e-issn: 2320-7078 aquatic insect community of river ......keywords: aquatic insects, brahmaputra,...

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Journal of Entomology and Zoology Studies 2017; 5(2): 1257-1265 E-ISSN: 2320-7078 P-ISSN: 2349-6800 JEZS 2017; 5(2): 1257-1265 © 2017 JEZS Received: 14-01-2017 Accepted: 15-02-2017

Arundhati Gogoi Research Scholar Department of Ecology and Environmental Science Assam University Silchar Silchar, Assam, India Susmita Gupta Department of Ecology and Environmental Science Assam University Silchar Silchar, Assam, India Correspondence Arundhati Gogoi Research Scholar Department of Ecology and Environmental Science Assam University Silchar Silchar, Assam, India

Aquatic insect community of River Brahmaputra near Dibru Saikhowa National Park, Assam, North

East India

Arundhati Gogoi and Susmita Gupta

Abstract A seasonal study was conducted on aquatic insect community of Brahmaputra River at three sites, Balijan ghat (BS1), Oakland (BS2) and Maijan ghat (BS3) of Dibrugarh district after its confluence with Dibru River near Dibru Saikhowa National Park. The sampling of insects and water was done in replicates during September 2014 to 2015. The study recorded 4 orders, 10 families, 12 genera and 12 species of aquatic insects. Highest density of order Hemiptera was found across the sites and seasons. Seasonal variations of environmental variables of water were found moderate. Density of aquatic insects showed significant negative relationship with Electrical conductivity, Total dissolved solid and Rainfall. The study revealed low Shannon Weiner Diversity Index values of aquatic insects (Shanon H՛< 1) in all the sites indicating perturbed condition of the water. Canonical Correspondence Analysis (CCA) showed that the species environment correlations of axis 1 and axis 2 were high and some of the species responded negatively to total alkalinity. This study though of preliminary nature contributed to the knowledge of faunal diversity of the area around Dibru Saikhowa National park. Keywords: Aquatic insects, Brahmaputra, density, diversity, relative abundance 1. Introduction Aquatic biodiversity forms one of the essential components of an aquatic ecosystem for maintaining stability and a means of coping with any environmental change [1]. Of the great variety of insects, aquatic forms, though of less variety, are important constituent of freshwater ecosystems. All over the world about 45000 species of insects are known to inhabit diverse freshwater ecosystems [2]. Aquatic insects are among the most prolific animals on earth, but are highly specialized and represent less than 1% of the total animal diversity [3]. They form an important component of the food chain and energy flow pathways and comprise of a high proportion of biomass in fresh water. They play an important role in ecosystem functioning and are used as bioindicator. Although there are a few studies on aquatic insects of lotic systems of north east India [4 ,5, 6] till date no study on aquatic insects of mighty River Brahmaputra is in record. An extremely dominant monsoon interacting with an active seismo-tectonic instability together with anthropogenic factors have moulded the Brahmaputra into one of the world’s most intriguing gigantic fluvial system [7, 8]. The River Brahmaputra originating from the Mansarovar Lake of Himalaya known there as the Tsangpo enters Arunachal Pradesh, India, named as the Dihang (or Siang) River and flows to Assam plain where two rivers, the Dibang and the Lohit originating from eastern flank of Himalaya, join the river and the combined flow is called the Brahmaputra. The river flows along the northern side of the Dibru Saikhowa National Park (27˚30 N to 27˚45՛N and 95˚10 E to 95˚45՛E), an ideal habitat for the one horned rhinoceros, Indo Gangetic Dolphin and a variety of birds and animals. This mighty river is also the habitat for a large number of aquatic fauna. Although studies are there on the ecology in relation to the fisheries of the river system [9-14], there is no study on aquatic insect community. A qualitative and quantitative assessment of the aquatic insect community and environmental variables of water of this part of the river will enrich the aquatic fauna data base and reflect the water quality status of the river as well.

