AQA(B) A2 Module 5:





Energy and Matter

Food Chains and Webs

Energy Flow Through Ecosystems

Material Cycles in Ecosystems

Population Ecology

Population Growth

Factors Affecting Population Size

The Ecological Niche


Impact of Farming






These notes may be used freely by A level biology students and teachers, and they may be copied and edited. I would be interested to hear of any comments and corrections.

Neil C Millar ( 10/6/01

Module 5 Specification


A population is all the organisms of one species in a habitat. Populations of different species form communities. These communities are found in a particular habitat and are based on dynamic feeding relationships. The relationship of pyramids of number, biomass and energy to their corresponding food chains and webs

Energy Flow through Ecosystems

Photosynthesis is the major route by which energy enters an ecosystem. Energy is transferred through the trophic levels in food chains and food webs and is dissipated. Quantitative consideration of the efficiency of energy transfer between trophic levels.

Material Cycles in Ecosystems

Complex organic molecules are broken down in an ecosystem by microorganisms. Carbon dioxide and inorganic ions are thus made available for re-use.

  • The role of microorganisms in the carbon cycle
  • The role of microorganisms in the nitrogen cycle in sufficient detail to illustrate the processes of saprophytic nutrition, deamination, nitrification, nitrogen fixation and denitrification. (Names of individual species not required.)

Population Ecology

An ecosystem supports a certain size of population of any one species. This population size may vary as a result of

  • the effect of abiotic factors
  • interactions between organisms
  • inter-and intra-specific competition
  • predation.

Ecological Niche

Within a habitat a species occupies a niche governed by adaptation to food and/or prevailing abiotic forces.


In natural and suitable conditions land will gradually become colonised by a range of herbaceous plants, then by shrubs and finally by trees as a climax community. There is change in the communities with time, because of the interaction between species and their environment. At each stage certain species can be recognised which change the environment so that it becomes more suitable for other species. Candidates should be able to describe one example of succession.

Ecological Impact of Farming

There is a balance of food production and conservation.

  • The impact of monoculture and the removal of hedgerows on the environment.
  • The effects of organic effluent, nitrates and phosphates on aquatic ecosystems, including eutrophication and effects on biochemical oxygen demand.
  • Biodegradable and non-biodegradable pesticides. The bioaccumulation of pesticides in food webs.
  • Farms may be managed in ways that help to ensure sustainability and reduce the impact on wildlife, such as the use of organic fertilisers, prevention of erosion, control of pesticide use and maintenance of habitat variety.

Evaluate evidence and make balanced judgements between the need to meet the demands for increased food production by agriculture and the need to conserve the environment.

Practical Ecology

Studied an ecosystem in the field and be familiar with the uses, roles and limitations of

  • frame quadrats
  • line transects
  • measurement of abiotic factors such as pH, light and temperature.

Candidates should understand the principles involved in the use of standard deviation and the chi-squared test in reporting the results of ecological studies.


Ecology is the study of living organisms and their environment. Its aim it to explain why organisms live where they do. To do this ecologists study ecosystems, areas that can vary in size from a pond to the whole planet.


A reasonably self-contained area together with all its living organisms.


The physical or abiotic part of an ecosystem, i.e. a defined area with specific characteristics where the organisms live, e.g. oak forest, deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc.


The living or biotic part of an ecosystem, i.e. all the populations of all the different species living in one habitat.


Any living or biological factor.


Any non-living or physical factor.


The members of the same species living in one habitat.


A group of organisms that can interbreed and produce fertile offspring.

Energy and Matter

Before studying ecosystems, it is important to appreciate the difference between energy and matter. Energy and matter are quite different things and cannot be inter-converted.

All living organisms need energy and matter from their environment. Matter is needed to make new cells (growth) and to create now organisms (reproduction), while energy is needed to drive all the chemical and physical processes of life, such as biosynthesis, active transport and movement.

