1 Introduction

Wetlands are the most lucrative resource having environmental and ecological values on the earth [1].Wetlands provide ecological as well as hydrological functions such as flood mitigation, stabilization of local climate, nutrient retention, ground water recharge and support biodiversity [1,2,3,4,5]. Wetlands also provide wide range of ecosystem services like food, fiber, fuel and greenhouse gas regulation through carbon sequestration [6, 7]. Due to prerequisite hydrological and meteorological condition associated with high water table, high productivity and low decomposition rate wetlands act like carbon sinks which is helpful for mitigation of global warming [6, 8, 9]. Like ecosystem services wetlands support non- market goods and services such as landscape maintenance and sustenance of rural people [7]. But in recent few decades this productive ecosystem [1, 10, 11] have been facing alterations which leads to wetland degradation [12]. Wetlands are important on earth supporting diverse and unique habitat for organisms as well as providing fresh water, fisheries, agriculture, irrigation, industrial activities, and recreation on which many humans largely depend [11, 13]. Economic development of any region results into the increasing pressure on wetland hydrology and affect wetland aquatic environment. Water surface temperature is an influential parameter to examine the existing wetland processes such as evaporation, water quality and oxygen concentration in water [14, 15]. In this connection a complete study requires to plan strategies for their protection and preservation.

The study of aquatic environment of wetlands reveals ecological importance of floodplain [16,17,18,19] which is mainly the outcome of geomorphology and hydrology [19]. Floodplain, complex and diverse ecosystem [20] stores huge amount of sediments [21]. Shifting of river courses, ox-bow lakes, scrolls are the common features associated with the floodplain [21] and the frequency of these features depend on many factors like flood occurrence, soil, channel pattern, precipitation over catchment area, sediment transportation etc. [21, 22]. These natural wetlands which are locally known as beel or baor in Assam, West Bengal are ecologically very productive [23, 24]. Floodplain wetlands providing enormous growth of fish food organisms are very much potential for capture fisheries as well as culture based fishery [25]. These natural landscapes with strategic importance to human society [25, 26] have been facing threats from anthropogenic modifications [27], often leading to unbalanced floodplain conditions such as increased sedimentation and siltation processes; eutrophication through various anthropogenic sources [28, 29]. Wetland hydrology plays very significant role to define its nature whether seasonal, dry or perennial [3]. To assess the physico-chemical and biological properties of the wetlands, routine monitoring of Dissolved oxygen, total dissolved solids, conductivity, salinity, temperature, pH are the important parameters [30,31,32,33]. Monitoring wetland hydrology also allows for the detection of local climate change [5]. Wetlands though act like a ‘kidney of landscape’ [34] but contaminated water can negatively affect wetland biota and ecosystem [10, 35]. Thus, surface water quality of floodplain wetland can be determined by both the natural processes (precipitation rate, soil erosion, weathering processes) and the anthropogenic influences such as agricultural and industrial activities, urbanization and over exploitation of water resources [28, 36]. High amount of sediments from the catchment area increases turbidity which affects wetland biota negatively [20, 37]. But these nutrients amplify the growth of some aquatic weeds like water hyacinth and cause hindrance for the light penetration into the wetland and affect wetland ecosystem [38]. In recent few decades the siltation and various anthropogenic activities are to be found as a major reason behind the gradual depletion of ecological productivity in floodplain wetland [28, 39].

The Sikkim-Bhutan Himalayan foothill region is the geomorphological hotspot over which processes of alterations are occurring in gradual rates. To the south of the Sikkim-Bhutan Himalaya, the northern plain has been divided into three distinctive parts Bhabar, Tarai and Alluvial plain [40]. In the Bhabar, due to the coarser sediments the rivers disappear and get reemerged into the Tarai. This Tarai region consists of alternative layer of sand, silt and clay which results into marshy land and luxurious growth of vegetation.The study area encompasses 127 villages of Tufanganj I and Tufanganj II C.D. Blocks in Coochbehar District [41], situated on the eastern border of West Bengal with a latitudinal and longitudinal extension of 26° 9ʹ 22ʺ N to 26° 27ʹ 36ʺ N and 89° 31ʹ 55ʺ E to 89° 52ʹ 48ʺ E bordered by Assam in East, Alipurduar district of West Bengal in the north, Dinhata block of the Coochbehar district in the south and Coochbehar I and Coochbehar II in west (Fig. 1).