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Journal of Entomology and Zoology Studies

2. Materials and Methods For this study three sites of River Brahmaputra after its confluence with Dibru River in the south –west of Dibru Saikhowa National Park were selected. The sites are Balijan ghat (BS1 27˚55՛ N and 95˚16՛ E), Oakland ghat (BS2- 27˚55՛N and 95˚16E) and Maijan ghat (BS3-27˚50՛N and 94˚93՛E) (Fig. 1). Aquatic insects were collected seasonally in replicates by a hand net having a mesh size of 60µm fixed to a square wrought iron frame by “1 minute Kick” method [15] whereby the vegetation was disturbed and the net was dragged through the system for a unit time [16]. Three such drags constituted a sample. They were sorted, enumerated and preserved in 70% ethyl alcohol and identification was done using Magnus (Olympus Model MSZ-TR) and Stereozoom microscope (Motic, Model- SMZ-168). Taxonomic identification was carried out using standard keys [17-28]. For analysing dissolved oxygen (DO), water samples were collected in 300 ml BOD bottle in replicates from each site. For analysing free CO2, pH, electrical conductivity (EC), total dissolved solid (TDS), total alkalinity (TA), nitrate (NO3

-) and phosphate (PO4

3-) water samples were collected in polyethylene bottles from each sampling site. Air temperature (AT) and water temperature (WT) were recorded itself in the site by mercury bulb thermometer. Rainfall (RF) data were collected from Sessa Tea state, Dibrugarh district, Assam. pH was determined with the help of pH meter (Digital pH meter MK VI). Electrical conductivity (EC) was estimated by conductivity meter. DO, F CO2, TA, NO3

- and PO43-were

estimated by standard methods [29-31]. Soil samples were also collected from the selected sites and analysis was done by hydrometer method [32]. Diversity indices like Margalef Index (M), Shannon –Wiener Index (H/), Evenness Index (J), and Berger Parker Index of Dominance (d) for the insect community were worked out using the package Biodiversity Professional Version 2 for Windows 1997 (The Natural History Museum and Scottish Association for Marine Science). The dominance status of insect species was evaluated following Engelmann’s scale based on relative abundance [65]. SIGNAL (Stream Invertebrate Grade Number-Average Level), a family-level water pollution index based on the known tolerances of aquatic macro-invertebrate families to various pollutants was worked out by the standard method [33]. BMWP (Biological Monitoring Working Party) and ASPT (Average Score Per Taxon) scores were computed by standard methods [34]. HKHbios (Hindu Kush-Himalayan biotic Score) was worked out using standard methods [35]. Statistical analyses were done by using package SPSS 20.0 for Windows 7. Canonical Correspondence analysis (CCA) was done to see the relationship of insect abundance to environmental variables in the four seasons using CANOCO package for windows 4.5 [36] 3. Results 3.1. Diversity and density The study recorded 4 orders, 10 families, 12 genera and 12 species of aquatic insects. The orders were Coleoptera, Hemiptera, Odonata and Ephemeroptera. The families were Elmidae, Dytiscidae, Dryopidae, Hydrophilidae, Noteridae, Gerridae, Veliidae, Corixidae, Coenagrionidae and Baetidae. (Table 1)

