Townsend C.R., Begon M., Harper J.L. Essentials of Ecology

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of carbon ﬂux to the deep ocean ﬂoor. Some of this organic material is consumed

by deep-sea animals, some is mineralized to inorganic form by bacteria and recir-

culated, and a small proportion becomes buried in the sediment. Figure 11.14 is

essentially the ocean equivalent of the forest system in Figure 11.13. In contrast

to the atmospheric source of carbon, nutrients such as phosphorus come from

two sources – river inputs and water welling up from the deep. Phosphorus atoms

in the surface water follow a similar set of pathways to carbon atoms, with about

1% of detrital phosphorus being lost to the deep sediment during each oceanic

mixing cycle.

All water bodies receive nutrients, in inorganic and organic form, in the water

draining from the land. It is no surprise, therefore, that human activities are

responsible for dramatic changes in nutrient ﬂuxes both locally (Box 11.3) and

globally. We turn to global biogeochemical cycles in the next section.

Chapter 11 The ﬂux of energy and matter through ecosystems

379

11.3 TOPICAL ECONCERNS

11.3 Topical ECOncerns

The excess input of nutrients from sources such

as agricultural runoff and sewage has caused many

‘healthy’ oligotrophic lakes (low nutrients, low plant

productivity with abundant water weeds, and clear

water) to switch to a eutrophic condition where high

nutrient inputs lead to high phytoplankton productiv-

ity (sometimes dominated by toxic bloom-forming

species), making the water turbid, shading out large

plants and, in the worst situations, leading to anoxia

and ﬁsh kills. This process of cultural eutrophication

of lakes has been understood for some time. But

it was only recently that people noticed huge ‘dead

zones’ in the oceans near river outlets, particularly

those draining large catchment areas such as the

Mississippi in North America and the Yangtze in China.

The following extracts are from a news item posted by

Associated Press on March 29, 2004.

Ocean dead zones on the increase

So-called ‘dead zones’, oxygen-starved areas of

the world’s oceans that are devoid of ﬁsh, top the

list of emerging environmental challenges, the

United Nations Environment Program [UNEP]

warned Monday in its global overview.

The new ﬁndings tally nearly 150 dead zones

around the globe . . . The main cause is excess

nitrogen run-off from farm fertilizers, sewage and

industrial pollutants. The nitrogen triggers blooms

of microscopic algae known as phytoplankton.

As the algae die and rot, they consume oxygen,

thereby suffocating everything from clams and

lobsters to oysters and ﬁsh.

‘Human kind is engaged in a gigantic,

global, experiment as a result of inefﬁcient and

often overuse of fertilizers, the discharge of

untreated sewage and the ever rising emissions

from vehicles and factories’, UNEP Executive

Director Klaus Toepfer said in a statement.

‘Unless urgent action is taken to tackle the

sources of the problem, it is likely to escalate

rapidly.’

(All content © MMIV The Associated Press and

may not be published, broadcast, rewritten or redis-

Suggest some ‘urgent actions’ that could be taken to

alleviate the problem.

Nutrient enrichment of aquatic ecosystems: a major problem for

lakes and oceans

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11.6 Global biogeochemical cycles

Nutrients are moved over vast distances by winds in the atmosphere and by

the moving waters of streams and ocean currents. There are no boundaries, either

natural or political. It is appropriate, therefore, to conclude this chapter by

moving to an even larger spatial scale to examine global biogeochemical cycles.

11.6.1 The hydrological cycle

The principal source of water is the oceans; radiant energy makes water evaporate

into the atmosphere, winds distribute it over the surface of the globe and pre-

cipitation brings it down to the Earth’s surface (with a net movement of atmospheric

water from oceans to continents), where it may be stored temporarily in soils, lakes

and iceﬁelds (Figure 11.15). Loss occurs from the land through evaporation and

transpiration or as liquid ﬂow through stream channels and groundwater aquifers,

eventually to return to the sea. The major pools of water occur in the oceans (97.3%

of the total for the biosphere), the ice of polar icecaps and glaciers (2.06%), deep

in the ground water (0.67%) and in rivers and lakes (0.01%) (Berner & Berner,

1987). The proportion that is in transit at any time is very small – water drain-

ing through the soil, ﬂowing along rivers and present as clouds and vapor in the

atmosphere – together this constitutes only about 0.08% of the total. However,

this small percentage plays a crucial role, both by supplying the requirements for

the survival of living organisms and for community productivity and because so

many chemical nutrients are transported with the water as it moves.

The hydrological cycle would proceed whether or not a biota was present.

However, terrestrial vegetation can modify the ﬂuxes that occur. Vegetation can

intercept water at two points on this journey, stopping some from reaching the

ground water and moving it back into the atmosphere, by: (i) catching some

in foliage from which it evaporates; and (ii) preventing some from draining from

the soil water by taking it up via the roots into the plant’s transpiration stream.

