HEAVY METALS IN WATER, P E R I P H Y T O N I C ALGAE, DETRITUS, AND INSECTS F R O M T W O STREAMS IN SHILLONG, N O R T H E A S T E R N INDIA A. GUPTA Dept. of Zoology, St. Edmund's College, Shillong 793 003, India
(ReceivedJanuary 1995; revisedJune 1995) Abstract. Cadmium, copper, manganese,lead and zinc concentrationswere determinedin water, periphytonic algae, detritus, and larvae of three aquatic insects, viz. Baetis sp. (Ephemeroptera: Baetidae), Hydropsyche sp. (Trichoptera:Hydropsychiidae),and Chironomus ramosus Choudhury and Das (Diptera: Chironomidae),during two surveysfrom an unpollutedand a polluted stream in Shillong, MeghalayaState, NortheasternIndia. Metal concentrationswere higher in all the samples from the pollutedstream,possiblyreflectingthe contributionsfromvariousnon-pointsourcesof metal contaminationpresentin its catchment.Amongthe threeinsects,Baetis sp. was foundto accumulate cadmium, copper, and zinc, and Hydropsyche sp. manganese,to concentrationssignificantlyhigher than those found in the other taxa. The concentrationsof all the metals were higher in fine detritus than in C. ramosus, althoughCd and Zn concentrationswere elevatedin Baetis as comparedto those in periphytonicalgae and fine detritus, indicating possible bioconcentrationof these metals by this animal.
1. Introduction Contamination of freshwater ecosystems by heavy metals and the accumulation of these elements by invertebrate animals have been studied at length in temperate latitudes (e.g. Enk and Mathis, 1977; Brown, 1977; Namminga and Wilhm, 1977; Burrows and Whitton, 1983; Krantzberg, 1989; Alikhan et al., 1990; Keenan and Alikhan, 1991; Mwangi and Alikhan, 1993). In contrast, information from tropical and sub-tropical lakes and rivers is very poor and incomplete (Kakulu and Osibanjo, 1986; Ajmal et al., 1987; Kakulu et al., 1987; Biney and Beeko, 1991). The present study records the concentrations of cadmium, copper, manganese, lead, and zinc in the water, periphytonic algae, detritus, and the larvae of three aquatic insects from two streams in Shillong, Meghalaya State, India. Heavy metal contamination in Shillong, and in the northeastern region of India in general, occurs to a large extent from diffuse, non-point sources such as untreated sewage from houses, small industries, agricultural lands, road surfaces, stone-sand quarries, and automobile workshops and service centres (Nath and Gupta, 1993; Gupta, 1995). The objective of this study was, therefore, to assess the extent of contamination and bioconcentration of metals solely from non-point sources, as opposed to those from large-scale mining and major industrial activities prevalent in developed countries and in the heavily industrialized regions of India as well. Of the three species of aquatic insects investigated in the present study, only C h i r o n o m u s rarnosus Choudhury and Das could be identified to the species level. Environmental Monitoring and Assessment 40:215-223, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.
216
A. GUPTA
However, our earlier studies (Gupta and Michael, 1983, 1992; Gupta, 1993) had confirmed that larvae designated as Baetis sp. and Hydropsyche sp. represented a single species, although no specific name could be assigned to either of them due to the lack of suitable taxonomic keys.
