The ecology of a land drainage channel—I. Oxygen balance

Water R¢,eurch VoL 15. pp. 1075 to 1085. 1981 Printed m Great Brztam A[I rights reserved

0043-1354, 8l 0ql075-I 1502.00.0 Copyright C I981 Pergamon Press Ltd

THE ECOLOGY OF A LAND DRAINAGE CHANNEL--I. OXYGEN BALANCE E. J. P. MARSHALL* Department of Applied Biology, University of Wales Institute of Science and Technology, King Edward VII Avenue, Cardiff CF1 3NU, Wales

(Received October 1980)

Abstract--ln shallow land drainage channels day-time solar heating during the summer produced vertical temperature and density gradients. These facilitated the development of marked gradients of dissolved oxygen, with maximum sub-surface values exceeding 300,°,0air saturation and deoxygenated water near the sediments. Night-time cooling promoted mixing of the water column. Rates of community photosynthesis and respiration, calculated from dissolved oxygen distributions by two methods, were high.

INTRODUCTION

Land drainage channels are of economic importance to the agriculture of large areas of Britain, and form an extensive aquatic habitat in England and Wales with a channel length of 1.3 x 105 km draining an area of 1.3 x 10 6 ha. Regular management of such channels is required to maintain drainage efficiency; approx. 3.0 x 104kin of arterial channel receive manual, mechanical and chemical maintenance each year (Marshall et al., 1978). Despite the extensive distribution of drainage channels and their potential importance as a refuge for aquatic floras and faunas, there are few studies of their biology and chemistry (Brooker, 1976a, b), In order to assess the importance of channels and to understand their ecology, several studies of ditches on the Monmouthshire levels, south Wales, have been conducted. The general drainage system of the area was described by Scotter et al. (1977), and studies of dred~ng effects (Wade, 1977), and macroinvertebrate distribution (Clare, 1978) have been completed. This paper is the first of two describing the ecology of short channel lengths, and the effects of submerged weed control using diquat (1,1t-ethylene-2,2~-dipyri dylium dibromide). The results of the present study would be relevant to the estimated 2000 km of arterial channel in England and Wales which receive chemical control of submerged weeds each year. SITE DESCmWrlON Studies of channel biology and chemistry were made in a reen (local term) near the village of Goldcliff(NGR: ST 378823)(Fig. 1). The channel runs for 1400 m behind a seawall which was built in 1967 to protect the area from tidal inundation. The reen is * Present address: ARC Weed Research Organization, Begbroke Hill, Yarnton, Oxford OX5 1PF, England.

divided by culverts into four sections of different dimensions, and is connected to the local drainage network all year by a single channel. Studies of oxygen balance were made at four stations, noted as A, B, C and E on Fig. I. Each station was located within a 50m study length whose physical dimensions are summarized in Fig. 2. Stretches A, B and E were of similar width and depth, while stretch C was both wider and deeper. The channel contained oligohaline eutrophic waters and stands of the plant species Potamogeton pectinatus and Myriophyllum spicatum. The duckweed Lemna minor and L. trisulca formed extensive summer cover in stretches A and B. Water in the channels was static for long periods in summer and flow was only observed during high rainfall.

PRACTICAL METHODS

To investigate the effects of diquat on oxygen economy, applications of the herbicide were made to stretches C and E on 19 August and 10 July respectively. Amounts of active ingredient applied were calculated to give a maximum concentration of 1 mg I-t. Oxygen balances in the Goldcliff channel were studied by examining changes in vertical distributions of dissolved oxygen and temperature during 24h periods. Diurnal investigation s were made at a single mid-channel station within each of the four 50 m study sections. Vertical water temperatures were recorded approximately hourly during sampling periods using thermistors. Isopleth diagrams of water temperature were drawn by interpolating between depth-time data points. Relative water density (gcm- 3) is directly related to water temperature and values were found from conversion tables. Seasonal patterns of surface water temperature were recorded from maximum-minimum thermometers. Meterorological data were obtained from stations 11 and 30 km from Goldcliff. Table 1 summarizes the location and times of the 24 h studies and records some environmental information. Dissolved oxygen measurement using oxygen electrodes was unsatisfactory, as steady readings could not be obtained under all conditions. Therefore oxygen measurements were made from water samples taken from points

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1077 MeQn demh Ot C I ~ e l cerllre ( nn }

A

58

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63

0.79

72

1.55

4.6

0.49

4 Fig. 2. Cross-sections, and mean width and mean depth at channel centre of experimental sites. within the water column. A sampling pole, similar to that used by Heaney (1974), was lowered through the water and samples taken at depths of 5, 20, 35 and at successive 15 cm intervals dependent on channel depth. Oxygen content was estimated chemically by a Winkler method (D.O.E., 1972). Plots of dissolved oxygen concentration against depth were drawn, and planimetry used to estimate dissolved oxygen mass {gin-z). Isopleths of concentration were drawn by interpolating between depth and time points.