We have seen earlier how cutting down the forest in a catchment in Hubbard

Brook (see Section 1.3.3) increased the throughput of water to streams, along

with its load of dissolved and particulate matter. It is small wonder that large-scale

Part III Individuals, Populations, Communities and Ecosystems

380

Runoff 0.037

Evaporation

0.073

Rivers and lakes (0.13)

Ground water (9.5)

Ice (29)

Precipitation

0.110

Vapor transport 0.037

Atmosphere (0.013)

Precipitation

0.386

Evaporation

0.423

Ocean (1370)

Figure 11.15

The hydrological cycle, showing

volumes of water in the ‘reservoirs’

of oceans, ice (polar and glacier),

rivers and lakes, ground water and

atmosphere (units of 10

), and

on the move as precipitation, runoff,

evaporation and vapor transport

(arrows: units of 10

−1

AFTER BERNER & BERNER, 1987

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deforestation around the globe, usually to create new agricultural land, can lead

to loss of topsoil, nutrient impoverishment and increased severity of ﬂooding.

Water is a very valuable commodity, and this is reﬂected in the difﬁcult political

exercise of dealing with competing demands – to divert river water for hydro-

electric power generation or agricultural irrigation as opposed to maintaining the

intrinsic values of an unmanipulated river.

The world’s major abiotic reservoirs for nutrients are illustrated in Figure 11.16.

We now consider these cycles in turn.

Chapter 11 The ﬂux of energy and matter through ecosystems

381

(a) The phosphorus cycle

Sewage

Fertilizers

Fishing

Deforestation

Atmosphere

Human activities Human activities

(b) The nitrogen cycle

Rock

Respiration

Terrestrial

communities

Aquatic

communities

Ocean

sediments

Water

in rivers,

lakes, and

oceans

soil

Atmosphere

Human activities

Rock

Terrestrial

communities

Aquatic

communities

Ocean

sediments

Water

in rivers,

lakes, and

oceans

soil

Atmosphere

Human activities

Rock

Terrestrial

communities

Aquatic

communities

Ocean

sediments

Water

in rivers,

lakes, and

oceans

soil

Atmosphere

Rock

Terrestrial

communities

Aquatic

communities

Ocean

sediments

Water

in rivers,

lakes, and

oceans

soil

Volcanic activity

Sea

spray

SO from

burning of

fossil fuels

dissolves

uptake in

photosynthesis

Combustion

of fossil fuels

Land

clearance

Organic carbon

in runoff

Photosynthesis

and organic uptake

Extraction of

fossil fuels

Combustion

increases NO

Land clearance,

agriculture,

fertilizers

/NH

Figure 11.16

The major global pathways of nutrients between the abiotic ‘reservoirs’ of atmosphere, water (hydrosphere) and rock and sediments (lithosphere),

and the biotic ‘reservoirs’ constituted by terrestrial and aquatic communities. Human activities (maroon arrows) change nutrient ﬂuxes in terrestrial

and aquatic communities by releasing extra nutrients into both atmosphere and water. Cycles are presented for four important nutrient elements:

(a) phosphorus, (b) nitrogen, (c) sulfur and (d) carbon. Insigniﬁcant compartments and ﬂuxes are represented by dashed lines.

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11.6.2 The phosphorus cycle

The principal stocks of phosphorus occur in the waters of soil, rivers, lakes and

oceans and in rocks and ocean sediments. The phosphorus cycle may be described

as a sedimentary cycle because of the general tendency for mineral phosphorus

to be carried from the land inexorably to the oceans where ultimately it becomes

incorporated in the sediments (Figure 11.16a).

A ‘typical’ phosphorus atom, released from the rock by chemical weathering,

may enter and cycle within the terrestrial community for years, decades or centuries

before it is carried via the ground water into a stream. Within a short time of

entering the stream (weeks, months or years), the atom is carried to the ocean.

It then makes, on average, about 100 round trips between surface and deep

waters, each lasting perhaps 1000 years. During each trip, it is taken up by surface-

dwelling organisms, before eventually settling into the deep again. On average,

on its 100th descent (after 10 million years in the ocean) it fails to be released as

soluble phosphorus, but instead enters the bottom sediment in particulate form.

Perhaps 100 million years later, the ocean ﬂoor is lifted up by geological activity

to become dry land. Thus, our phosphorus atom will eventually ﬁnd its way back via

a river to the sea, and to its existence of cycle (biotic uptake and decomposition)

within cycle (ocean mixing) within cycle (continental uplift and erosion).

