172
The phenomenon of sea level rise may lead to the phenomenon of shifting the
coordinates of mangrove zone. When the force of relative sea level rise is the predominant component causing change in the position of mangrove belt, landscapelevel response of mangroves to relative sea level rise, over a period of decades and
longer, can be predicted based on the reconstruction of paleo-environmental mangrove response to past sea level fluctuations (Ellison and Stoddart 1991; Woodroffe
1995; Ellison 1993, 2000; Gilman 2004). This prediction can be done on the basis
of (a) the sea level change rate relative to the mangrove surface, (b) the mangrove’s
physiographic setting (slope of the land adjacent to the mangrove, slope of the mangrove, and presence of obstacles to landward migration), and (c) erosion or progradation rate of the mangrove seaward margin (Ellison and Stoddart 1991; Ellison
1993, 2000; Woodroffe 1995; Alongi 1998; Gilman 2004). There are three general
scenarios for mangrove response to relative sea level rise, given a landscape-level
scale and time period of decades or longer (Fig. 6.10).
1. No change in relative sea level
When sea level is not changing relative to the mangrove surface, the mangrove
margins will remain in the same location (Fig. 6.10a), provided other factors (like
human intervention etc.) remain constant.
2. Relative sea level lowering
When sea level is dropping relative to the mangrove surface, the seaward and
landward boundaries of mangroves will tend to migrate seaward (Fig. 6.10b) and
depending on the topography, the mangrove may also expand laterally.
3. Relative sea level rising
If sea level is rising relative to the mangrove surface, the mangrove’s seaward
and landward margins retreat landward (Fig. 6.10c). However, under this situation,
the survival of mangroves will depend on getting proper environmental conditions
like aquatic salinity, soil quality, frequency and period of tidal inundation etc. If the
optimum conditions are not achieved, the mangroves may get restricted in a narrow
patch or fringe or be lost in course of time.
However, over small temporal and spatial scales and where natural and anthropogenic forces other than changing relative sea level exert a larger influence on
Table 6.5 Species – wise peat accumulation rate of true mangroves at Chotomollakhali Island
Species
Peat accumulation rate (mm/year)
(average of 5 consecutive years)
Sonneratia apetala
3.1
Avicennia alba
3.8
Avicennia marina
4.1
Avicennia officinalis
4.0
Bruguiera gymnorhiza
1.9
Excoecaria agallocha
3.5
6 Mangroves: A Potential Vegetation Against Sea Level Rise
The phenomenon of sea level rise may lead to the phenomenon of shifting the
coordinates of mangrove zone. When the force of relative sea level rise is the predominant component causing change in the position of mangrove belt, landscapelevel response of mangroves to relative sea level rise, over a period of decades and
longer, can be predicted based on the reconstruction of paleo-environmental mangrove response to past sea level fluctuations (Ellison and Stoddart 1991; Woodroffe
1995; Ellison 1993, 2000; Gilman 2004). This prediction can be done on the basis
of (a) the sea level change rate relative to the mangrove surface, (b) the mangrove’s
physiographic setting (slope of the land adjacent to the mangrove, slope of the mangrove, and presence of obstacles to landward migration), and (c) erosion or progradation rate of the mangrove seaward margin (Ellison and Stoddart 1991; Ellison
1993, 2000; Woodroffe 1995; Alongi 1998; Gilman 2004). There are three general
scenarios for mangrove response to relative sea level rise, given a landscape-level
scale and time period of decades or longer (Fig. 6.10).
1. No change in relative sea level
When sea level is not changing relative to the mangrove surface, the mangrove
margins will remain in the same location (Fig. 6.10a), provided other factors (like
human intervention etc.) remain constant.
2. Relative sea level lowering
When sea level is dropping relative to the mangrove surface, the seaward and
landward boundaries of mangroves will tend to migrate seaward (Fig. 6.10b) and
depending on the topography, the mangrove may also expand laterally.
3. Relative sea level rising
If sea level is rising relative to the mangrove surface, the mangrove’s seaward
and landward margins retreat landward (Fig. 6.10c). However, under this situation,
the survival of mangroves will depend on getting proper environmental conditions
like aquatic salinity, soil quality, frequency and period of tidal inundation etc. If the
optimum conditions are not achieved, the mangroves may get restricted in a narrow
patch or fringe or be lost in course of time.
However, over small temporal and spatial scales and where natural and anthropogenic forces other than changing relative sea level exert a larger influence on
Table 6.5 Species – wise peat accumulation rate of true mangroves at Chotomollakhali Island
Species
Peat accumulation rate (mm/year)
(average of 5 consecutive years)
Sonneratia apetala
3.1
Avicennia alba
3.8
Avicennia marina
4.1
Avicennia officinalis
4.0
Bruguiera gymnorhiza
1.9
Excoecaria agallocha
3.5
6 Mangroves: A Potential Vegetation Against Sea Level Rise