Density and relative abundance of order Hemiptera was found to be the highest in all the sites in all the seasons while order Coleoptera was the lowest recorded in pre monsoon in BS2. Hemipterans belonging to family Gerridae showed the highest relative abundance in BS2 during winter. Species like Cloeon inscriptum of family Baetidae and Microvelia sp.of family Veliidae were the common species found in all the seasons. Based on Engelmann’s scale, the eudominant species recorded during the study period were Gerris lepcha (Fabricius, 1794), Aquarius conformis (Schellenberg, 1800), Micronecta scutellaris (Stal, 1977), Microvelia sp.(Westwood, 1834), Pseudagrion sp. (Rambur,1842) and Cloeon inscriptum (Bengtsson,1914). 3.2 Diversity Indices The Shannon-Wiener (Hʹ) (0.44) was found highest in Pre monsoon and lowest (0.36) in Monsoon (Table 2). Margalef index (M) (6.1) value was found highest in Post monsoon with lowest (4.12) in Monsoon. Berger Parker Index of Dominance (d) value (0.60) was found highest in Monsoon and lowest (0.53) in Post monsoon. Highest value of Shannon Evenness (J) index (0.89) was recorded in Monsoon season with lowest value (0.78) in Winter. 3.3 Biomonitoring Scores The BMWP scores for the river ranged between 11.67 (Winter) to 19.3 (Pre monsoon) (Table 2). The ASPT score of the study site was found in the range of 3 (Winter) to 5.25 (Monsoon). Highest SIGNAL score (4.03) was found in Winter and lowest in Pre monsoon season (3.3). Highest and same HKH biotic score (6.1) was found in Winter and Pre monsoon with lowest (5.14) in Post monsoon 3.4 Environmental variables of Water The environmental variables of water were estimated in Post monsoon, Monsoon, Pre monsoon and Winter seasons (Table 3). AT was found to fluctuate within the range of 18.07˚ C ± 6.72 (Winter, BS 2) to 37.7˚C ± 2.25 (Monsoon, BS1). WT fluctuated within the range of 19.4˚C± 0.31 (Winter, BS1) to 28.1˚C± 0.46 (Pre monsoon, BS3). pH varied from 6.52±0.1(Monsoon, BS1) to 7.46± 0.06 (Post monsoon, BS1). EC in all the samples were observed in the range of 95.07±0.25 μS.cm-1(Winter, BS1) to 180.37±3.37 μS.cm-1 (Monsoon, BS1). The TA of stream water in the selected sites ranged from 66.67±1.15 mg.L-1(Winter BS3) to 97.33± 1.15 mg.L-1 (Post monsoon, BS2 and BS3). The range of NO3

- ranged from below detectable limit (BDL) to 0. 06 mg.L-1 and the range of PO4

3- was from 0.16 mg.L-1 to 0.53 mg.L-1. The DO values in different sites were good ranging from 7.67± 0.11 mg.L-1 (Pre monsoon, BS2 2) to 11.07± 0.12 mg.L-

1(Monsoon, BS2). Free CO2 varied from 4.6±1.15 mg.L-1(Post monsoon BS1) to 7.32±1.15 mg.L-1 (Winter, BS1). Density of insects showed significant negative relationship with TDS, EC and RF. Significant positive relationship of density of aquatic insects with species richness was recorded.

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Fig 1: Map of India followed by map of Assam showing the collection sites at Brahmaputra River near Dibru Saikhowa National Park

Fig 2: Seasonal variations in density (no. of ind/unit time)of different orders of aquatic insects in three sites of Brahmaputra River

Fig 3: Seasonal variations in relative abundance (%) of different families of aquatic insects in three sites of Brahmaputra River

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Table 1: Dominance status of different species of aquatic insect community found in Brahmaputra River based on Engelmann’s scale (1978). Relative Abundance < 1 % = Subrecedent (SR); 1.1 - 3.1 % = Recedent (R); 3.2 - 10 % = Subdominant (SD); 10.1 - 31.6 % =Dominant (D), > 31.7 % = Eudominant (ED).

Order Family Taxa Post monsoon Winter Pre monsoon Monsoon BS1 BS2 BS3 BS1 BS2 BS3 BS 1 BS2 BS3 BS1 BS2 BS3

Odonata Coenogrionidae Pseudagrion sp. (Rambur.1842)

27.27 D _ 40

ED 18.75

D _ _ _ _ _ _ _ _

Ephemeroptera Baetidae Cloeon inscriptum (Bengtsson,1914) 36.36 ED _ _ 18.75

D _ _ _ 20 D _ 42.86

ED 71.43 ED

66.67 ED

Hemiptera

Gerridae

Gerris lepcha (Fabricius,1794) 36.36 ED

83.3 ED _ 56.25

ED 52.38 ED

22.22 D __ 15

D _ _ _ _

Aquarius conformis (Schellenberg,1800) _ _ _ _ 42.86 ED

66.67 ED _ _ _ _ _ _

Cylindrostethus sp. (Mayr,1865) _ _ - _ 4.76 SD _ - - _ _ _ _

Corixidae Micronecta scutellaris (Stal,1977) _ _ 40 ED _ _ 11.11

D 15 D _ 29.41

D _ _ _

Veliidae Microvelia sp. (Westwood,1834) _ _ 20 D

6.25 SD _ _ 65

ED 60 ED

52.94 ED

21.43 D _ 33.33

ED Coleoptera Noteridae Suphisellus sp.