Food Chains and Webs

The many relationships between the members of a community in an ecosystem can be described by food chains and webs. Each stage in a food chain is called a trophic level, and the arrows represent the flow of energy and matter through the food chain. Food chains always start with photosynthetic producers (plants, algae, plankton and photosynthetic bacteria) because, uniquely, producers are able to extract both energy and matter from the abiotic environment (energy from the sun, and 98% of their matter from carbon dioxide in the air, with the remaining 2% from water and minerals in soil). All other living organisms get both their energy and matter by eating other organisms.

Although this represents a "typical" food chain, with producers being eaten by animal consumers, different organisms use a large range of feeding strategies (other than consuming), leading to a range of different types of food chain. Some of these strategies are defined below, together with other terms associated with food chains.


An organism that produces food from carbon dioxide and water using photosynthesis. Can be plant, algae, plankton or bacteria.


An animal that eats other organisms


A consumer that eats plants (= primary consumer).


A consumer that eats other animals (= secondary consumer).

Top carnivore

A consumer at the top of a food chain with no predators.


A consumer that eats plants or animals.


A human that chooses not to eat animals (humans are omnivores)


An organism that manufactures its own food (= producer)


An organism that manufactures its own food using light energy


An organism that manufactures its own food using energy derived from chemical reactions (e.g. sulphur reducing bacteria)


An organism that obtains its energy and mass from other organisms

(=consumers + decomposers)


Microscopic aquatic organisms.


"Plant plankton" i.e. microscopic aquatic producers.


"Animal plankton" i.e. microscopic aqautic consumers.


An animal that hunts and kills animals for food.


An animal that is hunted and killed for food.


An animal that eats dead animals, but doesn't kill them


Dead and waste matter that is not eaten by consumers


An organism that consumes detritus (= detrivores + saprophytes)


An animal that eats detritus.


A microbe (bacterium or fungus) that lives on detritus.


Organisms living together in a close relationship (= parasitism, mutualism, pathogen).


Two organisms living together for mutual benefit.


Relationship in which only one organism benefits


An organism that feeds on a larger living host organism, harming it


A microbe that causes a disease.

So food chains need not end with a consumer, and need not even start with a producer, e.g.:

Ecological Pyramids

In general as you go up a food chain the size of the individuals increases and the number of individuals decreases. These sorts of observations can be displayed in ecological pyramids, which are used to quantify food chains. There are three kinds:

1. Pyramids of Numbers.

These show the numbers of organisms at each trophic level in a food chain. The width of the bars represent the numbers, or the bars may be purely qualitative. The numbers should be normalised for a given area for a terrestrial habitat (usually m²), or volume for an aquatic habitat (m³). Pyramids of numbers are most often triangular shaped, but can be almost any shape. In the pyramids below, A shows a typical pyramid of numbers for carnivores; B shows the effect of a single large producer such as a tree; and C shows a typical parasite food chain.

2. Pyramids of Biomass

These convey more information, since they consider the total mass of living organisms (i.e. the biomass) at each trophic level. The biomass should be dry mass (since water stores no energy) and is measured in kg m-2. The biomass may be found by drying and weighing the organisms at each trophic level, or by counting them and multiplying by an average individual mass. Pyramids of biomass are always pyramid shaped, since if a trophic level gains all its mass from the level below, then it cannot have more mass than that level (you cannot weigh more than you eat). The "missing" mass, which is not eaten by consumers, becomes detritus and is decomposed.

3. Pyramids of Energy

Food chains represent flows of matter and energy, so two different pyramids are needed to quantify each flow. Pyramids of energy show how much energy flows into each trophic level in a given time, so the units are usually something like kJ m-2 y-1. Pyramids of energy are always pyramidal (energy cannot be created), and always very shallow, since the transfer of energy from one trophic level to the next is very inefficient The "missing" energy, which is not passed on to the next level, is lost eventually as heat.




Energy Flow in Ecosystems

Three things can happen to the energy taken in by the organisms in a trophic level:

These three fates are shown in this energy flow diagram:

Eventually all the energy that enters the ecosystem will be converted to heat, which is lost to space.

