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The use of chironomid pupal exuviae in the surveillance of sewage pollution within a drainage system
Water Research Vol. 13, pp. 887 to 894 Pergamon Press Ltd 1979. Printed in Great Britain
THE USE OF CHIRONOMID PUPAL EXUVIAE IN THE SURVEILLANCE OF SEWAGE POLLUTION WITHIN A DRAINAGE SYSTEM* J. D. McGILL, R. S. WILSON and A. M. BRAKE Department of Zoology, University of Bristol, England (Received 20 March 1979) Abstract--Collections of chironomid pupal exuvise were made during the summers of 1976 and 1977 abo~'e and below a number of sewage effluent discharges in the Bristol R. Avon drainage system. The effluents varied in their qualities, and discharged into rivers of different sizes. I t was found that the exuvial collections showed changes in their species composition and relative species abundance which matched the overall polluting effects of the various discharges, the most change occurring below effluents of poor quality discharging into small rivers. The pollutional effects were more marked during 1976 when the rivers had unusually low flow, than in 1977 when the river flows were higher. It is concluded that biological changes consequent on sewage effluent discharge into these rivers can be monitored by sampling the chironomid pupal exuviae above and below the site of discharge.
would be a drawback as most are visited only once or twice a year. To be widely applicable, the techHeUawell (1977), has suggested that no single macro- nique must be capable of providing an assessment invertebrate sampling method is applicable to all sizes of water quality on the basis of perhaps a single field of water course, as it is difficult to sample all aquatic collection. habitats e.g. weed, stones, mud, etc. with equal effecThe present study shows that although seasonaiity tiveness. However, the collection of chironomid pupal modifies the pattern of relative abundance observed, exuviae can give comparable data from sites through- a single collection, if taken at an optimum time of out the length of a river from source to mouth as year when the pattern of emergence is most stable samples are composed of material derived from and (late May/June to late August/September), can prointegrated over a stretch of river, and are not limited vide such an assessment. to any specific microhabitat. This allows for the direct •This paper deals with the interpretation of exuvial comparison between, for example, organically pol- collections as an estimation of the effect of organic luted sites of all sizes within a drainage basin, whether .~loading from a number of sewage treatment works these are on shallow, eroding or deeper depositing n o n the Bristol Avon drainage system (Avon, England). reaches. The study was aimed at investigating the seasonal Previous investigations using the potentially useful 'and long-term variation in chironomid emergence at chironomids as indicator organisms have been ham- two points of sewage discharge as well as "above and pered because they have concentrated on the larval below" differences at a total of seven sites. stages, which are often difficult to distinguish. IdentifiThe following collections were made: cation of the pupal exuviae or skins is much easier, (a) Over a one-year period from August 1976 to and in a previous paper (Wilson & McGilI, 1977k August 1977, at the Chew Stoke site on the River the collection of exuviae which accumulate with the Chew sampled previously (Wilson & McGiil, 1977), general flotsam of streams along the banks and where at 2-3 month intervals a sequence of closelyagainst obstructions, was put forward as a quick and spaced samples was taken spanning the effluent outeasy means of monitoring lotic chironomid communi- fall. ties in relation to water quality. (b) Over the spring and summer period" of 1977, Objections to the method were outlined, the major in the vicinity of the outfail from a large works on one being that, although all aquatic Chironomidae the lowland section of the Bristol Avon at Saltford. undergo eclosion at the water surface, the exuviai Here, pairs of "above and below" samples were taken samples are subject to seasonal variations dependent along extensive sections of the river upstream and on the emergence pattern of the various chironomid downstream from the point of discharge. taxa. (c) At a total of seven sewage treatment works In view of the large number of sites monitored rouwithin the Bristol Avondrainage system (including tinely by Water Authority biologists, such seasonality Chew Stoke and Saltford), with a wide range of river * The work was supported by a grant from the Natural flow/effluent flow dilution ratios. These were examEnvironment Research Council. ined in August 1976 and again in August 1977, when INTRODUCTION
887
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Fig. 1. Graph of monthly total rainfall falling within the Bristol Avon catchment over the three-year period 1975-1977. pairs of "above and below" samples were taken at each site on each occasion. Precipitation within the Bristol Avon catchment was markedly different over the two years of study as can be seen from Fig. 1. A total of only 256 mm of rain fell between October 1975-March 1976 which, coupled with the low rain-fall throughout the remainder of 1976, gave rise to the extremely low
river flows and general drought conditions which prevailed during the spring and summer months. However, from October 1976 through until March 1977 precipitation wa~ high, a total of 645 mm of rain falling within this period. It is evident that the first year (1976) of this study coincided with extremes of low river flow, high water temperatures and reduced effluent dilution during the
J Bristol Saltford ChewStoke
Bath
f/-
7-
t
Rode d ( (~
10 Km I
Dilton oMarsh
20 I
Fig. 2. Sketch map of the Bristol Avon drainage system, indicating the position of the sewage treatment works studied.
Surveillance of sewage pollution
889
Table 1. List of sewage treatment works in order of size of the receiving streams, together with effluent dilution ratios
Dilton Marsh Thingley Trowbridge Box Chew Stoke Rode Saltford
Minimum recorded river flow, MI day -1
Dilution ratio river: effluent
0.08 0.30 4.9 11 14 17 101
0.41 : 1 0.12:1 0.34:1 16 :1 10 :1 154 :1 4.5:1
prevailing drought conditions. It was therefore to be expected that the effects of the various discharges would be heightened in 1976, compared to 1977 which was a more normal year.