2. Materials and Methods 2.1. STUDYAREA Collections were made during January and August 1992 from two streams in and around Shillong city (25~ 91~ Meghalaya State, India. Shillong lies at an altitude of ca. 1500 m a.s.1, and receives an average annual rainfall of around 2000 mm. The rocks in the area are mostly quartzites with intrusions of granites, metamorphosed basic rocks called 'Khasi Greenstones', phyllites, and others (Mazumder, 1986). Collection site 1 was located on Wah Diengling stream, a tributary of Umshirpi stream, where it flows through a forest reserve in the outskirts of Shillong city, and site 2 on Umkhrah stream flowing inside the city (Figure 1). The catchment area of Umkhrah near site 2 has a dense urban build-up with roads carrying much motorized traffic, small metal fabrication units, automobile workshops and service centres. Untreated sewage is discharged into the stream at several points. The catchment also has a number of stone-sand quarries, while plots of agricultural land are located a few kilometres upstream. 2.2. SAMPLECOLLECTIONAND ANALYSIS Water samples for metal analysis were collected in clean, acid-washed PVC bottles, acidified with concentrated HNO3 (1 ml 1-1), stored at 4 ~ and analysed (without filtering) within 2-3 days. Fine detritus (only at site 2) was collected with a PVC scoop from deposits in and around Chironomus ramosus beds, and by scraping with a PVC scraper from stones on the stream bed. Periphytonic algae were collected with a brush from stone surfaces, while leaf detritus was collected manually from the interstices of stones. Baetis and Hydropsyche larvae were collected from both the sites by kick sampling into a net of 200 #m mesh, and C. ramosus larvae with a PVC scoop from site 1 only. All insects were taken alive to the laboratory and kept in filtered and aerated stream water from the corresponding sites for 12 hours to eliminate food in the digestive tract. Three to four replicate samples of water, algae, detritus, and each insect taxon were collected during each survey. Algae, detritus, and insect samples were dried at 70 ~ to constant weight, weighed and digested in 10 ml concentrated HNO3 to dryness, and redissolved in 20 ml 10% HNO3 (Mwangi and Alikhan, 1993). Analyses for Cd, Cu, Mn, Pb, and Zn were carried out on a Perkin-Elmer 2380 flame atomic absorption spectrophotometer. The readings were checked with those for standard solutions, and contamination errors minimized by using blanks, acid-washed glasswares,
217
HEAVY METALS IN SHILLONG, NORTHEASTERN INDIA
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Fig. 1. Map of Shillong showing: (a) collection sites 1 and 2, on Wah Diengling (W.D.), a tributary of the Umshirpi stream, and on the Umkhrah stream, respectively; (b) major traffic arteries shown as dotted lines; and (c) stone and sand quarries shown as triangles.
218
A. GUPTA
TABLE I Concentrations of Cd, Cu, Mn, Pb and Zn (mean 4- S. D.) in Water (unfiltered) from Wah Diengling (Site 1) and Umkhrah (Site 2) streams in Shillong, Meghalaya State, India, based on surveys made in January and August 1992
Site
Metal concentrations (#g 1- t) (n=8) Cd Cu Mn Pb
1
Hydropsyche
Baetis > Hydropsyche > C. ramosus
Cu Mn Pb Zn
Baetis > Hydropsyche
Baetis > Hydropsyche "~ C. ramosus
Hydropsyche > Baetis
Hydropsyche > Baetis > C. ramosus
Baetis ~_ Hydropsyche
Baetis ~- Hydropsyche ~ C. ramosus
Baetis > Hydropsyche
Baetis > Hydropsyche '~ C. eamosus
m e t a l i n p u t s f r o m p a r t i c u l a t e m a t t e r g e n e r a t e d b y v a r i o u s h u m a n a c t i v i t i e s in the c a t c h m e n t t h a t e n t e r s the s t r e a m a n d e v e n t u a l l y g e t s d e p o s i t e d o n t h e s t r e a m bed. 3.2.
