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The measurement of benthal respiration
H,'aler Rc~ear('h ~ol. ~,. pp t~55 Io ~St~ Pergamon Pres~ l v ' a Prlnled in Great Hr=taln
THE MEASUREMENT OF BENTHAL RESPIRATION A. JAMFS
Division of Public Health Engineering. Dcparmvent of Civil Engineering. University of Ne~castle upon T)ne. England IReceired 15 September t973}
Abstract-This paper reviews the methods used in the measurement of benthal respiration. The disadvantages of laboratory methods are stressed, in particular the technical problems in simulating flow over the substratum. Two m situ methods are described for use in lakes and streams and the results of these are compared with laboratory methods using an oxygen budget as an independent check.
Because the errors inherent in the laboratory appeared to be greater it was decided to try to improve the method of measuring benthal respiration by developing the in situ techniques.
I'~TRODL'CTION
The importance of benthal respiration in the oxygen regime of rivers and estuaries has been pointed out by many writers (e.g. [ I]). This paper reviews the difficulties that have been encountered in measurement and discusses the influence of environmental factors on the rate ofrespiration. In particular the paper draws attention to the importance of the hydraulic conditions and describes methods for minimizing the hydraulic interference in making in .~Jtu measurements.
MATERIALS
The difficulties of mcttsuring hclTthal respiration. The techniques so far devised for measuring benthal respiration fall into two categories: (a) laboratory methods which ha~,e culminated in the development of the mud-core respirometer [2]: Ib~ in sire methods which have developed as various forms of chamber respirometer e.g, [3]. [J,] and [5]. Both approaches have encountered difficulties: the former due to the disturbance of the mud in sampling and the problems of simulating natural conditions in the laboratory: the latter due to the interference with natural conditions caused in making the measurement. As there is no absolute measure of benthal respiration k is not easy to assess the magnitude of the errors in the above techniques. However it is possible by a mass balance calculation to compute the rate of benthal respiration. Using this method [6] found considerable discrepancies up to 48 per cent between the calculated values and measurement by a laboratory technique. This paper was received for the Paris Conference. but together gith several more was accepted for H'ater Research as there was no place for it on the Conference programme.
AND
METHODS
7ivo in situ teclmiques were derised and are described below (a) Tunnel respirometer. This was a modified version of the apparatus previously described [6]. It was constructed from a thermoplastic strip 30 m long by I m wide supported at I m intervals on mild steel hoops to form a tunnel on the bed of the stream las shown in Fig. 1). The tu'nnel was laid on a straight line on the bed of the stream parallel to the direction of flow. The metal hoops were inserted in the sheaths welded onto the underside of the tunnel and then the projecting parts of the hoops were pressed into the stream bed one at a time working downstream, the thermoplastic tunnel being held in tension during this process to prevent warping. The thermoplastic flaps at each side of the tunnel served to prevent exchange of water out of or into the tunnel and were accordingly pressed firmly into the bed of the stream. Once the tunnel had been fixed in place it was checked for leaks by injecting fluore.~cein into the tunnel mouth with an hypodermic syringe. When the tunnel was leak free. a further fluoreseien injection was used to measure the time of travel and then the flow in the tunnel was gauged by the sahdilution method [~7-J using Lithium chloride. Four BOD bottles were then filled with water at the entrance to the tunnel. The Dissolved Oxygen concentration ~'as determined in two of these immediately using the Azide modification of the Winkler technique.
955
956
A JA~,I~
, ,
!
'
i
I
,
i
,, "-.-<.~..'--..-..:
',
!
!(I!
l
i//i
I
:
,
,,,. ......