LOCALITY AND METHODS
All the sewage treatment works examined were within the Bristol Avon drainage system. Figure 2 is a sketch map of the study area showing the location of the sewage works together with major conurbations. Flotsam samples containing the pupal exuviae were taken from along the river margins with a fine mesh (250/an) long-handled pond net. The material was processed by wet sieving as previously described in Wilson & McGill (1977). Identification of the pupal skins was largely accomplished with the aid of keys to the pupal exuviae of the British Chirononmidae prepared by Mr P. H. Langton (unpublished).
RESULTS
Table 1 lists the sites in order of increasing stream discharge. It can be seen that the size range of the rivers examined is.large; the smallest sewage receiving water Course being the Allenbourne Stream at Dilton Marsh (0.08 MI day-t) and the largest, the Saltford site on the main River Avon (101 M1 day-1). Table 2 gives effluent quality data collected from May to August 1976. By comparing Tables 1 and 2 it can be seen that low quality effluents (i.e. those with a high suspended solids load and high BODs) were discharged into both large and small water courses. Although all exuviae were identified to species or species group, the data are produced in Figs. 345 in a simplified form, showing only the major sub-families, tribes and selected genera. The number code designations are a simplification of those proposed by Maitland (1977). (a) Chew Stoke sewage works A series of bar-charts depicting the results for the year-long study at Chew Stoke is given in Fig. 3. At
Table 2. Quality of sewage treatment works effluents, May-August 1976. All figures are ppm in the effluent
Dilton Marsh Thingley Trowbridge Box Chew Stoke Rode Salfford
max. mill. mean max. mm. mean max. mln. mgan max. mln. meall lnal. mm. mean max. mln. mean max. mln. mcatn
BOD5
SS
Ammonia
7 4 6 41 3 14 348 5 25 22 5 11 28 7 17 9 3 5 38 3 17
2O 11 15 117 3 18 147 4 37 30 4 21 60 11 34 17
4.O 1.0 2.2 11.0 0.1 1.9 38.0
0.2 2.4 2.0 0.2 1.1 4.0
0.6 2.3 1.0
8
0.4
13 49 11 24
0.8 14.0 1.2 5.5
J. D. McGILL, R. S. WILSONand A. M. BRAKE
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Fig. 3. Bar charts depicting the relative abundance of the major sub-families, tribes and Chironomus riparius (black shading) in seasonal sequences of samples spanning the effluent outfail at Chew Stoke. Stations were spaced at intervals of approximately 130m, upstream and downstream from the point of discharge. this site a dense population of Chironomus riparius Meigen larvae existed below the outfall. Larvae of C. riparius are often found in water courses receiving organic enrichment from sewage works (Edwards, 1957}; frequently achieving dominance within the polluted zone (Gower & Buckland, 1978; Learner & Edwards, 1966; Pain & Gaufin, 1956). The proportions of C. riparius are shown separately by the black shading in each sample. The proportions of the C. riparius exuviae taken in the August 1976 collections reflected the enrichment caused by the sewage, and the subsequent selfpurification of the system with increasing distance downstream. In the autumn collections the pattern of exuvial distribution was similar to that obtained in the summer. Although qualitative examination of the benthos by dredge sampling revealed that larvae were still present in April 1977 only a small irregularly distributed number of C. riparius exuviae were found to be hatching and this type of uneven distribution was maintained throughout the remainder of the study period. The August patterns for the two years 1976/1977 are therefore not "the same and the dominance of C. riparius was not recorded in the second year. (b) Salq'ord sewage works The samples taken at Saltford above and below the sewage outfall (Fig. 4) are for the most part simi-
lar to each other, except for the seasonal difference between the winter (February-March) and summer (May-August) exuvial patterns. A comparison of results from above and below the point of discharge indicates that this effluent had little effect on the benthic Chironomidae of the River Avon during 1977. Within the family, there was a tendency for the Tanypodinae and Chironomini to decrease in abundance below the outfall throughout the summer period although within the latter there is an increase in the abundance of Glyptotendipes sp. Changes can also be seen in two species of Orthocladiinae (Cricotopus bicinctus and C. sylvestris), which also increase in abundance below the outfall in summer. The latter three types have been associated with organic enrichment by Hynes (1960) and Hobart (1975). (c) Sewage works survey August 1976/1977 The results from the August 1976/1977 survey at all seven sewage treatment works are given as barcharts in Figs. 5 and 6. The two sets of samples allow for a comparison of results from two successive years. Despite the fact that some of the samples contain low numbers of exuviae, the pattern is reproducible although there is an overall shift in emphasis away. from those forms tolerant of organic pollution within the tribe Chironomini in the second year. The data showed marked differences between the above and below samples at certain sites (e.g. Thing-
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Fig. 4. Bar charts showing seasonal difference in relative abundance o f the major chironomid taxa in samples taken above and below the effluent ouffall at Saltford, between February and August 1977. For explanation of numbers, see text.
ley, Chew Stoke and Trowbridgek while at others (e.g. Box and Saltford), changes in the fauna are minor. The shift away from the organic pollution tolerant species in 1977 is apparent from the data. Most obviously samples contain fewer C. riparius than in 1976 and overall the 1977 samples reveal that less effect is being felt in the receiving streams from the sewage loading during that year of normal river flow, than in the previous drought year.