METALS IN INSECTS
T h e c o n c e n t r a t i o n s o f the d i f f e r e n t m e t a l s in B a e t i s sp. a n d H y d r o p s y c h e sp. (at sites 1 a n d 2) a n d in C. r a m o s u s (at site 2 o n l y ) are p r e s e n t e d in T a b l e III, w h i l e
HEAVY METALS IN SHILLONG, NORTHEASTERN INDIA
221
Table IV summarizes the metal accumulation patterns in the three taxa. When the same taxon (viz. Baetis or Hydropsyche) was present both at sites 1 and 2, the concentrations of all the metals in animals from the latter site were higher. Again, this was peralleled by higher concentrations in water and detritus at site 2. Similar findings were reported in some other investigations (Brown, 1977; Burrows and Whitton, 1983; Sumi et al., 1991). Table IV demonstrates that the degree of accumulation of different metals varied considerably among taxa at both the study sites. Thus, Baetis was found to accumulate cadmium, copper, and zinc to levels significantly higher than the others; Hydropsyche accumulated more of manganese; while none of the three taxa showed any clear order of preference for lead. Burrows and Whitton (1983) found that mayflies as a group accumulated higher amounts of zinc, cadmium, and lead than the other invertebrates, while Sumi et al. (1991) observed a higher affinity for zinc than copper and iron in the mayfly Baetis thermicus. They suggested that the relatively high zinc concentrations were likely to be due to its accumulation in all the tissues such as mid-gut epithelium, fat bodies and muscles, while copper and iron were more localized in specific areas of the mid-gut epithelium only. The present study may not be considered detailed enough to comment on the bioconcentration of metals by insects, especially as neither shredders nor carnivorous species were investigated. Nevertheless, metal levels were higher in fine detritus than in C. ramosus, suggesting an absence of bioconcentration by this insect. In contrast, Baetis sp., which is known to feed upon periphytonic algae and detrital deposits on stones (Gupta and Michael, 1981), exhibited some bioconcentration of cadmium and zinc (Tables I! and III). Several studies reveal that the exact relationship between metal concentrations in food materials and those in aquatic invertebrates differ according to metal and animal. Thus, copper, zinc, and lead in chironomids were found to be higher than in adjacent sediments, while chromium was lower (Namminga and Wilhm, 1977). Again, copper and zinc in leeches and isopod crustaceans were found to decrease with increasing sediment concentrations, while lead increased (Eyres and Pugh-Thomas, 1978). The present study also points towards the existence of different degrees of metal bioconcentration in different aquatic insects. However, neither size-dependent variations (Rainbow and Moore, 1986; Krantzberg, 1989) nor seasonal differences (Ritz et al., 1982; Ireland, 1984; Krantzberg, 1989) in metal concentrations within the same taxon were investigated in the present study as well as in most others on metal bioconcentration and biomagnification in aquatic insects. It might be prudent to assess the influence of these factors on food chain concentration of metals before making any generalization.
Acknowledgement I am thankful to Dr. Jason M. Weeks for kindly reviewing the manuscript.
222
A. GUPTA
References