CALCULATIONS

As there was substantially no flow in the channel during summer months community oxygen flux was estimated from changes in dissolved oxygen mass (Edwards, 1962), based on a single-curve technique. Oxygen resources within the water column were estimated by integrating concentration with depth, and the observed rate of oxygen change is given by the equation: (1)

Q=P+D-R

(2)

O = f ( C , - C)

where f = exchange coefficient (m h - t ) C, = saturated oxygen concentration (rag 1-~) C = measured sub-surface oxygen concentration (mg l-l). The exchange coefficient may be estimated from simultaneous equations for conditions at two times during darkness, when there is no photosynthesis and when values of (C, - C) and hence D are very different.

f =

(Q + R), - (Q + R),, (c,

-

c),

-

(c,

-

(3)

c).

where ( ), and ( ),, represent post-dusk and pre-dawn values respectively. Community respiration in equation (3) was calculated in two ways. Method I

where Q P D R

Diffusion, dependent on the saturation deficit at the water surface, is given by:

= = = =

Respiration is assumed to be constant and equation (3) is simplified to:

rate of change of oxygen mass (gm-2h - t) photosynthesis or gross oxygen production diffusion community respiration.

Q, - Q,.

f = (C, - C),, - (C, - C),"

(4)

Table 1. Dates and locations for 24 h dissolved oxygen sampling periods, meteorological conditions during such periods, and percentage cover of Lerana species in Sites A and B

Date 07-08.6.74 13-14.2.75 14-15.3.75 10-11.4.75 09-10.5.75 04-05.7.75 10.7.75 11-12.7.75 18-19.7.75 25-26.7.75 09-10.8.75 19.8.75 19-20.8.75 05-06.9.75

Air temperature Max. Min. (°C)

Hours Sun

15.7 3.2 9.1 -6.0 4.8 4.6 1.5 0.5 5.5 0.4 0 15.2 6.0 2.8 25.5 13.5 7.5 Diquat applied to Site E 21.5 14.5 1.8 22.9 13.5 8.8 20.1 9.8 I 1.0 30.4 15.0 5.8 Diquat applied to Site C 20.2 l 6.0 0 20.0 8.5 3.0

Solar radiation* (J cm-2 d a y - t )

Mean wind velocity (kin h - t)

2403 1651 899 811 1301 2123

5.13 9.04 4.25 5.50 7.58 3.63

A,C B A,C B A,C A,B,C,E

34.0

13.3 75.0

40.0

1126 2351 2735 1738

6.13 4.29 5.33 4.29

A,B,C,E A,B,C,E A,B,C,E A,B,C,E

68.0 63.5 62.0 75.0

30.0 22.0 21.0 32.5

81 I 1336

3.71 3.46

A,B,C,E A.B,C,E

80.0 70.0

40.0 45.0

* Data from Long Ashton Research Station, 30 km away.

Sites sampled

Lemna cover (~o)

Site A

Site B

1.0 15.0 5.0

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F ~ P \,l~,l,:,;H~LI

(A)

Dissolved oxygen (mg ! 12

)

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(B) Temperature ( ° C ) 12

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06

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20-

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40Relative water density ( g ml -~) 12 18 l

. h 24

(96

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12

O. 99 8 6

"6

40Fig. 3. D e p t h - t i m e isopleth d i a g r a m s of dissolved oxygen, w a t e r t e m p e r a t u r e and relative w a t e r density from site E. 04-05.7.75.

The ecology of a land drainage channel--I Respiration may then be estimated from night-time oxygen changes corrected for diffusion, and averaged over the period of darkness. This value may be used in equation (1t to calculate photosynthetic rate.

rates (P) were then calculated from values of Q. D and R using equation (1). The efficiency of solar energy conversation was estimated assuming the production of one mole of oxygen required 4.69 x 105 J (Edwards & Owens, 1962). Values of solar radiation reaching the Goldcliff channel (Table 1) were assumed to be similar to those at a site 11 km away, where sunshine records were kept, a regression equation being established between sunshine and solar radiation recorded at another neighbouring site (30 km).