(Cretch,1875) _ 16.67

D _ _ _ _ _ _ _ 28.57 D

28.57 D _

Dryopidae Helichus sp. (Erichson,1847) _ _ _ _ _ _ 5 R _ 17.6

D _ _ _

Dytiscidae Hydrovatus sp. (Motschulsky,1855) _ _ _ _ _ _ _ 5 R _ _ _ _

Elmidae Stenelmis sp. (Dufour1835) _ _ _ _ _ _ 15 D _ _ _ _ _

Hydrophilidae Cercyon sp. (Leach,1815) _ _ _ _ _ _ _ _ _ _ 7.14 SD _

Table 2: Seasonal variations in diversity indices and biomonitoring scores of Brahmaputra River

Seasons Shannon H' Log Base 10. Evenness’(J) Berger-Parker Dominance (d) Margaleff M Base 10 BMWP ASPT SIGNAL HKHbios Post monsoon 0.38 0.86 0.53 6.1 13.67 5.1 3.43 5.14

Winter 0.41 0.78 0.58 4.76 11.67 3 4.03 6.1 Pre monsoon 0.44 0.8 0.59 4.7 19.33 5.17 3.33 6.1

Monsoon 0.36 0.89 0.60 4.12 13.67 5.25 4.2 5.93 BMWP score: 0-16=Poor water quality; 17-50=Moderate water quality; 51-100=Good water quality; 101-150=High water quality; 151+=Very high water quality ASPT score: >6= Clean water, 5-6= Doubtful quality, 4-5 = Probable moderate pollution, <4 = Probable severe pollution SIGNAL Score: Greater than 6= Healthy habitat, between 5 and 6= Mild pollution, between 4 and 5=Moderate Pollution, Less than 4=Severe pollution HKHbios: ≥7.5= Reference/good; ≥6.5= good/moderate;≥5.3=moderate/poor; ≥4= poor/bad

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Table 3: Seasonal variations in environmental variables (Mean±SD) of water of River Brahmaputra during 2014-15

Post-monsoon, 2014 Winter, 2015 Pre-monsoon, 2015 Monsoon, 2015 BS1 BS2 BS3 Mean ±SD BS1 BS2 BS3 Mean ±SD BS1 BS2 BS3 Mean ±SD BS1 BS2 BS3 Mean±SD

AT (˚C) 32.9± 2.89 32.8± 4.28 25.5± 6.27 32.9 ± 4.5 21.9±9.2 18.07± 6.72

23.17 ±6.73 21.07±7.6 30.9 ±3.79 28.10 ±3.46 34.93 ±1.81 31.3 ± 3.0 37.7 ±2.25 36.4 ±2.41 34.84

±0.25 36.3±1.6

WT (˚C) 22.07 ± 0.61 22.3± 0.67 21.87 ±0.91 22.06± 0.7 19.4±0.3 19.6 ±0.46 19.47 ±0.15 19.5±0.3 27.43 ±0.15 27.6 ±0.46 28.1 ±0.46 27.7± 0.4 25.4 ±0.21 24.9 ±0.36 25.3 ±0.49 25.2±0.4

pH 7.46± 0.06 7.15± 0.05 7.12 ±0.03 7.3 ±0.0 7.1±0.09 6.65 ±0.15 7.19 ±0.13 7± 0.1 6.54 ±0.21 6.6± 0.09 6.56 ±0.04 6.6 ±0.1 6.5 ±0.1 6.6 ±0.05 6.63 ±0.06 6.6 ± 0.1 EC µS cm-1 157.9 ± 3.08 153.9 ± 2.08 151.4 ± 0.9 155.9±2.0 95± 0.25 96.23

±0.55 97.67 ±0.61 96.3± 0.5 99± 0.75 100.1 ±0.40 98.77 ±0.55 99.3±0.6 180.3

±3.37 178 ±5.30 175.6 ±4.74 178± 4.5

TDS mgL-

1 99.57 ± 2.48 100.9± 0.25 100.2 ±0.7 100.3±1.1 63±0.83 63.7 ±1.14 64.6 ±0.6 63.8±0.9 64.1 ±0.45 65.2 ±0.45 65.27±0.51 64.9 ±0.5 115.9 ±2.59 114.2 ±3 113.7

