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Tidal range and sediment supply are two critical indicators of mangrove
response to sea-level rise. Mangrove communities in macro-tidal, sediment rich
areas (e.g., mangrove communities in northern Australia; Semeniuk 1994;
Woodroffe 1995) may be better able to withstand and survive sea level rise than
those in micro-tidal sediment starved areas (e.g., mangroves in Caribbean islands;
Parkinson et al. 1994).
The tidal profiles in different Indian estuaries are ideal for the survival of mangroves (Fig. 6.9), but the ideal condition is attained in Ganga-Brahmaputra basin,
where the sediment load is extremely rich. The Ganga-Brahmaputra basin comprises an area of about 10.98 lakh sq. km and the delta area is about 1,50,000 sq. km.
Each year Ganga and Brahmaputra rivers bring around 166.70 crore tonnes of silt
and it is this silt which has created the world’s largest delta and the delta-building
process is still ongoing (Banerjee 1999).
The migration of mangroves is also a function of geo-physico-chemical nature of
the substratum. It has been observed that carbonate settings are often associated
with coral atolls and islands, where landward migration of mangroves to escape the
effects of sea level rise is not possible and sediments are often limited; thus mangrove communities in carbonate islands are considered extremely vulnerable to sea
level rise (UNEP 1994). Therefore, sea level rise is expected to decrease the geographic distribution and species diversity of mangroves on small islands with microtidal sediment-limited environments (IPCC 1997). Mangroves with access to
allochthonous sediments, such as riverine mangroves, are more likely to survive sea
level rise than those with low external inputs (Woodroffe 1990; Pernetta 1993). It is
important to note that although access to sediment is critical for mangroves to survive sea level rise, too much sediment (e.g., resulting from poor agricultural practices) can bury their pneumatophores and kill mangroves (Ellison and Stoddart
1991). In addition to varying sediment input rates, sediment accumulation rates also
differ for mangrove ecosystems worldwide. According to Ellison and Stoddart
(1991), through accretion, low island mangroves can keep pace with sea level rise
of up to 1.2 mm/year, while high island mangroves can keep pace with rates of
4.5 mm/year, depending on sediment supply. As mentioned above, global projections of sea level rise are between 1.0–8.8 mm/year, thus mangroves may not survive sea level rise in some areas. In the low-lying island mangroves in Bermuda, the
rate of sediment accretion under mangroves has been 0.8–1.1 mm per 100 years
over the last 2000 years, but the present rate of sea level rise in Bermuda exceeds
2 mm per year (Ellison 1993), clearly out-pacing the sediment accretion rate.
Furthermore, the seaward margin of the mangroves has retreated and eroded significantly; 26% of the largest mangrove area at Hungry Bay, Bermuda, has been lost
over the last century due to retreat of its seaward edge. On Kosrae Island, Micronesia,
most mangrove habitats developed by accumulating mangrove peat with a gradual
sea level rise of 1.0–2.0 mm/ year (Fujimoto et al. 1996). A rapid rate of relative sea
level rise of about 10 mm/year occurred between 4100 and 3700 B.P. and caused
mangroves to retreat landward and stop accumulating peat (Fujimoto et al. 1996).
Therefore, if sea level rise exceeds 10 mm/year, mangroves on Pacific Islands may
move landward and quickly reduce in number. Using Holocene stratigraphic records
6 Mangroves: A Potential Vegetation Against Sea Level Rise
Tidal range and sediment supply are two critical indicators of mangrove
response to sea-level rise. Mangrove communities in macro-tidal, sediment rich
areas (e.g., mangrove communities in northern Australia; Semeniuk 1994;
Woodroffe 1995) may be better able to withstand and survive sea level rise than
those in micro-tidal sediment starved areas (e.g., mangroves in Caribbean islands;
Parkinson et al. 1994).
The tidal profiles in different Indian estuaries are ideal for the survival of mangroves (Fig. 6.9), but the ideal condition is attained in Ganga-Brahmaputra basin,
where the sediment load is extremely rich. The Ganga-Brahmaputra basin comprises an area of about 10.98 lakh sq. km and the delta area is about 1,50,000 sq. km.
Each year Ganga and Brahmaputra rivers bring around 166.70 crore tonnes of silt
and it is this silt which has created the world’s largest delta and the delta-building
process is still ongoing (Banerjee 1999).
The migration of mangroves is also a function of geo-physico-chemical nature of
the substratum. It has been observed that carbonate settings are often associated
with coral atolls and islands, where landward migration of mangroves to escape the
effects of sea level rise is not possible and sediments are often limited; thus mangrove communities in carbonate islands are considered extremely vulnerable to sea
level rise (UNEP 1994). Therefore, sea level rise is expected to decrease the geographic distribution and species diversity of mangroves on small islands with microtidal sediment-limited environments (IPCC 1997). Mangroves with access to
allochthonous sediments, such as riverine mangroves, are more likely to survive sea
level rise than those with low external inputs (Woodroffe 1990; Pernetta 1993). It is
important to note that although access to sediment is critical for mangroves to survive sea level rise, too much sediment (e.g., resulting from poor agricultural practices) can bury their pneumatophores and kill mangroves (Ellison and Stoddart
1991). In addition to varying sediment input rates, sediment accumulation rates also
differ for mangrove ecosystems worldwide. According to Ellison and Stoddart
(1991), through accretion, low island mangroves can keep pace with sea level rise
of up to 1.2 mm/year, while high island mangroves can keep pace with rates of
4.5 mm/year, depending on sediment supply. As mentioned above, global projections of sea level rise are between 1.0–8.8 mm/year, thus mangroves may not survive sea level rise in some areas. In the low-lying island mangroves in Bermuda, the
rate of sediment accretion under mangroves has been 0.8–1.1 mm per 100 years
over the last 2000 years, but the present rate of sea level rise in Bermuda exceeds
2 mm per year (Ellison 1993), clearly out-pacing the sediment accretion rate.
Furthermore, the seaward margin of the mangroves has retreated and eroded significantly; 26% of the largest mangrove area at Hungry Bay, Bermuda, has been lost
over the last century due to retreat of its seaward edge. On Kosrae Island, Micronesia,
most mangrove habitats developed by accumulating mangrove peat with a gradual
sea level rise of 1.0–2.0 mm/ year (Fujimoto et al. 1996). A rapid rate of relative sea
level rise of about 10 mm/year occurred between 4100 and 3700 B.P. and caused
mangroves to retreat landward and stop accumulating peat (Fujimoto et al. 1996).
Therefore, if sea level rise exceeds 10 mm/year, mangroves on Pacific Islands may
move landward and quickly reduce in number. Using Holocene stratigraphic records
6 Mangroves: A Potential Vegetation Against Sea Level Rise