Method II Respiration of plants and sediments is assumed to vary with both temperature and oxygen concentration. The effects of oxygen concentration are given by: R

=

aC b

(5)

Constants (a = 0.84; b = 0.38) derived by Owens & Marls (1964) were used to estimate plant respiration rates, which were corrected for temperature effects using a Qto of 2.0. Differences between plant respiration calculated for mean night oxygen concentration and temperature, and mean nocturnal respiration found in Method I, were assumed to represent mud respiration. Edwards & Rolley (1965) calculated b = 0.45 for several river sediments. Values of constant a for mud respiration were found from the equation: log a = log R, - b log C

I079

RESULTS Dissolred oxygen distributions

During daylight in summer m o n t h s considerable vertical differences in temperature were recorded. A typical summer diurnal pattern of oxygen, temperature and density distributions is shown in Fig. 3. Oxygen concentrations were continuously low above the sediments although sub-surface concentrations briefly exceeded 300% air saturation. Thermal density gradients were disturbed by nocturnal cooling and mixing occurred. Sub-surface oxygen concentrations declined in early evening and, after mixing, did not appreciably improve the oxygen status of deeper

(6)

where C = mean nocturnal oxygen concentration (mg 1-t) R, = residual respiration (total less plant respiration). Equation (5) was then used to estimate plant and residual respiration throughout the 24 h period, correcting for temperature effects using a Q~o of 2.0. Gross photosynthetic 30

(A)

o

Site A

•

Site C

i...

E

20

I

I

I

I

I

I

I

I

2O

o

I

F

l

M

I

A

I

M

i

d

I

d

i

A

I

S

Fig. 4. (A) Diurnal ranges of subsurface dissolved oxygen concentration at two channel sites between February and September 1975. (B) Maximum and minimum surface water temperatures at site C taken over fortnightly periods.

E . J . P . MARSHALL

1080

siderable L. minor cover {Table l}, supported Lower oxygen masses than site E, a channel of similar depth {Fig. 5).

water. Water close to the sediments commonly contained less than 2 mg I-t dissolved oxygen for long periods during the summer. Daytime relative density gradients gave stability to the water column and these helped maintain vertical oxygen gradients. Dissolved oxygen producers, the floating and submerged macrophytes and algae are not distributed uniformly within a water column (Westlake, i966). Most photosynthesis occurs towards the water surface, especially where floating plants occur {e.g. sites A and B). Major oxygen losses will be to the sediments, and under saturated conditions, to the atmosphere. Under stable conditions oxygen movement will be limited to diffusion, and so gradients might be expected between regions of oxygen production and utilization, i.e. upper layers and the sediments. There are seasonal differences in the physico-chemical behaviour in the channels. Figure 4 shows the maximum and minimum sub-surface dissolved oxygen concentrations in two sites, during the period February-September, and illustrates the seasonal patterns of maximum and minimum surface water temperature. During summer, greater variation in oxygen concentrations was recorded while in colder months concentrations approximated to air saturation and also showed little variation with depth, Water temperatures were lower in colder months, and also showed little depth variation in winter. U ttsch (1973) reported lower oxygen concentrations below floating plants compared with open water, and Morris & Barker (1977) have noted that a proportion of the oxygen produced by floating plants is lost directly to the atmosphere. While concentrations as high as 310°; air saturation (26 mg 1- ~) were recorded amongst Lemna minor in the Goldcliff channels, concentrations below these floating plants were generally lower than those at comparable depth in sites without floating plant species. Thus site A, which had con-

Oxygen metabolism

Estimated rates of ~oss oxygen production and community metabolism exhibited seasonal increases (Fig. 6). The ratio of rates of photosynthesis to respiration (P:R ratio) also showed seasonal variation with values generally greater than unity early in the year, and lower values in August and September. Such a trend followed macrophyte growth early in the year, and subsequent senescence. Rates of photosynthetic oxygen production, community respiration and net diffusion calculated by two methods, are compared in Fig. 7. Method II gave lower estimates of the surface exchange coefficient, and lower daily rates of net diffusion. Consequently, results from Method II showed marginally higher rates of respiration in summer, and therefore higher rates of gross oxygen production. In Method I, oxygen demand calculated at night is assumed constant throughout the 24 h. However, oxygen concentration and temperature affect respiration, and as Method II includes such effects results from these calculations are considered further. Herbicide effects

The effects of the herbicide diquat on oxygen economy were investigated in sites C and E. Diquat was applied to site E on 10 July 1975, and to site C on 19 August 1975 to control submerged plants. In both lengths of channel the maximum diquat concentration was 1.0 mg t- i For a short period following spraying vertical oxygen gradients were less pronounced than in untreated sites, and rates of community metabolism were affec-

I0

o

Site

A

o

Site

E

E

g

I)

0

)< 0

0

I

I

JULY

I

" "

AUG

SEPT

Fig. 5. Diurnal ranges of dissolved oxygen mass (g m-2) in a site containing Lemna minor, and a site of similar depth without floating plants.