:.~....\
Sheath
" S t e e l
hoop
Flap
Fig. I. The other two were incubated in the stream for the time of travel of the water in the tunnel and then fixed and titrated by the Winkler method. A further two samples were collected at the downstream end of the tunnel at the end of the travel time and were fixed and titrated immediately. (b) The respiration chamber. This consists of a metal box open at the base which can be lowered onto the bed of a lake or stream to enclose a small volume of water in contact with the mud. The shape and size of the metal chamber are shown in Fig. 2. The vertical flanges serve to anchor the chamber on the bed and confine the mud and the horizontal Banges serve to fix the depth of the vertical penetration. The flanges and the box have been painted with an epoxy resin to minimize corrosion. Apart from the external flanges there are also internal flanges in the tapering entrance and exit to the box. Both shape and flanges help to minimize the turbulence generated in the flow entering and leaving the box. Holes in the side wall of the chamber accommodate a dissolved oxygen electrode and a thermistor probe.
A submersible pump which is fastened to the top of the chamber induces flow in the box via two 25 mm plastic hoses. There is a screw valve in the outlet line from the pump, which is used to regulate the flow in the box. Horizontal velocities in the range of 0"1 to 0.4 m s- ' arc obtainable. At the beginning of a test the electrode and thermistor are fixed in the chamber and the hose between the pump oudet and chamber is disconnected. With the pump running the apparatus is submerged until all the air "is expelled. The hose is then reconnected underwater, the screw ,,'alve is set and the apparatus is lowered gently to the bottom, so that it sinks into the mud under its own weight. Regular measurements (usually 10rain intervals) are made of the dissolved oxygen concentration and temperature, after an initial period of 15 min to allow disturbed sediment to settle.
TEST PROCEDURE The aim of the experiment was to compare the rate of benthal respiration as measured by the mud-core.
~ _ //./
/ Screw valve
7
Pump K ~
~
y~!
" .~Thermistor "
Fig. 2.
The measurement of benthal respiration
957
Table 1. Location of the study sites Site number
Name
Location
I
Cunsey Beck
2
Moors Burn
3
River Ankcr
4
Lea Marston Lake
5 6
Esthwaite Water Blelham Tarn
7
Loch Leven
,.
Lake District. Stream flowing south from Esthwaite Water County Durham. Stream flowing into River ~.:ear at Chester-I:-Street Nottinghamshire. Tributary of the River Trent Nottinghamshire. Artificial impoundment of the River Tame near Nottingham Lake District near Hawkeshead Lake District near NW corner of Lake Windermere Scotland. Approximately 30 km North of Edinburgh
tunnel and chamber respirometer and as computed from an oxygen balance calculation. It was clearly not practical to use the tunnel in lakes but apart from this limitation the rates of benthal respiration were measured as far as possible by all three techniques at the seven sites listed in Table 1. The mass balance technique used at sites 1.2 and 3 was based on hourly changes in oxygen concentration at two stations at the upstream and downstream end of the test reach. At the lake sites 4. 5 and 6 a similar approach was used based on average diurnal changes observed at a number of stations in each lake. At Site 7 the oxygen balance was not possible to compute due to the difficulties in obtaining su~cient observations in so large a body of water. RESL'LTS
The results of the oxygen consumption measurements are summarized in Table 2. "DISCUSSION Since the earl)' work on the tunnel respirometer [6"1 the technique has been improved by the use of dilution
gauging {within Lithium chloride) to estimate the flow in the tunnel. This has improved the accuracy of the method so that the results lie within the range of _ 15 per cent of the values calculated from the oxygen balance (see Table ~. It is doubtful if any further improvement in accuracy is obtainable in this method because of the variability of the substratum. Even without further improvement the tunnel respirometer gives the best agreement with the calculated uptake values and the minimum interference with natural conditions consistent with obtaining a measurement. The chief disadvantage of the tunnel respirometer is the necessity to obtain a significant drop in the dissolved oxygen concentration (around 1 mg 1- 1) which means that under conditions of moderate oxygen uptake rates (I-2 g O_, m-" day)a tunnel length of 30 m is required. Also the placing of the tunnel is somewhat laborious and as previously noted is not normally possible in depths of > I r a or in velocities of > if3 m s- ~" However. man)' of the stream situations where benthal respiration is important fall within these constraints.
