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Pakistan Journal of Marine Sciences, Vol.1(2), 145-153, 1992.
REVIEW ARTICLE
THE MARKED REDUCTION OF THE INDUS RIVER FLOW
DOWNSTREAM FROM THE KOTRI BARRAGE: CAN THE
MANGROVE ECOSYSTEMS OF PAKISTAN· SURVIVE IN THE
RESULTING HYPERSALINE ENVIRONMENT?
Saiyed I. Ahmed
School of Oceanography, University of Washington, Seattle, WA 98195, U.SA.
APPLICATION OF KNOWLEDGE ,OF MANGROVE ECOSYSTEMS FROM
AROUND THE WORLD TO THE UNDERSTANDING OF THE PRESENT
STATUS OF MANGROVES OF PAKISTAN.
The global inventory of obligate mangroves consists of 54 species belonging to 20
genera in 16 families and are estimated to occupy about 23 million hectares of sheltered coastal intertidal land (Tomlinson, 1986; Lugo and Snedaker, 1974; Chapman,
1975, 1976). These are regarded as 11 obligate11 mangroves as they are 11 restricted11 to
coastal saline intertidal environments compared to 11 facultative 11 mangroves which may
develop in non-coastal environments. In a general classification scheme basically two
groups of mangroves can be identified: (a) the Eastern Group: mangroves on the
coasts of Indian and Western Pacific Oceans, (b) the Western Group: Those on the
coasts of the Americas, West Indies and West Africa.
Generally speaking, the Eastern Group of mangroves is richer in species diversity
with mangroves in India and southeast Asia exhibiting species diversity of > 20 with
generally healthy and luxurious plant growth. A vicennia and Rhizophora are the only
two common to both the old and the new world and therefore, are probably the first
two mangrove genera to have evolved.
A close examination of Australia can be quite fruitful from the point of view that it
possesses a very diverse mangrove distribution pattern. The Australian mangroves
occupy 11,500 km 2 and extend 6000 km along the coasts (Bunt et al., 1982; Hutchings
and Saenger, 1987). The mangrove floras of the (wet) eastern and (dry) western
coasts of Australia have been rather extensively studied (see Tomlinson, 1986). The
vegetation distribution indicates that the mangroves of northeast Australia were richer
in the past, but with increasing aridity of Australia in recent geologic time, the less
adaptable species are dying out (Bunt et al. 1982). The densest mangrove forests
occur near Innisfall (rainfall between 350 and 400 em annually with a species diversity
of > 20) (Macnae, 1968). In subhumid regions with less than 100 em per year of
rainfall, mangroves have less diversity, i.e. about 12 species. Southward as the climate
becomes semi-arid, 8 species are common and in arid regions of Australia (southwest Australia) only 3-4 species or less are found (Semeniuk, 1983). Thus the arid
southwest coast of Australian mangroves resemble the current mangrove distribution
Contribution No.1946 from the School of Oceanography, University of Washington, Seattle, U.S.A.
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Pakistan Journal of Marine Sciences, Vol.1(2), 1992
patterns in the Indus River Delta of Pakistan in terms of species diversity. As stated
in previous paper (Ahmed, 1992), seawater is not an absolute requirement for the
growth of mangroves. Since mangroves are slow growing halophytes they can compete better with the faster growing glycophytes in intertidal coastal environments.
Also the individual species differ in their tolerance to salinity (see Table I), which has
been used as an argument for the metabolic basis for zonation (Lugo et al., 1975,
Table I: Highly salt-tolerant species of mangroves (Adapted from Chapman,
1975, 1976)
Species
A vicennia marina
Lumnitsera racemosa
Rhizophora mangle
Laguncularia racemosa
Ceriops tagal
R. stylosa
R. mucronata
Sonneratia spp.
