Ecological assessment of a subtropical floodplain wetland of the Ganga basin in the context of changing climate using GIS tools

Wetlands occupy approximately 6% of the terrestrial surface and provide a variety of ecological services to the surrounding population (Mao et al. 2018). Tropical floodplain wetlands are typically situated next to rivers, lakes, and oceans and formed through hydrological circumstances (Kingsford 2000). These are rich, biologically sensitive ecosystems that support unique aquatic biodiversity and play a vital role in providing livelihoods and nutritional security to a large segment of the population in developing countries (Pathak et al. 2004). However, they are disappearing globally as a result of increased human activity and climate change, particularly in Asia and several mid- and high-latitude regions (Zhang et al. 2017). Besides the direct and indirect effects of rising temperature, changes in rainfall intensity and frequency, extreme climatic events such as drought, flooding, and the frequency of storms, floodplains have been converted for intensive agricultural exploitation in developing countries (Naiman & Decamps 1997). Freshwater ecosystems and the associated fish diversity and fisheries are changing at the same pace under the changing climate (Woodward et al. 2010; Sarkar et al. 2019). Global wetlands have lost up to 87% of their original area since 1700 (Hu et al. 2017).

Floodplain wetlands play an important role in the economic, social, and cultural activities of rural India. However, wetland fishery resources of the country are under continuous threat of environmental degradation due to natural as well as anthropogenic activities, resulting in ecosystem imbalance, shrinkage in water spread area, a decline in biodiversity and a decrease in fish production at an alarming rate (Sarkar & Borah 2018). Wetlands of India have been subjected to acute conversion, hydrologic modification (dam/barrage building), and chronic loss (alteration of wetlands, degradation of water quality, groundwater depletion, invasion of alien species and extinction of natural biota) causing loss of floodplain wetland biodiversity and degradation causing a drop in fish production (Phukan & Saikia 2014; Das 2015). Lack of consistency in government policy governing economics, environment protection, and development planning, as well as a lack of sound governance in scientific management have contributed to the degradation of these water bodies (Turner et al. 2000; Kumar et al. 2013).

Inland floodplain wetlands in India cover a 0.55 Mha area, which is one of the major resources for freshwater fish and is indispensable for different economically important and migratory fish species (Sarkar & Borah 2018). Typically, the Gangetic plains of West Bengal have a considerable number of natural wetlands (area: 42,500 ha) in India contributing to the economy and food security of the state. Inland water bodies of West Bengal harbour around 190 varieties of native fishes, which is about 23% of total freshwater fishes found in India (Mahapatra et al. 2014). Productive floodplain wetlands play an important role in fisheries, livelihood, and the nutritional security of the rural population of West Bengal (Sarkar et al. 2020a, 2020b, 2020c). The alarming reduction in fish yield has had a significant impact on the lives and livelihoods of indigenous fishers as well as other stakeholders that rely on these resources directly or indirectly. Human encroachment for habitation and agricultural use resulted in habitat degradation and the contraction of wetland areas throughout India, accelerating the deterioration of the Gangetic planes in terms of its ecology (Sarkar et al. 2020a, 2020b, 2020c). Flood control measures like the formation of dams as well as sluice-gate near the feeding river area can also affect the productivity of wetlands by preventing or limiting auto-stocking. Uncontrolled weed infestations, particularly water hyacinth, are a threat to floodplain wetland fisheries as the spreading plant accelerates eutrophication in wetlands that lead to a variety of problems in navigation, fishing, boating tourism, water quality, and species diversity (Lahon et al. 2023). Aside from water hyacinth, Ipomoea infestation is frequent in the studied region, causing wetland ecosystem degradation and shrinkage of the water spread area through eutrophication. The lack of effective equipment and gear particularly built for floodplain wetlands also makes it difficult to maximise the potential of wetland resources.