Material Cycles in Ecosystems

Matter cycles between the biotic environment and in the abiotic environment. Simple inorganic molecules (such as CO2, N2 and H2O) are assimilated (or fixed) from the abiotic environment by producers and microbes, and built into complex organic molecules (such as carbohydrates, proteins and lipids). These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers. Without either producers or decomposers there would be no nutrient cycling and no life.

The simple inorganic molecules are often referred to as nutrients. Nutrients can be grouped as: major nutrients (molecules containing the elements C, H and O, comprising >99% of biomass); macronutrients (molecules containing elements such as N, S, P, K, Ca and Mg, comprising 0.5% of biomass); and micronutrients or trace elements (0.1% of biomass). Macronutrients and micronutrients are collectively called minerals. While the major nutrients are obviously needed in the largest amounts, the growth of producers is usually limited by the availability of minerals such as nitrate and phosphate.

There are two groups of decomposers:

Detailed material cycles can be constructed for elements such as carbon, nitrogen, oxygen or sulphur, or for compounds such as water, but they all have the same basic pattern as the diagram above. We shall only study the carbon and nitrogen cycles in detail.

The Carbon Cycle

As this diagram shows, there are really many carbon cycles here with time scales ranging from minutes to millions of years. Microbes play the major role at all stages.



The Nitrogen Cycle

Microbes are involved at most stages of the nitrogen cycle:

Nitrogen Fixation. 78% of the atmosphere is nitrogen gas (N2), but this is inert and can’t be used by plants or animals. Nitrogen fixing bacteria reduce nitrogen gas to ammonia (N2 + 6H Õ  2NH3), which dissolves to form ammonium ions (). This process uses the enzyme nitrogenase and ATP as a source of energy. The nitrogen-fixing bacteria may be free-living in soil or water, or they may live in colonies inside the cells of root nodules of leguminous plants such as clover or peas. This is an example of mutualism as the plants gain a source of useful nitrogen from the bacteria, while the bacteria gain carbohydrates and protection from the plants. Nitrogen gas can also be fixed to ammonia by humans using the Haber process, and a small amount of nitrogen is fixed to nitrate by lightning.

Nitrification. Nitrifying bacteria can oxidise ammonia to nitrate in two stages: first forming nitrite ions then forming nitrate ions . These are chemosynthetic bacteria, which means they use the energy released by nitrification to live, instead of using respiration. Plants can only take up nitrogen in the form of nitrate.

Denitrification. The anaerobic denitrifying bacteria convert nitrate to N2 and NOx, which is then lost to the air. This represents a constant loss of "useful" nitrogen from soil, and explains why nitrogen fixation by the nitrifying bacteria and fertilisers are so important.

Ammonification. Microbial saprophytes break down proteins in detritus to form ammonia in two stages: first they digest proteins to amino acids using extracellular protease enzymes, then they remove the amino groups from amino acids using deaminase enzymes.

Population Ecology

Population Ecology in concerned with the question: why is a population the size it is? This means understanding the various factors that affect the population.

Population Growth

When a species is introduced into a new environment its population grows in a characteristic way. This growth curve is often seen experimentally, for example bees in a hive, sheep in Tasmania, bacteria in culture. The curve is called a logistic or sigmoid growth curve.

The growth curve has three phases, with different factors being responsible for the shape of each phase. The actual factors depend on the ecosystem, and this can be illustrated by considering two contrasting examples: yeast in a flask (reproducing asexually), and rabbits in a field (reproducing sexually).


Yeast in a flask

Rabbits in a field

1. Lag phase

Little growth while yeast starts transcribing genes and synthesising appropriate enzymes for new conditions.

Little growth due to small population. Individuals may rarely meet, so few matings. Long gestation so few births.

2. Rapid Growth Phase

Rapid exponential growth. No limiting factors since relatively low density.

Rapid growth, though not exponential. Few limiting factors since relatively low density.

3. Stable Phase

Slow growth due to accumulation of toxic waste products (e.g. ethanol) or lack of sugar.

