Eco-Energetics and Nutrient cycling P.1
Eco_Energy Flow and Nutrient Cycling in the Ecosystem
T
he whole ecosystem can be regarded as one machine which keep recycling n________ by an
input of e______ from the Sun. Nutrients are derived originally from the abiotic components
of the ecosystem, to which they eventually return, either as waste products or as dead bodies.
Both living and non-living components are
involved in the constant recycling of
nutrients, so the cycles are called
b__g__c_________ cycles.
The e_____ to drive these cycles is supplied
ultimately by the Sun. Energy transfer
starts with photo______ organisms, they
utilise the Sun's energy directly and pass it on
to other organisms.
The net result is a f____ of energy and a c_______ of nutrients in the ecosystem.
I)
Energy Flow
Nutritional groups
Producers
Autotrophic organisms (green plants, algae and phytoplankton) responsible for
synthesis of or_____ food.
Consumers
Heterotrophic organisms (mainly animals) which con_____ other organisms or
their products.
Decomposers Saprophytic organisms (mainly bacteria and fungi) responsible for b______ down
waste and dead organic matter into simple in______ compounds which can be used
by the green plants again.
Food chains
The transfer of food en______ from plants through a series of organisms in a feeding relationship.
eaten by
i.e. producer
primary consumer
secondary consumer
tertiary consumer.
Food web
Food chains in an ecosystem are not isolated, but are interconnected with one another, such a number
of interconnected f____ c_____ is known as a food web.
Trophic levels
Organisms in a food chain occupy different trophic levels, which indicate their place in the energy
f____ through the community. Organisms whose food is obtained from plants by the same number
of steps in the food chain belongs to the same trophic level.
First trophic level
Second trophic level
Third trophic level
Fourth trophic level
Producers
Primary Consumers
Secondary Consumers
Tertiary Consumers
Green plants
Herbivores
Carnivores
Secondary Carnivores
Eco-Energetics and Nutrient cycling P.2
Ecological pyramids
The feeding relationships between organisms at different trophic level within a community.
a)
Pyramids of number
The progressive l___ of energy at each trophic level of a food chain results in a natural decrease in
the total number of living organisms that exist at each suc________ level in the chain. The number
of organism in each level is usually smaller than that of the one they are feeding on, and thus can be
expressed in the form of a pyramid, called the pyramid of numbers.
Boy 1
Cows 5
Grasses
2 x 107
The pyramid of number has limited value because it contains no information about the s___ or m___
of the organisms in each trophic level. An in______ pyramid can appear if :
1) the producer is large such as a tree, it can support life of hundred of insects.
Insects 200
Oak tree 1
2) it is a parasitic food chain.
Hyperparasites
Parasites
Host
b)
Pyramid of Biomass
The difficulty in the pyramid is partially overcome by using the pyramid of biomass, which show the
total d___ weight (biomass) of living materials at each trophic level at any one time.
a boy 30 Kg
Beef
1000 Kg
Grasses
80000 Kg
However, inverted pyramid of
biomass do occurs in open water
ecosystem where the phytoplankton
with smaller biomass can support the
zooplanktons with larger biomass.
This can be explained as follows:

The pyramid of biomass only
show the st_____ biomass at
certain time of the year. For
such s___-living minute
organisms as the planktons, their
populations do flu_____
according to se_____ factors,
and the inverted pyramids is only
a tem______ phenomenon.

The biomass does not indicate
the prod________(rate of
production) of the organisms.
The phytoplanktons because of
their much higher rate of
rep_______, can support a
larger biomass of zooplanktons.
Eco-Energetics and Nutrient cycling P.3
c)
Pyramid of Energy
At each transfer of energy from one trophic level to the next, there is always a l___ of energy.
pyramid of energy is always of the ‘correct’ shape.
So a
a pyramid of energy shows the total amount of chemical e______ utilized by the organisms in
different trophic level in a given a____ (square metre) , over a given period of t____ (productivity).