±3.06 114.6± 2.9

DO mgL-1 8.47± 0.12 9.07 ± 0.12 8.47 ±0.12 8.8 ±0.1 8.6± 0.20 8.27 ±0.12 8.53 ±0.23 8.4± 0.2 7.93 ±0.23 7.67 ±0.11 7.8 ± 0.2 7.8± 0.2 10.9 ±0.12 11.07 ±0.12 10.7 ±0.12 10.9± 0.1

F-CO2 mgL-1 4.66± 1.15 5.3± 1.16 6.66± 1.15 5.0 ±1.2 7.32 ±1.1 5.32 ±1.15 6.66 ±1.15 6.4 ±1.2 5.3 ±1.15 4.66 ±1.16 5.32 ±1.15 5.1±2 4.7± 1.15 6.66 ±1.15 5.3± 2.31 5.5± 1.5

TA mgL-1 94.67 ± 4.16 97.3± 1.15 97.33 ± 1.15 96± 2.2 83.1 ±0.06 68±2 66.67 ±1.15 45.9±1.1 77.3 ±3.06 78±2. 81.3 ±3.06 78.9± 2.7 82.0 ±2 84.67

±2.31 85.3 ±2.31 84± 2.2

NO3-

mgL-1 0.06±0 0.02 ±0 0.05 ±.02 0±0.0 0.0±0.01 0.05 ±0.01 0.04 ±0.01 0.04±0.0 0.01 ±0.01 0.02 ±0.01 0.03 ±0.01 0.0±0 BDL BDL BDL BDL

PO43-

mgL-1 0.29± 0.07 0.37± 0.12 0.45 ±.07 0.3 ±0.1 0.16±0.07 0.33 ±0.07 0.53 ±0.07 0.34±0.1 0.16 ±0.07 0.30 ±0.12 0.29± 0.07 0.3± 0.1 0.2 ±0.14 0.29 ±0.07 0.3 ±0.07 0.3 ±0.1

RF (mm) 115.05±163.09 115.05±163.09 115.05±163.09 115.05±163.09 20.57±24.5 20.57 ±24.5

20.57 ±24.5 20.57±24.5 192.3±157.02 192.3±157.02 192.3±157.02 192.3 ±

157.02 354.4

±52.27 354.4

±52.27 354.4

±52.27 354.4±52.27

Sand (%) 92±1 93.6±1.15 92.67±2.08 92± 1.4 93±2 92 ± 1.00 92.3 ±1.53 92.4±1.5 93.7 ±2.52 92±1 93 ±2 92.9±1.8 95.7 ±0.58 92.0 ±1.73 95.0 ±1.73 94.2±1.3 Silt (%) 3.67± 0.58 3.33 ±0.58 3.67± 0.58 3.7±0.6 3.3± 0.58 3.67 ±0.58 3.3 ±0.58 3.44±0.6 2.3 ±0.58 3.±0 3.3 ±1.53 2.9 ±1.8 1.7 ±0.58 3.3 ±1.53 2.3 ±0.58 2.4±0.9 Clay (%) 4.33± 1.53 3.00±1.00 3.67± 1.53 4.3±1.4 3.67 ±1.53 4.33±1.15 4.33±1.15 4.1± 1.3 4.0 ±2.0 5.±1 3.7 ±0.58 4.2± 1.2 2.7 ±1.15 4.67 ±1.15 2.7 ±1.15 3.3 ±1.2 (AT= Air temperature, WT= Water temperature, EC= Electrical Conductivity, TDS= Total Dissolve Solid, DO= Dissolve Oxygen, TA= Total Alkalinity, NO3

-= Nitrate, PO43-= Phosphate,

RF= Rainfall, BDL= Below Detectable Limit)

Table 4: Significant correlations of aquatic insect density and species richness with environmental variables of water of Brahmaputra River

Variables R value ID vs SR 0.707**

ID vs TDS - 0.444** ID vs EC - 0.455** ID vs RF -0.468**

ID-Insect density; SR-Species richness; TDS - Total dissolved solid; EC – Electrical conductivity; RF- Rainfall **. Correlation is significant at the 0.01 level (2-tailed)

