Biology of Freshwater PollutionPollution can be defined as “The introduction by Man into the environment of substancesor energy harmful to plant or animal life, or which damage amenity or structure”. ThisFactsheet concentrates on the biological effects of common types of freshwater pollution.

1. Organic PollutionOrganic pollution includes sewage waste, silageeffluent (silage is rotted grass used to feed cattlein winter), Paper mill waste and dairy wastes.

Organic pollution consists of organic (carbon-containing) compounds which can be brokendown (oxidised) by microorganisms.Consequently, this type of pollution alwaysresults in an increased demand for oxygen in thewater. One measure of this is given by theBiochemical Oxygen Demand (BOD).

The BOD tells us how much oxygen is needed bythe organisms in the water for organicdecomposition, hence it is an index of waterquality.

The lower the BOD, the less organic pollutionthere is in it.

Fig 1 shows the biological effects of an organiceffluent (eg. sewage) on a river and the changeswhich occur downstream of the pollution incident.

1. Bacterial populations increase because theorganic matter contains and provides foodfor bacteria

2. Bacteria use up oxygen to break downorganic matter therefore creating high BOD

3. Oxygen levels therefore fall rapidly

4. The population of fish and clean waterinvertabrates decreases rapidly because

of declining oxygen levels (bacterialactivity) and due to the release of toxinsfrom the organic effluent.

5. Species such as sludgeworms (genusTubificidae) increase in number becauseorganic matter provides plentiful food anda tunnelling medium.

6. Bacterial population decreases as organicmatter is used up and protozoa begin to feedon the bacteria.

7. Protozoan population increases accordingly.

8. Sewage fungus population increases and thendecreases as organic matter is used up. Largerplants (macrophytes) decline because of thesmothering effect of silt or sewage fungus.

9. Algal population decreases initially becauseof declining light penetration due to sedimentand because oxygen concentration declinesdue to bacterial activity. However, thedecomposition of organic matter releasesnutrients and allows more light to penetrate,therefore algal population increases.

2. Nitrates in Drinking WaterNitrate concentrations in freshwater havedramatically increased since 1945 because ofthe increasing use of nitrogenous fertilisers.

Nitrates are very soluble and therefore leachinglosses can be high. The nitrate may move downthrough the soil horizons into groundwaterreserves, which later become part of the watersupply. Large areas of the UK receive drinkingwater which regularly exceeds the World HealthOrganisation’s (WHO) 50mg per litre limit.

Once ingested, nitrates may be converted tonitrites which can then combine with infanthaemoglobin, reducing normal oxygentransport. This is known as “blue babysyndrome” or methaemoglobinaemia.

High nitrate concentrations may also lead to theproduction of nitrosamines in the body, someof which are known carcinogens (cancer-causing chemicals).

Bio FactsheetJanuary 1998 Number 10

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Figure 1: Biological effects of an organic effluent

Sludgeworms

Oxygen

BOD

Algae

Sewage fungus

Clean water fauna

Distance downstream

BacteriaProtozoa

Effluent

EXAM HINT - This is a very straightforwardtopic which often comes up as a prose (essay-style) question.

2

3

1

7

6

9

8

4 5

3. Acidification of Fresh WaterSince carbon dioxide dissolves in atmosphericmoisture to form carbonic acid, rain is naturallyacidic (pH 5.6). Fossil fuel combustion in powerstations and vehicles releases acidic gases whichintensify and accelerate the process of acidification(Figure 2).

Acid rain is a misnomer. Acid deposition is a moreaccurate term since it includes dry deposition eg.sulphur which falls relatively near to the pollutionsource and wet deposition which can be carriedthousands of miles before being deposited i.e. itis a trans-boundary pollutant.

Acid rain affects fresh water ecosystems bothdirectly and indirectly. Sudden changes in waterpH, as may result from the rapid influx of a snowmelt, may be lethal to invertebrates and fish.More seriously, when the pH of acidified soilsfalls below 4.2, aluminium becomes soluble andmay enter aquatic ecosystems. High aluminiumconcentrations:

1. Adversely affect the ability of fish gills toregulate cations such as sodium. The resultingosmotic imbalance can be fatal.

2. Causes excess mucus production which leadsto clogging of the gills and suffocation.

3. Interferes with calcification of the skeletons offish fry and therefore recruitment (the percentageof young fish which develop into adults) decreasesand the population decreases.

Even gradual acidification will have serious effectson species diversity. A typical progression isshown in Table 2.

EutrophicationEutrophication is the enrichment of fresh water by excess nutrients, usually nitrogen andphosphorus. It is a natural process which humans have greatly accelerated. The nutrient statusof lakes increases naturally as sediment constantly reaches it in streams or through direct soilerosion. Thus an oligotrophic (low nutrient, low productivity) lake will inevitably change intoa eutrophic one. Accelerated eutrophication has occurred as a result of the following:

1. Increased use of phosphate-containing detergents

2. Increased leaching and run-off from agricultural land

3. Drainage or washings from intensive animal units

4. Bank erosion caused by the swash of boats

5. Increased soil erosion eg. as a result of deforestation

Whereas nitrates are very soluble, phosphates are not and so it usually enters the water as a resultof erosion from land. It is, however, a common limiting factor in fresh water and it is usuallythe extra phosphorus which results in the excess growth of plants so characteristic ofeutrophication.