Slow growth due to intraspecific competition for food/territory, predation, etc.

At the end of phase 3 the population is stable. This population is called the carrying capacity of the environment (K), and is the maximum population supported by a particular ecosystem.

Factors Affecting Population Size

Many different factors interact to determine population size, and it can be very difficult to determine which factors are the most important. Factors can be split into two broad group: abiotic factors and biotic factors. We’ll look at 7 different factors.

1. Abiotic Factors

The population is obviously affected by the abiotic environment such as: temperature; water/humidity; pH; light/shade; soil (edaphic factors); mineral supply; current (wind/water); topography (altitude, slope, aspect); catastrophes (floods/fire/frost); pollution. Successful species are generally well adapted to their abiotic environment.

In harsh environments (very cold, very hot, very dry, very acid, etc.) only a few species will have successfully adapted to the conditions so they will not have much competition from other species, but in mild environments lots of different species could live there, so there will be competition. In other words in harsh environments abiotic factors govern who survives, while in mild environments biotic factors (such as competition) govern who survives.

2. Seasons

Many abiotic factors vary with the seasons, and this can cause a periodic oscillation in the population size.

This is only seen in species with a short life cycle compared to the seasons, such as insects. Species with long life cycles (longer than a year) do not change with the seasons like this.




3. Food Supply

A population obviously depends on the population of its food supply: if there is plenty of food the population increases and vice versa. For example red deer introduced to an Alaskan island at first showed a population increase, but this large population grazed the vegetation too quickly for the slow growth to recover, so the food supply dwindled and the deer population crashed.

4. Interspecific Competition

Interspecific competition is competition for resources (such as food, space, water, light, etc.) between members of different species, and in general one species will out-compete another one. This can be demonstrated by growing two different species of the protozoan Paramecium in flasks in a lab. They both grow well in lab flasks when grown separately, but when grown together P.aurelia out-competes P.caudatum for food, so the population of P.caudatum falls due to interspecific competition:

  1. Intraspecific Competition

Intraspecific competition is competition for resources between members of the same species. This is more significant than interspecific competition, since member of the same species have the same niche and so compete for exactly the same resources.

Intraspecific competition tends to have a stabilising influence on population size. If the population gets too big, intraspecific population increases, so the population falls again. If the population gets too small, intraspecific population decreases, so the population increases again:

Intraspecific competition is also the driving force behind natural selection, since the individuals with the "best" genes are more likely to win the competition and pass on their genes. Some species use aggressive behaviour to minimise real competition. Ritual fights, displays, threat postures are used to allow some individuals (the "best") to reproduce and exclude others (the "weakest"). This avoids real fights or shortages, and results in an optimum size for a population.

6. Predation

The populations of predators and their prey depend on each other, so they tend to show cyclical changes. This has been famously measured for populations of lynx (predator) and hare (prey) in Canada, and can also be demonstrated in a lab experiment using two species of mite: Eotetranchus (a herbivore) and Typhlodromus (a predator). If the population of the prey increases, the predator will have more food, so its population will start to increase. This means that more prey will be eaten, so its population will decrease, so causing a cycle in both populations:

7. Parasitism and Disease

Parasites and their hosts have a close symbiotic relationship, so their populations also oscillate. This is demonstrated by winter moth caterpillars (the host species) and wasp larvae (parasites on the caterpillars). If the population of parasite increases, they kill their hosts, so their population decreases. This means there are fewer hosts for the parasite, so their population decreases. This allows the host population to recover, so the parasite population also recovers:

A similar pattern is seen for pathogens and their hosts.


The Ecological Niche

A population’s niche refers to its role in its ecosystem. This usually means its feeding role in the food chain, so a particular population’s niche could be a producer, a predator, a parasite, a leaf-eater, etc. A more detailed description of a niche should really include many different aspects such as its food, its habitat, its reproduction method etc, so gerbils are desert seed-eating mammals; seaweed is an inter-tidal autotroph; fungi are asexual soil-living saprophytes. Identifying the different niches in an ecosystem helps us to understand the interactions between populations. Members of the same population always have the same niche, and will be well-adapted to that niche, e.g. nectar feeding birds have long thin beaks.