The similarities between stations above and below the outfalls over the two-year period can be seen in the dendrograms of cluster analysis performed on the relative abundance data for all species from each site (Fig. 7). The levels of similarity between stations are low in both years, except for the grouping of Chew Stoke, Trowbridge and Thingley with high similarity below their effluents in 1976 and Of Chew Stoke and Thingley in 1977. 6:~rrrt:76
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Fig. 5. Bar charts showing relative abundance of the major chironomid taxa in samples taken above and below all seven sewage treatment works on 6 August 1976. For key to shadinl see Fig. 4.
892
J.D. McG[LL, R. S. WmSONand A. M. BRAKE 8:'o'Trr: 77
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Fig. 6. Bar charts showing relative abundance of the major chironomid taxa in samples taken above and below all seven sewage treatment works on 8 August 1977. For key to shading see Fig. 4.
DISCUSSION The phonological patterns obtained from Chew Stoke and S~ltford are typical of the seasonal changes in chironomid emergence which have been outlined for North American and British drainage systems by e.g. Coffman (1973) and Wilson (1977) respectively. Throughout the winter months, emergence from water courses of all, sizes is composed of a restricted
number of species within the Orthocladiinae and Tanytarsini. In the swing, there is a steep increase in the number of emerging taxa of all types, but it is not until late May or early June that, where present, the Chironomini and to a lesser extent the Tanypodinae emerge in any numbers. The authors consider it essential in the context of monitoring organic enrichment by the exuvial
1976
1977 Below
Above 7
6342
7
56
23
Above I 4
627:5
Below 4
7 6 523
14
75
5O
%
[
25
r
I Chew Stoke
4 Dilton marsh
2 Trowbridge
5 Rode
:3 ThinQley
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Fig. 7. Dendrograms of cluster analysis performed on the relative abundance data for all taxa taken from above and below the seven sewage treatment works in 1976 and 1977.
Surveillance of sewage pollution
893
method, that the Chironomini should be included in exert similar effects on both eroding and depositing the sample collections, as many forms favoured by river reaches, by blanketing out the bed and ialilling organic enrichment belong to this tribe. Even on spaces between larger substrate elements. The assemrapid, eroding reaches, particulate matter deposited blage of chironomid species favoured by the imposiby the discharge provides a suitable microhabitat for tion of such conditions may be considered to form these largely mud-living forms (Hynes, 1960; Oliver, a tolerant "pollutional-community". In the rivers 1971). For this reason it is recommended that exuvial studied it is composed of types within the genera Crisampling programmes aimed at the long-term moni- cotopus, Chironomus (especially C. riparius), Giyptotentoring of organically loaded point discharges should dipes and in some cases Microspectra. The effects noted within this study ranged from a be focused on the period from late May or June to gross influx of C. riparius where more extreme condiSeptember. At both Chew Stoke and Saltford, the Chironomini tions prevailed, to a shift of balance in favour of one constitute a major proportion of exuvial collections or more of the other groups. Changes in the composition of the chironomid throughout the summer months. C. riparius was dominant at the former site, but at Saltford it was fauna detected by the exuvial surveillance method inreplaced by a species of Glyptotendipes. Members of dicate that the generally increased river discharges this genus are often associated with C. riparius in seen in 1977 resulted in the effluents examined having enriched conditions and it has been considered to be less effect on the rivers in the second year. There was indicative of polluted situations in its own right by a reduction in relative abundance of those chironoKimerle & Anderson (1971). At Saltford it is however mid taxa associated with organic loading. The exuvial method of sampling the chironomid only present in low numbers, the more common taxa within the Chironomini being Parachironomus sp. and fauna of various water bodies has been used previously in taxonomic and ecological studies (Brundin, Limnochironomus sp. The chironomid communities monitored in this 1966; Coffman, 1973; Fittkau, 1962; Hirvenoja, 1973; study show a variable response to the alteration of Humphries, 1938; Lehmann, 1971; Reiss, 1968; conditions between the two years. In particular, those Thienemann, 1910; Wilson, 1977), usually in conjuncstations exhibiting a large C. riparius population in tion with standard larval collections. None of these 1976, i.e. Chew Stoke, Thingley and Trowbridge workers have put forward the method as a means (those with the worst effluent quality in Table 2 and of either typing rivers, an application proposed by forming an adjacent group with high similarity in Wilson & Bright (1973) or assessing the effect of a Fig. 7) have a lower percentage of this type in 1977. polluting discharge as outlined in Wilson & McGill The change at Trowbridg¢ is the most spectacular, (1977). However, the exuvial technique can detect both there being no C. riparius exuviae in the 1977 sample. It is evident that prevailing conditions favoured Glyp- short and long-term changes in water quality and is totendipes sp. which increased from a relative abun- likely to provide a useful adjunct to standard surveildance of 31% in 1976 to 82% in 1977. lance methods, especially at deep water sites where The cluster analysis dendrograms shows an in- benthic