Ajmal, M., Raziuddin, and Khan, A. U.: 1987, 'Heavy Metals in Water, Sediments, Fish and Plants of River Hindon, U.P., India', Hydrobiologia 148, 151-157. Alikhan, M. A., Bagatto, G. and Zia, S.: 1990, 'The Crayfish as a "Biological Indicator" of Aquatic Contamination by Heavy Metals', Water Res. 24, 1069-1076. Biney, C. A. and Beeko, C. A.: 1991, 'Trace Metal Concentrations in Fish and Sediment from the WIWI: Small Urban River in Kumasi, Ghana', Trop. Ecol. 32, 197-206. Brown, B. E.: 1977, 'Effects of Mine Drainage on the River Hayle, Cornwall. A) Factors Affecting Concentrations of Copper, Zinc and Iron in Water, Sediments and Dominant Invertebrate Fauna', Hydrobiologia 52, 221-233. Burrows, I. G. and Whitton, B. A.: 1983, 'Heavy Metals in Water, Sediments and Invertebrates from a Metal-Contaminated River Free of Organic Pollution', Hydrobiologia 106, 263-273. Elwood, J. W., Hildebrand, S. G. and Beauchamp, J. J.: 1976, 'Contribution of Gut Contents to the Concentration and Body Burden of Elements in Tipula spp. from a Spring-Fed Stream', J. Fish. Res. Board Canada 33, 1930-1938. Enk, M. D. and Mathis, B. J.: 1977, 'Distribution of cadmium and Lead in a Stream Ecosystem', Hydrobiologia 52, 153-158. Eyres, J. P. and Pugh-Thomas, M.: 1978, 'Heavy Metal Pollution of the River Irwell (Lancashire U.K.) Demonstrated by Analysis of the Substrate Materials and Macro-Invertebrate Tissue', Environ. Poll. 16, 129-136. Frrstner, U. and Wittmann, G. T. W.: 1981, Metal Pollution in the Aquatic Environment, SpringerVerlag, Berlin, Heidelberg. Gupta, A.: 1993, 'Life histories of Two Species of Baetis (Ephemeroptera: Baetidae) in a Small North-East Indian Stream', Archivfiir Hydrobiologie 127, 105-114. Gupta, A.: 1995, 'Impacts of Urbanization on Fresh Water Systems With Special Reference to NorthEast India', in J. B. Ganguly (ed.), Urbanization and Decelopment in North-East India - Trends and Policy Implications, Deep and Deep Publications, New Delhi, pp. 77-81. Gupta, A. and Michael, R. G.: 1981, 'Population Ecology and Feeding Propensities of Two CoExisting Species of Baetidae (Insecta: Ephemeroptera)', Ecology of Animal Populations, Zoological Survey of India, Pt. 2, 95-104. Gupta, A. and Michael, R. G.: 1983, 'Seasonal Differences and Relative Abundance Among Populations of Benthic Aquatic Insects in a Moderately High Altitude Stream', High Altitude Entomology and Wildlife Ecology, Zoological Survey of India, 21-28. Gupta, A. and Michael, R. G.: 1992, 'Diversity, Distribution and Seasonal Abundance of Ephemeroptera in Streams of Meghalaya State, India', Hydrobiologia 228, 131-139. Hewitt, C. N. and Candy, G, B. B.: I990, 'Soil and Street Dust Heavy Metal Concentrations in and Around Cuenca, Ecuador', Environ. Poll. 63, 129-136. Ireland, W. P.: 1984, 'Seasonal Changes in Zinc, Manganese, Mercury, Copper, and Calcium Content in the Digestive Gland of the Slug Arion ater L.', Comp. Biochem. Physiol. 78a, 855-858. Kakulu, S. E. and Osibanjo, O.: 1986, 'A Baseline Study of Mercury in Fish and Sediments in the Niger Delta Area of Nigeria', Environ. Poll. 11, 315-322. Kakulu, S. E., Osibanjo, O. and Ajayi, S. A.: 1987, 'Trace Metal Content of Fish and Shell Fishes of the Niger Delta Area of Nigeria', Environment Inter 13, 247-251. Keenan, S. and Alikhan, M. A.: 1991, 'Comparative Study of Cadmium and Lead Accumulations in Cambarus bartoni (Feb.) (Decapoda, Crustacea) from an Acidic and a Neutral Lake', Bull. Environ. Contamin. Toxicol. 47, 91-96. Krantzberg, G.: 1989, 'Metal Accumulation by Chironomid Larvae: the Effects of Age and Body Weight on Metal Body Burdens', Hydrobiologia 188/189, 497-506. Linberg, S. E. and Harris, R. C.: 1974, 'Mercury Enrichment in Estuarine Plant detritus', Mar. Poll. Bull. 5, 93-95. Mazumder, S. K.: 1986, 'The Precambrian Framework of Part of the Khasi Hills, Meghalaya', Geol. Survey oflndia Records, 117, (pt.2), 55 pp.
