Mangrove Spatial Distribution in the Indian Sundarbans : Predicting Salinity-Induced Migration in a Changing Climate

This study contributes to understanding the physical and economic impacts of progressive, climate-driven aquatic salinization on the spatial distribution of mangrove species in the Indian Sundarbans, which accounts for about two-fifths of the 10,200 km tidal-wetland forest delta. To estimate future mangrove distribution, a five-step analysis was undertaken, using high-resolution spatial assessments. A current (2015) basemap and overlays of salinity tolerance for major mangrove species and their assemblages and projected location-specific aquatic salinity for 2050 were used to predict salinity-induced migration. The results show gain-and-loss patterns, with salt-tolerant species predominating at the expense of freshwater species. These changes are likely to reduce the flow of ecosystem services, adversely affecting the livelihood options of poor people in adjacent areas. Effective management will require establishing baseline data for monitoring system changes over time, protocols for maintaining species health, and support for mangrove regeneration and restoration. Resources should also be directed to alternative livelihoods for mangrove-dependent households. The study recommends an integrated policy approach, focused on rising salinity, changes in mangrove dynamics, and the welfare of mangrove-dependent communities.


Introduction
The mangrove ecosystems of the Sundarbans-the world's largest remaining contiguous mangrove forest situated along coastal segments of Bangladesh (60 percent) and India (40 percent) in the Bay of Bengal-are at increasing risk from the impacts of climate-driven sea-level rise.In this 10,200 km 2 tidal-wetland forest delta, as in other globally important mangrove forests, sea-level rise may even threaten the survival of mangrove species.In the past, mangroves have shown considerable resilience to sea-level fluctuations (Alongi, 2002(Alongi, , 2008;;Gilman et al., 2006;Erwin, 2009); however, their future rate of adaptation and migration may not keep pace (Ellison & Stoddart, 1991;Semeniuk, 1994;United Nations Environment Programme, 1994;McLeod & Salm, 2006;Lange et al., 2010).In the case of the Sundarbans, the extent of permanent inundation is uncertain since sedimentation is still occurring in the active Ganges-Brahmaputra Delta.That said, climate-driven sea-level rise will inevitably have significant implications for the many poor people who depend on the mangrove forest for their livelihoods.(Note 1)

Challenges to Healthy Mangroves in the Sundarbans
In the Sundarbans, the two greatest threats to the health of mangroves in a changing climate are progressive aquatic salinization and shortage of nutrients from freshwater flows (Dasgupta, Kamal, Khan, Choudhury, & Nishat, 2015a;Dasgupta, Hossain, Huq, & Wheeler, 2015b;Institute of Water Modeling, 2003;Peterson & Shireen, 2001;Soil Resources Development Institute, 2000, 2010;United Kingdom Department of Environment, Food & Rural Affairs, 2007).It is expected that alteration of riverine flows from the Himalayas, combined with sea-level rise, will intensify salinity intrusion as climate change continues (Dasgupta et al., 2015a, b