The ecology of a land drainage channel--I 20

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1081

Site A

R

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Date

Fig. 6. Rates of gross oxygen production (P) and community respiration (R) in three channel sites.

ted. Photosynthetic oxygen production had fallen to 2.9 g m-2 day-1 in site E 24 h after spraying. At the same time, unsprayed site C values remained high at 1 5 . 0 g m - 2 d a y - ' (Fig. 6). Community respiration, which was 9.1 g m -z d a y - t a week before application in site E, declined to 6.1 g m -z d a y - ~. The P:R ratio and solar conversion efficiency in site E also declined from 1.0 to 0.5 and from 0.6 to 0.4% respectively. Ten days after diquat application the P:R ratio in site E was similar to unsprayed channels, though rates of production and respiration remained lower than site C. Fifteen days after spraying, rates of community metabolism, probably resulting from increased algal productivity, were comparable with unsprayed sites. The decrease in photosynthetic rate was caused by macrophyte death, and supported previous reports (Brooker & Edwards, 1973).

Diquat was applied to site C after seasonal maximum plant biomass had been achieved. Oxygen measurements were taken during the 24 h immediately after herbicide application as opposed to the following day, and gave similar photosynthetic rates to other sites (Fig. 6). However an increase in respiration rate to 18.5 g m -z d a y - 1 was recorded, giving a low P:R ratio of 0.4. Photosynthetic efficiency was not obviously affected by the herbicide over the first 24 h after application. The lack of marked effects might reflect several factors, notably that the herbicide may not have had sufficient time to affect the macrophytes. Diquat application was made by knapsack sprayer during the morning before oxygen measurements were taken. It is likely that column stability prevented mixing until the evening, and diquat was limited to the upper water layers before

E J

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Community respiration (gin-2 day-: )

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Method I

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+ + -HI

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Method I Fig. 7. Comparisons between results of Method I and Method II c o m m u n i t y metabolism calculations. (A) Gross oxygen production. (B) C o m m u n i t y respiration. (C) Net diffusion into water column. (D) Surface exchange coefficient.

Table 2. Estimated rates (g 02 m -z d a y - tl of plant (R~) and residual respiration (R,) of three drainage channel sites on several sampling dates. Macrophyte standing crop data (g dry weight m -2) were estimated from crop samples cut Once a m o n t h and cover maps of species distributions (Marshall, 1981) Site

A

C

Date

Macrophyte standing crop

Rr

7.6.74 14.3.75 9.5.75 4.7.75 11.7.75 18.7.75 25.7.75 9.8.75 19.8.75 5.9.75

100 88 85 109 109 109 109 107 107 95

3.36 1.52 2.54 3.50 2.83 3.01 3.18 2.35 1.66 1.89

E

Rr

Macrophyte standing crop

Ro

4.77 2.19 4.91 5.82 1.69 4.99 3.69 5,00 5.75 2.02

103 142 132 91 91 91 82 54 54 53

3.57 2.70 3.64 2.66 2.28 2.68 2.29 1.64 1.29 1.51

R,

Macrophyte standing crop

Ro

R,

2.86 1.29 1.02 3.94 9.34 13.24 20.20 t0.27 17.16 15.63

---50 50 50 35 20 20 20

---1.61 1.23 1.64 1.23 0.62 0.61 0.69

---7.47 4.84 9.08 22.79 13.23 13.57 17.42

The ecology of a land drainage channel--I cooling occurred. Effects on oxygen economy would have become obvious during the second day after application. Macrophyte biomass was declining when diquat was applied (Table 2), and reduced amounts of photosynthetic material in the site may have contributed to lack of effects on oxygen balance. DISCUSSION