HEAVYMETALSIN SHILLONG,NORTHEASTERNINDIA
223
Mwangi, S. M. and Alikhan, M. A.: 1993, 'Cadmium and Nickel Uptake by Tissues of Cambarus bartoni (Astacidae, Decapoda, Crustacea): Effects on Copper and Zinc Stores', Water Res. 27, 921-927. Namminga, H. and Wilhm, J.: 1977, 'Heavy Metals in Water, Sediments, and Chrironomids', J. Water Poll. Control Fed. 49, 1725-1731. Nath, H. and Gupta, A.: 1993, 'Status of Environmental Pollution in Shillong (1981-91) - an Overview', in B. Datta Ray and S. K. Agnihotri (eds.), Dynamics of a Tribal Society - A MicroStudy, Uppal Publishing House, New Delhi (India), pp. 41-64. Rainbow, P. S. and Moore, P. G.: 1986, 'Comparative Metal Analysis in Amphipod Crustaceans', Hydrobiologia 141,273-289. Ritz, D. A., Swain, R. and Elliot, N. G.: 1982, 'Use of the Mussel Mytilus edulis planulatus (Lamarck) in Monitoring Heavy Metal Levels in Seawater', Austral. J. of Marine and Freshwater Res. 33, 491-506. Sumi, Y., Fukuoka, H. R., Murakami, T., Suzukui, T., Hatakeyama, S. and Sujuki, K. T.: 1991, 'Histochemical Localization of Copper, Iron and Zinc in the Larvae of the Mayfly Baetis thermicus Inhabiting a River Polluted with Heavy metals', Zool. Sci. 8, 287-293.
(ReceivedJanuary 1995; revisedJune 1995) Abstract. Cadmium, copper, manganese,lead and zinc concentrationswere determinedin water, periphytonic algae, detritus, and larvae of three aquatic insects, viz. Baetis sp. (Ephemeroptera: Baetidae), Hydropsyche sp. (Trichoptera:Hydropsychiidae),and Chironomus ramosus Choudhury and Das (Diptera: Chironomidae),during two surveysfrom an unpollutedand a polluted stream in Shillong, MeghalayaState, NortheasternIndia. Metal concentrationswere higher in all the samples from the pollutedstream,possiblyreflectingthe contributionsfromvariousnon-pointsourcesof metal contaminationpresentin its catchment.Amongthe threeinsects,Baetis sp. was foundto accumulate cadmium, copper, and zinc, and Hydropsyche sp. manganese,to concentrationssignificantlyhigher than those found in the other taxa. The concentrationsof all the metals were higher in fine detritus than in C. ramosus, althoughCd and Zn concentrationswere elevatedin Baetis as comparedto those in periphytonicalgae and fine detritus, indicating possible bioconcentrationof these metals by this animal.
1. Introduction Contamination of freshwater ecosystems by heavy metals and the accumulation of these elements by invertebrate animals have been studied at length in temperate latitudes (e.g. Enk and Mathis, 1977; Brown, 1977; Namminga and Wilhm, 1977; Burrows and Whitton, 1983; Krantzberg, 1989; Alikhan et al., 1990; Keenan and Alikhan, 1991; Mwangi and Alikhan, 1993). In contrast, information from tropical and sub-tropical lakes and rivers is very poor and incomplete (Kakulu and Osibanjo, 1986; Ajmal et al., 1987; Kakulu et al., 1987; Biney and Beeko, 1991). The present study records the concentrations of cadmium, copper, manganese, lead, and zinc in the water, periphytonic algae, detritus, and the larvae of three aquatic insects from two streams in Shillong, Meghalaya State, India. Heavy metal contamination in Shillong, and in the northeastern region of India in general, occurs to a large extent from diffuse, non-point sources such as untreated sewage from houses, small industries, agricultural lands, road surfaces, stone-sand quarries, and automobile workshops and service centres (Nath and Gupta, 1993; Gupta, 1995). The objective of this study was, therefore, to assess the extent of contamination and bioconcentration of metals solely from non-point sources, as opposed to those from large-scale mining and major industrial activities prevalent in developed countries and in the heavily industrialized regions of India as well. Of the three species of aquatic insects investigated in the present study, only C h i r o n o m u s rarnosus Choudhury and Das could be identified to the species level. Environmental Monitoring and Assessment 40:215-223, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.