samples are more difficult to secure. crease in similarity of the chironomid community Acknowledgements---We are grateful to Mr P. H. Langton below the worst polluted outfalls as compared with for kind permission to use his unpublished keys and to that above. All the other stations group horizontally Mr K. Roberts, Chief Executive of the Wessex Water Authin accordance with the overall size and nature of the ority for permission to use chemical data on the quality of sewage effluent, river water flows and rainfall. receiving stream and its flow regime. Conditions throughout the catchment in 1976 REFERENCES favoured the mud-living warm adapted Chironomini at the expense of the largely cold-adapted Orthocla- Brundin L. (1966) Transantarctic relationships and their diinae (Oliver, 1971). The trend away from the Chirsignificance, as evidenced by chironomid midges. K. svenska Vetensk Acad. HandL 11, 1-472. onomini in 1977 is best seen in the Chew Stoke data (Fig. 3), although it is a shift which occurs in all the Coffman W. P. (1973) Energy flow in a woodland stream ecosystem. II. The taxonomic composition and phenosamples taken in the second year. This was probably logy of the Chironomidae as determined by the collec-' due to the influence of higher flows and lower temtion pupal exuviae. Arch. Hydrobiol. 71,281-322. peratures thereby giving greater effluent dilutions and Edwards R. W. (1957) Vernal sloughing of sludge deposits in a sewage effluent channel. Nature, Lond. 180, 100. reducing oxygen stress. E. J. (1962) Die Tanypodinae. Akademie Verlag, The effect of an organically loaded effluent on the Fittkau Berlin. benthic community of the receiving stream will vary Gower A. M. & Buckland P. J. (1978) Water quality and in relation to the quality of the effluent, the dilution the occurrence of Chironomus riparius Meigen (Diptera: Chironomidae) in a stream receiving sewage effluent. ratio of efflent to stream flow, and the flow regime Freshwat. Biol. 8, 153-164. and bed topography of the receiving stream. HowHellawell J. (1977) Biological Surveillance and Water Quaever, as Hynes (1960) points out, organic inflows conlity Monitoring. In: Biological Monitorin0 of Inland Fishtaining a high suspended solids load, tend to favour eries. Edited by J. S. Alabaster. pp. 69-85. Applied the formation of stereotyped communities as they Science Publishers, London.
894
J.D. McGILL, R. S. Wn.SO~ and A. M. BRAKE
Hirvenoja M. (1973) Revision der Gattung Cricotopus van der Wulp und ihrer Verwandten (Diptera, Chironomidae). Ann. Zool. Fennici 10, 1-363. Hobart A. R. (1975) Some aspects of the biology and ecology of immature Chironomidae in the River Lee, a partially canalised river. Ph.D. Thesis, North east London Polytechnic, London, England. Humphries C. F. (1938) The Chironomid fauna of the Grosser Planer See, the relative density of its members and their emergence period. Arch. Hydrobiol. 33, 535-548. Hynes H. B. N. (1960) The Biolooy of Polluted Waters. Liverpool University Press, England. Kimerle R. A. & Anderson N. H. (1971) Production and bioenergetic role of the midge Glyptotendipes barbipes (Staeger) in a waste stabilization lagoon. Limnol. Oceanogr. 16, 646-659. Learner M. A. & Edwards R. W. (1966) The distribution of the midge Chironomus riparius Meigen in a polluted river system and its environs. Air. Wat. Pollut. Int. J. 10, 757-768. Lehmann J. (1971) Die Chironomiden der Fulda. Arch. HydrobioL Suppl. 37, 466-555. Maitland P. S. (1977) A Coded Check List of Animals
Occurring in Fresh Water in The British Isles. pp. 1-76. Natural Environment Research Council. Oliver D. R. (1971) Life history of the Chironomidae. A. Rev. Ent. 16, 211-230. Pain G. H. Jr. & Gaufin A. R. (1956) Aquatic Diptera as indicator of pollution in a mid-western stream. Ohio J. Sci. 56, 291-304. Reiss F. (1968) ()kologisehe und systematische Untersuchungen an Chironomiden (Diptera) des Bodensees. Arch. Hydrobiol. 64, 176-323. Reiss F. & Fittkau E. J. (1971) Taxonomie und Okologie europiiish verbreiterer Tanytarsus-Arten (Chironomidae, Diptera). Arch. Hydrobiol. Suppl. 40, 75-200. Thiennemann A. (1910) Das Sammeln yon Puppenh~iuten der Chironomiden. Eine Bitte un Mitarbeit. Arch. Hydrobiol. 6, 213-214. Wilson R. S. (1977) Chironomid pupal exuviae in the River Chew. Freshwat. Biol. 7, 9--17. Wilson R. S. & Bright P. L. (1973) The use of pupal exuviae for characterizing streams. Freshwat. Biol. 3, 283-302. Wilson R. S. & McGill J. D. (1977) A new method of monitoring water quality in a stream receiving sewage effluent, using Chironomid pupal exuviae. Water Res. !1, 959-962.
THE USE OF CHIRONOMID PUPAL EXUVIAE IN THE SURVEILLANCE OF SEWAGE POLLUTION WITHIN A DRAINAGE SYSTEM* J. D. McGILL, R. S. WILSON and A. M. BRAKE Department of Zoology, University of Bristol, England (Received 20 March 1979) Abstract--Collections of chironomid pupal exuvise were made during the summers of 1976 and 1977 abo~'e and below a number of sewage effluent discharges in the Bristol R. Avon drainage system. The effluents varied in their qualities, and discharged into rivers of different sizes. I t was found that the exuvial collections showed changes in their species composition and relative species abundance which matched the overall polluting effects of the various discharges, the most change occurring below effluents of poor quality discharging into small rivers. The pollutional effects were more marked during 1976 when the rivers had unusually low flow, than in 1977 when the river flows were higher. It is concluded that biological changes consequent on sewage effluent discharge into these rivers can be monitored by sampling the chironomid pupal exuviae above and below the site of discharge.