Table 2. Summary of measurements of benthal respiration (the numbers in the Table are the arithmetic mean values of all results from each site) Ox2,gen uptake rate of bottom mud I'gO.,m--' h) Site number I 2 3 4 5 6 7
Respiration tunnel
Respiration chamber
Mud core respirometer '
Calculated from oxygen balance
0-22 1"59 0-42
0-16 1"13
0-15 0-92 .0-22
0"19 1"84 0"34 0"03 0-35 6-32
0-15 0-40 0-35 0-13
0"39 0-22 0"07
958
A. JA.~l~.s
7 ¸
/
E ,t. O
/
4
@
!
,b
In the chamber respirometer the maximum velocity obtainable was higher but still below that in the Cunsey Beck and the Moors Burn. It was therefore conc[uded that neither of these were really suitable for as~ssing the oxygen uptake in fast flowing streams unless the measured value was corrected in some way for the velocit,, differential between apparatus and stream. There are many aquatic situations, notabl~ deep ri,:~.~s, estuaries and lakes, where the tunnel respirometer cannot be used. In these situations a comp a , s o n was made between results from the chamber respirometer, the mud core respirometer and values calculated from the oxygen balance. The results presented in Table 2 only referring to work in various lakes show reasonable agreement between calculated values and the mud.core respirometer results for all the four lakes studied although there was an overall tendency for the respirometer to underestimate the uptake. As there was no problem of simulating ,,elocitics over the mud surface the most likely explanation of the discrepancy appeared to be the disturbance of the mud in the sample collection. Measurements made in a laboratory tank showed that some compaction of the mud occurred during core removal causing up to 15 per cent reduction in the depth of mud. This inevitably resulted in expression of some of the interstitial water and thus disturbance of the concentration gradients within the mud. The cores were aerated overr.i~ht prior to the measurement of oxygen uptake rates and during this period some restoration of the gradients would have occurred but it seemed likely from the, esults obtained that disturbance of the mud had reduced its uptake potential. Replicate measurements carried out after 1.5 and 10 day intervals gave uptake rates which varied by up to 20 per cent due to internal changes in the mud.
/
X
x
g2
I 0-01
I OOZ
1
r
003
004
VelOclf y,
I OO5
I
[
006
O,Or
m s "I
Fig. 3. Table 2 also shows a comparison of the results obtained for stream situations by the tunnel and chamber respirometers. This shows that the chamber respirometer tended to underestimate the o x y ~ n consumption though not by as great a percentage as the mud-core respirometer. In both cases the low value for oxygen consumption appeared to be due to a difficulty in obtaining the correct range of water velocities over the mud surface. It was found experimentally that even at low velocities there was a linear increase in uptake with increasing velocity (Fig. 3). and with higher velocities the rate of increase became exponential (Fig. 4). This effect was probably due to increased resuspension tending to increase the interracial area. The maximum velocity obtainable in the mud-core respirometer was calculated as 0-024 m s- ~.
/ / / / Oxygen ba|ancef / E
& ResDira*ion~ tunnel x/
="
/ / Yy"
Respiration c Core x 0
chamber resc)trometer x x .=~.x x ~ x r
~
O0 00I
~ ,~
.~7~
I
d
0.05 '~/ot er
velocity,
Fig. 4,
i 0 ~0 •~1 S - !
0.20
The measurement of benthal respiration The results obtained from the chamber respirometer in the four lakes were also in reasonable agreement but tended to overestimate the rate of oxygen uptake. This particularly applied in the case of Lea Marston, It was found to be due to disturbance of the mud during the final stages of lowering the chamber onto the bed. Resuspension of fine particles occurred, and unless a period of 30 min was allowed for quiescent settling, the particles remained suspended and gave increased uptake rates. Once this artefact had been discovered, and the technique altered to allow a period of settling. the results obtained from the chamber v,ere in close agreement with the calculated values.
