The Damodar and Subarnarekha Rivers are the first and second largest running waters of Jharkhand, having variable river bed morphology with naturally hard substrates. The riverine biodiversity is outstanding, as it includes endemic taxa on species and subspecies levels. Aquatic habitats are mostly colonized by native species. Macrozoobenthic animals are important bioindicators of the habitat quality of the river and are used for rapid assessment of river quality globally. In order to have preliminary information on the biodiversity of macrobenthos, the first survey was undertaken to find the impact of domestic pollution, mining, and thermal power plants on the aquatic life of the Damodar in May 2004. On the basis of our observations and species records, systematic field sampling for macro-benthic invertebrates in the Damodar and Subarnarekha Rivers was conducted from 2007 to 2012 during different hydrological conditions to cover the seasonal variation. Synoptic sampling was done to allow comparison between the two rivers. Aquatic invertebrate fauna has been inventoried with 124 identified taxa. The highest proportion of functional feeding groups recorded were detritivores, followed by carnivores and herbivores, respectively. This present study demonstrates the seasonal disappearance of macrozoobenthic invertebrates caused by urban and industrial pollution and that the fauna of the two main rivers of Jharkhand are similar in species inventory. This reflects the ecological conditions of the shared ecoregion known as Chota Nagpur Dry Forests. The results of the present research provide an essential database to evaluate the future environmental impact of restoration and changes in water quality.

Introduction

Use of macrobenthic invertebrates (MIs) as bioindicators is more accurate than chemical and microbial analyses (Kumar et al., 2013) and hence MI species inventory is the most popular biological method in assessment of freshwater bodies receiving domestic and industrial effluents (Odiete, 1999). MIs comprise a multiphyletic assemblage of organisms, representing different functional feeding groups (FFGs) and feeding modes. The majority of MIs utilize debris deposited on the bottom of rivers and are fed upon by bottom feeder fish and other invertebrates.

Information about the riverine biodiversity is more complete for vertebrates than for invertebrates (Balian et al., 2008). It is also more complete within the temperate waters than those of the tropics. Previous knowledge about MIs of the studied rivers is limited to the few records of identified aquatic Mollusca (Preston, 1915; Subba Rao, 1989; Dey, 2007), aquatic bugs (Insecta: Heteroptera) (Thirumalai, 2007), and collections of adult Dragonflies and Damselflies (Prasad and Varshney 1988; Mitra, 2003) from the region. Being consumers at intermediate trophic levels, MIs play a pivotal role in transferring carbon through the trophic cascade of riverine ecosystems. They are also responsible for recycling of nutrients by constituting the link between the nutrients trapped in detritus and useful nutrients in nektonic forms and fish (Bell and Woodin, 1984). Therefore, analyses of the functional compositions of macroinvertebrate communities and the relative proportions of different functional feeding groups have great implications for ecosystem functioning of the riverine ecosystems (Dudgeon, 2012; Kumar et al., 2013).

Functional feeding groups as indicators of biological water quality

Analyses of foraging tactics and trophodynamics involve FFGs and provide information on the balance of feeding strategies (food acquisition and morphology). The distributional patterns of FFGs are related to the physicochemical characteristics of the river attesting River Continuum Concepts and are used as bioindicators and as part of the Index of Trophic completeness (Pavluk et al., 2000). Various FFGs belonging to MIs comprise the gatherers (consume fine detritus deposited on the substrate); filterers (consuming fine detritus suspended in the water column); predators (consuming live animals); scrapers (exploiting the epilithic layer that grows on the surfaces of substrates); and shredders (consuming the coarse detritus, composed mainly of leaves falling down from riparian vegetation).

In a lotic ecosystem, processes vary spatially and temporally between and within aquatic habitats (Merritt et al., 2005). The knowledge of FFGs at spatial and temporal scales is essential to understand ecosystem process and integrity and the River Continuum Concept (Maiolini and Bruno, 2007). Very little information is available on FFGs of MI communities on these scales in Indian rivers originating from the plateau. This study was undertaken for species inventory and to understand the general distribution patterns of FFGs in two rivers—Damodar and Subarnarekha—originating in the Chota Nagpur Plateau region.