R. apiculata
Brnguiera parviflora
Max. salinity
tolerance (%)
9.0
9.0
8.0
8.0
6.0
5.5
5.5
3.5
3.0
3.0
Geographical
distribution
Pakistan (all areas)
Southeast Asia
American Continent
American Continent
Pakistan (Miani Hor, Keti Bundar)
Pakistan (Miani Hor)
Southeast Asia
Southeast Asia
Southeast Asia
Southeast Asia
Snedaker, 1982). However, no mangrove species is capable of withstanding NaCl
concentrations of greater than 90 ppt (see Table I, also Scholander et al., 1962) on a
permanent basis. This is because at this level of salinity water transport into the plant
is almost totally shut off (Tomlinson, 1986). Before this limit in water absorption and
transport is reached at about a 90 ppt NaCllevel, a very high metabolic cost is associated with the saline water transport. First there is the cost associated with developing
an ultrafiltration system for removing undesirable salts and taking up water and other
nutrients. Then there are processes associated with sequestering the salt that gets in
at suitable locations, making desirable organic solutes for proper functioning of
enzymes and finally getting rid of some of the salt stored in tissues, mainly leaves
through the formation and maintenance of salt glands (Fahn and Shimony, 1977;
Fahn, 1979). Development of leaf succulence which is also associated with maintaining salt balance is another metabolic requirement. All of these processes require
expenditure of energy which otherwise could be available for normal growth and
development. Connor (1969) in fact has shown that optimum growth of Avicennia
marina occurred in culture solutions with a sodium concentration about half that of
seawater. Soto and Jimenez (1982) have also shown the decline in the stature of
Ahmed: Mangrove ecosystem of Pakistan
147
mangroves along gradients of increasing salinity. Thus mangroves have a fairly low
limit to their salt tolerance compared with strict, herbaceous halophytes, which are
usually much more succulent but not tall. Because of these reasons, some periodic
freshwater input in the establishment of mangrove vegetation is deemed so important,
and they develop in areas of either high annual rainfall and/or surface water runoff
from upland water sheds (Pool et al., 1977; Cintron, 1981). As a matter of fact a
comparison between e~teril and western coasts of Australia shows that at identical
latitudes the diversity of mangroves speciation is significantly different which can be
directly correlated to the level of precipitation. It is for this reason that in Pakistan,
the largest areas of greatest mangrove development are located in the Indus River
Delta region south of Karachi and north of the international border with India
(Snedaker, 1984). Since the Indus River is the sixth largest river in the world and the
Indus Delta is the second largest in area in the subcontinent, it is this spilled freshwater flow from the Indus River that has played such a significant role in the development of mangrove forests of this region. Otherwise, the annual rainfall in the Indus
Delta is too scant to have supported the growth and development of the mangrove
forests. This evidence is corroborated by a historical record for British India and for
Pakistan (Khan, 1966) which indicated that the speciation as well as the distribution of
mangroves have significantly changed during the past several hundred years and this
change appears to have accelerated in recent times particularly since the damming
and barraging of the Indus River. Before this activity began, the Indus had a large,
river-dominated estuary (Schubel, 1984). As a result of the diversion of the Indus for
irrigation, the Indus River now has an estuary only in•the flood or summer monsoon
season. During the remainder of year it has no estuary. As has been also pointed out
by Milliman et al., (1984), up to the 1940's the Indus River apparently discharged
more than 600 million tons (net) of suspended sediment annually from the Himalaya
mountains of which some 250 million tons reached the Indus Delta annually. Since
the 1980's (post damming, barraging activity) less than 50 mt of sediment reaches the
estuary and it is estimated to be further drastically reduced in the late 1990's. This
greatly decreased sediment and water flow is expected to have a profound effect on
the coastal environments in general and mangrove ecosystems in particular, in two
major ways: (1) by decreasing freshwater flow, it greatly exposes the mangroves to a
decreased supply of dissolved nutrients as well as an increase in porewater salinities,
(2) since no new sediments are being deposited through riverine transport, mangroves
are entirely at the mercy of tidal action for the reworking of their sediments, with
accretion and erosion and thus overall geomorphology being almost entirely determined by tidal action.