11.6.3 The nitrogen cycle

The atmospheric phase predominates in the global nitrogen cycle, in which nitrogen

ﬁxation and denitriﬁcation by microbial organisms are of particular importance

(Figure 11.16b). However, nitrogen from certain geological sources may also be

locally signiﬁcant in fueling productivity in terrestrial and freshwater communities

(Holloway et al., 1998, Thompson et al., 2001). The magnitude of the ﬂux in

streamﬂow from terrestrial to aquatic communities is relatively small, but it is by

no means insigniﬁcant for the aquatic systems involved. This is because nitrogen

is one of the two elements (along with phosphorus) that most often limit plant

growth. Finally, there is a small annual loss of nitrogen to ocean sediments.

11.6.4 The sulfur cycle

Three natural biogeochemical processes release sulfur to the atmosphere: the

formation of seaspray aerosols, anaerobic respiration by sulfate-reducing bacteria

and volcanic activity (relatively minor) (Figure 11.16c). Sulfur bacteria release

reduced sulfur compounds, particularly H

S, from waterlogged bog and marsh

communities and from tidal mudﬂats. A reverse ﬂow from the atmosphere

involves oxidation of sulfur compounds to sulfate, which returns to earth as both

wetfall and dryfall.

The weathering of rocks provides about half the sulfur draining off the land into

rivers and lakes, the remainder coming from atmospheric sources. On its way to

the ocean, a proportion of the available sulfur (mainly dissolved sulfate) is taken

up by plants, passed along food chains and, via decomposition processes, becomes

available again to plants. However, in comparison to phosphorus and nitrogen,

a much smaller fraction of sulfur takes part in internal recycling in terrestrial and

aquatic communities. Finally, there is a continuous loss of sulfur to ocean sediments.

Part III Individuals, Populations, Communities and Ecosystems

382

the life history of a

phosphorus atom

the nitrogen cycle has an

atmospheric phase of

overwhelming importance

the sulfur cycle has an

atmospheric phase and a

lithospheric phase of

similar magnitude

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11.6.5 The carbon cycle

Photosynthesis and respiration are the two opposing processes that drive the

global carbon cycle. It is predominantly a gaseous cycle, with carbon dioxide as

the main vehicle of ﬂux between atmosphere, hydrosphere and biota. Historically,

the lithosphere played only a minor role; fossil fuels lay as dormant reservoirs of

carbon until human intervention in recent centuries (Figure 11.16d).

Terrestrial plants use atmospheric carbon dioxide as their carbon source for

photosynthesis, whereas aquatic plants use dissolved carbonates (i.e. carbon from

the hydrosphere). The two subcycles are linked by exchanges of carbon dioxide

between atmosphere and oceans. In addition, carbon ﬁnds its way into inland

waters and oceans as bicarbonate resulting from weathering (carbonation) of

calcium-rich rocks such as limestone and chalk. Respiration by plants, animals

and microorganisms releases the carbon locked in photosynthetic products back

to the atmospheric and hydrospheric carbon compartments.

11.6.6 Human impacts on biogeochemical cycles

It goes almost without saying that human activities contribute signiﬁcant inputs

of nutrients to ecosystems and disrupt local and global biogeochemical cycles.

For example, the amounts of carbon dioxide and oxides of nitrogen and sulfur

in the atmosphere have been increased by the burning of fossil fuels and by car

exhausts, and the concentrations of nitrate and phosphate in stream water have

been raised by agricultural practices and sewage disposal. These changes have

far-reaching consequences, which will be discussed in Chapter 13.

Chapter 11 The ﬂux of energy and matter through ecosystems

383

the opposing forces of

photosynthesis and respiration

drive the global carbon cycle

Patterns in primary productivity

Primary production on land is limited by a variety of

factors – the quality and quantity of solar radiation, the

availability of water, nitrogen and other key nutrients,

and physical conditions, particularly temperature.

Productive aquatic communities occur where, for

one reason or another, nutrient concentrations are

unusually high and the intensity of radiation is not

limiting.

The fate of primary productivity

Secondary productivity by herbivores is approxim-

ately an order of magnitude less that the primary

productivity on which it is based. Energy is lost at

each feeding step because consumption efﬁciencies,

assimilation efﬁciencies and production efﬁciencies

are all less than 100%. The decomposer system

processes much more of a community’s energy and

matter than the live consumer system. The energy

pathways in the live consumer and decomposer

systems are the same, with one critical exception –

feces and dead bodies are lost to the grazer system

(and enter the decomposer system), but feces and

dead bodies from the decomposer system are simply

sent back to the dead organic matter compartment

at its base.