Pollution and macrobenthic invertebrate community structure

Although the main purpose of the present study was to elucidate the MI community structure, the study additionally compares MIs inhabiting polluted and unpolluted stretches of the rivers to gain insight into the MI community structure in relation to land driven pollution along both the rivers. Subarnarekha is the lifeline of tribal communities inhabiting the Chota Nagpur belt. These communities used to obtain their livelihood and resources from the river's fish and gold. The River Subarnarekha at Ghatshila is constantly being polluted by the effluent discharged from Hindustan Copper Limited (HCL) and Uranium Corporation of India Limited.

The River Damodar has become highly impacted by the industrial waste, mainly from the use of fertilisers in Sindri, the super phosphate factory, and the associated cement company (Kumar et al., 2005). In addition, wastes from coal washing water also find their way into the river. Bokaro thermal power plants frequently release huge amounts of suspended coal particles into the river. The objective of the present study was to investigate biodiversity of MIs and to identify the polluted and unpolluted zones of these two high-ecological value rivers using MIs as indicators of the biological and water quality classes.

Materials and methods

Study area

The Damodar (Dam) and Subarnarekha (Sub) Rivers are the first and second largest running waters of Jharkhand, having variable riverbed morphology with natural hard substrates. Both the rivers originate in the Chota Nagpur Plateau region. This is a continental plateau—an extensive area of land thrust above the general land. The plateau's ancient origin is substantiated by the Gondwana substrates. This ecoregion was the first area of contact with Eurasia.

The River Damodar originates on the Chota Nagpur Plateau west of Chandawa (elevation of 512 m) in the Latehar district, and flows eastward for about 592 km through the states of Jharkhand and West Bengal to the estuary of the River Hooghly. The source of River Subarnarekha is a spring on the plateau at Nagri near Ranchi, at an elevation of 705 m. The river flows eastward and turns to the south for a total length of about 470 km, running through the Indian states of Jharkhand, West Bengal, and Orissa. The mouth of the river, which flows into the Bay of Bengal, is located near Kirtania port. The details of sampling stations of River Dam and of River Sub are depicted in Figure 1.

For the current study, the habitat was classified according to longitudinal and lateral terminology described by Illies (1961), Illies and Botosaneanu (1963), and Amoros and Roux (1988). The MI fauna of the Damodar and Subarnarekha Rivers was investigated from June 2007 to January 2012. The study was carried out during different hydrological conditions over several seasons and samples were taken from along the left and right banks of both rivers. Altogether, 26 sites were visited ten times between June 2007 and January 2013 for sample collection from different habitats (boulders, sand, silt and mud substrate); see Table 1. At each site a stretch of approximately 50–100 m was sampled for between 30 and 60 min. In addition, 20 selected microhabitat sub-samples were gathered in the shallow zone of the rivers using standard hand nets with mesh size of 500 μm and 1 mm. Later, all 20 sub-samples were combined into a single MI sample. The sampling efforts were proportionally distributed over all microhabitats. This includes the natural or artificial substrates (organic, organic debris, clay, silt, sand, gravel and boulder) and organic substrates (macrophytal, mainly Potamogeton crispus, Potamogeton pectinatus and Chara species, floating or submerged) in the water. Each aquatic habitat was explored—either with a hand net or manually—to collect the highest possible diversity of MIs. Arthropod samples were preserved in a mixture of 70% ethanol and 4% formaldehyde for hardening of the materials. Annelid specimens were preserved in 70% ethanol; Leeches were usually relaxed in 15% ethanol, and then transferred into 70% ethanol for long term preservation. Molluscs and decapods were washed from the sediment samples at the spot and, if necessary, preserved in 4% formaldehyde. Usually only empty shells of large bivalves were collected and living specimens were released. The preliminary sorting and identification was done with the help of a Nikon dissecting microscope, and further identification of the material was carried out using digital imaging under a Leica-stereo zoom Fluorescence Microscope (Model: M205FA+DFC310FX+AF6000).