Human tissue 2x104J
Beef 5x106J
Grasses
6.3x107J
Solar energy received
2.6x1011J
Food Chains and Energy Lost
There are two kinds of food chains in nature:
1. the grazing food chain --food chain based on living plants
2. the detritus food chain.--food chain based on non-living particulate organic material (detritus).
Energy may be lost
from the grazing food
chain by :
H____ loss in respiration
and other metabolic
activities.

Ex________ waste
and indig_______
materials, etc.

D______
On average about ___% of the energy is passed from one trophic level to the next.-- The ‘10% law’.
Because of this loss of energy, the number of steps in a food chain is limited, usually to _____.
Some of the dead remains are
consumed by the detritus
feeders, that is, pass along
the de_____ food chain, the
remaining are broken down
by the decom_______, while
some even end up as fo___
fuels.
Eventually all chemical
energy will be dissipated as
h___ and lost to the
atmosphere.
Thus there is a fl__ of energy
through the ecosystem but
NOT a recycling of energy.
The continual in___ of
energy from the s___ is
essential for maintaining the
ecosystem of the Earth.
Eco-Energetics and Nutrient cycling P.4
Ecological Energetics
Biological productivity is the rate at which biomass is produced by an ecosystem. it has two
components:


Primary productivity, the production of new organic matter by green plants (autotrophs), and
Secondary productivity, the production of new organic matter by consumers (heterotrophs).
Both of these can be divided into


a)
gross productivity (the total amount of organic matter produced) and
net productivity (the organic matter of the organism after respiration and metabolism have been fuelled).
Primary productivity of Natural Ecosystems
a. Name 4 factors on which the primary
productivity of a natural ecosystem
depends.
b. A large part of the Earth's surface is of
very low primary productivity, mostly
because of the lack of water and nutrients
to support plant growth (deserts, for
example). Productivity of the oceans is
even lower than that of the land. Suggest 3
reasons why this is so ?
c.
Nitrogen is also scarce near the surface waters of the open ocean and
deep lakes because most of it is in bottom sediments. What does this
explain about the productivity of open ocean compared to estuaries ?
Eco-Energetics and Nutrient cycling P.5
b)
Secondary productivity
Secondary productivity is the accumulation of
organic matter in the con______ in the food
chain.
It is illustrated for the bullock in the figure .
More than half the food eaten passes straight
through. Most of the food assimilated is used
in res______ /metabolism; only about 11 per
cent is incorporated into new tissue.
The amount of energy passed on at each step in
the food chain is about 10 per cent, so the
amount of energy available at each step
de_______ exponentially.
The implication for us is that where food is in
short supply, many more veg_______ can feed
than those who must have meat.
Q. The figure shows the energy flow via a small part of a grassland ecosystem. The figures are given in kJm -2 yr-1.
a. What is the gross primary production of grasses and herbs?
b. What is the photosynthetic efficiency (that is the efficiency of conversion of incident solar energy to gross primary
production)?
c. What is the net production of the seed-eating birds, spiders and common green grasshoppers respectively?
d. How much energy is lost via respiration and faeces by field mice?
e. Which of the organisms are producers, primary consumers and secondary consumers?
f.
Which of the organisms are heterotrophic?
g. What are the 'other pathways' likely to be? (Name three.)
Eco-Energetics and Nutrient cycling P.6
Food chains and Human Food production
a. Compare the assimilation efficiency of the carnivores and the herbivores.
b. Try to explain the differences.
c. Compare the respiration-to production ratios of the poikilothermic and homeothermic animals. Why are they so
different ?
Generalizations about Human Food Chains:
a) For animals of similar weight, homoiotherms are less efficient than poikilotherms. e.g. rearing fish or
invertebrate may be ecologically more efficient than rearing pigs and cattle.
b) And long food chains are less efficient than short ones. e.g. cutivation of protein-rich crops such as soy
bean plants is a more efficient way to obtain proteins than animal husbandry.