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Traditionally, the intertidal zone where mangroves are found has been well-suited for breeding and rearing a variety of fish, crustacean, and mollusk species.(Note 12) But progressive aquatic salinization is expected to impact the food web, taking its toll on economically important fish catches with specific salinity-tolerance limits.Also, with changes in mangrove species combinations, honey collectors ("Moulis") may not fetch as high a price due to variations in the honey's fragrance and viscosity.In addition, reduced diversity of mangrove species could diminish tourism in the region.Furthermore, altered species combinations could increase salinity ingression, affecting the water table and thus water sources; women would shoulder a greater burden since they spend more hours collecting fuelwood and drinking water.
Engineering attempts to control rising salinity in the Indian Sundarbans are unlikely to succeed.Concerns over increasing aquatic salinity have not yet been incorporated into regional management protocols despite the Sundarbans' treaty-protected ecological status, which is widely acknowledged.Eastward meandering of the Ganges and Brahmaputra Rivers has already reduced freshwater inflows significantly.Even at the current sea level, strong tides may travel long distances upstream since the land is quite flat.Continuing sea-level rise will only exacerbate such tidal effects.As long as these dynamics continue, efforts to improve local ecological conditions through changes in hydrological regime (e.g., river training) will likely prove futile (Potkin, 2004).
The Indian Sundarbans is a UNESCO World Heritage site, and effective conservation management will require establishing baseline data for monitoring system changes over time, management protocols to maintain mangrove health, and support for mangrove regeneration and restoration.Location-specific baseline data is needed for tree-stand structures, tree abundance, species richness and diversity, export of nutrients, hydrological patterns, rates of sedimentation, and relative sea-level rise (McLeod & Salm, 2006).Management protocols should include connectivity between mangrove systems and nearby river sources, as well as maintenance of upland freshwater catchments.Also, areas that are likely to survive climate-driven sea-level rise should be identified.In addition, monitoring of tidal fluctuations, varying pH, and salinity is needed to support regeneration and colonization of suitable species, where necessary.Furthermore, restoration of currently degraded areas should be undertaken.(Note 13) Sea-level rise will continue beyond 2100, even if greenhouse gas emissions are stabilized in the near future.The impacts on globally-important mangrove ecosystems and the socioeconomic implications for vulnerable populations are substantial, suggesting that resources should also be directed to developing alternative livelihoods for mangrove-dependent households.It is hoped that this research will promote more widespread efforts to develop conservation and sustainable development policies that integrate rising salinity, changes in mangrove dynamics, and the welfare impacts on poor communities.Note 6. Bad bands were removed while converting Digital Number (DN) value to radiance using the radiometric calibration tool.The output was converted to band-interleaved-by line (BIL) radiance image with floating point values as Fast Line-of-sight Atmospheric Analysis of Spectral Hypercube (FLAASH) correction module use BIL format.
Note 7.During the acquisition of Hyperion data, vertical striping occurs at times due to poor calibration of push broom sensors.For this analysis, de-striping was performed by filling up the DN value of the gap line with an average DN value from the previous and the next column (Farooq & Govil, 2014).Atmospheric correction was performed using the FLAASH package available in ENVI TM .
Note 8. Gain and bias corrections of satellite data through radiometric calibration are prerequisites for the classification and detection of change from the multi-temporal images (Duggin & Robinove, 1990).
Note 9.At present, there is no geo-coded database on aquatic or soil salinity for the Indian Sundarbans.Data was compiled from field measurements taken by the Nature Environment & Wildlife Society (NEWS) and World Wildlife Fund-India (WWF-India).

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Table 1 .
Ecosystem services and common uses of mangrove species in the Indian Sundarbans (Islam and Gnauck, 2008)et (1996)ute.(2010).SoilPfeffer, Harper, & O'Neel, 2008;Rahmstorf, 2007;Dasgupta et al., 2009;Brecht et al., 2012)d Kingdom Department ofEnvironment, Food and Rural Affairs.(2007).Investigating the impact of relative sea-level rise on coastal communities and their livelihoods in Bangladesh.Dhaka, Bangladesh: Institute of Water Modeling and Center for Environment and Geographic Information Services.Current scientific estimates are that sea level may rise by 1 m or more in this century, globally affecting some 1 billion people by 2050(Hansen et al., 2011; Vermeer & Rahmstorf, 2009;Pfeffer, Harper, & O'Neel, 2008;Rahmstorf, 2007;Dasgupta et al., 2009;Brecht et al., 2012); a rise of 3 m or more by 2100 is feared in light of new evidence on ice-cliff instability of the Antarctic.https://www.nature.com/articles/nature17145;http://www.nature.com/news/antarctic-model-raises-prospect-of-unstoppable-ice-collapse-1.19638;https://climatefeedback.org/evaluation/antarctica-doomsday-glaciers-could-flood-coastal-cities-grist-eric-holthaus/Note 2.Rao (1987)reported that the Indian mangroves consisted of 60 species, whileNaskar (1988)reported 35 true mangroves andBlasco, Saenger, & Janodet (1996)reported 58 species.Note 3. Freshwater flow has become increasingly restricted since the 1975 construction of the Farakka Barrage Township; between 1962 and 2006, water discharge of the Ganges fell from 3,700 m 3 per second to 364 m 3 per second, strangling an already parched ecosystem and thus making the distributary networks more dependent on tidal flow bringing in sea water from the Bay of Bengal(Islam and Gnauck, 2008).Note 4. The groundwater is also saline, except for a few meter-thick, confined aquifers.Note 5.The acquisition date of the Landsat 8 OLI data is March 18, 2015 and the path/row is 138/45.Landsat 8 OLI has 9 spectral channels, ranging from visible to shortwave infrared bands.The spatial resolution is comparable to the ETM+.Temporal resolution of Landsat 8 is 16 days.Hyperspectral data from Hyperion were processed and used for the spectral signature generation of various mangrove species.The acquisition dates of the Hyperion data are September 10, 2011, November 23, 2014, and November 13, 2016 and the path/row is 138/45.Hyperion images have 242 bands that include both Visible and Near Infrared (VNIR) and Shortwave Infrared (SWIR), having a spectral range of 357 to 2,576 nm with a spectral interval of 10 nm.