Each taxon was assigned to a specific FFG based on the definitions of Vannote et al. (1980), Williams and Feltmate (1992), and Merritt and Cummins (1996). The community structure and relative proportion of FFGs were estimated in terms of numerical abundance, and species richness as follows: (a) Herbivore-detritivore filter- or suspension-feeders; (b) Detritivore sediment- or deposit- (surface-) feeders; (c) Gathering collectors of omnivorous behaviour; (d) Predators (carnivore engulfers and carnivore piercers); (e) Parasites (carnivore suckers); (f) Scrapers/grazers (herbivore organic scrapers; detritivore, herbivore, and omnivore mineral scrapers); (g) Combined scrapers and filter-feeders; (h) Shredders (detritivore chewers; herbivore miners). We calculated ratios of the various FFGs on the basis of numerical abundance.

To evaluate patterns in community structure for Annelida, Arthropoda, and Mollusca species, occurrence × sampling station matrices were analyzed with multivariate analyses and hierarchical clustering was done using the Bray–Curtis distance measurement. The Indicator Value Index (IndVal) of different taxa occurring at more than 10% in each cluster was estimated (Table 2) using the formula given in Kumar et al. (2013). To see the results as per major taxonomic unit visit use https://www.researchgate.net/profile/Ram_Kumar2. Details about cluster analyses and indicator species analyses are given in Kumar et al. (2013) and Roy et al (2013), respectively. Data analysis was done using PRIMER-V software.

Several books and keys of the Indian subcontinent and Southern Asia, including taxonomically updated keys were used for final identification. These included: Sessile Invertebrates: Annandale (1911) and Wood et al. (2006); Annelida: Harding and Moore (1927), Gates (1972), Chandra (1983), Mandal (2004), and Naidu (2005); Crustacea: Battish (1992), Mariappan et al. (2003), Brandis and Sharma (2005), Ng et al. (2008), and Valarmathi (2009); Insecta: Prasad and Varshney (1988), Mitra (2003), and Thirumalai (2007); Mollusca: Preston (1915), Subba Rao (1989), Dey (2007), Nesemann et al. (2007) and Sharma (2011).

Results

In total, 124 taxa of benthic macroinvertebrates were recorded and identified in the present study. The species richness recorded in the river Damodar was higher, with 108 identified taxa, than that in the Subarnarekha River (Figure 2), which had 87 identified taxa. For the complete species-list, visit https://www.researchgate.net/profile/Ram_Kumar2. Altogether, of the 69 families, the arthropod genera dominated the benthic-invertebrate composition. Aquatic insects are the most prevalent fauna, with 61 identified taxa. Of these, nymphs of Dragonflies and Damselflies, larvae of two-winged flies, and water bugs are the most common groups. High diversity was found, with 33 species of Molluscs and 17 species of Annelids. The crustacean fauna included five taxa of Crabs, Prawns, Shrimp, Mysid Shrimp, Isopods (Malacostraca), and one Clam Shrimp (Branchiopoda). The Flatworms (Turbellaria) and Flukes (Trematoda) were recorded in lower abundance. The two rivers differed in relative abundance of different groups, as insects were the most abundant group, representing ∼50% of total macroinvertebrate diversity in the Subarnarekha River, whereas in the Damodar River, the gastropod Molluscs (Prosobranchia and Pulmonata) were the most highly abundant group, representing ∼45% of the total macroinvertebrate diversity (Figure 2).

Functional feeding groups

Functional feeding groups at the family level are summarized in Figures 3 and 4 for 77 recorded taxa identified during the study. Six major FFGs were identified in the present study. These included shredders and scrapers (Gastropoda) with all sediment- and filter-feeders (collectors). Altogether 46–48% of the families could be assigned to this group. The true carnivores represented 37–40% of the qualitative faunal composition, indicating high diversification and prey specialization (Figures 3 and 4). Herbivores (>4–10%) represented the minor group in the rivers, and this reflects the scattered occurrence of vascular aquatic plants along the pools.