1. With low levels of nutrient input, plant species diversity and abundance may increase.Faunal diversity may also increase because more plants means more food.

2. Microscopic plants (algae) proliferate rapidly causing algal blooms. Although thealgae photosynthesise and therefore release some oxygen into the water, by blanketingthe surface they severely reduce the amount of light which reaches the lower depths andthis reduces the number of larger plants (macrophytes).

3. Zooplankton (microscopic fauna) use macrophytes to escape predation by fish so asmacrophyte numbers decrease more zooplankton are eaten so their numbers decrease.

4. As zooplankton numbers decrease, less algae are eaten so algal numbers increase further.

5. Algae have a high turnover rate (productivity and death rate are both high). Dead algaeare broken down by aerobic bacteria which use up much of the oxygen in the water (highBOD).

6. Declining oxygen levels lead to the death of many aerobes (both plants and animals).Many food chains collapse.

7. Dead algae and zooplankton increase the turbidity of the water. Detritus forms sediment.

Biology of Freshwater Pollution Bio Factsheet

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pH Effect6.0 Crustaceans and molluscs die5.8 Salmon, roach and trout die5.5 Whitefish die5.0 Perch and pike die4.5 Eels die

Table 2: Sensitivities of aquaticorganisms to lowered pH

Figure 2: Formation of Acid Rain

1. NO2 from

vehicles

2. NO, NO2, SO

2 emitted as a

result of combustion processespower stations

3. drydeposition of

acidic particles

5. wetdeposition

4.sulphur dioxide sulphurous + sulphuric

acid acidnitrogen dioxide nitrous + nitric

acid acid

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may be thousands of miles

Acknowledgements;This Factsheet was researched and written byKevin Byrne

Curriculum Press, Unit 305B, The Big Peg,120 Vyse Street, Birmingham. B18 6NF

Bio Factsheets may be copied free of charge byteaching staff or students, provided that theirschool is a registered subscriber.

No part of these Factsheets may bereproduced, stored in a retrieval system, ortransmitted, in any other form or by any othermeans, without the prior permission of thepublisher.

ISSN 1351-5136

Biology of Freshwater Pollution Bio Factsheet

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Tolerance of increasing temperatures varieswidely between different species and betweendifferent life stages of any one species. However,there are four general effects:

1. Hot effluent kills most bacteria, invertebrates,fish and plants. Some bacteria survive and beginto break down these dead organisms, hence raisingthe BOD. The rate of metabolic activity increasesas temperature increases (effect on enzymes)therefore oxygen is used up faster.

2. Increasing temperature also increases thetoxicity of any poisonous substances present inthe water.

3. Increasing metabolic activity releases morecarbon dioxide which raises the lower oxygenlimit which fish can tolerate.

4. As water temperature increases, oxygensolubility decreases therefore oxygen levels dropfurther.

5. Toxic ChemicalsThe most important categories of toxic chemicalsare heavy metals and organic compounds such aspolychlorinated biphenyls (PCBs) and pesticides.

Although its use in the UK has been banned formany years, the organochlorine pesticide DDT isstill widely distributed in the aquatic environment.Such chemicals have been implicated in the seriousdecline of some top carnivores eg the otter. Sub-lethal concentrations reduce reproductive successin fish and fish-eating birds.

PCBs are chlorinated hydrocarbons which havebeen used in transformers, heat exchangers,lubricating oils, paints and inks. They have enteredthe aquatic environment from leaking landfills andsewage effluent. As with other organochlorines,PCBs seriously reduce the reproductive ability ofmammals.

The lethal effects of toxic chemicals often resultfrom biomagnification - the increasingconcentration of a substance up through a foodchain (see Factsheet 14 Bioaccumulation andBiomagnification).

Thus the concentration of toxin found in thetissues of the organisms at the top of the foodchainmay be thousands of times greater than theconcentration which was released into theenvironment. Such toxins are often stored in fattytissue. When this is metabolised to provide energythe toxins are released into the organism'sbloodstream, often with fatal effects.

4. Thermal Pollution Practice questions

(a) Name one pollutant which could have causedthis effect. (1 mark)

(b) Suggest an explanation for the effect of the

pollutant on the numbers of:

(i) bacteria (2 marks)

(ii) clean water invertebrates (3 marks)

Answers(a) Credit any organic pollutant eg. sewage/

silage/milk

(b)

(i) organic matter proides food sourceorganic matter contains bacteria;so number increase;numbers then fall because of predation;

(ii) Oxygen levels fall. BOD increases;Result of high bacterial numbers;which use oxygen matter;Numbers recover as oxygen increases/bacteria decline;

1. The Figure shows the effect of an effluent on a river.

Oxygen

BOD

Algae

Sewage fungus

Clean water fauna

Distance downstream

BacteriaProtozoa

Effluent