Species with narrow niches are called specialists (e.g. anteater). Many different specialists can coexist in the same habitat because they are not competing, so this can lead to high diversity, for example warblers in a coniferous forest feed on insects found at different heights. Specialists rely on a constant supply of their food, so are generally found in abundant, stable habitats such as the tropics.

Species with broad niches are called generalists (e.g. common crow). Generalists in the same habitat will compete, so there can only be a few, so this can lead to low diversity. Generalists can cope with a changing food supply (such as seasonal changes) since they can switch from one food to another or even one habitat to another (for example by migrating).




The niche concept was investigated in some classic experiments in the 1930s by Gause. He used flasks of different species of the protozoan Paramecium, which eats bacteria.

Experiment. 1:

Conclusion: These two species of Paramecium share the same niche, so they compete. P. aurelia is faster-growing, so it out-competes P. caudatum.

Experiment. 2:

Conclusion: These two species of Paramecium have slightly different niches, so they don't compete and can coexist.

It is important to understand the distribution in experiment 2. P. caudatum lives in the upper part of the flask because only it is adapted to that niche and it has no competition. In the lower part of the flask both species could survive, but only P. bursaria is found because it out-competes P. caudatum. If P. caudatum was faster-growing it would be found throughout the flask.

The niche concept is summarised in the competitive exclusion principle: Two species cannot coexist in the same habitat if they have the same niche.


Ecosystems are not fixed, but constantly change with time. This change is called succession. Imagine a lifeless area of bare rock. What will happen to it as time passes?

  1. Very few species can live on bare rock since it stores little water and has few available nutrients. The first colonisers are usually lichens, which are a mutualistic relationship between an alga and a fungus. The alga photosynthesises and makes organic compounds, while the fungus absorbs water and minerals and clings to the rock. Lichens are such good colonisers that almost all "bare rock" is actually covered in a thin layer of lichen. Mosses can grow on top of the lichens. Between then these colonisers start to erode the rock and so form a thin soil. Colonisers are slow growing and tolerant of extreme conditions.
  2. Pioneer species such as grasses and ferns grow in the thin soil and their roots accelerate soil formation. They have a larger photosynthetic area, so they grow faster, so they make more detritus, so they form better soil, which holds more water.
  3. Herbaceous Plants such as dandelion, goosegrass ("weeds") have small wind-dispersed seeds and rapid growth, so they become established before larger plants.
  4. Larger plants (shrubs) such as bramble, gorse, hawthorn, broom and rhododendron can now grow in the good soil. These grow faster and so out-compete the slower-growing pioneers.
  5. Trees grow slowly, but eventually shade and out-compete the shrubs, which are replaced by shade-tolerant forest-floor species. A complex food web is now established with many trophic levels and interactions. This is called the climax community.

These stages are called seral stages, or seral communities, and the whole succession is called a sere. Each organism modifies the environment, so creating opportunities for other species. As the succession proceeds the community becomes more diverse, with more complex food webs being supported. The final seral stage is stable (assuming the environment doesn’t change), so succession stops at the climax stage. In England the natural climax community is oak or beech woodland (depending on the underlying rock), and in the highlands of Scotland it is pine forests. In Roman times the country was covered in oak and beech woodlands with herbivores such as deer, omnivores such as bear and carnivores such as wolves and lynxes. It was said that a squirrel could travel from coast to coast without touching ground.

Humans interfere with succession, and have done so since Neolithic times, so in the UK there are few examples of a natural climax left (except perhaps small areas of the Caledonian pine forest in the Scottish Highlands). Common landscapes today like farmland, grassland, moorland and gardens are all maintained at pre-climax stages by constant human interventions, including ploughing, weeding, herbicides, burning, crop planting and grazing animals. These are examples of an artificial climax, or plagioclimax.