CONCLUSIONS
From the above rest, Its it was concluded that: la) the respirometer tunnel and the chamber provided useful field techniques for measuring the oxygen consumption of bottom muds: (b) the physical limitations of the tunnel technique restrict its use to shallow rivers and streams depth <1 m and velocities <0"3ms-~. Within these constraints it appears to oftbr the most accurate method of measurement: Ic) the chamber respirometer is not suitable for measurement in velocities >0.4 m s-~ but for slow-flowing streams and lakes oilers a better method of measurement tlaan the mudcore respirometer: 4d) the mud-core respirometer can
95'~
be used in almost an~ aquatic situation but becau~ of the limitations described above it tends to underestimate the demand. REFERENCES
I'll Downing A. L. i19711 Forecasting the effects of polluting discharges on natural ~,,aters. Ira. J. Enrironmt'mal S:udies 2. 101. [-2-] Knowles G.. Edwards R. W. and Briggs R. t19021 Poiarographic measurement of the rate of respiration of natural sediments. Limnol. Ocean,,er. 7, 4~1. 1"3] Department of Scientific and Industrial Research 11964) £tf,'cts ,~I Pollutin~ Di~ciiar.ovs on lhe Tham,,s Estuary. Wat. Pollut. Res. Tech. Paper No. 11. H.M.S,O. London. [.4] Stein J. E. and Dennison J. G. (1966~ hl.siru Benthal Oxygen Demand of Cellulosic Fibres. Proc. 3rd Ira. Conl~ tlbL Polhu. Rex. Munich 1966. [5] Edbcrg N. and Hofsten B. V. 119731 Oxygen uptake of bottom sediments studied h]-situ and in the laborafore. Water Rcs. 7. 1285. [6] Bradle) R. and James A. (196~SlA new method for the measurement of oxygen consumption in polluted rivcrs. J. It]it. Pollur. Crmtro! 67. 462. [7"] Collingc V. editor (1966) Dihaio, Gau~in.q. Bulletin No. I5 Civil Engineering Dept.. The Universlt) of Newcastle upon Tyne. [8] Owens M. [1969) Measurement of primar) productivity in flowingwaters, In Measuremt,,r o.lPrimary Productirity. IBP. Handbook lEdited by Vollenweider) Black~oods. 1"9"] Odum H T and Hoskins C. M. 1195Sl Comparative studi~s on the metabolism of marine ,xaters. Publ. Inst. Mar. SoL ~,xa.s. 5.
THE MEASUREMENT OF BENTHAL RESPIRATION A. JAMFS
Division of Public Health Engineering. Dcparmvent of Civil Engineering. University of Ne~castle upon T)ne. England IReceired 15 September t973}
Abstract-This paper reviews the methods used in the measurement of benthal respiration. The disadvantages of laboratory methods are stressed, in particular the technical problems in simulating flow over the substratum. Two m situ methods are described for use in lakes and streams and the results of these are compared with laboratory methods using an oxygen budget as an independent check.
Because the errors inherent in the laboratory appeared to be greater it was decided to try to improve the method of measuring benthal respiration by developing the in situ techniques.
I'~TRODL'CTION
The importance of benthal respiration in the oxygen regime of rivers and estuaries has been pointed out by many writers (e.g. [ I]). This paper reviews the difficulties that have been encountered in measurement and discusses the influence of environmental factors on the rate ofrespiration. In particular the paper draws attention to the importance of the hydraulic conditions and describes methods for minimizing the hydraulic interference in making in .~Jtu measurements.
MATERIALS
The difficulties of mcttsuring hclTthal respiration. The techniques so far devised for measuring benthal respiration fall into two categories: (a) laboratory methods which ha~,e culminated in the development of the mud-core respirometer [2]: Ib~ in sire methods which have developed as various forms of chamber respirometer e.g, [3]. [J,] and [5]. Both approaches have encountered difficulties: the former due to the disturbance of the mud in sampling and the problems of simulating natural conditions in the laboratory: the latter due to the interference with natural conditions caused in making the measurement. As there is no absolute measure of benthal respiration k is not easy to assess the magnitude of the errors in the above techniques. However it is possible by a mass balance calculation to compute the rate of benthal respiration. Using this method [6] found considerable discrepancies up to 48 per cent between the calculated values and measurement by a laboratory technique. This paper was received for the Paris Conference. but together gith several more was accepted for H'ater Research as there was no place for it on the Conference programme.