Integrating all specimens from both the rivers, the following FFGs were recorded:

Collectors:

  • Herbivore-detritivore filter- or suspension-feeders: Spongillidae, Plumatellidae, Hislopidae, Unionidae, Mysidae, Cyclestheriidae, Culicidae, and Hydropsychidae.

  • Detritivore sediment- or deposit- (surface-) feeders on decomposing particulate organic matters: Naididae, Tubificidae, Octochaetidae, Corbiculidae, and Sphaeriidae.

  • Gathering collectors (omnivorous): Caenidae, Leptophlebiidae, Ecnomidae, Ceratopogonidae, Chironomidae (in part), Syrphidae, Palaemonidae, Atyidae, and Gecarcinucidae.

Predators:

  • Carnivore engulfers of whole animals or parts: Planariidae, Salifidae, Aeshnidae, Gomphidae, Lindeniinae, Libellulidae, Corduliidae, Macromiidae, Coenagrionidae, Protoneuridae, Platycnemididae, Ecnomidae, Psychomyidae, Rhyacophilidae, Hydrophilidae (larvae), Dytiscidae, Gyrinidae, Athericidae, Tabanidae, Limoniidae, semi-aquatic Pisauridae and semi-aquatic Lycosidae.

  • Carnivore piercers of cell and tissue fluids: Glossiphoniidae, Belostomatidae, Nepidae, Pleidae, Naucoridae, Notonectidae, Hydrometridae, Gerridae, Veliidae and Mesoveliidae.

Parasites:

  • Carnivore suckers of blood and tissue fluids from swimming larger aquatic animals (Malacostraca, Pisces): Corallanidae as ecto-parasites on prawns and Diplostomatidae as endo-parasites of fish.

  • Scrapers/Grazers:

  • Herbivore organic scrapers on Algae: Corixidae and Micronectidae.

  • Detritivore, herbivore, and omnivore mineral scrapers: Perlidae, Baetidae, Leptophlebiidae (in part), Chironomidae (in part), Scirtidae, Psephenidae, Neritidae, Stenothyridae, Thiaridae, Pleuroceridae, Lymnaeidae, Planorbidae, Physidae and Succineidae.

  • Combined scrapers and filter-feeders: Viviparidae and Bithyniidae.

Shredders:

  • Detritivore chewers on decomposing vascular plant tissue: Leptophlebiidae, Lepidostomatidae, Blaberidae, Pyralidae, Chironomidae (in part), Hydrophilidae (adults), Curculionidae and Ampullariidae.

  • Herbivore miners on living vascular plant tissue: Pyralidae (in part).

Patterns of benthic macroinvertebrate community structure

The abundance of major taxa (Figures 5a and 6a) was higher in the middle and lower stretches of the two rivers, with increased Mollusca density. Insects are more abundant in upper Dam (Figure 5a) but the class is equally distributed throughout upper and lower Sub (Figure 6a). The average number of taxa reached its maximum values in the middle and lower stretches of the two rivers. The highest diversity was observed in March of 2012, with 28–33 taxa per sampling site in Dam and 29–32 taxa per sampling site in Sub. Similarly, the indices of richness and diversity recorded the highest values at the sampling sites with the maximum taxa number. The relative abundance (%) of Mollusca in Dam was higher than that of Insecta (Figure 2a). In contrast, a higher percentage of Insecta was recorded than that of Mollusca (Figure 2) in Sub. The sampling stations SR St 03–05 (Ranchi) recorded less faunal inventory, and the domestic sewage in this part of the river was mainly supporting and feeding a typical low-oxygen community with red Chironomus riparius larvae, the Tube Worms Limnodrilus hoffmeisteri and the pulmonate-breathing Physidae Snail, Haitia mexicana. In contrast, the middle reach of Dam recorded higher proportion of Leeches and the Physidae Haitia mexicana at the sampling stations DR St 05 and DR St 06 (Figure 5a).

