In recent years, we increasingly hear about global climate change. The global sea-level rise has increasingly been reported as a major outcome of climate change. Intergovernmental Panel on Climate Change (IPCC) projected an average 3-4 mm annual sea-level rise in the 21st century.
Recently, Dr. Daniel M. Alongi of Australian Institute of Marine Science showed that unlike many other mangrove forests The Sundarbans has not been efficient in adjusting its growing surface (elevation) to the rising sea-level. This is an alarming result. Dr. Alongi demonstrated that subsidence (sinking) has been happening in the Sundarbans (both India and Bangladesh sides) due to greater rise in sea-level relative to the elevation of Sundarbans’ growing surface. The Sundarbans is a unique forest ecosystem called mangroves. Historically, mangrove forests have the ability to adjust its growing surface to the rising sea level through sediment accretion and thickening peat deposition by biotic activities.
The sediment accretion is the process by which soil particles (sediments) add to the landmass (tectonic plate), and vertically increase the land area. A reduction in sediment accretion is associated with coastal erosion due to cyclones, floods, and tsunamis, and reduced supply of sediment particles in suspension into mangrove forests from coastal waters during tides. While reviewing literature Dr. Alongi demonstrated a reduced sediment accretion in the Sundarbans due at least partly to upstream dams and persistent erosion by coastal cyclones.
Mangrove forests can also vertically expand their growing surface through thickening the peat deposits. Peat is an accumulation of partially decayed vegetation materials. In mangrove forests, all vegetation materials such as leaves, branches, deadwoods etc. decompose slowly due to low soil oxygen availability. When the influx of vegetation materials to mangrove floor is higher relative to the rate of decomposition, vegetation materials accumulate, causing the vertical expansion of peat deposits. Beside above-ground vegetation materials, trees of mangrove forests develop extensive root networks due to soil nutrient deficiency and tides. These extensive root networks also increase the thickness (vertical expansion) of peat deposits.
If the sea-level rises exceeds the rises of mangrove (Sundarbans) forest floor, mangrove species are likely to have a landward migration, resulting in competition with mainland species and deterioration of the whole ecosystem, including the food chain. The rising sea-level will also increase soil salinity, erosion, and destruction of mangrove plant root networks.
Therefore, a continuous monitoring on both elevation of sea-level and of mangrove floor is necessary. We also need to assess the vulnerability of coastal ecosystems while incorporating the effect of mangrove plants migration to the mainland, and how that migration will affect the growth and survivability of both mangrove and mainland plant species. It is also necessary to improve our knowledge of how rising sea level will affect the nurseries of fish population and habitats of wildlife as they are essential parts of the Sundarbans food chains.