AND
METHODS
7ivo in situ teclmiques were derised and are described below (a) Tunnel respirometer. This was a modified version of the apparatus previously described [6]. It was constructed from a thermoplastic strip 30 m long by I m wide supported at I m intervals on mild steel hoops to form a tunnel on the bed of the stream las shown in Fig. 1). The tu'nnel was laid on a straight line on the bed of the stream parallel to the direction of flow. The metal hoops were inserted in the sheaths welded onto the underside of the tunnel and then the projecting parts of the hoops were pressed into the stream bed one at a time working downstream, the thermoplastic tunnel being held in tension during this process to prevent warping. The thermoplastic flaps at each side of the tunnel served to prevent exchange of water out of or into the tunnel and were accordingly pressed firmly into the bed of the stream. Once the tunnel had been fixed in place it was checked for leaks by injecting fluore.~cein into the tunnel mouth with an hypodermic syringe. When the tunnel was leak free. a further fluoreseien injection was used to measure the time of travel and then the flow in the tunnel was gauged by the sahdilution method [~7-J using Lithium chloride. Four BOD bottles were then filled with water at the entrance to the tunnel. The Dissolved Oxygen concentration ~'as determined in two of these immediately using the Azide modification of the Winkler technique.
955
956
A JA~,I~
, ,
!
'
i
I
,
i
,, "-.-<.~..'--..-..:
',
!
!(I!
l
i//i
I
:
,
,,,. ......
:.~....\
Sheath
" S t e e l
hoop
Flap
Fig. I. The other two were incubated in the stream for the time of travel of the water in the tunnel and then fixed and titrated by the Winkler method. A further two samples were collected at the downstream end of the tunnel at the end of the travel time and were fixed and titrated immediately. (b) The respiration chamber. This consists of a metal box open at the base which can be lowered onto the bed of a lake or stream to enclose a small volume of water in contact with the mud. The shape and size of the metal chamber are shown in Fig. 2. The vertical flanges serve to anchor the chamber on the bed and confine the mud and the horizontal Banges serve to fix the depth of the vertical penetration. The flanges and the box have been painted with an epoxy resin to minimize corrosion. Apart from the external flanges there are also internal flanges in the tapering entrance and exit to the box. Both shape and flanges help to minimize the turbulence generated in the flow entering and leaving the box. Holes in the side wall of the chamber accommodate a dissolved oxygen electrode and a thermistor probe.
A submersible pump which is fastened to the top of the chamber induces flow in the box via two 25 mm plastic hoses. There is a screw valve in the outlet line from the pump, which is used to regulate the flow in the box. Horizontal velocities in the range of 0"1 to 0.4 m s- ' arc obtainable. At the beginning of a test the electrode and thermistor are fixed in the chamber and the hose between the pump oudet and chamber is disconnected. With the pump running the apparatus is submerged until all the air "is expelled. The hose is then reconnected underwater, the screw ,,'alve is set and the apparatus is lowered gently to the bottom, so that it sinks into the mud under its own weight. Regular measurements (usually 10rain intervals) are made of the dissolved oxygen concentration and temperature, after an initial period of 15 min to allow disturbed sediment to settle.
TEST PROCEDURE The aim of the experiment was to compare the rate of benthal respiration as measured by the mud-core.
~ _ //./
/ Screw valve
7
Pump K ~
~
y~!
" .~Thermistor "
Fig. 2.
The measurement of benthal respiration
957
Table 1. Location of the study sites Site number
Name
Location
I
Cunsey Beck
2
Moors Burn
3
River Ankcr
4
Lea Marston Lake
5 6
Esthwaite Water Blelham Tarn
7
Loch Leven
,.