would be a drawback as most are visited only once or twice a year. To be widely applicable, the techHeUawell (1977), has suggested that no single macro- nique must be capable of providing an assessment invertebrate sampling method is applicable to all sizes of water quality on the basis of perhaps a single field of water course, as it is difficult to sample all aquatic collection. habitats e.g. weed, stones, mud, etc. with equal effecThe present study shows that although seasonaiity tiveness. However, the collection of chironomid pupal modifies the pattern of relative abundance observed, exuviae can give comparable data from sites through- a single collection, if taken at an optimum time of out the length of a river from source to mouth as year when the pattern of emergence is most stable samples are composed of material derived from and (late May/June to late August/September), can prointegrated over a stretch of river, and are not limited vide such an assessment. to any specific microhabitat. This allows for the direct •This paper deals with the interpretation of exuvial comparison between, for example, organically pol- collections as an estimation of the effect of organic luted sites of all sizes within a drainage basin, whether .~loading from a number of sewage treatment works these are on shallow, eroding or deeper depositing n o n the Bristol Avon drainage system (Avon, England). reaches. The study was aimed at investigating the seasonal Previous investigations using the potentially useful 'and long-term variation in chironomid emergence at chironomids as indicator organisms have been ham- two points of sewage discharge as well as "above and pered because they have concentrated on the larval below" differences at a total of seven sites. stages, which are often difficult to distinguish. IdentifiThe following collections were made: cation of the pupal exuviae or skins is much easier, (a) Over a one-year period from August 1976 to and in a previous paper (Wilson & McGilI, 1977k August 1977, at the Chew Stoke site on the River the collection of exuviae which accumulate with the Chew sampled previously (Wilson & McGiil, 1977), general flotsam of streams along the banks and where at 2-3 month intervals a sequence of closelyagainst obstructions, was put forward as a quick and spaced samples was taken spanning the effluent outeasy means of monitoring lotic chironomid communi- fall. ties in relation to water quality. (b) Over the spring and summer period" of 1977, Objections to the method were outlined, the major in the vicinity of the outfail from a large works on one being that, although all aquatic Chironomidae the lowland section of the Bristol Avon at Saltford. undergo eclosion at the water surface, the exuviai Here, pairs of "above and below" samples were taken samples are subject to seasonal variations dependent along extensive sections of the river upstream and on the emergence pattern of the various chironomid downstream from the point of discharge. taxa. (c) At a total of seven sewage treatment works In view of the large number of sites monitored rouwithin the Bristol Avondrainage system (including tinely by Water Authority biologists, such seasonality Chew Stoke and Saltford), with a wide range of river * The work was supported by a grant from the Natural flow/effluent flow dilution ratios. These were examEnvironment Research Council. ined in August 1976 and again in August 1977, when INTRODUCTION
887
888
J.D. MCGILL, R. S. WILSONand A. M. BRAKE
mm 200
1975
1976
1977
,,4,,
50
.... :,/...!/, F
M
A
M
J
J
A
S
0
N
O
d
F
M
A
M
d
d
A
S
0
N
0
J
F
RI
A
M
J
J
A
S
0
N
D
Fig. 1. Graph of monthly total rainfall falling within the Bristol Avon catchment over the three-year period 1975-1977. pairs of "above and below" samples were taken at each site on each occasion. Precipitation within the Bristol Avon catchment was markedly different over the two years of study as can be seen from Fig. 1. A total of only 256 mm of rain fell between October 1975-March 1976 which, coupled with the low rain-fall throughout the remainder of 1976, gave rise to the extremely low
river flows and general drought conditions which prevailed during the spring and summer months. However, from October 1976 through until March 1977 precipitation wa~ high, a total of 645 mm of rain falling within this period. It is evident that the first year (1976) of this study coincided with extremes of low river flow, high water temperatures and reduced effluent dilution during the
J Bristol Saltford ChewStoke
Bath
f/-
7-
t
Rode d ( (~
10 Km I
Dilton oMarsh
20 I
Fig. 2. Sketch map of the Bristol Avon drainage system, indicating the position of the sewage treatment works studied.
Surveillance of sewage pollution
889
Table 1. List of sewage treatment works in order of size of the receiving streams, together with effluent dilution ratios
Dilton Marsh Thingley Trowbridge Box Chew Stoke Rode Saltford
Minimum recorded river flow, MI day -1
Dilution ratio river: effluent
0.08 0.30 4.9 11 14 17 101
0.41 : 1 0.12:1 0.34:1 16 :1 10 :1 154 :1 4.5:1
prevailing drought conditions. It was therefore to be expected that the effects of the various discharges would be heightened in 1976, compared to 1977 which was a more normal year.
LOCALITY AND METHODS
All the sewage treatment works examined were within the Bristol Avon drainage system. Figure 2 is a sketch map of the study area showing the location of the sewage works together with major conurbations. Flotsam samples containing the pupal exuviae were taken from along the river margins with a fine mesh (250/an) long-handled pond net. The material was processed by wet sieving as previously described in Wilson & McGill (1977). Identification of the pupal skins was largely accomplished with the aid of keys to the pupal exuviae of the British Chirononmidae prepared by Mr P. H. Langton (unpublished).