The health of inland water bodies is directly connected to aquatic biodiversity, and even minor changes have major impacts on it. The effect of unpredictable climatic events such as shifts in rainfall patterns, recurrent floods, droughts, and cyclones is the major constraint for sustaining the healthy ecology of the wetlands, fisheries, and livelihoods of fishers (Sarkar & Borah 2018). Extreme climatic events such as floods and droughts are becoming more frequent, and hurt socioeconomic growth (Faye 2022). The impact of climate change was observed by various authors (Chen et al. 2018; Mehvar et al. 2019; Saintilan et al. 2019; Das Sarkar et al. 2020) on various types of wetland ecosystems and in diverse agro-climatic zones using prediction models (Ekwueme 2022) and other tools. However, the application of the geographic information system (GIS) to understand and manage wetland fisheries has not been adequately explored in India. The GIS is an important tool for collecting, storing, reviewing, processing, combining, analysing, and displaying data spatially related to lakes and their basins (De Mers 1997). It was found during the literature review that the scientific reports on assessing the overall health and importance of tropical floodplain wetlands in India are scanty. There is a dearth of comprehensive studies and literature on the spatio-temporal change of floodplain wetland resources in the face of ecological degradation, encroachment, and area shrinkage due to harmful climate change and anthropogenic activities. Especially in Asia, the wetland management plans are in their nascent phase (Sarkar et al. 2021). To fill in this gap, there is an urgent need to produce region- or location-specific studies indicating the all-around present conditions of wetlands with respect to their ecology and productivity.

On this ground, the present study was conducted by incorporating a systematic investigation of climatic and hydrological influences on floodplain wetland fish diversity as well as fish production trends. To the best of our knowledge, this is the first study to demonstrate the spatial analysis of a threatened tropical floodplain wetland using the Kriging geospatial technique. The first aim of the present study was to comprehensively analyse the historic pattern of climate change in the selected region in relation to its impact on the wetland ecology including the fish diversity and production trends. The paper also highlights the role of the GIS for ecosystem-based fisheries management and research needs for conserving biodiversity and sustainably increasing the productivity. The second aim of the study was to estimate the influence of changing climates on the ecosystem services provided by the wetland by utilising stakeholders' perceptions. The present account also suggests several climate-resilient adaptation measures for the improvement of the ecosystem health of the wetland in relation to the benefit of fishers.

Fish and environmental samples were collected in triplicate from each site, monthly (Sarkar et al. 2020a, 2020b, 2020c). Water temperature, pH, salinity, total dissolved solids (TDS), and conductivity were determined with the multi-parameter Testr™ 35 series (OAKTON). The depth of the water column was estimated through the Hondex™ digital depth sounder. Transparency was measured using a 20-cm diameter standard black-and-white Secchi disk (Strickland & Parsons 1972). DO was measured by a digital DO meter (Ultron DO5510). Total alkalinity (TA), hardness, available phosphate (AP), nitrate (N), free carbon dioxide (FCO₂), and chlorophyll-a (Chl-a) were measured using standard methods (APHA 1998). Primary productivity (gross primary productivity: GPP and net primary productivity: NPP) was measured through in situ analysis of community respiration following Vollenweider (1969). The Trophic State Index (TSI) was calculated following Lamparelli (2004). After conducting experimental fishing, samples were brought and identified in the laboratory based on specific morphometric and meristic characteristics following Talwar & Jhingran (1991) and Vishwanath et al. (2011).

The monthly climatic data (mean air temperature and total annual rainfall) of the concerned districts (North 24 Parganas, West Bengal, India) were collected (1985–2020) from the Indian Metrological Department (IMD), Alipore, Kolkata, India for the study period. On the other hand, the yearly fish production statistics were collected from the existing fishermen cooperative society's record book.