Fig. 1
figure 1

Study area

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The northern portion of the study area lies under Tarai whereas the rest of the portion is under alluvial plain. The main rivers Raidak I, Raidak II, Gadadhar, Kaljani have frequently shifted their courses and formed ox-bow lakes, scrolls, paleo-channel. The total area of Tufanganj I and Tufanganj II is 582.69 km2 out of which 579.48 km2 constitute urban whereas 3.21 km2 is rural. Total water area of Tufanganj I and Tufanganj II Blocks are 6.19% and 8.62% of Coochbehar district out of the total water area beels have been found in maximum portion i.e. 261.81 ha. and 178.52 ha. respectively [58]. The main objectives of the study is to (1) find out the ecological importance of wetlands in Tufanganj C.D. Blocks (2) find out the deviation of temperature from wetland to the surroundings (3) identify anthropogenic sources of toxic elements in the wetland of Tufanganj C.D. Blocks.

2 Materials and methods

2.1 Elevation and geomorphological set up

ASTER DEM (Digital Elevation Model) of the study area has been downloaded from https://earthexplorer.usgs.gov/ and reprojected in Universal Transverse Mercator (UTM), Zone 45 N from Geographic Coordinate system (WGS 1984) to get the actual ground elevation. The Dem consist of pixels having 90 m spatial resolution. Using spatial analysis tool, elevation has been extracted automatically and geomorphological and geohydrological map of Coochbehar District have been collected from Geological survey of India and the study area has been extracted to know the slope and physiographic settings of the region (Fig. 2) as geomorphology and hydrology act like a driving processes for the formation and maintenance of floodplain wetlands [18]. All maps are prepared using Arc GIS 10.6 software.

Fig. 2
figure 2

Geomorphology and geohydrology of the study area

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2.2 Water sample test

Water samples (Fig. 3) in the study area have been collected from different beels. The total number of water samples for this assessment study is seventeen in number. Only those beels have been selected which are wet throughout the year. Using multi-parameter (Oakton 35425-00, 35425-05, 35425-10; Eutech instruments) pH, total dissolved solids (ppm), temperature in  °C, conductivity (μs), salinity (ppm) have been measured on the spot to know the physico-chemical properties of wetlands. With the help of DO meter (WTW DO meter model 3310, Xylem Germany) dissolved oxygen in mg/ltr has also been measured on the spot to determine the oxygen level for the aquatic life (Fig. 4). All water samples are measured during post monsoon season. Correlation of the parameters has been calculated in Table 1 to study the interrelationship among parameters. Garmin GPS Etrex 30x instrument has also been used to locate the sample beels of the study area (Fig. 3).

Fig. 3
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Location of water sample sites

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Table 1 Correlation study of the parameters
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Fig. 4
figure 4

Photo plates of some beels during water sample test a 26°18ʹ46.61ʺN, 89°41ʹ13.97ʺE, b 26°15ʹ48.02ʺN, 89°38ʹ09.78ʺE, c 26°19ʹ57.26ʺN, 89°37ʹ4.23ʺE

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2.3 Wetland index

Wetland index developed by National Aquatic Ecosystem Health Monitoring Programme (NAEHMP), previously known as River Health programme (RHP) is an effective tool to examine the ecological and environmental assessment of the floodplain wetland system [18, 42,43,44]. Wetland index is designed for the rapid assessment of floodplain wetlands to understand the present ecological status [18, 43]. This index is also easy to use for any non-wetland specialists. For this purpose, six water quality parameters with the standard score [45] prepared by Fishery Department, WHO and BIS [46] have been selected which are Temperature, pH, DO, TDS, Conductivity and salinity. These various components of water quality have great influence on productivity of floodplain wetland as a result of variety of impacts/activities in the catchment [18, 45]. The principal component analysis has been used to evaluate the ecological importance of wetlands as well as identification of the sources of anthropogenic interventions. The PC result reveals what parameter is in better position and what are lagging behind for the assessment of wetlands (Table 2). The calculation and formulation of wetland index [42, 47] involved the following steps:

Table 2 Score calculation for wetland Index
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  1. 1.

