Geoscience Reference
In-Depth Information
Table 15-1.
Percentage of total worldwide mangrove
coverage in the top ten countries, which represent
approximately two-thirds of all mangroves. Source FAO
(2007).
contained within Bangladesh and the rest in
India. The region experiences typically humid
subtropical climate with a mean annual high tem-
perature of 34 °C. Both the Southwest summer
and Northeast winter Monsoons affect the region.
A majority of rainfall is from the Southwest Mon-
soons and occurs between June and October. It
brings an annual average of between 1600 and
1800 mm to the wetlands depending on location.
The Sundarbans may be divided into two
broad ecological zones (Fig. 15-1). The Sundar-
bans mangroves cover the edge zone of the delta
immediately adjacent to the Bay of Bengal and
comprise mudl ats, islands, creeks, channels,
estuaries, shallow water bodies, and grasslands
that experience periodic inundation. The Sundar-
bans fresh-water and brackish swamps lie north
of the mangroves farther inland within the delta
and comprise mixed tropical species. Over the
centuries, this latter zone has witnessed wide-
spread clearing to make way for dense human
settlement and agricultural activities. A color-
infrared space-shuttle photograph clearly reveals
the extent of preserved mangroves (Fig. 15-2).
The mangroves are indicated in red, and sur-
rounding agriculture is clearly noticed in pale
gray tones. Shrimp farming and intensive agri-
culture are common around the protected man-
grove ecosystem. The entire region is dotted
with islands and crisscrossed by numerous chan-
nels, streams and rivers, which not only provide
important inl uxes of fresh water but also deposit
sediment into the Bay of Bengal.
Pollen and sediment analysis from radiocar-
bon dating indicates a brackish estuarine swamp
environment across the delta over the past 9000
years with variable periods of expansion and
retreat of mangrove forests possibly due to sea
transgression and high sedimentation rates
(Hait and Behling 2009). More recently, neotec-
tonic activity causing the delta to tilt eastward
and the shifting courses of the Ganges and
Brahmaputra rivers over the past several centu-
ries have played a crucial role in the hydrology
and morphology of the delta and inl uenced the
sedimentation, accretion, and erosion patterns
witnessed across the region (Gopal and Chauhan
2006). Moreover, tidal action, the inl uence of
which may be observed up to 50 km inland,
Country
% of total mangroves
Indonesia
19
Australia
10
Brazil
7
Nigeria
7
Mexico
5
Malaysia
4
Cuba
4
Myanmar
3
Bangladesh
3
India
3
The name “Sundarbans” likely arises from the
local name for a mangrove species, sundari
(
Heritiera fomes
), which was once widespread
across the region but is now threatened and in
decline.
The Sundarbans is just one stretch of man-
groves found at latitudes below 25° N and S.
Coastal mangrove systems are observed across
tropical regions in Asia and Oceania, Africa,
North and Central America, and total some 15.2
million hectares (152,000 km
2
) (Food and Agri-
culture Organization 2007). As Table 15-1 shows,
the top 10 countries in terms of mangrove area
coverage are not limited by geography but
spread across the tropical belt encircling the
globe. A recent report, however, suggests that
some 20 percent of this, or 3.6 million hectares
(36,000 km
2
), have been lost to development and
other activities since 1980 (Food and Agriculture
Organization 2007). Mangrove ecosystems gen-
erally comprise trees and shrubs that have
developed special adaptations to saline condi-
tions and variable periods of inundation. They
provide numerous economic benei ts to resident
populations and perform important roles in sta-
bilizing coastlines, mitigating the impact of
storms and l oods, and providing habitats for
numerous invertebrates, crustaceans, i sh, and
other mammals and birds.
The Sundarbans extends between 21° 30
′
to
22° 40
E longi-
tude, straddling the countries of India and Bang-
ladesh with roughly 60 percent of the wetlands
′
N latitude and 88° 05
′
to 89° 55
′