Long period average (LPA) was utilised as a baseline to measure the change in rainfall and temperature in the present study (Lianthuamluaia et al. 2023). One-way analysis of variance (ANOVA) followed by Tukey's post hoc test was carried out to investigate potential differences in the environmental parameters across the stations of the wetland. Data on fish diversity were initially collected and then organised according to taxonomic orders. The conservation status has been documented according to the IUCN Red List of Threatened Species (IUCN 2020). The richness and evenness of fish species at each station were also determined following standard indices (Table 1). To investigate the relationship between environmental conditions and fish community structure, Canonical Correspondence Analysis (CCA) at a significance threshold of p < 0.05 was done using PAST3.26. We utilised the CCA for each site to estimate the relative value of each site, which links fish abundance and environmental characteristics. The CCA was applied to the total fish data matrix as well as the environmental data matrix, resulting in a direct environmental interpretation of the generated ordination axes. Trend analysis of the available production data was carried out to assess the pattern of fish production in the wetlands. The present fish species richness (number of different species) was compared with the previously (before three decades) found fish species richness by cross-verification with the local fishermen through a standard questionnaire prepared by ICAR-CIFRI (2020), India.

Table 1

Different species diversity and evenness indices used in the study

Index .	Equation .	Reference .
Shannon–Weiner diversity index (H′)		Shannon & Weiner (1949)
Simpson diversity index (DSim)		Whittaker (1965)
Pielou's Evenness index (J′)		Pielou (1975)

Index .	Equation .	Reference .
Shannon–Weiner diversity index (H′)		Shannon & Weiner (1949)
Simpson diversity index (DSim)		Whittaker (1965)
Pielou's Evenness index (J′)		Pielou (1975)

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In the present study, a Landsat 8 Operational Land Imager (OLI) multispectral image (path 148, row 38, resolution 30 m) with less than 10% cloud coverage was downloaded from the USGS Earth Explorer website to extract the water spread area for the wetland. The GPS coordinates of the points were collected during on-field sampling. The water area of the wetland was digitised, and the percentage of the area was measured. Layer stacking and radiometric correction processes have been used during data processing (Abd El-Kawy et al. 2011; Dibs et al. 2015). After the image correction, false colour composition (FCC) and the composite band were created for better and clearer visualisation of the image. The spatial interpolation maps were created through the ‘Kriging’ interpolation method, an advanced geostatistical analysis (Isaak & Srivastava 1989; ESRI 2013). Kriging is a superior approach and differs from others in that it exploits spatial correlation between sampled locations during the process to interpolate values in the spatial field (Chakraborty et al. 2022). The interpolation was applied concerning the different environmental datasets by transferring spatial data into the GIS platform using ArcGIS 10.4.

Wetland ecology and biota are primarily impacted by wetland habitat factors. Water's physicochemical qualities are influenced by hydrological, geological, climatic, and anthropological variables (Bartram & Balance 1996). The temperature of the surface water in the examined wetland upsurges from pre-monsoon to monsoon season. DO in water decreases when water temperature rises (Bouslah et al. 2017). Water pH is a crucial factor in determining the health of any wetland, and it plays a significant part in aquatic species' life cycles by controlling metabolic activities (Palit et al. 2018). During the investigation, the alkaline pH of the water fluctuated significantly (p < 0.05). TDS levels were found to be high during the monsoon season, which could be attributed to the runoff of residential waste, garbage, and wastewater (Verma et al. 2012; Choudhary et al. 2014). During the monsoon season, the concentration of DO was lower, which could be attributed to increased rainfall throughout the research period. A similar range of DO concentrations was found in Kailash Khal, a Sundarban tropical wetland (Gogoi et al. 2019). Transparency is a significant limiting factor for the productivity of the wetland and usually depends on suspended clay, silt, particle organic matter, dispersed aquatic organisms, and pigments created by the decomposition of existing organic matter (Michael 1969). The current study observed the least transparency during the summer, which is related to the effects of wind, suspended organic debris, phytoplankton enrichment, and increasing temperature. Michael (1969) and Kumar (1985) both documented seasonal variations in lake water transparency. The recent investigation discovered that seasonal changes in the water column were significant and largely depended on rainfall. According to Sugunan et al. (2000), the majority of wetlands in West Bengal are subject to water stress, and reduced rainfall is the primary reason for water balance challenges in closed wetlands. According to Lamparelli's (2004) TSI, a Media wetland can be classified as a mesotrophic wetland (range: 44–54). However, according to Das Sarkar et al. (2020), most of the wetlands in the studied region were found to be early eutrophic according to Carlson's TSI and were advanced to eutrophic according to Lamparelli's TSI. Kumari et al. (2021) discovered that sewage-fed wetlands had a higher TSI than floodplain wetlands.