    In the first step, each of the six water quality parameters has been assigned to a rank and weightage value on the basis of productivity as well as human interferences of the beels depending on the previous studies [18, 42, 47]. All these parameters do not have the same capacity to influence aquatic environment. On the basis of importance and sensitivity to change aquatic environment the weightage has been assigned (Table 2).

  2. 2.

    In the second step, the each variable has been awarded numerical score to a maximum of 10 and its score has been distributed proportionately among the components of each variable on the basis of impact factor of the parameter on biological productivity of the beels (Table 2).

  3. 3.

    After given weightage to each component of all the parameters the overall score index (SIi) for each component has been obtained by multiplying the original score by assigned score (Table 2).

  4. 4.

    Finally, after calculation of overall score index (SIi) of each component of six parameters the wetland index has been calculated by following equation:

    $${\text{WI}} = \mathop \sum \limits_{i = 1}^{n} {\text{SI}}i$$

    where WI = wetland index, n = number of parameters, SIi = score of individual parameter.

The output scores from Wetland index model are categorized to understand the present ecological state of productivity and suitability for aquatic life form as well as to identify the anthropogenic sources of toxic elements within wetland.

2.4 Land surface temperature

The measurement of surface temperature if done manually is quite cost effective and time consuming. To avoid such barriers remote sensing is an eminent tool to understand the ecological as well as environmental assessment of wetland [48, 49].Landsat imageries having the thermal bands are useful for surface temperature measurement [50]. Thermal bands of Landsat imagery i.e. Band 10 and 11 of Landsat 8 OLI/TIRS (wavelength 10.60–12.51 μm.) for Jan, 2019 (post monsoon season) have been used for surface temperature measurement (USGS 2019). It includes three step procedures [51, 52]. After calculating LST of band 10 and 11, cell statistics from spatial analysis tool on Arc GIS 10.6 software has been incorporated to get the mean temperature of two bands.

  1. 1.

    Conversion to Top Of Atmospheric(TOA) radiance:

    $${\text{L}}_{\uplambda} = {\text{M}}_{\rm{L}} {\text{Q}}_{\rm{cal}} + {\text{A}}_{\rm{L}}$$

    where Lλ = TOA spectral radiance (Watts/(m2 * srad * μm), ML = Band specific multiplicatice rescaling factor from the metadata (Radiance multiple band x, where x is the band number), AL = Band specific additive rescaling factor from metadata (Radiance add band x, where x is the band number), Qcal = quantized and calibrated standard product pixel values (DN)

  2. 2.

    Conversion to top of atmosphere brightness temperature in  °C:

    $${\text{TB}} = \left[ {{\text{K}}2/{\text{Ln}}\left( {{\text{K}}1/{\text{L}}_{\uplambda} + 1} \right)} \right] - 272.15$$

    where T = top of atmosphere brightness temperature (in  °C), Lλ = TOA spectral radiance (Watts/(m2 * srad * μm), K1 = Band specific thermal conversion constant from the metadata (K1 constant band x, where x is the thermal band number), K2 = Band specific thermal conversion constant from the metadata (K2 constant band x, where x is the thermal band number).

  3. 3.

    Conversion from Atmospheric Satellite temperature to Land surface temperature:

    $${\text{T}} = {\text{T}}_{\text{B}} /\left[ {1 + \left( {\uplambda*{\text{T}}_{\text{B}} /{\text{C}}2} \right)*{\text{Ln}}\left( {\text{e}} \right)} \right]$$

    where λ = wavelength of emitted radiance, c2 = h * c/s = 1.4388 * 10−2 m k = 14,388 μm, h = Planck’s constant = 6.626 * 10−34 Jsk, s = Boltzmann constant = 1.38 * 10−23 J/K, c = velocity of light 2.988 * 101 ms.

