tto meett th
the plant’s
l t’
own needs.
Thus: Net photosynthesis = Gross photosynthesis - Respiration
Respiration accounts for most of the energy trapped by plants;
the net result is an overall photosynthetic efficiency in the order
of 3 to 6 percent of total available solar energy.
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Energy Flow in Ecosystems
Net Primary Production
Accumulated net production by photosynthesis is
measured in terms of biomass, and is usually expressed as
the dry weight of organic matter per unit of surface area
within the ecosystem; for example
example, as kilograms per square
metre (kg m2) or tonnes per hectare (t ha-1; 1 ha =104 m2).
From the viewpoint of ecosystem productivity, what is
important is the annual yield of useful energy produced by
the ecosystem, or the net primary production (NPP).
Annual Net Primary Production in
Terrestrial Ecosystems
Average Annual Net Primary
Production of the Oceans
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Biomass as an Energy Source
The use of biomass as an energy source involves releasing
solar energy that has been fixed in plant tissues through
photosynthesis.
This p
process can take p
place in a number of ways
y — the
simplest is direct burning of plant matter as fuel.
Other approaches involve the generation of intermediate
fuels from plant matter; for example, charcoal from wood,
ethanol from grains, and methane from anaerobic digestion
of organic wastes.
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Biogeochemical Cycles in the Biosphere
Matter moves through ecosystems under the influence of both
physical and biological processes.
Each substance follows a specific biogeochemical cycle
(material cycle or nutrient cycle) that consists of various pools
interconnected by flow pathways.
A pool refers to any area or location of material concentration.
There are two types of pools — active pools, where materials
are in forms and places easily accessible to life processes, and
storage pools, where materials are more or less inaccessible to
living systems.
Biogeochemical Cycles in the Biosphere
Matter moves through ecosystems under the influence of both
physical and biological processes.
Each substance follows a specific biogeochemical cycle
(material cycle or nutrient cycle) that consists of various pools
interconnected by flow pathways.
A pool refers to any area or location of material concentration.
There are two types of pools — active pools, where materials
are in forms and places easily accessible to life processes, and
storage pools, where materials are more or less inaccessible to
living systems.
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Biogeochemical Cycles in the Biosphere
Biogeochemical cycles exists in two forms: gaseous cycles
and sedimentary cycles.
In a gaseous cycle, the element or compound can be
converted directly into a gas - the primary constituents of
living matter —carbon,
carbon hydrogen
hydrogen, oxygen
oxygen, and nitrogen
nitrogen—all
all
move through gaseous cycles.
In a sedimentary cycle, weathering releases the compound
or element from rock - the cycle is completed when the
rock is uplifted and exposed to weathering.
Biogeochemical Cycles in the Biosphere
Nutrient Elements in the Biosphere
Fifteen elements are commonly present in living matter.
The three principal components of a carbohydrate—hydrogen,
carbon, and oxygen — account for 99.5 percent of all living
matter.
In addition to these macronutrients are secondary nutrients and
micronutrients including nitrogen, calcium, potassium,
magnesium, sulphur, and phosphorus.
Biogeochemical Cycles in the Biosphere
The Carbon Cycle
The movements of carbon through the life layer are of great
importance because all life is composed of carbon
p
of one form or another.
compounds
Of the total carbon available, most lies in storage pools as
carbonate sediments below the Earth’s surface.
Only about 0.2 percent is readily available to organisms as
CO2 or as decaying biomass in active pools.
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Biogeochemical Cycles in the Biosphere
The Oxygen Cycle
The largest active pool of the oxygen cycle is found in the atmosphere,
but a small active pool is also present in the oceans.
The complete picture of the cycling of oxygen includes its movements
and storage when combined with carbon as
CO2 and as organic and inorganic compounds.
Oxygen enters the active pool through release in photosynthesis,
both in the oceans and on land.
Each year, a small amount of new oxygen comes from volcanoes
through out-gassing, principally as CO2 and water.
