Rapid Bioassessment of Magura Haor (floodplain Wetland), Cachar District, Assam, India Using Aquatic Insects

Investigation on the water quality of the floodplain wetland, Magura through rapid assessment survey using aquatic insects for the four seasons were done during 2013-14. Aquatic insects were collected from the two sites and were identified up to family level. All together 5 orders (Hemiptera, Coleoptera, Odonata, Ephemeroptera and Diptera) and 21 families were recorded from the wetland. 17 families from site 1 (Hemiptera-7 families, Coleoptera-4 families, Odonata-3 families, Ephemeroptera-1 family and Diptera-2 families) and 18 families from site 2 (Hemiptera-6 families, Coleoptera-6 families, Odonata-2 families, Ephemeroptera-1 family and Diptera-3 families) were recorded. Four family level biotic indices-SIGNAL 2, ASPT, BMWP and FBI were used to determine the status of the water quality. According to SIGNAL 2 and BMWP scores it was moderately polluted, whereas ASPT showed doubtful quality but FBI proved very good (site 1) and good water condition (site 2). This study revealed that only one biotic index cannot give proper justification of the water quality status of a wetland.

Rapid bioassessment survey is the quick and easy method of analyzing water quality of any water body by using benthic macroinvertebrates.Benthic macroinvertebrates are common inhabitants of freshwater systems and are also sensitive elements of aquatic biota 1 .Therefore, changes in its taxonomic richness and composition are considered sensitive tools for perceiving alterations in aquatic ecosystems 2 .Therefore, rapid procedures for assessing the biotic communities of surface waters become widely used in recent years where they allow large number of sites to examine at a low cost 3 .In general, rapid bioassessment can be defined as an evaluation of the condition of a water body using biological surveys and other direct measurements of the resident biota in surface waters 4 .Resident biota in water body are natural monitors of environmental quality and can reveal the effects of episodic as well as cumulative pollution and habitat alteration 5 .Biotic indices such as Trent Biotic index 6 ; BMWP score 7 , ASPT score 7 , Wisconsin Biotic Index (BI) and Family Biotic Index (FBI) for macro-invertebrates are used to detect and monitor water quality and human perturbations in varying ecosystems like streams 8 , rivers 9 and lakes 10 .
The present study is focused on one floodplain wetland (locally called as 'haor') named Magura in Cachar district of Assam, India.This study is aimed to establish ecological status of the wetland and classify the sampling sites with the help of different biotic indices where aquatic insects have been used as bioindicator.