Thermal density gradients were maintained by solar heating of surface layers promoted by the lack of shading and of water flow at the sites. The extremes of oxygen concentration recorded would suggest that such vertical gradients were capable of withstanding any disturbance caused by lowering the sampling apparatus through the water column. Vertical dissolved oxygen gradients, as found in this study, have been recorded previously (Tailing, 1957), and evidence of a diurnal overturn in shallow tropical waters was shown by Newell (1957). However, while Happey (1970) demonstrated vertical differences in oxygen concentration in a pond ( > 3 . 5 m deep), no reports have shown such extreme dissolved oxygen gradients over small depth ranges in temperate waters. The diurnal fluctuation of oxygen resources is likely to affect both invertebrate distribution and composition (Clare, 1979). Method II community metabolism calculations relied upon several assumptions. Nevertheless, comparison of results from Methods I and II (Fig. 7) indicated that such assumptions were acceptable. A major assumption in calculating respiration was that mean oxygen concentration in the water column represented the vertical gradients actually present. Vertical differences in plant and residual respiration rates, dependent on oxygen concentrations, would be expected. In addition, plant respiration [equation (5)] did not account for total respiration, particularly in summer when there were marked differences between residual and plant respiration rates (Table 2). Sediment respiration, a component of residual respiration, would have been negligible for long periods in summer, when water above the mud was deoxygenated. Therefore, either a large proportion of residual respir-

1083

ation was caused by biological components in the water column, or plant respiration was not adequately represented by equation (5), or a combination of these factors. Further uncertainties in the use of equation (5) arise from the fact that oxygen concentrations in the channels were often outside the range (2-10 mg 1-t) for which the equations were derived. Also, a temperature correction using a Qto of 2.0 took no account of coefficient variation which was shown by Owens & Maris (1964). Despite the assumptions used in Method II, rates of community respiration were broadly similar to those of Method I (Fig. 7). Initial attempts to model the dissolved oxygen patterns within these shallow channels were reported by Edwards et al. (1978). In order to simulate dissolved oxygen gradients a layered model was constructed, and oxygen flux was calculated by surface exchange, diffusion between layers and total mixing at night. Bubble losses from leaf surfaces and anaerobic sediments, and their unquantified mixing effect, were ignored. Dissolved oxygen distributions were calculated for each layer, using published formulae for respiration and photosynthesis (Owens & Maris, 1964; Edwards & Rolley, 1965; Westlake, 1966). Computergenerated oxygen distributions were well matched by field data, which indicated that oxygen transport was satisfactorily modelled by diffusion and nocturnal mixing. Published relationships for photosynthesis and respiration also approximated to conditions in the channels (Edwards et al., 1978). The latter observation is surprising in view of the differences noted here between plant and residual respiration. Rates of gross oxygen production and community respiration (Method II) in the channels were in broad agreement with published figures for several aquatic habitats (Table 3), although maximum rates were somewhat higher than most previously reported values. Exchange coefficients, which were also in agreement with published figures (Gameson & Truesdale, 1959), were not apparently affected by wind velocity, as only a weak negative correlation was found between these variables. However, a plot of L e m n a spp cover against exchange coefficient for sites A and

Table 3. Cumulative gross oxygen production and community respiration recorded in various aquatic environments Gross

Location Site A Site B Site C Site E River Itchen River Ivel Holding pond Sanguin pond Barry reservoir

oxygen Community production respiration (gO~ m - z day- t) 2.5-10.0 2.8-16.2 5.6-22.4" 2.9-26.5 0.4--14.0 3.2-17.6 7.5-23.9 6.8-20.3 1.3-8.9

3.7-9.3 2.0--15.3 4.0--22.5 6.1-24.0 4.2-18.6 6.7-15.4 8.5-30.2 4.8-16.8 0.6-10.8

References Current study Current study Current study Current study Odum (1956) Edwards & Owens (1962) Copeland & Dorris (1962) Goulder (1970) Brooker & Edwards (1973)

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0 IO •

0.08

,= E

8

0.06

SJte

8

A :

• =-057.

P- 0.05

B:

•

P = 0.01

=-0.83,

r

o 04

g, c

hl

0.02

I

0

20

40

60

80

i00

% cover of Lernna

Fig. 8. Relationships between the surface exchange coefficient and cover of Le,nna species in sites A and B.