Lake District. Stream flowing south from Esthwaite Water County Durham. Stream flowing into River ~.:ear at Chester-I:-Street Nottinghamshire. Tributary of the River Trent Nottinghamshire. Artificial impoundment of the River Tame near Nottingham Lake District near Hawkeshead Lake District near NW corner of Lake Windermere Scotland. Approximately 30 km North of Edinburgh
tunnel and chamber respirometer and as computed from an oxygen balance calculation. It was clearly not practical to use the tunnel in lakes but apart from this limitation the rates of benthal respiration were measured as far as possible by all three techniques at the seven sites listed in Table 1. The mass balance technique used at sites 1.2 and 3 was based on hourly changes in oxygen concentration at two stations at the upstream and downstream end of the test reach. At the lake sites 4. 5 and 6 a similar approach was used based on average diurnal changes observed at a number of stations in each lake. At Site 7 the oxygen balance was not possible to compute due to the difficulties in obtaining su~cient observations in so large a body of water. RESL'LTS
The results of the oxygen consumption measurements are summarized in Table 2. "DISCUSSION Since the earl)' work on the tunnel respirometer [6"1 the technique has been improved by the use of dilution
gauging {within Lithium chloride) to estimate the flow in the tunnel. This has improved the accuracy of the method so that the results lie within the range of _ 15 per cent of the values calculated from the oxygen balance (see Table ~. It is doubtful if any further improvement in accuracy is obtainable in this method because of the variability of the substratum. Even without further improvement the tunnel respirometer gives the best agreement with the calculated uptake values and the minimum interference with natural conditions consistent with obtaining a measurement. The chief disadvantage of the tunnel respirometer is the necessity to obtain a significant drop in the dissolved oxygen concentration (around 1 mg 1- 1) which means that under conditions of moderate oxygen uptake rates (I-2 g O_, m-" day)a tunnel length of 30 m is required. Also the placing of the tunnel is somewhat laborious and as previously noted is not normally possible in depths of > I r a or in velocities of > if3 m s- ~" However. man)' of the stream situations where benthal respiration is important fall within these constraints.
Table 2. Summary of measurements of benthal respiration (the numbers in the Table are the arithmetic mean values of all results from each site) Ox2,gen uptake rate of bottom mud I'gO.,m--' h) Site number I 2 3 4 5 6 7
Respiration tunnel
Respiration chamber
Mud core respirometer '
Calculated from oxygen balance
0-22 1"59 0-42
0-16 1"13
0-15 0-92 .0-22
0"19 1"84 0"34 0"03 0-35 6-32
0-15 0-40 0-35 0-13
0"39 0-22 0"07
958
A. JA.~l~.s
7 ¸
/
E ,t. O
/
4
@
!
,b
In the chamber respirometer the maximum velocity obtainable was higher but still below that in the Cunsey Beck and the Moors Burn. It was therefore conc[uded that neither of these were really suitable for as~ssing the oxygen uptake in fast flowing streams unless the measured value was corrected in some way for the velocit,, differential between apparatus and stream. There are many aquatic situations, notabl~ deep ri,:~.~s, estuaries and lakes, where the tunnel respirometer cannot be used. In these situations a comp a , s o n was made between results from the chamber respirometer, the mud core respirometer and values calculated from the oxygen balance. The results presented in Table 2 only referring to work in various lakes show reasonable agreement between calculated values and the mud.core respirometer results for all the four lakes studied although there was an overall tendency for the respirometer to underestimate the uptake. As there was no problem of simulating ,,elocitics over the mud surface the most likely explanation of the discrepancy appeared to be the disturbance of the mud in the sample collection. Measurements made in a laboratory tank showed that some compaction of the mud occurred during core removal causing up to 15 per cent reduction in the depth of mud. This inevitably resulted in expression of some of the interstitial water and thus disturbance of the concentration gradients within the mud. The cores were aerated overr.i~ht prior to the measurement of oxygen uptake rates and during this period some restoration of the gradients would have occurred but it seemed likely from the, esults obtained that disturbance of the mud had reduced its uptake potential. Replicate measurements carried out after 1.5 and 10 day intervals gave uptake rates which varied by up to 20 per cent due to internal changes in the mud.
