Ecological risk assessment of Sundarbans mangrove forest due to sea level rise

Abstract

With a total area of over 10,000 square kilometers, the SlUldarbans constitute the World's largest contiguous mangrove ecosystem, The floristic composition of the Sundarbans is rich compared to many other mangroves orihe world. In recognition of this richness in biodiversity, both Indian and the Bangladesh Sundarbans were declared world heritage sites by UNESCO. Mangrove offers both tangible and intangible benefits to the people. It plays an important role in the national economy of Bangladesh. Since the viability of the Sundarbans rests on the hydrology of the Ganges and its tributaries, climate change is expected to have significant impact on the SWldarbans. A rise in sea level would occur under climate change which would cause increased back"aler effect in the major distributaries of the Ganges and tend to push the saline front further inland. The backwater effect would also reduce the discharge of freshwater flow from the northern reaches of the tributaries of the Ganges resulting in a relatively prolonged inundation of the forest land. The effects of climate change on the Sundarbans would be considerably more critical during the dry season. Climate models predict a decrcase in precipitation during this period which might further reduce freshwater flows. This reduction in freshwater Inflows Into the Sundarbans could be exacerbated by increaSed evapo-transpiration losses and water use on account of rising winter temperatures. Reduced freshwater flows coupled with sealevel rise would consequently further enhance the dry season salinity levels in the Sundarbans. Northward penetration of the salinity front would rcsult in salinity induced succession problems in the Sundarbans and as a result, the symbiotic process in the entire ecosystem would change completely. Majority of the mesohaline areas v.ri1lbe transformed into polyhaline areas, while oligohaline areas would be reduced to mesohaline areas. This study has been conducted to assess the ecological risk of sea level rise to three ecological entities like ecological zone, ecological health in ternls of productivity and regeneration in the Sundarbans ecosystem. Ecological risk has been calculated as a function of probability of sea level rise and vulnerability of ecological entity. In this study, the Sundarbans has been divided into three ecological zones depending on salinity. The prepared maps of spatial distribution of salinity at base condition and 88cm sea level rise clearly indicate transformation of ecological zones. The probability of 88cm sea level rise has been calculated to be 20 percent. Due to 88cm sea levcl rise about 9 percent area of less saline water zone will be transferred to moderate saline water zone while about 3 percent area of modcrate saline water zone will be transferred to strong salinc water zone. The vulnerability to ecological zones transformation is 0.13 where the risk to zone transformation is 3 percent. Vulnerability of 88cm sea level rise to the ecological health of the Sundarbans has bccn calculated considering the compartments to be affected and it Is 0.07 and so the risk to ecological health is 1.45 percent. In the similar way, the vulnerability to regeneration has becn calculated and it will be 0.15 due the 88cm sea level rise and the risk to regeneration is 3 percent. Vulnerability to regeneration of Sundry (Heritiera fomes), most valuable trees in the Sundarbans, is 0.4 and thus the risk to regeneration of Heririerafomes is 8 percent.