Ecological Impact of Farming

One of the main reasons for studying ecology is to understand the impact humans are having on the planet. The huge increases in human population over the last few hundred years has been possible due to the development of intensive farming, including monoculture, selective breeding, huge farms, mechanisation and the use of chemical fertilisers and pesticides. However, it is apparent that this intensive farming is damaging the environment and is becoming increasingly difficult to sustain. Some farmers are now turning to environmentally-friendly organic farming. We’ll examine 5 of the main issues and their possible solutions.

1. Monoculture

Until the middle of the 20th century, farms were usually small and mixed (i.e. they grew a variety of crops and kept animals). About a third of the population worked on farms. The British countryside was described by one observer in 1943 as "an attractive patchwork with an infinite variety of small odd-shaped fields bounded by twisting hedges, narrow winding lanes and small woodlands". Today the picture is quite different, with large uninterrupted areas of one colour due to specialisation in one crop - monoculture. Monoculture increases the productivity of farmland by growing only the best variety of crop; allowing more than one crop per year; simplifying sowing and harvesting of the crop; and reducing labour costs.

However, monoculture has a major impact on the environment:

Some farmers are now returning to traditional crop rotations, where different crops are grown in a field each year. This breaks the life cycles of pests (since their host is changing); improves soil texture (since different crops have different root structures and methods of cultivation); and can increase soil nitrogen (by planting nitrogen-fixing legumes).


2. Hedgerows

Hedges have been planted since Anglo-Saxon times to mark field boundaries and to contain livestock. As they have matured they have diversified to contain a large number of different plant and animal species, some found nowhere else in the UK. Since the second world war much of the hedgerow has been removed because:

However it has now become clear that hedgerows served an important place in the ecology of Britain.

The importance of hedgerows is now being recognised, and farmers can now receive grants to plant hedgerows. However it takes hundreds of years for new hedgerows to mature and develop the same diversity as the old ones.

3. Fertilisers

Since the rate of plant growth in usually limited by the availability of mineral ions in the soil, then adding more of these ions as fertiliser is a simple way to improve yields, and this is a keystone of intensive farming. The most commonly used fertilisers are the soluble inorganic fertilisers containing nitrate, phosphate and potassium ions (NPK). Inorganic fertilisers are very effective but also have undesirable effects on the environment. Since nitrate and ammonium ions are very soluble, they do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes and causing eutrophication. They are also expensive.

An alternative solution, which does less harm to the environment, is the use of organic fertilisers, such as animal manure (farmyard manure or FYM), composted vegetable matter, crop residues, and sewage sludge. These contain the main elements found in inorganic fertilisers (NPK), but in organic compounds such as urea, cellulose, lipids and organic acids. Of course plants cannot make use of these organic materials in the soil: their roots can only take up inorganic mineral ions such as nitrate, phosphate and potassium. But the organic compounds can be digested by soil organisms such as animals, fungi and bacteria, who then release inorganic ions that the plants can use (refer to the nitrogen cycle). Some advantages of organic fertilisers are:

Some disadvantages are that they are bulky and less concentrated in minerals than inorganic fertilisers, so more needs to be spread on a filed to have a similar effect. They may contain unwanted substances such as weed seeds, fungal spores, heavy metals. They are also very smelly!

4. Pesticides

To farmers, a pest is any organism (animal, plant or microbe) that damages their crops. Some form of pest control has always been needed, whether it is chemical (e.g. pesticides), biological (e.g. predators) or cultural (e.g. weeding or a scarecrow). Chemicals pesticide include:


Pesticides have to be effective against the pest, but have no effect on the crop. They may kill the pests, or just reduce their population by slowing growth or preventing reproduction. Intensive farming depends completely on the use of pesticides, and some wheat crops are treated with 18 different chemicals to combat a variety of weeds, fungi and insects. In addition, by controlling pests that carry human disease, they have saved millions of human lives. However, with their widespread use and success there are problems, the mains ones being persistence and bioaccumulation.