B (Fig. 8) showed significant negative correlation, indicating floating plants may reduce surface oxygen exchange rates (Morris & Barker, 1977). Solar energy conversion efficiencies of the channels (0.2-1.7%) were similar to those of a shallow reservoir investigated by Brooker & Edwards (1973), and a negative correlation (r = -0.51, P = 0.01) found between conversion efficiency and solar energy was also reported by these workers. Conversion efficiencies in sites A and B were lower than sites without floating Lemna minor, and rates of oxygen production were generally greater in sites C and E where depth of photosynthetic activity would not have been reduced by surface shading. A midsummer application of diquat for submerged plant control reduced oxygen resources for a short time. Values of photosynthetic oxygen production and respiration returned to rates comparable with unsprayed sites after 15 days (Fig. 6). A late summer application of diquat, when macrophyte biomass was declining, did not markedly affect community metabolism. This possibly reflected the effect of water column stability preventing herbicide mixing and therefore uptake, during the first day after spraying. Oxygen production and respiration was not obviously affected 16 days after application (Fig. 6), and only temporary effects on oxygen balance resulted from diquat treatment of these shallow channels. Deoxygenation following herbicide application can sometimes cause mortality amongst component species of an aquatic fauna (Brooker & Edwards, 1975). However, the fauna of the Goldcliff channels was not obviously affected by diquat applications (Marshall, in preparation) and was probably well adapted to the rigorous oxygen regime present before spraying. The current study illustrates the marked variability

of oxygen resources and the stability of the water column in shallow channels. It is likely that this behaviour is a feature of many channels and possibly of shallow ponds and lakes. Daytime thermal stability under conditions of no flow maintains oxygen gradients, and is likely to affect distribution of pH and other chemicals in the water. These factors may affect the channel fauna. The spatial variability of oxygen resources along the channels was not determined in detail, though measurements of surface oxygen concentration indicated local differences were present. Further studies are required. Column stability will affect the distribution and uptake of herbicides applied as a liquid. Such formulations are likely to remain in surface layers until diurnal mixing. If no flow occurs during the day, then the herbicide will be taken up throughout the water column. However, treatments are more vulnerable to wash-out where the herbicide is concentrated in upper layers. In addition, wind action may create movement of the surface layer but leaving lower layers. Herbicide loss from channel lengths which have been partially treated might easily occur under such conditions. Temperature-density differences may explain some failures to control submerged plants using surface-applied liquid herbicides. Water flow and bulk movement of thermal layers will modify oxygen balance and column stability, and require further investigation.

Acknowledgements--The author would like to thank Professor R. W. Edwards for help throughout the study and in the preparation of this paper. The work was supported by a grant from the Science Research Council and I.C.I. Plant Protection Ltd. The manuscript was kindly typed by Mrs M. Cox and Mrs J. Wallswoth.

The ecology of a land drainage channel--I REFERENCES

Brooker M. P. (1976a) The ecological effects of the use of dalapon and 2.4-D for drainage channel management. I. Flora and chemistry. Arch. Hvdrobiol. 78, 396--412. Brooker M. P. (1976b) The ecoiogical effects of the use of dalapon and 2,4-D for drainage channel management. II. Fauna. Arch. Hydrobiol. 78, 507-525. Brooker M. P. & Edwards R. W. (1973) Effects of the herbicide paraquat on the ecology of a reservoir. II. Community metabolism. Freshwat. Biol. 3, 383-389. Brooker M. P. & Edwards R. W. (1975) Aquatic herbicides and the control of water weeds. Water Res. 9, 1-15. Clare P. (1978) The distribution of aquatic macroinvertebrates in the Gwent Levels. Ph.D. Thesis, UWIST, University of Wales. Copeland B. J. & Dorris T. C. (1962) Photosynthetic productivity in oil refinery effluent holding ponds. J. Wat. Pollut. Control Fed. 34, 1104-1111. Department of the Environment (1972) Analysis of Raw, Potable and Waste Waters. London, HMSO. Edwards R. W. (1962) Some effects of plants and animals on the conditions in freshwater streams with particular reference to their oxygen balance. Int. J. Air Wat. Pollut. 6, 505-520. Edwards R. W. & Owens M. (1962) The effects of plants on river conditions. IV. The oxygen balance of a chalk stream, d. Ecol. 50, 207-220. Edwards R. W. & Rolley H. L. J. (1965) Oxygen consumption of river muds. J. Ecol. 53, 1-19. Edwards R. W., Duffleld A. N. & Marshall E. J. F. (1978) Estimates of community metabolism of drainage channels from oxygen distributions. Proc. EWRS 5th Syrup. on Aquatic Weeds, pp. 295-302. Gameson A. L. H. & Truesdale G. A. (19591 Some oxygen studies in streams, d. Instn Wat. Engrs 13, 175-187. Goulder R. (1970) Day-time variations in the rates of production by two natural communities of submerged freshwater macrophytes. ,I. Ecol. 58, 521-528.

w.R.

15.,9--D

1085

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