RESULTS
Table 1 lists the sites in order of increasing stream discharge. It can be seen that the size range of the rivers examined is.large; the smallest sewage receiving water Course being the Allenbourne Stream at Dilton Marsh (0.08 MI day-t) and the largest, the Saltford site on the main River Avon (101 M1 day-1). Table 2 gives effluent quality data collected from May to August 1976. By comparing Tables 1 and 2 it can be seen that low quality effluents (i.e. those with a high suspended solids load and high BODs) were discharged into both large and small water courses. Although all exuviae were identified to species or species group, the data are produced in Figs. 345 in a simplified form, showing only the major sub-families, tribes and selected genera. The number code designations are a simplification of those proposed by Maitland (1977). (a) Chew Stoke sewage works A series of bar-charts depicting the results for the year-long study at Chew Stoke is given in Fig. 3. At
Table 2. Quality of sewage treatment works effluents, May-August 1976. All figures are ppm in the effluent
Dilton Marsh Thingley Trowbridge Box Chew Stoke Rode Salfford
max. mill. mean max. mm. mean max. mln. mgan max. mln. meall lnal. mm. mean max. mln. mean max. mln. mcatn
BOD5
SS
Ammonia
7 4 6 41 3 14 348 5 25 22 5 11 28 7 17 9 3 5 38 3 17
2O 11 15 117 3 18 147 4 37 30 4 21 60 11 34 17
4.O 1.0 2.2 11.0 0.1 1.9 38.0
0.2 2.4 2.0 0.2 1.1 4.0
0.6 2.3 1.0
8
0.4
13 49 11 24
0.8 14.0 1.2 5.5
J. D. McGILL, R. S. WILSONand A. M. BRAKE
890 Lore Summer
Winter
Autumn I
Spring
~rly Su~
iiii!i]!il I iiiiiiiiiiiil
~ Su~
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liiiiii]iiiiill ~ ' lii!iiiiiiill ~ i [ii!iiiiiii] ~
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1
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,~]]]]]]]]]]]]1 ~
l~. B ~.']]]]]i] 26 :/~':77
_
!~ ~::::ii]::]::l
14:~I':771
2:~q1':77
Fig. 3. Bar charts depicting the relative abundance of the major sub-families, tribes and Chironomus riparius (black shading) in seasonal sequences of samples spanning the effluent outfail at Chew Stoke. Stations were spaced at intervals of approximately 130m, upstream and downstream from the point of discharge. this site a dense population of Chironomus riparius Meigen larvae existed below the outfall. Larvae of C. riparius are often found in water courses receiving organic enrichment from sewage works (Edwards, 1957}; frequently achieving dominance within the polluted zone (Gower & Buckland, 1978; Learner & Edwards, 1966; Pain & Gaufin, 1956). The proportions of C. riparius are shown separately by the black shading in each sample. The proportions of the C. riparius exuviae taken in the August 1976 collections reflected the enrichment caused by the sewage, and the subsequent selfpurification of the system with increasing distance downstream. In the autumn collections the pattern of exuvial distribution was similar to that obtained in the summer. Although qualitative examination of the benthos by dredge sampling revealed that larvae were still present in April 1977 only a small irregularly distributed number of C. riparius exuviae were found to be hatching and this type of uneven distribution was maintained throughout the remainder of the study period. The August patterns for the two years 1976/1977 are therefore not "the same and the dominance of C. riparius was not recorded in the second year. (b) Salq'ord sewage works The samples taken at Saltford above and below the sewage outfall (Fig. 4) are for the most part simi-
lar to each other, except for the seasonal difference between the winter (February-March) and summer (May-August) exuvial patterns. A comparison of results from above and below the point of discharge indicates that this effluent had little effect on the benthic Chironomidae of the River Avon during 1977. Within the family, there was a tendency for the Tanypodinae and Chironomini to decrease in abundance below the outfall throughout the summer period although within the latter there is an increase in the abundance of Glyptotendipes sp. Changes can also be seen in two species of Orthocladiinae (Cricotopus bicinctus and C. sylvestris), which also increase in abundance below the outfall in summer. The latter three types have been associated with organic enrichment by Hynes (1960) and Hobart (1975). (c) Sewage works survey August 1976/1977 The results from the August 1976/1977 survey at all seven sewage treatment works are given as barcharts in Figs. 5 and 6. The two sets of samples allow for a comparison of results from two successive years. Despite the fact that some of the samples contain low numbers of exuviae, the pattern is reproducible although there is an overall shift in emphasis away. from those forms tolerant of organic pollution within the tribe Chironomini in the second year. The data showed marked differences between the above and below samples at certain sites (e.g. Thing-
Surveillance of sewage pollution 15:T1:77
/
21:rrt:77
,
04 ~'l
22:]~:77
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A
o4 !
B
891
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04
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/
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m
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i ~,, \ nos 268 ~
241
;
II I
~44J4
....
10:5
115
142
Tonypodinoe ----10rthoclodllnoe [ ~
Cricotopus ~
122
Chironomini
102 ~
Chironomus ~
105
1:50 160
Tanytarsini
Chironomusriporius
Micfopsectro
Glyptotendlpes
Fig. 4. Bar charts showing seasonal difference in relative abundance o f the major chironomid taxa in samples taken above and below the effluent ouffall at Saltford, between February and August 1977. For explanation of numbers, see text.
ley, Chew Stoke and Trowbridgek while at others (e.g. Box and Saltford), changes in the fauna are minor. The shift away from the organic pollution tolerant species in 1977 is apparent from the data. Most obviously samples contain fewer C. riparius than in 1976 and overall the 1977 samples reveal that less effect is being felt in the receiving streams from the sewage loading during that year of normal river flow, than in the previous drought year.