MATERIALS AnD METHoDS
Cachar distict (25.0833°N, 92.9167° E) is the commercial district of Barak valley.Magura haor (N 24 0 36.910/ E 092 0 51.924 / ) is the floodplain wetland of River Rukni, one of the tributaries of River Barak.During monsoon its depth reached up to 150 cm.It covers an area of 1,87,146 sq.meters (Fig. 1).Major parts of the wetland get dried up during dry period and used for paddy cultivation and grazing of the cattles.The wetland is rich in macrophytes like Eichhornia crassipes, Ludwigia adscendens, Utricularia aurea, Pistia sp., Nymphoid indicum, and Myriophyllum spectrum.
Insect samples were collected seasonally during post-monsoon (2013) to monsoon (2014) from the two sites of the wetland-site 1 and site 2 in replicates.For the collection of aquatic insects "Kick method" was applied where vegetation was disturbed and a circular net (mesh size 60mm) was dragged around the vegetation for a unit of time [11][12] .Three such drags constituted one sample 13 .Collected insects were immediately sorted and preserved in 70% ethyl alcohol.They were identified upto family level using Magnus stereozoom microscope with the help of standard keys [14][15][16][17][18][19][20] .Different biotic indices like SIGNAL, ASPT, BMWP and FBI were used for the rapid bioassessment survey of the Magura haor.
Table 1 showed the SIGNAL 2 scores for site 1 and site 2 for the four seasons of Magura haor.The SIGNAL (Stream Invertebrate Grade Number-Average level) is a family level scoring system for macroinvertebrates sample where it gives the indication of water quality from where the sample was collected 21 .In site 1, highest score was shown in monsoon while lowest score was shown in pre-monsoon.Although species richness was found more in pre-monsoon, monsoon season scored highest because of the presence of more relatively sensitive groups.In site 2, again monsoon season secured highest score and post monsoon season had lowest score.In both the sites, weight factor of Baetidae family was found highest which also has highest SIGNAL 2 sensitivity grade and highest relative abundance in monsoon season (Fig. 2), thus increasing its SIGNAL 2 score as a whole.Both the sites showed severe pollution status (i.e., < 4).Similar status of SIGNAL 2 score was also recorded from other floodplain wetlands of Cachar district [22][23] .Though it is mainly used in streams and rivers where organic pollution is less, it can also be applied in wetlands and other lentic ecosystems.But the scores would be less as some of the macroinvertebrate families that have the highest SIGNAL 2 sensitivity grades are naturally rare in wetlands 21 .Therefore we can consider this wetland under moderate water pollution.
Table 2 showed the BMWP and ASPT scores for site 1 and site 2 for the four seasons of Magura haor.BMWP (Biological Monitoring Working Party) is a family level scoring system which represents organisms' tolerance to pollution.The greater their tolerance towards pollution, lower the BMWP score 27 .In site 1, BMWP score was found highest in pre-monsoon season and lowest in monsoon.The highest score reflected moderate condition of water according to BMWP grade.In site 2, winter had highest score and had moderate condition and monsoon had lowest score and had poor water condition.Overall we can say, according to BMWP score this wetland was moderately polluted.Similar studies at other parts of the world also revealed moderate pollution according to BMWP score [28][29] .ASPT (Average Score Per Taxon) is also a family level scoring system for the average tolerance level of all the taxa within the community and was calculated by dividing the BMWP by the number of families represented in the sample 30 .In case of ASPT score, for site 1 winter season had highest score and in site 2 monsoon season had highest score.Both the highest scores came under doubtful quality.Previous studies also revealed doubtful quality for ASPT score [22][23] .
Table 3 represented the Family Biotic Index (FBI) for site 1 and site 2 for the four seasons of Magura haor.FBI is a family biotic index which provides a single tolerance value which is the average of the tolerance values of all species within the benthic arthropod community.The Biotic Index was subsequently modified to the family-level with tolerance values ranging from 0 (very intolerant) to 10 (highly tolerant) based on their tolerance to organic pollution thus creating FBI 26 .FBI is different from the other two scores in the interpretation of tolerance status.In this lower the score more clean is the system.In site 1, post-monsoon season had the lowest score (2.88) and so it came under 'very good' condition and in site 2 pre-monsoon had lowest score (4.58) which had a 'good' water condition.Thus, both the sites represented less pollution condition according to FBI. Figure 2 showed relative abundance graph of the wetland of site 1 and site 2. In site 1 Baetidae was found most abundant in Monsoon followed by Culicidae in Pre-monsoon whereas in site 2 most abundant families were Hydraenidae in Post-monsoon and Baetidae in Monsoon followed by Notonectidae in Pre-monsoon.Family Baetidae was recorded as most abundant family in both the sites of this wetland.Though the family Baetidae belongs to order Ephemeroptera which is a part of EPT sensitive group, occurrence of insects belonging to this family indicate moderate pollution condition of water [31][32][33] .During dry period intensive farming practices carried out by the inhabitants might have contributed to the deterioration of water quality.A study on aquatic insects of one oxbow lake of South Assam also revealed similar result 34 .

ConCLUSIon
This study confirmed that rapid bioassessment survey can be a very useful tool in the determination of water quality of a system.Different biotic indices used in this study revealed that the system is moderately polluted although according to FBI it is in good or very good condition.We found that use of only one biotic index cannot give proper justification of the water quality of a system.There is need to analyze several indices which can lead to a conclusive remark.Therefore, it can be concluded that although the wetland had some impact of pollution, it is still under condition of reviving to its original state if intervention is made in time.The need of the hour is environmental awareness among the inhabitants regarding water quality and its possible consequences.