Q.
Give 3 justifications for obtaining proteins from meat rather than from plants.
c) Ecological efficiency is higher in domestic animals that in the wild. Animal husbandry sometime
approaches 20% efficiency in energy conversion. This is done by both environmental and genetic
means. Energy assimilated by an animal can be seen as rationed in three portions : maintainence, work
and growth. By reducing the energy spend in maintainence and workdone, it is possible to channel more
assimilated energy towards growth (fattening):
Q.
Suggest 4 ways by which the higher net secondary productivity is achieved :
d) Energy subsidy in agricultural crops.
Agriculture is based upon the primary productivity of various crops, mostly cultivated grasses. Crop yields
are improved by irrigation, removal of pests and the application of fer_____. Improvements to biological
productivity in 'industrialised' agriculture make it about ten times more prod____ than subsistence
agriculture. Yet this is sustained at the expense of fossil fuels used to build and drive farm mac____,
produce pes_____ and fer_____, pu___ water and breed high-yi____ varieties. There is a huge en____
subsidy to indu______ agriculture that may be more apparent when fossil fuel becomes scarce.
The apparent efficiency of modern agriculture is thus partly illusory. It is because it depends on an
enormous input of energy in terms of f____ fuels. We are exchanging fossil fuels for food energy;
people in the industrial nation are 'eating' petroleum!
Eco-Energetics and Nutrient cycling P.7
Rational cropping to feed the human population-- Sustainable yield
Cr______ is the removal of any organism from an ecosystem for food. Whether plant or animal.
Rational cropping is using the ecosystem to produce food in an efficient and sus_______ways.
Increasing productivity of plant crops may be achieved by adding fer_____ to the soil, and by
adding w____ to the soil by irrigation. Disadvantages of carrying out these processes are that, over a
long period of time, the use of man-made fertilisers can lead to deterioration of the s___ structure,
which will eventually lead to a decrease in prod_______, and addition of fertilisers or water is
often energy-exp______.
Decreasing the effects of pred_____, generally insects and birds in the case of plant crops, and
disease is usually carried out by the selection of res_____ genetic strains of the crop or the use of
pes______. Using such chemicals must be done with care; persistent chemicals, those not rapidly
broken down in the ecosystem, may have disastrous effects on other trophic levels, and effective
pesticides used too frequently may bring about outbreaks of resistant strains of the p___, as in the
attempts to control malarial mosquito in Africa with DDT.
The use of C4 plants, such as sugarcane and maize, rather than C3 plants in conditions of relatively
high light intensity and temperature is an example of using crops better suited to the env________.
C4 plants photosynthesise more efficiently, and therefore have a higher productivity, than C3 plants.
A further example is the possible use of wild ungulates rather than domestic livestock in animal
farming in East Africa. Q. Why?
An important concept in the management of animal 'crops' which take several years to reach sexual
maturity is that of Maximum sustainable yield. This is the rate at which individuals can be
cropped from the population without
affecting the f______ prod_______ of
the population.
It requires an understanding of the
dynamics of the pop_______,
particularly b____ rate as the higher this
rate the higher the rate of cropping
possible, and also of the population
st________.
This is important as it shows which
a___-groups have the greatest value in
producing y_____ for future
generations so that the other age-groups
can be selected for cropping.
Q. In Britain there is a legal minimum mesh size for fishing nets in commercial fishing. Suggest how this measure help to
ensure a sustainable yield of fish catch in the future
this way only the larger and
Eco-Energetics and Nutrient cycling P.8
II)
Mineral / Nutrients cycles (Biogeochemical cycles)
Living organisms require both organic and
inorganic nut_____. These are obtained from
other organisms or from the abiotic
environment.
When an organism dies, the nutrients contained
within its body are made available to other
organisms, by the action of de________.