The present study area has been experiencing an increased average annual temperature and a decrease in total annual rainfall. Sarkar et al. (2019, 2022) also recorded an increase in air temperature and a decrease in rainfall trend along the middle and lower stretch of the Ganga River basin. A similar study was carried out in the West Bengal districts of Maldah and Murshidabad by Naskar et al. (2022), and their findings are in parallel with the present study. Several other investigations in the Ganga Basin have been conducted (Das et al. 2013; Rathore et al. 2013), and it was established that variations in climatic characteristics such as rainfall pattern and temperature have a significant impact on the water quality parameters or micro-habitats found in associated wetlands. Seasonal fluctuations in wetland vegetation and fauna indicate that the wetland resources are climate-dependent for their viability (Rathore et al. 2013; Sarkar & Borah 2018). According to Sarkar et al. (2018), floodplain wetlands are biologically sensitive habitats that are thought to be the most impacted ecosystem by climate change. A shift in climatic conditions can actively alter the breeding season of the wetland fish community. The temperature varies substantially depending on the climatic conditions of the geographic location, and it also varies greatly during the day. In comparison to the current study, Bhowmik (1988) reported that minimum and maximum temperatures in West Bengal wetlands ranged from 17.5 to 32.0 °C.

In the present study, 45 species were described from the Media wetland indicating a declining trend in species diversity during the last few decades. Aziz et al. 2021 noticed the active participation of fishermen, and identified 63 species of fish and shellfish, belonging to 12 taxonomic orders in HakalukiHaor in 2018. Edwards et al. (2012) studied the fish fauna in Canyon Reservoir, and species diversity was also reduced. This kind of observation is common in other studies (Naskar et al. 2022; Sarkar et al. 2022). Mondal & Kaviraj (2009) recorded 49 fish species in the Gopalnagar and Dumur wetland of North 24 Parganas, West Bengal. Chakraborty (2002) observed 35 species from the Matura wetland and Ghosh & Biswas (2017) recorded 33 species from the wetland of Nadia district West Bengal, which is lower than the present finding. The fish species diversity decreases in the present study might be due to several reasons like habitat degradation, loss of river connectivity, climate change, pollution, anthropogenic activity like jute retting, agriculture run off, and sewage water disposal in the wetland (Sarkar & Borah 2018). In this study, the family Cyprinidae was found to be the major contributor to the overall wetland fish diversity. A similar pattern of observation was found by Sarkar et al. (2020a, 2020b, 2020c) and Mistry (2016). The conservation status shows few species in the NT and EN categories which need attention for their conservation. The species richness has declined in the last decades, Shannon–Weiner diversity index in the present study was significantly lower than that found by Sarkar et al. (2020a, 2020b, 2020c) in three similar floodplain wetlands (2.89–3.09), whereas the Simpson's index (0.049) was found to be less than previous studies that range from 0.93 to 0.94 (Sarkar et al. 2020a, 2020b, 2020c). Suriya et al. (2022) observed in Sago Palm Wetlands, Thailand, 62 species belonging to 24 families, and Cyprinidae was found to be the major family. The species diversity index was in the range of 0.66–2.67, the evenness index was in the range of 0.64–0.95, and the species richness index ranged from 0.36 to 6.86, indicating an intermediate–low uniformity, with a medium species number in the area. The value of evenness in the present study was quite good and reflects the homogeneous distribution of fishes throughout the wetland. Similar findings were reported from Coochbehar, West Bengal by Das (2018).