216
A. GUPTA
However, our earlier studies (Gupta and Michael, 1983, 1992; Gupta, 1993) had confirmed that larvae designated as Baetis sp. and Hydropsyche sp. represented a single species, although no specific name could be assigned to either of them due to the lack of suitable taxonomic keys.
2. Materials and Methods 2.1. STUDYAREA Collections were made during January and August 1992 from two streams in and around Shillong city (25~ 91~ Meghalaya State, India. Shillong lies at an altitude of ca. 1500 m a.s.1, and receives an average annual rainfall of around 2000 mm. The rocks in the area are mostly quartzites with intrusions of granites, metamorphosed basic rocks called 'Khasi Greenstones', phyllites, and others (Mazumder, 1986). Collection site 1 was located on Wah Diengling stream, a tributary of Umshirpi stream, where it flows through a forest reserve in the outskirts of Shillong city, and site 2 on Umkhrah stream flowing inside the city (Figure 1). The catchment area of Umkhrah near site 2 has a dense urban build-up with roads carrying much motorized traffic, small metal fabrication units, automobile workshops and service centres. Untreated sewage is discharged into the stream at several points. The catchment also has a number of stone-sand quarries, while plots of agricultural land are located a few kilometres upstream. 2.2. SAMPLECOLLECTIONAND ANALYSIS Water samples for metal analysis were collected in clean, acid-washed PVC bottles, acidified with concentrated HNO3 (1 ml 1-1), stored at 4 ~ and analysed (without filtering) within 2-3 days. Fine detritus (only at site 2) was collected with a PVC scoop from deposits in and around Chironomus ramosus beds, and by scraping with a PVC scraper from stones on the stream bed. Periphytonic algae were collected with a brush from stone surfaces, while leaf detritus was collected manually from the interstices of stones. Baetis and Hydropsyche larvae were collected from both the sites by kick sampling into a net of 200 #m mesh, and C. ramosus larvae with a PVC scoop from site 1 only. All insects were taken alive to the laboratory and kept in filtered and aerated stream water from the corresponding sites for 12 hours to eliminate food in the digestive tract. Three to four replicate samples of water, algae, detritus, and each insect taxon were collected during each survey. Algae, detritus, and insect samples were dried at 70 ~ to constant weight, weighed and digested in 10 ml concentrated HNO3 to dryness, and redissolved in 20 ml 10% HNO3 (Mwangi and Alikhan, 1993). Analyses for Cd, Cu, Mn, Pb, and Zn were carried out on a Perkin-Elmer 2380 flame atomic absorption spectrophotometer. The readings were checked with those for standard solutions, and contamination errors minimized by using blanks, acid-washed glasswares,
217
HEAVY METALS IN SHILLONG, NORTHEASTERN INDIA
L 0
2 Km.
4
11
t%
/ }
,
i
/
I
/
/"
ri ' S T R E A
I
p.i
p~ t
it ./' 111 z r s l
~
~
-
.~....
"-'"
I. Ir
I
/
~-\
/
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~--J
,,,~/
1
',
',
~
~
I
%
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,"
,"
/
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,'/'+-
_ "'~?-.:;"
/
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//
]
,"'1
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/4'
/
/
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J
(I
...~
J
'7 ,'.D.
\\
UMSHIRPI STREAM
./
I
,q .,*'l
// \ ,," 7
/;;z,.:'..
#~v// IP v / I
_.,
/ -,:"; --""--"-2
Fig. 1. Map of Shillong showing: (a) collection sites 1 and 2, on Wah Diengling (W.D.), a tributary of the Umshirpi stream, and on the Umkhrah stream, respectively; (b) major traffic arteries shown as dotted lines; and (c) stone and sand quarries shown as triangles.