An organism may be killed and eaten, thus
transferring nutrients to a higher t______
level.
The figure summarises the dependence of
organisms on each other, and on the abiotic
environment, for nutrients.
A)
The Carbon Cycle
1) Name a) a biological process, b) a human activity and c) a non-biological process, causing the release of carbon dioxide
to the atmosphere
.2) a) What compound does carbon dioxide readily form in water?
b) What organisms are responsible for carbon fixation in the oceans?
c) Under what conditions may decomposition be very slow, such as at the bottom of a stagnant pond?
Eco-Energetics and Nutrient cycling P.9
3) Can you suggest how some human interventions have caused disruption to the Carbon cycle:
B)
The Nitrogen Cycle
The atmosphere contains 79% by volume of nitrogen, yet nitrogen is relatively scarce in combined
(fixed) form because it is rather inert chemically. Nitrogen is an essential constituent of a____ acids,
and hence proteins, and it l
the supply of food available in ecosystems more than any other plant
nutrient.
The only way in which it can be made available to living organisms is via nitrogen f_______, an
ability mostly confined to certain prok______, although the techniques of genetic engineering might
eventually lead to introduction of the relevant genes into green plants.
a)
Nitrogen Fixation
In Biological N-fixation, nitrogen is first incorporated into amm_______ (NOT nitrate), this is then
used to make nitrogen containing organic compounds, e.g. nu_____ acids and p______.
Nitrogen fixation is e______-consuming because the nitrogen atoms of the nitrogen molecule must
first separated. Nitrogen-fixers achieve this by an e_______ (nitrogenase), using energy from
r________ (ATP). Non-enzymatic separation requires the much greater energy of industrial
processes or of ionising events in the atmosphere, such as l________ and cosmic radiation.
Nitrogen is so important for soil fert_____, and the demand for food production so great, that colossal
amounts of ammonia are produced industrially each year to be used mainly for nitrogen f_________
such as ammonium n_____ NH4NO3 and u____ CO(NH2)2.
Eco-Energetics and Nutrient cycling P.10
The amounts of nitrogen fixed commercially are
now roughly equal to the amounts fixed naturally.
A relatively small amount of fixed nitrogen
(5-10%) is formed by ionising events in the
atmosphere. The resulting nitrogen oxides
dissolve in rain, forming nitrates.
The leg____, such as clover, soybean and pea,
are probably the greatest natural source of fixed
nitrogen. Their roots possess characteristic
swellings called nod___ which are caused by
colonies of n________-fixing bacteria (genus
Rhizobium) living within the cells.
The relationship is mut_______ because the plant
gains fixed nitrogen in the form of amm_______
from the bacteria and, in return, the bacteria gain
car_________, from the plants.
In a given area leg_____ can contribute as much as 100 times more fixed nitrogen than free-living
bacteria and blue-green algae. It is not surprising, therefore, that they are frequently used to add
nitrogen to the s___, especially since they have the added benefit of making good fodder crops.
Q. Farmers often say that legumes are 'hard on the soil', meaning that they place a large demand on soil minerals. Why
should this be so?
Eco-Energetics and Nutrient cycling P.11
b)
Decay and Nitrification
The sequence from proteins to nitrate is a series of ox_________, requiring oxygen and involving a
number of aer____ bacteria.
When an organism dies, organic nitrogen (such as proteins) is decomposed by saprophytic bacteria
and fungi via amino acids to a_______. Animal wastes and excreta are similarly decomposed.
This
is called decay / dec___________.
Chemosynthetic bacteria then oxidise ammonia to n_______, a process called nitrification.
Q. In which of the nutritional categories would you place bacteria and fungi which are decomposers?
c)
Denitrification
Nitrification can be reversed by de_________ bacteria (denitrification) whose activities can therefore
reduce soil f______. They only do this under an_______ conditions, when nitrate is used instead of
oxygen as an oxidising agent (electron acceptor) for the oxidation of organic compounds. Nitrate itself
is reduced. The bacteria are therefore facultative aerobes.
