Environmental Engineering Reference
In-Depth Information
Table 2.9 Mangrove-rich countries in the Indian Ocean
region
Country
(i) Salt excretion : Mangrove plants take saline
water as such through roots. However, in the
tissues of some species of mangroves, only
water molecules and essential salts are
retained. Excess salts are excreted through
salt glands that are present in the leaves. The
salt-excreting species of mangrove commu-
nity such as
Area (in km 2 )
Indonesia
42,500
Myanmar
6,950
Malaysia
6,410
India
4,871
Avicennia alba
,
Aegiceros
NW Australia
4,513
corniculatum
,
Acanthus ilicifolius
,
Aegialitis
Bangladesh
4,500
rotundifolia
regulate their internal salt levels
through foliar glands. In salt-secreting
(excreting) mangroves, the NaCl concentra-
tion of xylem sap is relatively high, about
one-tenth of the concentration of salt in sea
water. So, the salt-excreting species allow
more salt into the xylem than do the non-
excretors, but still exclude about 90 % of the
salts (Scholander et al. 1962 ; Azocar et al.
1992 ). Salt is only partially excluded at the
roots. The absorbed salt is primarily excreted
metabolically via specialized salt glands in
the leaves. The salt in solution can crystallize
by evaporation and either can be blown away
or washed off. Since, in salt-excreting man-
groves, super
Madagascar
4,200
Mozambique
4,000
Pakistan
2,600
Thailand
1,900
The mangrove forests are highly productive
ecosystems with productivity about 20 times more
than the average oceanic production (Gouda and
Panigrahy 1996 ). Moreover,
'
detritus-
it
is a
based
ecosystem unlike other coastal ecosys-
tems, which are usually 'plankton-based' . The
detritus supplied by the ecosystem saturates the
ambient water with nutrients, which triggers the
growth and development of planktonic commu-
nity in the water bodies on which the
'
shery
resource is also dependent. The greatest concen-
tration of mangrove species is observed usually at
the mouth of tidal creeks and rivers where salt and
fresh water mix in ideal proportion and
uous salts are excreted by
guttation through special salt glands, all these
salt-excreting halophytes are often referred to
as crinohalophytes .
It is interesting to note that salt excretion is an
active process, as evidenced by ATPase
activity in the plasmalemma of the excretory
cells (Drennan and Pammenter 1982 ). The
process is probably regulated by leaf hypo-
dermal cells, which may store salt as well as
water (Balsamo and Thomson 1995 ).
(ii) Salt exclusion : In some of the mangrove
plants the roots possess an ultra-
fl
oodwa-
ters deposit plenty of material to build up the
banks. This unique coastal ecosystem of the world
sustains a rich spectrum of
fl
oral and faunal
community in and around its vicinity. The man-
groves enrich the coastal waters with nutrients,
yield commercial forest products, protect coast-
lines and support coastal
fl
sheries (Kathiresan and
Bingham 2001 ). Generally, the mangrove vege-
tations are well adapted to extreme conditions of
salinity, tides, winds and temperature, although
they show a preference for fresh water. There are
no
ltration
mechanism called reverse osmosis by which
water and salts in the sea water are separated
in the root zone itself and only water is taken
inside and the salts are rejected. Many
mangrove species can exclude 90 % of salt
in the ambient sea water or estuarine system
( http://www.epa.qld.gov.au/nature_conserva
tion/habitats/mangroves_andwetlands/mang-
roves ).
oral groups in the plant kingdom, which
possess such well-organized and highly devel-
oped morphological, biological and physiological
as well as ecological adaptations to extreme
environmental conditions.
Mangrove plants tolerate salinity of the soil
and water through three basic processes as listed
here:
fl
Rhizophora mucronata
,
Ceriops
decandra
,
Bruguiera gymnorrhiza
,
Kandelia
candal
, etc., are few salt excluders of
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