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Fig 4: Triplot of Canonical correspondence analysis (CCA) showing the association of aquatic insects species (∆), environmental variables (→) and selected sites with seasons (0) of Brahmaputra River during 2014-15. Abbreviations used for different aquatic insect species are: Psudg=

Pseudagrion sp., Cln= Cloeon inscriptum, Suphs= Suphisellus sp., Gers= Gerris lepcha, Mcnta= Micronecta scutellaris, Mcv= Microvelia sp., Aqs= Aquarius conformis, Cyndths= Cylindrostethus sp., Hchs= Helichus sp., Stml= Stenelmis sp., Hdvt= Hydrovatus sp., Crcn= Cercyon sp.

Environmental variables are: AT= Air Temperature, WT= Water Temperature, EC= Electrical Conductivity, CO2= Carbon dioxide. Abbreviations for seasons are: PO= Post monsoon, W= Winter, PR= Pre monsoon, M=Monsoon

Table 5: Summary statistics of CCA between aquatic insect species and environmental variables for first four axes in four seasons of three sites

of Brahmaputra River

Axis 1 Axis 2 Axis 3 Total Eigen values 0.764 0.608 0.339 2.256

Cum. Percentage variance of species data 33.9 60.8 75.8 of species-environment relation 33.9 60.8 75.8

of species-environment correlation 1.00 1.00 1.00

Plate 1: Images of some aquatic insects found in River Brahmaputra during the study period (1. Micronecta scutellaris; 2. Gerris lepcha; 3. Microvelia sp.; 4. Cloeon inscriptum; 5. Helichus sp.; 6. Stenelmis sp.)

4. Discussion Experiences from USA and European programs have demonstrated that benthic macro-invertebrates are most useful in monitoring fresh-water ecosystems [37, 38]. In the present study density and relative abundance of order Hemiptera were recorded highest in all the seasons. Further out of six eudominant species four are from order Hemiptera. A study in

the lower reach of Moirang River in Manipur, N.E. India also showed high Hemiptera diversity and density [5]. Highest species richness (7 taxa out of 12 taxa) was recorded during Pre monsoon and lowest (4 taxa) was recorded during Monsoon. The causes of fluctuations in insect abundance and distribution include macroclimatic and microclimatic changes and variation in the availability of food resources [39-45].

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Eudominance of Pseudagrion sp. of order Odonata might be because they are tolerant to several anthropogenic impacts [46]. Another eudominant species recorded in this study is Cloeon inscriptum generally recorded in moderately polluted water [47] inspite of being included in the EPT (Ephemeroptera, Plecoptera and Trichoptera) [48]. Diversity indices provide important information about rarity and commonness of species in a community which in turn helps to understand community structure. Shannon- Wiener diversity index (Hʹ) values were found to be much less than 1(0.376 to 0.443) in all the seasons indicating polluted nature of river water [49]. This river receives heavy loads of municipal sewage and solid waste from the towns, agricultural waste from the surroundings and pollutants from the nearby tea gardens besides inflow of nutrients from settlements around the river. In winter the system was assembled by more dominant groups encountering highest Berger-Parker index of Dominance (0.603). Although Shannon H/ indicated distressed condition of the river at the selected sites, values of Margalef water quality index were found more than 3 indicating good condition of water [50]. Score values for individual families reflect their pollution tolerance based on the current knowledge of distribution and abundance. Pollution intolerant families have high BMWP scores, while pollution tolerant families have low scores [51]. All the biomonitoring scores in this study indicated poor to moderate condition of water indicating anthropogenic disturbance. The BMWP score was from13.67- 19.33 in all the seasons which indicated poor to moderate water quality of the river. However the ASPT score in the present study ranged from 3-5.25 which indicated polluted to doubtful water quality of the river [52, 53]. The SIGNAL scores of the sites ranged from 3.43 to 4.2 indicated that the river was severely to moderately polluted [54]. The range of pH (6.52 to 7.46) was found between the acceptable range as per BIS [55] and WHO [56]. In the present study, Bhutani et al [57] recorded similar pH value in the water of River Ganga at Haridwar, Uttarakhand. The EC values are an index to represent the concentration of soluble salts in water. The range of EC (95µS cm-1 to 18µS cm-1) agreed to the values recorded by Boruah and Biswas [58] in the water of upper Brahmaputra. Semwal and Akolkar [60] found the value of conductivity in between 172.75 and 175.0 µS cm-1 while working on the River Ganga. In the present study, a significant negative correlation of species density with electrical conductivity was recorded. Alkalinity in water is due to the presence of carbonates, bicarbonates and hydroxide constituents which may be derived from rocks, salts or sediments [61]. The range of TA (66.67 mgL-1 to 97.3 mgL-1), TDS (63 mgL-1 to 115.9 mgL-1) were within the acceptable limit of drinking water. TDS is a direct measure of all the dissolved particles, both organic and inorganic ions in water. In the present study although the range of TDS (63 mgL-1 to 115.9 mgL-1) was found within the desirable limit, it showed a significant negative correlation with species density. Similar trends of TDS values were also recorded by Boruah and Biswas [58] in Brahmaputra basin. DO did not show much fluctuation and highest DO values were found in all the three sites during monsoon. Similar DO values in River Ganga and Yamuna were reported by several workers [57, 62, 63]. It could be said that the increased DO of the system favoured the life of most taxa. The ranges of PO4