The hierarchical classification for Annelida (Figure 7a), Arthorpoda (Figure 7b), and Mollusca (Figure 7c) revealed higher river-specific variations in species abundance than the variations seen between stations or seasons. The annelid species occurred at stations 2, 11, 12 and 14 of the Subarnarekha River grouped together (Figure 7), with Limnodrilus hoffmeisteri as a highly indicative species.

Stations 3, 6 and 9 of the Subarnarekha River, and 4, 5 and 7–12 of the Dam River did not show any affinity with other stations and were completely segregated at the first hierarchical level (Figure 7). Station 6 of Dam and 7 of Sub represented cluster II, with Alboglossiphonia heteroclite as a highly indicative species. Stations 1, 3, 4, 8 and 10 of Sub, and stations 1, 2 and 3 of Dam showed similar results and were segregated from other stations at the third hierarchical level, with Branchiura sowerbyi as a highly indicative species. Arthropods showed differential grouping patterns spatially. Stations 8, 11 and 12 segregated from the remaining stations, whereas stations 5 and 7 of the Subarnarekha River grouped together at the first hierarchical level with, Chironomidae as a highly indicative taxa.

River-specific differences were not prominent for Mollusca. Stations 5 of Sub and 6 of Dam grouped together and separated from the remaining stations at the first hierarchical level, with Haitia mexicana as the most indicative species. Stations 2 and 7 of Sub, and 7, 8, 9 and 11 of Dam grouped together at the second hierarchical level. This group recorded higher diversity of Molluscan fauna with genus Digoniostoma as the highly indicative species. Stations 3, 4, 5 and 10 of Dam, and the middle and lower stretches of Sub recorded genus Thiara as a highly indicative species.

Overall, higher species abundance at stations 5–8 of the Damodar River exhibit clear spatial variation in species dominance patterns (Figure 5). Station 5 was dominated by Insecta, which was replaced by Pulmonate gastropods at St. 6 and Prosobranchia gastropods at Sts. 7 and 8, respectively (Figure 5). All stations of the Sub were mainly dominated by Insecta, except the lowest station (St. 14), which was dominated by Pulmonate gastropods (Figure 6). The cluster grouping of sampling stations for Annelida (Figure 7) shows the sampling sites of group I with low anthropogenic pollution and the presence of river species of Naididae, followed by the middle reaches of Dam and Sub as group II being the main habitat of predatory Leeches as feeders of Tubificidae, Chironomidae and various Molluscs. The third group (III) includes a variety of different sampling stations, which are inhabited by the Leech Barbronia weberi, and the largest Tube Worm Branchiura sowerbyi. These species have no clear preference for any particular type of running water in subtropical conditions and are found from the spring regions downstream towards the estuaries.

Clustering of sampling stations for Arthropoda (Figure 7) distinguishes the heavily polluted sites of the Subarnarekha River as group I with mass occurrence of Dipteran larvae. The sampling sites of groups II and III are characterized by typical river inhabitants such as Prawns (Macrobrachium sp.) and sediment burrowing Dragonfly nymphs (Macrogomphus spp.) in the middle reaches of the rivers. The last group (IV) includes various lotic habitats with coarse particulate sand as the main substrate of the dragonfly species Paragomphus lineatus, which is an indicator of fairly good sediment conditions.

For Mollusca, the clustering of sampling stations (Figure 7c) permits identification of at least five different associations. The first group (I) is characterized by the Physidae Snail Haitia Mexicana, and includes the most polluted sampling stations of Dam and Sub. In group II, all sampling stations with a distinct presence of lentic fauna are included. They are represented by the families Bithyniidae (Digoniostoma spp.) and Planorbidae (Gyraulus spp., Indoplanorbis exustus). These species are predominantly found in the shallow wetlands of the artificial reservoirs, mostly in dense macrophytes. The third group (III) of sampling stations with Mollusca taxa has a typical river community with Thiara lineata, Bellamya bengalensis and the clam Corbicula striatella. The association of coastal rivers with Thiara riqueti is represented by the sampling stations of cluster group IV. These characteristic species of the investigated areas are restricted to large rivers close proximity to the sea, and do not enter into far upstream inland waters. The remarkable group V of the cluster has all the uppermost sampling stations in the unpolluted streams at elevations above 500–700 m. This is the main habitat of small bivalves like Pisidium nevillianum and the stream-inhabiting Lymnaeidae Snail, Radix luteola. Two of the Gastropoda are widespread in different permanent water bodies, namely Bellamya bengalensis and Melanoides tuberculatus. These species are found in groups II, III, IV and V sampling stations, covering a large part of the catchment areas.