The similarities between stations above and below the outfalls over the two-year period can be seen in the dendrograms of cluster analysis performed on the relative abundance data for all species from each site (Fig. 7). The levels of similarity between stations are low in both years, except for the grouping of Chew Stoke, Trowbridge and Thingley with high similarity below their effluents in 1976 and Of Chew Stoke and Thingley in 1977. 6:~rrrt:76
1976 Dilton marsh
//
t °
Thlngley A
B
A.
A
B
Trowbridge B
A
B
N-..N..
04
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ili
Box
Chew Stoke
Rode A
Soltford B
A
B
105
270
o4
o, 04
.....
I
:::::
I
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?.4.
':'
ii! ...
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• ,•. . ~ .
nos.
23
113
8
293
99
231
I01
63
I0
118
iiii 51
125
Fig. 5. Bar charts showing relative abundance of the major chironomid taxa in samples taken above and below all seven sewage treatment works on 6 August 1976. For key to shadinl see Fig. 4.
892
J.D. McG[LL, R. S. WmSONand A. M. BRAKE 8:'o'Trr: 77
1977 Dilton marsh
Chew Stoke
Thingley
A
A
B
Box
B
A
B
A
B
\N
A
B '
O4 04
Salt ford
Rode
Trowbridge
A
B
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I /
04
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Ilillll
...
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..... ..... ..... .....
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.....
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I
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27
97
141
141
51
144
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.a4
112
131
139
151
. . . . .
.....
: : : : :
:::::
: : : : :
164
145
1 i
130
160
Fig. 6. Bar charts showing relative abundance of the major chironomid taxa in samples taken above and below all seven sewage treatment works on 8 August 1977. For key to shading see Fig. 4.
DISCUSSION The phonological patterns obtained from Chew Stoke and S~ltford are typical of the seasonal changes in chironomid emergence which have been outlined for North American and British drainage systems by e.g. Coffman (1973) and Wilson (1977) respectively. Throughout the winter months, emergence from water courses of all, sizes is composed of a restricted
number of species within the Orthocladiinae and Tanytarsini. In the swing, there is a steep increase in the number of emerging taxa of all types, but it is not until late May or early June that, where present, the Chironomini and to a lesser extent the Tanypodinae emerge in any numbers. The authors consider it essential in the context of monitoring organic enrichment by the exuvial
1976
1977 Below
Above 7
6342
7
56
23
Above I 4
627:5
Below 4
7 6 523
14
75
5O
%
[
25
r
I Chew Stoke
4 Dilton marsh
2 Trowbridge
5 Rode
:3 ThinQley
6 Box 7 Saltford
Fig. 7. Dendrograms of cluster analysis performed on the relative abundance data for all taxa taken from above and below the seven sewage treatment works in 1976 and 1977.
Surveillance of sewage pollution
893
method, that the Chironomini should be included in exert similar effects on both eroding and depositing the sample collections, as many forms favoured by river reaches, by blanketing out the bed and ialilling organic enrichment belong to this tribe. Even on spaces between larger substrate elements. The assemrapid, eroding reaches, particulate matter deposited blage of chironomid species favoured by the imposiby the discharge provides a suitable microhabitat for tion of such conditions may be considered to form these largely mud-living forms (Hynes, 1960; Oliver, a tolerant "pollutional-community". In the rivers 1971). For this reason it is recommended that exuvial studied it is composed of types within the genera Crisampling programmes aimed at the long-term moni- cotopus, Chironomus (especially C. riparius), Giyptotentoring of organically loaded point discharges should dipes and in some cases Microspectra. The effects noted within this study ranged from a be focused on the period from late May or June to gross influx of C. riparius where more extreme condiSeptember. At both Chew Stoke and Saltford, the Chironomini tions prevailed, to a shift of balance in favour of one constitute a major proportion of exuvial collections or more of the other groups. Changes in the composition of the chironomid throughout the summer months. C. riparius was dominant at the former site, but at Saltford it was fauna detected by the exuvial surveillance method inreplaced by a species of Glyptotendipes. Members of dicate that the generally increased river discharges this genus are often associated with C. riparius in seen in 1977 resulted in the effluents examined having enriched conditions and it has been considered to be less effect on the rivers in the second year. There was indicative of polluted situations in its own right by a reduction in relative abundance of those chironoKimerle & Anderson (1971). At Saltford it is however mid taxa associated with organic loading. The exuvial method of sampling the chironomid only present in low numbers, the more common taxa within the Chironomini being Parachironomus sp. and fauna of various water bodies has been used previously in taxonomic and ecological studies (Brundin, Limnochironomus sp. The chironomid communities monitored in this 1966; Coffman, 1973; Fittkau, 1962; Hirvenoja, 1973; study show a variable response to the alteration of Humphries, 1938; Lehmann, 1971; Reiss, 1968; conditions between the two years. In particular, those Thienemann, 1910; Wilson, 1977), usually in conjuncstations exhibiting a large