3- and NO3- were also found

within desirable limit as per BIS [55] and WHO [56]. The increased values of phosphate in all the sites of the river could be because of the run-off from catchment area including some agricultural fields. High concentration of phosphate was also

recorded in the Yamuna River, Doon valley, Uttarakhand, India [64]. Canonical Correspondence Analysis (CCA) was computed taking the data of environmental variables and species to find out possible correspondence between them. Canonical Correspondence Analysis is a multivariate method to elucidate the relationship between biological assemblages of species and their environment [36]. Eigen values associated with each axis equals the correlation coefficient between species scores and site scores [66, 67]. Thus an eigen value close to 1 represent a high degree of correspondence between species and sites and an eigen value close to zero will indicate very little correspondence [68]. Environmental variables with long arrows are more strongly correlated with the ordination axes than those with short arrows [36]. In the present study the correspondence was measured among insect species, sites with seasons and environmental variables. Thus the total eigen value greater than one indicated high degree of correspondences between species and sites with seasons. In the present study, a total eigen value (2.26) was found which indicated a high degree of correspondence between aquatic insect species, environmental variables and seasons with sites at Brahmaputra River. The eigen values 0.764, 0.608 and 0.339 for the axes 1, 2 and 3 explained 33.9%, 60.8% and 75.8% of variance respectively and the species environment correlations of CCA axis1 was found to be 1.00 (Table 5) The CCA association of the insect species, environmental variables and seasons with sites of Brahmaputra River are graphically displayed in Figure 4. The CCA ordination diagram revealed that species and sites with seasons are placed in four groups differentiated by dry and wet seasons. The species, Pseudagrion sp. and Gerris lepcha were mostly governed by DO, NO3

- and pH in dry seasons. Wet seasons were found to be governed by EC, TDS and RF and the associated insects are Suphisellus sp., Cloeon inscriptum and Cercyon sp. They also showed dominant status (Engelmann’s scale) [65] in BS1 and BS3. All these facts confirmed the association of these insects with moderately polluted water. During winter in BS2, governing parameters were phosphate and silt, and the associated species were Aquarius conformis and Cylindrostetus sp. 5. Conclusions This study presents a first glimpse of aquatic insects at Brahmaputra River of Dibrugarh district near Dibru Saikhowa National park. A total of 12 species belonging to12 genera, 10 families and 4 orders were recorded. A detail long term study might add some more species in the list of inventory. The values of diversity indices and Biotic indices indicated perturbed and moderately polluted nature of the water body. Hence proper management intervention is needed for arresting further degradation of this mighty dynamic freshwater ecosystem. 6. Acknowledgement We thank the Head, Dept. of Ecology and Environmental Science for providing laboratory facilities. Sincere thanks go to those who have helped during collection. 7. References 1. Vinson, MR, Hawkins, CP. Biodiversity of streams:

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