The sessile filter- and suspension-feeders (sponges and bryozoans), occur in limited numbers in the reservoirs. Molluscs were the dominating fauna in Dam, whereas insects were more abundant in Sub. Large freshwater bivalves (Unionidae) are restricted to the middle and lower course of the two rivers with moderate current velocity and stable bed sediments, whereas true Pea-clams, Sphaeriidae (Musculium indicum, Pisidium nevillianum), were found frequently in the uppermost stream regions at elevations above 600 m in Sub. The Odonata play an important role in both rivers, with sand-inhabiting burrowing Gomphidae (Paragomphus lineatus, Macrogomphus spp.) and large Macromiidae (Macromia spec., Epophthalmia vittata).

The impact of pollution is clearly visible in both rivers with the increasing number of a few tolerant Gastropoda (Physidae: Haitia mexicana) and Annelida (Tubificidae: Limnodrilus hoffmeisteri), followed by their predators, e.g. Glossiphoniidae (Alboglossiphonia spp.) and Salifidae (Barbronia weberi) Leeches.

Discussion

Functional feeding groups

In terms of FFGs, changes were recorded in the benthic along the rivers, with predators being the dominant functional group at the largest number of sites. The dominance of predators among the macrobenthic community indicates a heterotrophic-dominant food web in this system; whereas collectors, as the second most dominant group, indicate the importance of seston transport in the water column. Our analyses attest to the observation that linkages in both rivers exist between coarse particulate organic matter (CPOM, particles >1 mm), fine particulate organic matter (FPOM, particles <1 mm and >0.45 µm), periphyton, and prey in the headwater streams. The relative abundance of functional groups reflects anthropogenic impact (Merritt et al., 2005; Cummins et al., 2005; Merritt and Cummins, 2006) on the river systems. Several taxa are able to shift their feeding responses to changes in land use and riparian conditions (Li and Dudgeon, 2008, 2009). The feeding of shredders on riparian litter affects detrital processing in aquatic systems. About 30% of the conversion of CPOM leaf litter to FPOM has been attributed to shredder feeding (Petersen and Cummins, 1974), and this may affect the growth of FPOM feeding collectors (Short and Maslin, 1977). Shredder feeding enhances the release of dissolved organic matter (DOM; Meyer and O'Hop, 1983). Such analyses link the balance between food resource categories and the predictable response of aquatic insect assemblages. Lower diversity of shredders have also been recorded from other riparian systems in tropical environments (e.g. in Brazil: Gonçalves et al., 2006; Colombia: Mathuriau and Chauvet, 2002; Costa Rica: Irons et al., 1994; Hong Kong: Li and Dudgeon, 2008; Papua New Guinea: Yule, 1996). The loss of riparian vegetation has resulted in a shift in the community organization of macroinvertebrates in riverine ecosystems (Jinggut et al., 2012). The loss of riparian forests increases river temperatures through loss of shade (Baxter et al., 2005), which in turn diminishes the inputs of leaf litter affecting the relative differences between wet and dry seasons (Wantzen et al., 2008; Arimoro et al., 2012). The relative proportions of FFGs are used as surrogates for determining ecosystem health of the river (Vannote et al., 1980; Merritt et al., 2005; Merritt and Cummins, 2006). Further study is required to estimate the balance between autotrophy and heterotrophy (p/r) and the linkage between riparian inputs and stream food webs (CPOM/FPOM). The higher proportion of predators in both rivers (Predator/prey ratio) >0.2 indicates an overabundance of these species. The true carnivores represent 37–40% of the qualitative faunal composition, indicating high diversification and prey specialization. Herbivores (>4–10%) were the minor group in both rivers, and this reflects the scattered occurrence of vascular aquatic plants along the pools. Some scrapers among the gastropods, especially Lymnaeidae, and miners among the insects, e.g. Pyralidae larvae of Moths, mainly utilize live Algae and macrophytes.