3 Results and discussion

3.1 Physiographic settings for the formation of wetland

The study area consists of the alluvial fan deposit (Fig. 2) with alternative layer of sand, silt and clay. The direction of groundwater flow from the north-west to the south-east direction towards Bangladesh (Fig. 2) indicates the slope of the study area. The average annual rainfall is 3500 mm [53]. These physiographic settings and climatic condition lead to the occurrence of flash flood every year. It results into the alteration in channel planform and movement of sediments. The region is fluvio-hydrologically dynamic area. These physiographic settings are resulting into the formation of large number of meander scrolls, paleo channels, neck and chute cut- offs due to the lateral oscillation of Kaljani, Gadadhar, Raidak I, Raidak II river which are collectively known as Beel or Baor.

3.2 Role of temperature for the assessment of wetland

Land surface temperature map (Fig. 5) has been prepared to know the deviation of temperature from wetland to the land area. The maximum temperature is 26.5 °C whereas minimum temperature is 16.5 °C. The pixel values show that the wetland area shows the lowest temperature whereas the land area shows the highest temperature in different pockets of Tufanganj I and Tufanganj II blocks of Coochbehar District. The deviation of temperature from wetland to surrounding region is almost 10 °C. The surface temperature derived from satellite images reveals that the wetlands reduce temperature of its own as well as its surrounding areas by 9 °C–10 °C. This temperature plays a significant role for the distribution of aquatic life [54] as there is a symbiotic relation with temperature and all parameters.

Fig. 5
figure 5

Land surface temperature

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3.3 Wetland assessment through wetland index

Wetland index (Table 3) has been prepared on the basis of ecological productivity of the beels. This index is very much efficient to study the ecological importance as well as the identification of anthropogenic sources on the beels in the study area. After calculation the score of the sample beels (Table 3) reveals that the Masankura beel has the lowest score i.e. 46.03 whereas the highest score has been found in Rasikbeel and Bamnir Chara i.e. 87.69 (Table 3).The higher the score the higher the productivity and suitability for the aquatic environment (Table 4). The wetland index score having more than 75 indicates that the beels are suitable for fish and aquatic life with less human intervention. Rasikbeel wetland complex with highest index score is one of the promising biodiversity reserves in this region. Every year Rasikbeel wetland complex witnesses a large population of residential as well as migratory birds such as cormorants, egrets, herons, snipes, kingfishers, Lesser-whistling teals, Poachard etc. Total 51 species of avifauna are recorded in this wetland. On the contrary, the beels having lowest index score (< 55) are facing threats from human intervention. The lowest wetland indexed beels, surrounded by agricultural land have no such fish production and are also harmful for other aquatic life due to the mixing of inorganic fertilizers through agricultural run-off profusely used in paddy cultivation in this region. The pH of the sample beels ranges from 7.1 to 10 indicating slightly alkaline nature. The optimal range of pH for aquatic life is 6.5–9.0 [55] for irrigation; it is 7.0–8.5 [46, 56]. The value of pH is associated with the photosynthetic activities on beels [57]. The permissible limit of pH, temperature and DO for fish production is 6.5–7.5, 25 °C–32 °C and 4–5 mg/ltr [58]. The pH of Jaigir Chilakhana Beel 1 and Jaigir Chilakhana Beel 3 is 9.1 and 10 respectively indicating these two beels are unsuitable for the growth and production of fish and other aquatic life. The variation in pH has occurred probably due to the effluents from the adjacent catchment area. Though the temperature of the beels is within the permissible limit but the other parameters show some variability in some of the beels. Fluctuation of temperature from the optimal range may become a threat for the aquatic life as oxygen level decreases and pollutants become toxic in higher temperature. Low water temperature and better transparency may result into the DO level slightly rising high during post-monsoon than monsoon and pre-monsoon. However, it can rapidly decrease due to the discharge of oxygen demanding wastes like ammonia, nitrate, iron etc. from agricultural land. Low DO content results into the heavy contamination of organic matters and adversely affects fish and other aquatic life. The DO level in the study area ranges from 4 to 8.5 mg/ltr indicating healthy aquatic environment for fish and other aquatic life. Conductivity is the parameter which determines the total dissolved ions in the beels. The desirable limit for conductivity and TDS for aquatic life is 1000 μs and 500 ppm respectively [46, 55]. It has been observed from the study that none of the beels has crossed the desirable limit for the growth of aquatic life. In present study lower TDS has been recorded in Jaigir Chilakhana beel 3 i.e. 10 ppm whereas the highest is in Uttat Balabhut beel i.e. 161 ppm. The increase of TDS value indicates the higher amount of nutrients present in water. The increase of TDS in beel is not good for the health of aquatic life. Agriculture is the prime focus of both the blocks and the beels are mostly used for boro cultivation due to the higher amount of water requirement. The use of fertilizers in the fields is also very common. Through the agriculture run-off these toxic elements enter into the beels and affect the wetland biota negatively. The recorded value of the physico-chemical parameters in limnological studies [4, 30, 42] in some parts of the beels reveal the favorable aquatic environment for the fish production and provide livelihood to rural poor.