218
A. GUPTA
TABLE I Concentrations of Cd, Cu, Mn, Pb and Zn (mean 4- S. D.) in Water (unfiltered) from Wah Diengling (Site 1) and Umkhrah (Site 2) streams in Shillong, Meghalaya State, India, based on surveys made in January and August 1992
Site
Metal concentrations (#g 1- t) (n=8) Cd Cu Mn Pb
1
Hydropsyche
Baetis > Hydropsyche > C. ramosus
Cu Mn Pb Zn
Baetis > Hydropsyche
Baetis > Hydropsyche "~ C. ramosus
Hydropsyche > Baetis
Hydropsyche > Baetis > C. ramosus
Baetis ~_ Hydropsyche
Baetis ~- Hydropsyche ~ C. ramosus
Baetis > Hydropsyche
Baetis > Hydropsyche '~ C. eamosus
m e t a l i n p u t s f r o m p a r t i c u l a t e m a t t e r g e n e r a t e d b y v a r i o u s h u m a n a c t i v i t i e s in the c a t c h m e n t t h a t e n t e r s the s t r e a m a n d e v e n t u a l l y g e t s d e p o s i t e d o n t h e s t r e a m bed. 3.2.
METALS IN INSECTS
T h e c o n c e n t r a t i o n s o f the d i f f e r e n t m e t a l s in B a e t i s sp. a n d H y d r o p s y c h e sp. (at sites 1 a n d 2) a n d in C. r a m o s u s (at site 2 o n l y ) are p r e s e n t e d in T a b l e III, w h i l e
HEAVY METALS IN SHILLONG, NORTHEASTERN INDIA
221
Table IV summarizes the metal accumulation patterns in the three taxa. When the same taxon (viz. Baetis or Hydropsyche) was present both at sites 1 and 2, the concentrations of all the metals in animals from the latter site were higher. Again, this was peralleled by higher concentrations in water and detritus at site 2. Similar findings were reported in some other investigations (Brown, 1977; Burrows and Whitton, 1983; Sumi et al., 1991). Table IV demonstrates that the degree of accumulation of different metals varied considerably among taxa at both the study sites. Thus, Baetis was found to accumulate cadmium, copper, and zinc to levels significantly higher than the others; Hydropsyche accumulated more of manganese; while none of the three taxa showed any clear order of preference for lead. Burrows and Whitton (1983) found that mayflies as a group accumulated higher amounts of zinc, cadmium, and lead than the other invertebrates, while Sumi et al. (1991) observed a higher affinity for zinc than copper and iron in the mayfly Baetis thermicus. They suggested that the relatively high zinc concentrations were likely to be due to its accumulation in all the tissues such as mid-gut epithelium, fat bodies and muscles, while copper and iron were more localized in specific areas of the mid-gut epithelium only. The present study may not be considered detailed enough to comment on the bioconcentration of metals by insects, especially as neither shredders nor carnivorous species were investigated. Nevertheless, metal levels were higher in fine detritus than in C. ramosus, suggesting an absence of bioconcentration by this insect. In contrast, Baetis sp., which is known to feed upon periphytonic algae and detrital deposits on stones (Gupta and Michael, 1981), exhibited some bioconcentration of cadmium and zinc (Tables I! and III). Several studies reveal that the exact relationship between metal concentrations in food materials and those in aquatic invertebrates differ according to metal and animal. Thus, copper, zinc, and lead in chironomids were found to be higher than in adjacent sediments, while chromium was lower (Namminga and Wilhm, 1977). Again, copper and zinc in leeches and isopod crustaceans were found to decrease with increasing sediment concentrations, while lead increased (Eyres and Pugh-Thomas, 1978). The present study also points towards the existence of different degrees of metal bioconcentration in different aquatic insects. However, neither size-dependent variations (Rainbow and Moore, 1986; Krantzberg, 1989) nor seasonal differences (Ritz et al., 1982; Ireland, 1984; Krantzberg, 1989) in metal concentrations within the same taxon were investigated in the present study as well as in most others on metal bioconcentration and biomagnification in aquatic insects. It might be prudent to assess the influence of these factors on food chain concentration of metals before making any generalization.
Acknowledgement I am thankful to Dr. Jason M. Weeks for kindly reviewing the manuscript.
222
A. GUPTA
References
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