Biological water quality

Station–wise, differences in community structure reflect differential pollution inputs along both the rivers. The heavy pollution in the rivers has adversely affected the water quality at several sampling sites, e.g. Tupudana and Namkum near Ranchi, and downstream Jamshedpur on the river Sub. The most instable fauna in Dam was observed at Fushro (St. 05), Telmacho (St. 06), and Patardih (St. 07). Large amounts of industrial and domestic effluents are discharged into the river at Fushro due to a cement factory and coal field. The downstream river section provides stable conditions for the sensitive species of large rivers, such as long-living large freshwater Mussels (Unionidae), sessile Sponges and Bryozoans. This includes the potential recovery zones of the Damodar where the pollution-disturbed species can colonize. These valuable habitats should be the focus of conservation efforts and continuous biomonitoring.

In the Subarnarekha River, the main source of pollution is discharged from the urban capital of Ranchi into the upper course of the river. We recorded lower diversity but higher abundance (most indicative) of the exotic, invasive, pollution-tolerant gastropod Haitia Mexicana (indicator value: 96.77) near the Telmacho bridge in Dam and Namkum in Sub. Both the sampling sites were downstream of discharge sites. Telmacho Bridge is downstream of the cement factory and coal field area, whereas the Namkum site of Sub is situated at the eastern end of the state capital, Ranchi. Higher abundance of Limnodrilus hoffmeisteri at sampling sites near Moubhandar (Ghatshila) and Janshola indicate impacts of effluents from the industrial cities. Moubhandar harbours Asia's first copper mines and the world's second deepest mines. Therefore, a higher abundance of a few tolerant Gastropoda (Physidae: Haitia mexicana) and Annelida (Tubificidae: Limnodrilus hoffmeisteri), followed by their predators, e.g. Glossiphoniidae (Alboglossiphonia spp.) and Salifidae (Barbronia weberi) Leeches clearly establish the impacts of industrial and urban discharge on macrobenthic invertebrate community structure. Domestic sewage provides the main source of food for the typically low-oxygen-community, including red Chironomus riparius larvae, Tube Worms, Limnodrilus hoffmeisteri, and the pulmonate breathing Physidae Snail, Haitia mexicana. In contrast, the industrial pollution along the middle reach of the Damodar River is caused by irregular wastewater discharge from coal mines and hydrothermal power stations. The impact on the fauna is mostly visible in the high percentage of Leeches and the Physidae Haitia mexicana at the sampling stations DR St. 05 and DR St. 06 (Figure 7).

Endemism and regional taxa

The large freshwater Mussels have drawn early attention because of their high specialization in accordance with geologically old watersheds (Preston, 1915). Parreysia rajahensis (Lea, 1841) and Radiatula keraudreni (Eydoux, 1838) (Family Unionidae) have a limited distribution range and are endemic local taxa. P. rajahensis is restricted to the middle and lower stretches of the river Subarnarekha. It is well distinguished from the more widely distributed Parreysia favidens (Benson, 1862) of the river Damodar. The prosobranchia Snail Thiara (Sermyla) riqueti (Grateloup, 1840) (Family Thiaridae) is a coastal inhabitant of brackish water bodies. It is usually absent from inland waters and the distribution in the Ganga River System is restricted to the two running waters of the Hugli and Damodar Rivers. Remarkable among the insects seen during this study is the stonefly nymph of Gibosia spec. (Family Perlidae), found in the middle stretch of the Subarnarekha River, where the species inhabits fast-flowing riffles. The majority of stoneflies (Plecoptera) are confined to cold water streams and rivers of higher mountains.