The process of decomposition

Decomposition results in complex, energy-rich

molecules being broken down by their consumers

SUMMARY

Summary

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Part III Individuals, Populations, Communities and Ecosystems

384

(decomposers and detritivores) into carbon dioxide,

water and inorganic nutrients. Ultimately, the incor-

poration of solar energy in photosynthesis, and the

immobilization of inorganic nutrients into biomass, is

balanced by the loss of heat energy and organic nutri-

ents when the organic matter is decomposed. This is

brought about partly by physical processes, but mainly

by decomposers (bacteria and fungi) and detritivores

(animals that feed on dead organic matter).

The ﬂux of matter through ecosystems

Nutrients are gained and lost by communities in a

variety of ways. Weathering of parent bedrock and

soil, by both physical and chemical processes, is the

dominant source of nutrients such as calcium, iron,

magnesium, phosphorus and potassium, which may

then be taken up via the roots of plants. Atmospheric

carbon dioxide and gaseous nitrogen are the prin-

cipal sources of the carbon and nitrogen content of

terrestrial communities while other nutrients from the

atmosphere become available as dryfall or in rain,

snow and fog. Nutrients are lost again through release

to the atmosphere or in the water that feeds into

streams and rivers. Aquatic systems (including the

stream communities themselves, and ultimately the

oceans) gain nutrients from streamﬂow and ground-

water discharge and from the atmosphere by diffusion

across their surfaces.

Global biogeochemical cycles

The principal source of water in the hydrological cycle

is the oceans; radiant energy makes water evaporate

into the atmosphere, winds distribute it over the surface

of the globe and precipitation brings it down to the

Earth’s surface. Phosphorus derives mainly from the

weathering of rocks (lithosphere); its cycle may be

described as sedimentary because of the general

tendency for mineral phosphorus to be carried from

the land inexorably to the oceans where ultimately it

becomes incorporated in the sediments. The sulfur

cycle has an atmospheric phase and a lithospheric

phase of similar magnitude. The atmospheric phase is

predominant in both the global carbon and nitrogen

cycles. Photosythesis and respiration are the two

opposing processes that drive the global carbon cycle,

while nitrogen ﬁxation and denitriﬁcation by microbial

organisms are of particular importance in the nitrogen

cycle. Human activities contribute signiﬁcant inputs of

nutrients to ecosystems and disrupt local and global

biogeochemical cycles.

REVIEW QUESTIONS

Review questions

Asterisks indicate challenge questions

1 A large proportion of the open ocean is, in

effect, a marine desert. Why?

2* Describe the general latitudinal trends in net

primary productivity. Suggest reasons why

such a latitudinal trend does not occur in the

oceans.

3* Table 11.2 presents the results of a study that

contrasted the productivity of a deciduous

beech forest (Fagus sylvatica) with that of a

nearby evergreen spruce forest (Picea abies).

The beech leaves photosynthesized at a greater

rate (per gram dry weight) than those of spruce,

and beech ‘invested’ a considerably greater

amount of biomass in its leaves each year.

But net primary productivity of the beech forest

was lower than spruce forest. Why? If these

species were grown together, which would

you expect to come to dominate the forest?

What factors other than productivity might

inﬂuence the relative competitive status of the

two species?

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Chapter 11 The ﬂux of energy and matter through ecosystems

385

4 What evidence suggests that the productivity

of many terrestrial and aquatic communities is

limited by nutrients?

5* In both aquatic and terrestrial communities,

secondary productivity by herbivores is

approximately one-tenth of the primary

productivity upon which it is based. This has

led some to suggest the operation of a 10%

law. Do you subscribe to this view?

6 Account for the observation that in most

communities much more energy is processed

through the decomposer system than through

the live consumer system.

7 Outline the role played by bacteria and fungi

(decomposers) in the ﬂux of energy and matter

through a named ecosystem. Imagine what

would happen if bacteria and fungi were

magically removed – describe the resulting

scenario.

8 Energy cannot be cycled and reused but

matter can. Discuss this assertion and its

signiﬁcance for ecosystem functioning.

9 Is the ocean simply a large lake in terms of

patterns of ﬂux of energy and matter?

10 The hydrological cycle would proceed whether

or not a biota was present. Discuss how the

presence of vegetation modiﬁes the ﬂow of

water through an ecosystem.

Table 11.2

Characteristics of representative trees of two contrasting

species growing within 1 km of each other on the Solling

Plateau, Germany.

NORWAY

BEECH SPRUCE

Age (years) 100 89

Height (m) 27 25.6

Leaf shape Broad Needle

Annual production of leaves Higher Lower

Photosynthetic capacity per Higher Lower

unit dry weight of leaf

Length of growing season 176 260

(days)

Net primary productivity 8.6 14.9

(metric tons of carbon

per hectare per year)

AFTER SCHULZE, 1970; SCHULZE ET AL., 1977A, 1977B

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PART FOUR

Applied Issues in Ecology

12 | Sustainability 389

13 | Habitat degradation 423

14 | Conservation 455

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