Table 3 Wetland assessment through wetland index
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Table 4 Productivity according to wetland index
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PCA of the beels (n = 17) explaining 40.3% of total variance in respective water quality datasets reveals that most of the variations in water quality are explained by the anthropogenic pollution sources. Correlation coefficient study of the different parameters reveals that there is a strong relationship between salinity and conductivity (r = 0.99), salinity and TDS (r = 0.89), conductivity and TDS (r = 0.79) (Fig. 6). Conductivity determines the concentration of dissolved salts i.e. the higher the salinity the higher is the conductivity [55]. Highest concentration of TDS has been found in those beels which have been surrounded by agricultural practices. And there is no such fish production. However, PCA results may be convincingly presumed that the beels in this floodplain regime, the aquatic environment is mainly affected through the agricultural run-off, idol immersion and different recreational activities like bathing, washing clothes etc. All the water quality parameters for the floodplain wetland (n = 17) assessment through wetland index show significant relationship with the ecological productivity (p < 0.05, t test).

Fig. 6
figure 6

Correlation between different parameters a total dissolved solids and Conductivity, b total dissolved solids and Salinity c

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4 Conclusion

The study reveals that the physiographic set up of the region and the climatic condition have resulted in the formation of many wetlands within meander belt due to the lateral oscillation of the River Kaljani, Gadadhar, Raidak I and Raidak II. While the Land surface temperature reveals that the presence of beels is effective to temperature from wetland to land area, the wetland Index shows that the beels are ecologically very much productive for fish culture and are excellent habitat for migratory birds. Rasikbeel wetland complex has been declared as Wetlands of National Importance and is under the National Wetlands Conservation Programme. Wetland assessment through wetland index provides a composite score of individual beel to determine the present ecological state as well as identify the anthropogenic sources hampering the ecological productivity of the beels at a glance [18]. PCA helped in identifying sources responsible for the variation in growth and production of fish and aquatic life in the beels. Of all the six water quality parameters in wetland index -TDS; conductivity and salinity have the greater influence on the aquatic environment indicating the anthropogenic interferences of the beels. Not only the anthropogenic interferences but also natural short term processes like rainfall, temperature, frequency of flooding and regional set up affect wetland habitat. Except rainy season; these beels have been excessively used by the villagers for boro crop cultivation thereby posing a threat to the ecological balance of the beels. Floodplain wetlands have been altered with the intensification of agricultural modernization. Any economic pressure may hamper the biological productivity i.e. influenced by temperature and morphometric features. The planned exploration of these floodplain wetland resources which provide favorable aquatic environment for the growth of fish and fishery may support livelihood for society. Therefore, development of this resource will generate suitable rural economic environment and employment. Thus, this study presents usefulness of wetland index model through water quality parameters, RS GIS techniques and statistical techniques in ecological assessment, environmental assessment and identification of anthropogenic sources with a view to get better aquatic environment as well as productivity in the floodplain wetland and proper monitoring strategies for the sustainable use of wetlands.