The two rivers differ only in the relative abundance and dominance patterns of species of the macrobenthic community; however, they do not differ in overall biodiversity of the macrobenthic invertebrate community. Similarity in biodiversity is attributed to the origin and course of the two rivers studied. This ecoregion is drier than neighboring regions. However, differential species dominance patterns are due to differences in the river basin (the Subarnarekha River basin is smaller than most multi-state river basins in India), and hence to differential land-driven discharge. The present results provide important baseline data for further investigation on the river continuum concept and the impacts of industrial and domestic waste discharges on the community structure and relative proportion of various FFGs and macrobenthic invertebrates in rivers from a plateau region.

Conclusions

The macrobenthic invertebrate (MI) community is a reliable bioindicator of riverine health and species inventories of MI community have become the standard biomonitoring method of river ecosystems. MI community represents various functional feeding groups at different trophic levels, showing a range of habitat quality specific community structure. The present study compares spatio-temporal patterns of MI community along two rivers (Damodar and Subernarekha) of the shared Eco region (Chota Nagpur Dry Forest). Synoptic field samplings were conducted in both rivers during June 2007 to January 2013, covering all seasons and different hydrological conditions at 26 sites. The sampling sites were proportionally distributed over all micro-habitat types including organic, organic debris, clay, silt, sand, gravel and boulder) natural or artificial substrate, macrophyte (phytal, mainly Potamogeton crispus, Potamogeton pectinatus and Chara spec. floating or submerged) in the water. Community structure patterns for Annelida, Arthropoda and Mollusca, were analyzed with multivariate analyses and using the Bray-Curtis distance measurement. Indicator Value Index (IndVal) of different taxa occurring more than 10% in each cluster was estimated using the formula given in Kumar et al. (2013). Identified taxa were grouped on the basis of their feeding mode of particular functional feeding groups (FFGs) following Vannote et al. (1980), Williams and Feltmate (1992) and Merritt and Cummins (1996). Comprising all samples a total of 124 MI taxa were identified including six major Functional Feeding groups including collectors, predators, Parasites, Scrapers/Grazers, Shredders and Herbivore miners. Altogether, 46–48% of the families were grouped as shredders and scrapers (Gastropoda) with all sediment- and filter feeders (collectors) whereas 37–40% of the identified fauna were represented by true carnivores. The species richness recorded in the river Damodar was higher with 108 identified taxa than that in River Subarnarekha with 87 identified taxa. The river-specific variations in species abundance were significantly higher than that of station or season specific for Annelida and Arthorpoda. In both rivers the impact of pollution was visibly indicated by the increasing number of a few tolerant Gastropoda (Physidae: Haitia mexicana) and Annelida (Tubificidae: Limnodrilus hoffmeisteri), followed by their predators e.g. Glossiphoniidae (Alboglossiphonia spp.) and Salifidae (Barbronia weberi) Leeches. The heavy pollution in the rivers has adversely affected the water quality at several sampling sites, e.g. Tupudana and Namkum near Ranchi and downstream Jamshedpur on River Sub. Higher abundance of Limnodrilus hoffmeisteri at sampling sites near Moubhandar (Ghatshila: Asia's first copper mines and the world's second deepest mines) and Janshola indicates impacts of effluents from the industrial city. The downstream river section provides stable conditions for the sensitive species of large rivers such as long-living large freshwater Mussels (Unionidae), sessile Sponges and Bryozoans. The impact on the fauna is mostly visible in the high percentage of Leeches and the Physidae Haitia mexicana at the sampling stations DR St 05 and DR St 06 of Damodar. The present results offer important baseline data for proper investigation of the river continuum concept and impacts of industrial and domestic effluents on MI community structure in rivers of a plateau region.

Acknowledgements

Authors are thankful to Dr. A. P. Sharma, for inviting RK and HN to the author's workshop. We thank Dr. Dilip Kumar and Dr. M. Munawar for their suggestions on a previous version of the manuscript. We acknowledge support from Zoological Survey of India; Department of Biotechnology, Govt. of India and Central University of South Bihar. Comments made by the two anonymous reviewers are highly appreciated.

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