C. riparius population in tion with standard larval collections. None of these 1976, i.e. Chew Stoke, Thingley and Trowbridge workers have put forward the method as a means (those with the worst effluent quality in Table 2 and of either typing rivers, an application proposed by forming an adjacent group with high similarity in Wilson & Bright (1973) or assessing the effect of a Fig. 7) have a lower percentage of this type in 1977. polluting discharge as outlined in Wilson & McGill The change at Trowbridg¢ is the most spectacular, (1977). However, the exuvial technique can detect both there being no C. riparius exuviae in the 1977 sample. It is evident that prevailing conditions favoured Glyp- short and long-term changes in water quality and is totendipes sp. which increased from a relative abun- likely to provide a useful adjunct to standard surveildance of 31% in 1976 to 82% in 1977. lance methods, especially at deep water sites where The cluster analysis dendrograms shows an in- benthic samples are more difficult to secure. crease in similarity of the chironomid community Acknowledgements---We are grateful to Mr P. H. Langton below the worst polluted outfalls as compared with for kind permission to use his unpublished keys and to that above. All the other stations group horizontally Mr K. Roberts, Chief Executive of the Wessex Water Authin accordance with the overall size and nature of the ority for permission to use chemical data on the quality of sewage effluent, river water flows and rainfall. receiving stream and its flow regime. Conditions throughout the catchment in 1976 REFERENCES favoured the mud-living warm adapted Chironomini at the expense of the largely cold-adapted Orthocla- Brundin L. (1966) Transantarctic relationships and their diinae (Oliver, 1971). The trend away from the Chirsignificance, as evidenced by chironomid midges. K. svenska Vetensk Acad. HandL 11, 1-472. onomini in 1977 is best seen in the Chew Stoke data (Fig. 3), although it is a shift which occurs in all the Coffman W. P. (1973) Energy flow in a woodland stream ecosystem. II. The taxonomic composition and phenosamples taken in the second year. This was probably logy of the Chironomidae as determined by the collec-' due to the influence of higher flows and lower temtion pupal exuviae. Arch. Hydrobiol. 71,281-322. peratures thereby giving greater effluent dilutions and Edwards R. W. (1957) Vernal sloughing of sludge deposits in a sewage effluent channel. Nature, Lond. 180, 100. reducing oxygen stress. E. J. (1962) Die Tanypodinae. Akademie Verlag, The effect of an organically loaded effluent on the Fittkau Berlin. benthic community of the receiving stream will vary Gower A. M. & Buckland P. J. (1978) Water quality and in relation to the quality of the effluent, the dilution the occurrence of Chironomus riparius Meigen (Diptera: Chironomidae) in a stream receiving sewage effluent. ratio of efflent to stream flow, and the flow regime Freshwat. Biol. 8, 153-164. and bed topography of the receiving stream. HowHellawell J. (1977) Biological Surveillance and Water Quaever, as Hynes (1960) points out, organic inflows conlity Monitoring. In: Biological Monitorin0 of Inland Fishtaining a high suspended solids load, tend to favour eries. Edited by J. S. Alabaster. pp. 69-85. Applied the formation of stereotyped communities as they Science Publishers, London.
894
J.D. McGILL, R. S. Wn.SO~ and A. M. BRAKE
Hirvenoja M. (1973) Revision der Gattung Cricotopus van der Wulp und ihrer Verwandten (Diptera, Chironomidae). Ann. Zool. Fennici 10, 1-363. Hobart A. R. (1975) Some aspects of the biology and ecology of immature Chironomidae in the River Lee, a partially canalised river. Ph.D. Thesis, North east London Polytechnic, London, England. Humphries C. F. (1938) The Chironomid fauna of the Grosser Planer See, the relative density of its members and their emergence period. Arch. Hydrobiol. 33, 535-548. Hynes H. B. N. (1960) The Biolooy of Polluted Waters. Liverpool University Press, England. Kimerle R. A. & Anderson N. H. (1971) Production and bioenergetic role of the midge Glyptotendipes barbipes (Staeger) in a waste stabilization lagoon. Limnol. Oceanogr. 16, 646-659. Learner M. A. & Edwards R. W. (1966) The distribution of the midge Chironomus riparius Meigen in a polluted river system and its environs. Air. Wat. Pollut. Int. J. 10, 757-768. Lehmann J. (1971) Die Chironomiden der Fulda. Arch. HydrobioL Suppl. 37, 466-555. Maitland P. S. (1977) A Coded Check List of Animals
Occurring in Fresh Water in The British Isles. pp. 1-76. Natural Environment Research Council. Oliver D. R. (1971) Life history of the Chironomidae. A. Rev. Ent. 16, 211-230. Pain G. H. Jr. & Gaufin A. R. (1956) Aquatic Diptera as indicator of pollution in a mid-western stream. Ohio J. Sci. 56, 291-304. Reiss F. (1968) ()kologisehe und systematische Untersuchungen an Chironomiden (Diptera) des Bodensees. Arch. Hydrobiol. 64, 176-323. Reiss F. & Fittkau E. J. (1971) Taxonomie und Okologie europiiish verbreiterer Tanytarsus-Arten (Chironomidae, Diptera). Arch. Hydrobiol. Suppl. 40, 75-200. Thiennemann A. (1910) Das Sammeln yon Puppenh~iuten der Chironomiden. Eine Bitte un Mitarbeit. Arch. Hydrobiol. 6, 213-214. Wilson R. S. (1977) Chironomid pupal exuviae in the River Chew. Freshwat. Biol. 7, 9--17. Wilson R. S. & Bright P. L. (1973) The use of pupal exuviae for characterizing streams. Freshwat. Biol. 3, 283-302. Wilson R. S. & McGill J. D. (1977) A new method of monitoring water quality in a stream receiving sewage effluent, using Chironomid pupal exuviae. Water Res. !1, 959-962.