Biogeochemical Cycles in the Biosphere
The Nitrogen Cycle
Nitrogen moves through the biosphere in the gaseous nitrogen cycle in
which the atmosphere acts as a vast storage pool.
Nitrogen in the atmosphere
atmosphere, in the form N2, is an inert gas
gas, and most
plants or animals cannot assimilate it directly.
The process by which nitrogen is converted into nitrogen compounds,
such as ammonia (NH3) and nitrates (NO3-), is called nitrogen fixation only certain microorganisms possess the ability to use nitrogen directly.
In these forms, nitrogen is then available for various biochemical
processes.
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Biogeochemical Cycles in the Biosphere
Dead Zones
The long-term impact of large amounts of nitrogen on the Earth’s
marine ecosystems remains uncertain, although nitrogen is implicated
as the cause of the dead zones that have been reported in many
coastal regions - characterized by hypoxia, a condition in which oxygen
i almost
is
l
t entirely
ti l d
depleted.
l t d
Hypoxia develops because of high biological demand from decomposer
bacteria that flourish on the abundant but short-lived algal blooms,
which develop as a result of excess nutrients in the water.
Thus, dead zones are linked either to areas of high population density
or to watersheds that deliver large quantities of fertilizers and other
nutrients to the oceans.
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Biogeochemical Cycles in the Biosphere
Dead Zones
The long-term impact of large amounts of nitrogen on the Earth’s
marine ecosystems remains uncertain, although nitrogen is implicated
as the cause of the dead zones that have been reported in many
coastal regions - characterized by hypoxia, a condition in which oxygen
i almost
is
l
t entirely
ti l d
depleted.
l t d
Hypoxia develops because of high biological demand from decomposer
bacteria that flourish on the abundant but short-lived algal blooms,
which develop as a result of excess nutrients in the water.
Thus, dead zones are linked either to areas of high population density
or to watersheds that deliver large quantities of fertilizers and other
nutrients to the oceans.
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Biogeochemical Cycles in the Biosphere
The Sulphur Cycle
Most of the Earth’s sulphur is tied up in rocks and ocean
sediments - a small amount is present in the atmosphere.
Sulphur
S
l h originates
i i t ffrom iigneous rocks,
k such
h as pyrite
it
(FeS2), and is also found in gypsum (CaSO4 · 2H2O) and
other sedimentary deposits.
Long-term storage of sulphur occurs in both organic and
inorganic forms, from which it is released by weathering
and decomposition.
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Biogeochemical Cycles in the Biosphere
The Sulphur Cycle
Sulphur in mineral form can be mobilized through oxidation of
sulphides to sulphate (SO42-), which may then go into solution
and be transported to the ocean in runoff - can also enter the
atmosphere as sulphur dioxide (SO2) and hydrogen sulphide
(H2S).
S)
Volcanic activity releases sulphur gases to the atmosphere H2S, dimethyl sulphide (DMS), and carbonyl sulphide (COS)
enter the atmosphere through biological activity.
Biogeochemical Cycles in the Biosphere
Sedimentary Cycles
Many other elements move in sedimentary cycles; that is,
from the land to ocean in running water, returning after
millions of years in uplifted terrestrial rocks.
Mineral nutrients, derived mainly from weathering of soil
minerals and decomposition of organic residues, move in
sedimentary cycles.
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Biogeochemical Cycles in the Biosphere
Sedimentary Cycles
Some are stored in nutrient pools within the soil, from
which they may, to varying degrees, be extracted by plants
or lost by leaching.
Nutrients that are held as ions on the surfaces of soil
colloids are readily available to plants; however, in other
forms, they may be relatively insoluble and slowly become
available over a long period of time.
A Look Ahead
Chapter 21 discusses the processes that determine the
distributions of individuals and species, including organism
– environment relationships and dynamic processes such
as species dispersal,
dispersal migration
migration, and extinction
extinction.
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