It should not be assumed that their activities on a global scale are detrimental to the biosphere because
it has been estimated that most of the atmospheric nitrogen might now be in the oceans or locked up
in sediments were it not for denitrification.
Q.1
What natural areas or situations might favour denitrification?
Q.2
Why should good drainage and ploughing increase soil fertility?
d)
Q.3
Human intervention in the Nitrogen Cycle
Consider the diagram above, in what ways are human activities interfering with the natural N cycle?
Eco-Energetics and Nutrient cycling P.12
 Urbanization, industrialization, and intensive agriculture have altered certain portions of the natural
human activities interfering with the natural N cycle. In particular, they have greatly increased the production
of biologically reactive nitrogen, primarily through activities such as:



fuel combustion, which adds nitric oxide to the atmosphere
the use of nitrogen fertilizers, which contain ammonia made from nitrogen gas
the cultivation of legumes and other crops, which increases biological nitrogen fixation beyond
natural levels.

Emission of large quantities of nitric oxide into the atmosphere when wood or any fuel is burned. The
nitric oxide then combines with oxygen gas in the atmosphere to form nitrogen dioxide (NO 2) gas, which can
react with water vapor in the atmosphere to form nitric acid (HNO3). This acid is a component of acid
deposition (acid rain), which is damaging trees and killing fish in parts of the world.

Mining mineral deposits of compounds containing nitrate and ammonium ions for use as commercial
inorganic fertilizers.

Depleting nitrate ions and ammonium ions from soil by harvesting nitrogen-rich crops.

Adding excess nitrate ions and ammonium ions to aquatic ecosystems in runoff of animal wastes
from livestock feedlots, runoff of commercial nitrate fertilizers from cropland, and discharge of
untreated and treated municipal sewage. This excess supply of plant nutrients stimulates rapid growth of
algae and other aquatic plants. The breakdown of dead algae by aerobic decomposers depletes the water of
dissolved oxygen gas, killing great numbers of fish.
III)
Movement of energy and nutrients between Ecosystems
Until now, ecosystems have been regarded as closed systems. We have seen that energy enters,
usually as sunlight, and es____ in the form of heat. Because the nutrients are re_____, It might be
expected that the quantity of nutrients in an ecosystem to remain con____. The following examples
show that, in reality, energy and nutrients may move from one eco_______ to another.
A problem of Deforestation
The soils of some types of forest are poor in nutrients, much of these being present in the t____
themselves. The Figure below shows the distribution of potassium. If the trees are removed by
deforestation, there may be insufficient nutrients rem________ to allow the forest to re-establish
itself. In addition, soil er_____ may greatly increase in the absence of trees, causing nut______ to
be transferred from the forest soil to a neighbouring river, and so to the s___, very often with
undesirable effects.
Eco-Energetics and Nutrient cycling P.13
Q.1
State Three possible external source (input) of nutrients to the forest --may not be shown in the figure.
Q.2
State Two ways shown in the figure by which nutrients might be lost from the forest.
Q.3
State One other way not shown in the figure by which nutrient might be lost from the forest.
Q.4
Which is more important in terms of the amount of potassium reserved in it, the biotic component or the abiotic
component of the forest ?
Q.5
Suggest some of the undesirable effects of extra nutrients and sediment load in rivers and other water bodies that
result from deforestation.
END
Eco-Energetics and Nutrient cycling P.14
Additional Exercise on Energetics & Nutrient cycling
I)
Nutrient cycling in lakes and oceans
In many temperate seas
and lakes there is a
characteristic annual
cycle in the abundance
of phytoplankton and
zooplankton in the
upper layers of the
water.
This cycle may partly be
due to the changes in the
thermal stratification
of the water throughout
the year.