GIS and remote sensing will be useful for mapping the resources, and these maps will act as decision support systems for their long-term usage and management. Hu et al. (2020) used remote sensing imaging and land use models to quantify long-term wetland degradation in Hangzhou Bay from 1984 to 2016 and projected the spatial locations of wetland degradation until 2046 under various scenarios. Sarkar et al. (2021) used GIS technologies to conduct a spatio-temporal change analysis of floodplain wetlands in West Bengal and reported a reduction of the potential wetland coverage by 37.20–57.68%. A similar observation was found in the present study. The GIS-based study was also carried out in large reservoirs like Pong, Himachal Pradesh, India (Chakraborty et al. 2022). Regional changes in climatic variability have not only impacted the hydrodynamics of the wetland ecosystem of Indo-Gangetic plains but also impacted the biological diversity of wetlands (Sarkar et al. 2019). The wetlands of West Bengal, especially the lower Gangetic plains (North 24 Parganas), have witnessed chronic macrophyte infestation with increased eutrophication (Karnatak et al. 2022). Our findings are also in line with the same.

It has been found that regional climate variability may influence the reproductive behaviour of fish (Lianthuamluaia et al. 2023). In the current study, the CCA revealed that environmental parameters such as rainfall, temperature, algal biomass, dissolved inorganic carbon, depth, conductivity, FCO₂, and dissolved organic carbon influenced fish diversity. Similar findings were recorded by Aziz et al. (2021), who demonstrated how the climatic and anthropogenic variables are responsible for the loss of fish diversity in Hakaluki Haor, an ecologically significant wetland of northeast Bangladesh. Environmental influence was also obtained on fish assemblage patterns in Chandil reservoir, Jharkhand (Lianthuamluaia et al. 2019).

In the present study, the ESI score by the RAWES method depicts a moderate level of ecosystem services provided by the wetland and a similar observation was documented by Mandal et al. (2021) at Indrakpur, Ramavchandrapur, Kasthahali, and Purbasthali wetlands, which belongs to the same region as the current study. Kumari et al. (2023) documented nearly the same ecological services from the Bhomra wetland, Nadia, India.

Although specific information on assessing the impact of climate change on floodplain wetlands is limited, it is clear that climate change will cause irreversible changes in the ecology of the natural hydrological network (Sarker 2022), as evidenced by a study on potential changes in floodplain inundation and connectivity between river and floodplain wetlands under projected future climates conducted in the river catchment area of Western Australia (Karim et al. 2016). According to Chen et al. (2015), inland natural wetlands, particularly those in dry and semiarid regions, would be influenced by changes in precipitation, runoff, temperature, and evapotranspiration owing to changes in precipitation, runoff, temperature, and evapotranspiration. As anticipated by the International Panel on Climate Change (IPCC 2023), increased flood intensity may result in erosion of floodplain wetland embankments, changes in sediment intake and nutrient load, and an accelerated hydro-cycle impacting notably closed wetlands. According to a predictive study using dynamic modelling, climate change manifested by increased temperature may have a profound impact on the lower Gangetic floodplain wetlands, as rising temperatures may cause erratic rainfall patterns, which may have ramifications on river hydrology (Whitehead et al. 2015). Any changes in climatic circumstances, particularly the rainfall pattern and temperature, have a significant impact on the water spread, turbidity, and aquatic vegetation in wetlands. Many scientists have found seasonal fluctuations in wetland flora and fauna, indicating that wetland resources are climate-dependent for their sustainability (Sarkar & Borah 2018; Kumari et al. 2023). Natural fish population abundance and species variety are expected to be particularly vulnerable to climatic perturbations, since lower water levels and a lack of rainfall may restrict the number of individuals able to spawn successfully. Heavy rainfall is a fundamental physiological trigger that causes fish to migrate and spawn. On the other hand, increased temperature during breeding and early larval development stages may potentially result in a sex-biased fish population, which may influence natural population increase in the future (Baroiller & Cotta 2001; Angienda et al. 2010). However, minor temperature increases throughout the winter in tropical wetlands were found to benefit cultivable fish through accelerated gonadal development and reproduction (Borah & Bujarbaruah 2016).