/
X
x
g2
I 0-01
I OOZ
1
r
003
004
VelOclf y,
I OO5
I
[
006
O,Or
m s "I
Fig. 3. Table 2 also shows a comparison of the results obtained for stream situations by the tunnel and chamber respirometers. This shows that the chamber respirometer tended to underestimate the o x y ~ n consumption though not by as great a percentage as the mud-core respirometer. In both cases the low value for oxygen consumption appeared to be due to a difficulty in obtaining the correct range of water velocities over the mud surface. It was found experimentally that even at low velocities there was a linear increase in uptake with increasing velocity (Fig. 3). and with higher velocities the rate of increase became exponential (Fig. 4). This effect was probably due to increased resuspension tending to increase the interracial area. The maximum velocity obtainable in the mud-core respirometer was calculated as 0-024 m s- ~.
/ / / / Oxygen ba|ancef / E
& ResDira*ion~ tunnel x/
="
/ / Yy"
Respiration c Core x 0
chamber resc)trometer x x .=~.x x ~ x r
~
O0 00I
~ ,~
.~7~
I
d
0.05 '~/ot er
velocity,
Fig. 4,
i 0 ~0 •~1 S - !
0.20
The measurement of benthal respiration The results obtained from the chamber respirometer in the four lakes were also in reasonable agreement but tended to overestimate the rate of oxygen uptake. This particularly applied in the case of Lea Marston, It was found to be due to disturbance of the mud during the final stages of lowering the chamber onto the bed. Resuspension of fine particles occurred, and unless a period of 30 min was allowed for quiescent settling, the particles remained suspended and gave increased uptake rates. Once this artefact had been discovered, and the technique altered to allow a period of settling. the results obtained from the chamber v,ere in close agreement with the calculated values.
CONCLUSIONS
From the above rest, Its it was concluded that: la) the respirometer tunnel and the chamber provided useful field techniques for measuring the oxygen consumption of bottom muds: (b) the physical limitations of the tunnel technique restrict its use to shallow rivers and streams depth <1 m and velocities <0"3ms-~. Within these constraints it appears to oftbr the most accurate method of measurement: Ic) the chamber respirometer is not suitable for measurement in velocities >0.4 m s-~ but for slow-flowing streams and lakes oilers a better method of measurement tlaan the mudcore respirometer: 4d) the mud-core respirometer can
95'~
be used in almost an~ aquatic situation but becau~ of the limitations described above it tends to underestimate the demand. REFERENCES
I'll Downing A. L. i19711 Forecasting the effects of polluting discharges on natural ~,,aters. Ira. J. Enrironmt'mal S:udies 2. 101. [-2-] Knowles G.. Edwards R. W. and Briggs R. t19021 Poiarographic measurement of the rate of respiration of natural sediments. Limnol. Ocean,,er. 7, 4~1. 1"3] Department of Scientific and Industrial Research 11964) £tf,'cts ,~I Pollutin~ Di~ciiar.ovs on lhe Tham,,s Estuary. Wat. Pollut. Res. Tech. Paper No. 11. H.M.S,O. London. [.4] Stein J. E. and Dennison J. G. (1966~ hl.siru Benthal Oxygen Demand of Cellulosic Fibres. Proc. 3rd Ira. Conl~ tlbL Polhu. Rex. Munich 1966. [5] Edbcrg N. and Hofsten B. V. 119731 Oxygen uptake of bottom sediments studied h]-situ and in the laborafore. Water Rcs. 7. 1285. [6] Bradle) R. and James A. (196~SlA new method for the measurement of oxygen consumption in polluted rivcrs. J. It]it. Pollur. Crmtro! 67. 462. [7"] Collingc V. editor (1966) Dihaio, Gau~in.q. Bulletin No. I5 Civil Engineering Dept.. The Universlt) of Newcastle upon Tyne. [8] Owens M. [1969) Measurement of primar) productivity in flowingwaters, In Measuremt,,r o.lPrimary Productirity. IBP. Handbook lEdited by Vollenweider) Black~oods. 1"9"] Odum H T and Hoskins C. M. 1195Sl Comparative studi~s on the metabolism of marine ,xaters. Publ. Inst. Mar. SoL ~,xa.s. 5.