In autumn and winter, the upper and lower layers of water are at approximately the same
temperature and considerable mixing occurs. Nutrients are cycled from the decomposers on the sea
bed to the upper layers, where algae are photosynthesizing. In spring and summer, however, the
upper layers of the water warm up. An illuminated aerobic layer of warm water, called the
epilimnion, lies over a dark, anaerobic layer of cold water, known as the hypolimnion. Between
them is a zone of rapid temperature transition, the thermocline.
The algae and cyanobacteria in the phytoplankton need nutrient ions to grow and reproduce. They
need in particular nitrates and phosphates, which they absorb from the water. When they die, their
bodies sink and leave the epilimnion, passing through the hypolimnion on their way to the sea bed.
This represents a loss of nitrates and phosphates from the epilimnion. These nutrients cannot be
replaced from below because in summer there is no exchange between the layers.
a
Why does the warmer water float on top of the cooler water? (1m)
b
What are the favourable factors that account for increase in the numbers of producers during March. (3m)
c
Suggest two reasons for the decline in the producers in May. (4m)
d
Account for the shape of the curve for primary consumers. (6m)
Eco-Energetics and Nutrient cycling P.15
e
Amongst the factors which might alter this idealized picture are : first, some species of algae and
cyanobacteria are move palatable and digestible to carnivores than others, and second, some cyanobacteria
fix nitrogen gas from the atmosphere. Speculate on the influence of these factors on the relative abundance of
the phytoplanton population at different times of the year. (5m)
II)
Nutrient cycling in the Amazonian tropical rain forest
Tropical rain forests occupy about ten per cent of the land area on Earth. These humid jungles cover
940 million hectares, but they are being destroyed at the rate of over ten million hectares a year. The
largest area of remaining forest (484 x 106 ha) is Amazonian rain forest, most of which is in Brazil.
In 1970, the Brazilian government announced plans to integrate the Amazonian region with the rest of
the country. The 3300km Transamazon highway was bulldozed through by 1975. By 1984 however,
only 1800 families had settled. The aim of unlocking the agricultural, mineral, and timber
resources of the area had only been partially successful.
The basic reason for the failure of the Brazilians to transplant agriculture to the Amazonian basin is
soil infertility. The rain forest is simply 'a desert covered by trees'. Over 75 per cent of the soils are
deep, well-drained, very acid (down to pH 1.6), and very poor in essential plant nutrients. The main
constraints which are likely to limit crop growth are phosphorus deficiency (90 per cent of the
area), aluminium toxicity (73 per cent), low potassium reserves (56 per cent), and poor drainage
and flooding (24 per cent). Only 6 per cent of the Amazonian basin has soils, mainly of volcanic
origin, which can support productive agriculture. Nevertheless, they cover over 32 million hectares !
Why are the Amazonian basin soils so poor in nutrients? A clue comes from an examination of the
nutrient contents of trees and soil from temperate zone and tropical forest ecosystems.
Nutrient elements (kg ha-1):
Temperate deciduous ash and oak
(Belgium)
(Biomass 380 tonnes ha-1)
Soil :
Vegetation:
Tropical deciduous
(Ghana)
(Biomass 333 tonnes ha-1)
Nitrogen
Phosphorus
Potassium
14000
1260
2200
95
767
624
Nitrogen
Phosphorus
Potassium
Soil :
4587
13
649
Vegetation:
1794
124
808
A Table to show the mass of certain elements in the soil and vegetation in two representative deciduous forests,
one temperate and the other tropical.