Given the uncertainties of climate change and anthropogenic stress, region-specific climate resilient adaptation and mitigation measures must be developed and popularised to address climate change-related challenges in the life and livelihood of the population that is directly or indirectly dependent on these ecosystems. Climate change and related stresses are aggravating the situation, putting fishers' livelihoods at risk (Naskar et al. 2018). The unpredicted timing of rainfall destabilises the water table of the wetland causing early or delayed breeding season for the associated aquatic fauna. To counter this problem, various strategies like pre-summer enclosure, pen culture, deep pool refuge, and submerged branch pile refuge can be beneficial (Sarkar et al. 2018). The application of balanced artificial feed in the pens is advised to manage eutrophication. The majority of urban and peri-urban wetlands get sewage, which increases biological production while degrading the environment (Sarkar et al. 2020a, 2020b, 2020c). In this context, the selection of appropriate fish species and the implementation of scientific management methods are critical for the long-term management of these ecosystems. In addition, the current study has uncovered and recorded several indigenous wetland fisheries management strategies that have been used for centuries, even before the phrase ‘climate change’ existed. A summary of the strategies for adapting to impending threats of climate change for the studied wetland/region is presented in Table 2. Appropriate adaptation and mitigation strategies would bring community empowerment in the face of climate change vulnerability. Protection of biological diversity and integrity of the wetland are important activities to improve the resiliency of wetland ecosystems so that they continue to provide important services under changed climatic conditions.

This is the first assessment of Media, a tropical floodplain wetland of India, in the context of the fisheries and climate-induced risks using improved approaches. The study established that regional climate variability may have a direct or indirect influence on the water quality, fish diversity, as well as the fish production of the wetland. The application of multiple approaches may be advantageous for building composite vulnerability analysis covering information on biotic and abiotic factors along with historic climate and biological data. The objectives of our study were satisfied and an improved systematic model framework was created for these categories of investigations. The GIS-based thematic maps of the wetland fisheries related to important environmental parameters could be applied for better management policy for the stakeholders. We recommend responsible fishing maintaining the natural breeding grounds of the indigenous fishes. On the other hand, an appropriate adaptation approach should be the restoration of the wetland habitat connectivity, creating deep pools, resolving siltation issues, and linking the water body to the main river channel to recover the natural breeding habitat and augment the natural fish population. The protection and maintenance of floodplain wetlands will help to address the worldwide concerns of livelihood security, water scarcity, and greenhouse gas emissions, as well as ensure a sustainable future. For future generations, a greener planet is preferable. Ideal management of the productivity of floodplain wetlands will significantly contribute to the achievement of the United Nations Sustainable Development Goals for the period 2016–2030, to eradicate poverty and hunger, and wellbeing for all ages of the global population.

Authors acknowledge the financial support from National Innovations for Climate Resilient Agriculture (NICRA), Indian Council of Agricultural Research (ICAR), New Delhi, India and mentoring support from the Director of ICAR - Central Inland Fisheries Research Institute (CIFRI), Barrackpore, Kolkata, India.

B.D.G. was involved in sampling, data generation, and manuscript preparation; S.D. was involved in sampling, data analysis, manuscript preparation, and revision; U.K.S. guided and monitored the research activities, B.K.D. did project administration, acquired funds, and supervised the study; M.P., C.J., and G.K. supervised the study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.