Both of these are illustrated by DDT (DichloroDiphenylTrichloroethane), an insecticide used against the malaria mosquito in the 1950s and 60s very successfully, eradicating malaria from southern Europe. However the population of certain birds fell dramatically while it was being used, and high concentrations of DDT were found in their bodies, affecting calcium metabolism and causing their egg shells to be too thin and fragile. DDT was banned in developed countries in 1970, and the bird populations have fully recovered. Alternative pesticides are now used instead, but they are not as effective, and continued use of DDT may have eradicated malaria in many more places.

Persistence. This refers to how long a pesticide remains active in the environment. Some chemicals are broken down by decomposers in the soil (they’re biodegradable) and so are not persistent, while others cannot be broken down by microbes (they’re non biodegradable) and so continue to act for many years, and are classed as persistent pesticides. The early pesticides (such DTT) were persistent and did a great deal of damage to the environment, and these have now largely been replaced with biodegradable insecticides such as carbamates and pyrethroids.

Bioaccumulation (or Biomagnification). This refers to the built-up of a chemical through a food chain. DDT is not soluble in water and is not excreted easily, so it remains in the fat tissue of animals. As each consumer eats a large mass of the trophic level below it, DTT accumulates in the fat tissue of animals at the top of the food chain. This food chain shows typical concentrations of DDT found in a food chain (in parts per million, ppm):

The high concentration of DDT in birds explains why the toxic effects of DDT were first noticed in birds.

5. Eutrophication

Eutrophication refers to the effects of nutrients on aquatic ecosystems. These naturally progress from being oligotrophic (clean water with few nutrients and algae) to eutrophic (murky water with many nutrients and plants) and sometimes to hypertrophic (a swamp with a mass of plants and detritus). This is in fact a common example of succession. In the context of pollution "eutrophication" has come to mean a sudden and dramatic increase in nutrients due to human activity, which disturbs and eventually destroys the food chain. The main causes are fertilisers leaching off farm fields into the surrounding water course, and sewage (liquid waste from houses and factories). These both contain dissolved minerals, such as nitrates and phosphates, which enrich the water.


Since producer growth is generally limited by availability of minerals, a sudden increase in these causes a sudden increase in producer growth. Algae grow faster than larger plants, so they show a more obvious "bloom", giving rise to spectacular phenomena such as red tides. Algae produce oxygen, so at this point the ecosystem is well oxygenated and fish will thrive.

However, the fast-growing algae will out-compete larger plants for light, causing the plants to die. The algae also grow faster than their consumers, so many will die without being consumed, which is not normal. These both lead to a sudden increase in detritus. Sewage may also contain organic matter, which adds to the detritus.

Decomposing microbes can multiply quickly in response to this, and being aerobic they use up oxygen faster than it can be replaced by photosynthesis or diffusion from the air. The decreased oxygen concentration kills larger aerobic animals and encourages the growth of anaerobic bacteria, who release toxic waste products.

Biochemical Oxygen Demand (BOD). This measures the rate of oxygen consumption by a sample of water, and therefore gives a good indication of eutrophication. A high BOD means lots of organic material and aerobic microbes, i.e. eutrophication. The method is simple: a sample of water is taken and its O2 concentration is measured using an oxygen meter. The sample is then left in the dark for 5 days at 20°C, and the O2 is measured again. The BOD is then calculated from: original O2 concentration – final O2 concentration. The more oxygen used up over the 5 days (in the higher the BOD, and the higher the BOD the more polluted the water is. This table shows some typical BOD values.



clean water

polluted water

cleaned sewage

raw sewage



20 (legal max)


Aquatic ecosystems can slowly recover from a high BOD as oxygen dissolves from the air, but long-term solutions depend on reducing the amount of minerals leaching into the water. This can be achieved by applying inorganic fertilisers more carefully, by using organic fertilisers, by using low-phosphate detergents, and by removing soluble minerals by precipitation in modern sewage plants. As a last resort eutrophic lakes can be dredged to remove mineral-rich sediment, but this is expensive and it takes a long time for the ecosystem to recover. This has been done in the Norfolk Broads.