Eco-Energetics and Nutrient cycling P.16
a
Calculate for each element at each site the ratio of its mass in the soil to its mass in the vegetation.(3m)
Temperate forest
Tropical forest
Nitrogen
11
2.5
Phosphorus
23
0.1
Potassium
1.23
0.8
b Is the felling of the temperate or the tropical forest move likely to produce soil more suitable for the growth of
plants ? Which of the three elements in the tropical soil is most likely to be deficient ? (2m)
Why do the plants in a tropical forest contain such a high proportion of certain nutrient elements
in the ecosystem? Some experimental evidence comes from plots in the Amazonian rain forest,
Radioactive 45Ca and 32P Were added both in leaves and twigs placed on the forest floor and as a
direct chemical spray, to simulate 'decomposed' litter, on the plots. In nine of the ten study plots, less
than 0.1 percent of the labelled nutrient leached past the root mat in six months. All leaching ceased
after one or two years. Analysis of root samples showed that the isotopes had indeed been taken up and
translocated by living roots.
The complex root mat therefore enables the forest to survive even on a poor soil. Many species also
produce 'canopy roots' from their branches and twigs. These adventitious roots take up nutrient ions
from the decomposing remains of epiphytic plants on their trunks and branches, and may also
trap nutrients from precipitation. Add this to the fact that the litter decomposition is probably five
times faster in the tropics than in temperate regions, and it does not seem surprising that the residence
time for nutrient ions in the soil is much shorter in tropical than in temperate forests.
c
If leaf litter decomposes five times move rapidly in tropical than in temperate forests, when leaves fall one
might expect five times the nutrient ions to be suddenly released in tropical soils than in temperate soils. Why,
then, is the nutrient concentration in tropical forest soils often lower than in temperate soils? (6m)
End
Eco-Energetics and Nutrient cycling P.17
Suggested Answers to Additional Ex. on Energetics & Nutrient cycling
I)
Nutrient cycling in lakes and oceans
A) Principle
In the upper layers of temperate oceans and lakes, the productivity of the phytoplankton may be limited by light
intensity, temperature and lack of nutrients such as nitrates and phosphates.
B) Answers
a. It is less dense.
b
Increased temperature, increased light intensity, abundant nutrients.
c
Overgrazing by herbivores. Competition for nutrient ions - the nutrient ions such as nitrates and
phosphates are lost from the epilimnion and cannot be replaced because of thermal stratification.
d
The curve rises in spring as their food supply, the primary producers, increases. It declines in summer
as primary producers decline.
The zooplankton are also eaten by predators in abundance.
Primary consumer numbers stabilize in August and September as autumn storms overturn the
thermocline and primary producers, freed from nutrient limitation, increase in numbers again.
Primary consumers then decrease as their food supply becomes scarce in winter.
e The palatable and digestible cells in the phytoplankton will be eaten preferentially by the primary consumers.
The less palatable species, such as the diatoms, will be avoided. The autumn peak in primary producers
contains a much higher proportion of the cells of unpalatable species than the spring peak.
The cyanobacteria fix nitrogen gas as well as photosynthesizing. Their reproduction is not limited by lack
of nitrate like the other phytoplankton. Cyanobacteria comprises a higher proportion of the cells in the
phytoplankton in autumn than spring.
II)
Nutrient cycling in the Amazonian tropical rain forest
A) Principle
Nutrient losses when tropical rain forests are felled are more serious than in their temperate counterparts
because in tropical forests more of the nutrients in the ecosystem are in the vegetation.
B)
a.
Answers
Temperate forest
Tropical forest
Nitrogen
11
2.5
Phosphorus
23
0.1
Potassium
1.23
0.8
b. Temperate forest soil would be more fertile, at least on the basis of its nitrogen and phosphorus content.
Phosphorus is the element which, in the tropical soil, is most likely to be deficient.
c
In tropical forests the roots and the mycorrhizal fungi exploit a greater proportion of the soil volume than
in temperate forests. They will intercept most of the nutrients which are released.
Uptake of nutrients by roots exposed to nutrients is also more rapid in tropical than in temperate soils
because at the higher temperatures roots grow faster. More energy from cellular respiration is also
available for ion uptake by cells.
In the tropics there is no cold season. In temperate zones, however, leaf fall often occurs just before the cold
season, in which the rates of decomposition, nutrient release, root growth, and nutrient uptake are very
slow.
END