Biological Oxygen Demand

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Number 138www.curriculumpress.co.uk

What is it?It is the amount of oxygen used by organisms in a body of water over aspecified time period. By definition, aerobic organisms use oxygen inrespiration. The more organisms there are at a particular site, for example, ina river, and the faster their rate of respiration, the more oxygen they will use.So, the BOD at any particular point in a river, for example, is determined by:• The number of aerobic organisms there are at that point;• Their rate of respiration.

Why is it useful?It can indicate whether a particular part of a river is polluted with organicmatter (e.g. sewage, silage, milk). This is because the presence of an organicpollutant will stimulate an increase in the population of organisms that willfeed on and break down the pollutant. In doing so, they respire and use up alot of oxygen. In other words organic pollutants cause a high BOD.

How is BOD measured?This is how an examiner's mark scheme would look:

1. Sample of water of measured volume is taken;2. Oxygen level is measured;3. Sample is incubated in dark to prevent photosynthesis (any microscopic

plants contained in the sample would photosynthesise, releasingoxygen which would give an artificially low BOD);

4. at 20 ºC;5. for 5 days;6. Oxygen level is re-measured;7. BOD = difference in the two measurements.

Certain species of organism are tolerant of organic pollution and the lowoxygen levels associated with it. They are found in high population densitieswhere an organic pollution incident occurs. Other species cannot toleratelow oxygen levels and, if organic pollution enters the river where they live,they move away (Fig 2). We can use both these groups as Indicator Species– indicators, i.e. of organic pollution.

An indicator species is one whose presence, absence or abundance can beused as an indicator of pollution. It doesn’t have to be water pollution –some species can be used, for example, to indicate particular soil types.

Certain species can be used to indicate air pollution, soil nutrient levelsand abiotic water characteristics (Table 1).

0

2

4

6

8

10

12

14

16

0 10 20 30 40

oxyg

en s

olub

ility

/mgd

m-3

Temperature 0C

Fig 1. Oxygen solubility

This problem is made even worse for aquatic invertebrates and fishbecause they can do little to regulate their body temperature. As watertemperature increases, so does their internal temperature and so doestheir rate of repiration. This means they need more oxygen – but theamount of oxygen disolved in the water is going down! The release ofwarm organic pollutants such as milk can therefore be devastating.

Organic pollutants can be more dangerous in SummerOrganic pollution can be lethal if it enters warm water. This is becausethe solubility of oxygen decreases as the water temperature increases(Fig 1).

Indicator species

Lichen (Usnea articulata)

Nettles (Urtica dioica)

Red Alga (Corallina officinalis)

What does it indicate?

Very low levels of sulphur dioxide

High phosphate levels in soils

Fully-saline rock pools but absentfrom brackish ones

Table 1. Indicator species

Stonefly nymph

Flattenedmayfly nymph

Caseless caddisfly larva

Swimming mayfly nymph

Freshwater nymph

Water louse

Blood worm (or midge larva)

Rat-tailed maggot

Tubifex

High river pollution

low river pollutionD

ecre

asin

g sp

ecie

s di

vers

ity

Fig 2. Invertebrate indicators of fresh water pollution

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138 Biological Oxygen Demandwww.curriculumpress.co.uk

1. Organic pollution enters the river

2. Population of certain bacteria, fungi and Tubifexincreases. These organisms feed on and begin tobreak down the organic matter

3. These organisms respire and use upa lot of oxygen - the BOD increases

4. By this point in the river, the amount oforganic matter has fallen by dispersal andbecause it has been broken down. BOD is lessthan at the point where pollutant entered

River

direction of flow

5. Pollutant now much reduced.BOD just above normal

6. BOD back at normalpoint - all organic matterbroken down

EffluentDistance downstream

Bacteria

BOD

Oxygen

Tubifex wormfeed on andtunnel in theeffluent

Mayflynymphs

Fig 3 shows how the population of some organisms is influenced by theentry of an organic effluent into a river. This graph is a common examquestion Make sure that you can explain the population changes in: bacteria,Tubifex worms and mayfly nymphs.

The graph illustrates how, for e.g., the Tubifex worms and the mayflynymphs could be used as indicator species in other rivers. The Tubifexworms feed on and tunnel into the effluent; their populations increaserapidly immediately downstream of the effluent entry.

A high population of these organisms in any river could indicate thatorganic pollution has occurred there. In contrast, the population of mayflynymphs crashes as soon as the effluent enters. They need clean water andat the point of entry are either killed or move away. Thus, the absence ofmayfly nymphs in a particular river might indicate organic pollution hasoccurred and large populations might indicate clean, unpolluted water. It isoften faster and cheaper to measure organisms such as Tubifex or mayflynymphs than it is to try to measure the concentration of specific pollutants– this is one of the advantages of using indicator species.

Fig 3. Effect of organic pollution

Acknowledgements:This Factsheet was researched and written by Kevin Byrne.Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in aretrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136