CURRENT DISTRffiUTION OF MANGROVES IN THE INDUS DELTA
In Pakistan, now only 8 mangrove species can be found in the Indus Delta region
with species belonging to A. marina dominating most areas to the extent of 95-99%.
Most stands of mangroves can be defined as monospecific forests of A. marina.
According to the evidence I presented earlier, A. marina is the most salt resistant
mangrove species; and thus its dominance is quite logical. Even under ideal conditions, Avicennia tree is slow growing, reaching full maturity in about 60 years with the
stem reaching a girth of about 50 em and a height of about 10 m with a canopy radius
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Pakistan Journal of Marine Sciences, Vol.1(2), 1992
of 6 m. However, an examination of the Indus Delta mangrove distributions would
indicate that only a very small portion of mangroves belong to this class. The majority
of the trees appear to be clearly stressed or stunted with vegetation height of no more
than 3-4 metres, and correspondingly small stem girth. Qureshi (1986) has undcrtak~
en a survey of the mangrove vegetation in collaboration with the SUPARCO of Pakistan using LANDSAT imagery. The growing stock of forest has been classified as (a)
dense mangroves (b) normal mangroves and (c) sparse vegetation.
Dense Mangroves: These would comprise about 17% of the vegetation area. The soil
in such areas is well drained and low lying and thus subjected to regular flushing by
tidal water. They are usually found on islands near tidal banks on freshly accreted
soils. Besides A vicennia, occasional local patches of Ceriops tagal and Ae~:.:iceras comiculatum are also found. Salt tolerance of the latter species is less than that of Avicennia or Ceriops.
Normal Mangroves: These occupy about 27% of the area. Avicennia is almost totally
dominant in this area.
Sparse Vegetation: Covered by about 49% of the surveyed area. The vegetation
mainly consists of shrubs and grasses. Although diversion of the freshwater from
Indus has had the major negative effect on the species composition and thus man~
grove diversity as well as the extent of coverage, clearly some of the other activities of
man such as lopping, and cutting for fuel and fodder and browsing and grazing by
animals in certain areas of easy accessibility, has also ext:rcised some negative effects.
However, these deleterious effects appear to be more easily amenable to improved
management practices such as has been recently instigated by the Coastal Forest
Division of the Sindh Forestry Department.
EVIDENCE FOR THE NEGATIVE EFFECfS OF THE INDUS RIVER FRESHWATER DIVERSION ON THE MANGROVE HABITAT:
Increased porewater salinities: We have measured porewater salinities at a number
of sites both in the mangrove soils growing near the creek banks as well as those
growing away from water at higher grounds. (Saleem et al., in preparation). The
creek water salinities in many sites visited by us often exceeded normal seawater salinities especially during the hot season (Khan et al., 1991). The porewater salinities
have also been correlated with mangrove growth and vigour at several sites (Saleem et
al., in preparation). In the above study in some areas porewater salinities of up to 7080 ppt were observed. Thus, high porewater salinities appear to have a major negative impact on mangrove growth in the Indus Delta. High porewater salinities combined with more anaerobic soils at higher ground elevations where water renewal is
less frequent and thus anaerobic soil conditions more prevalent, result in increased
costs of nutrient transportation. This increased level of metabolic energy expenditure
results in less resources being available for net growth and development and thus
significant stunting and dwarfism is observed, especially among the less mature vegetation.
Ahmed: Mangrove ecosystem of Pakistan
149
Soil erosion and accretion effects: Since almost no new soil is being deposited in the
mangrove areas as a result of the Indus River diversion, tidal action is the predominant forcing factor that is affecting soil geomorphology in and around the mangroves.
Thus certain creek banks where erosion is the predominant phenomenon, stunted,
dwarfed mangroves are to be found. Other areas where net soil accretion is taking
place exhibit more normal or even dense mangrove stands. Of course, other soil
hydrologic conditions such as nutrient availability, salinity, pH and Eh, Redox Potential and other related factors must also be considered when reviewing the health of
the mangroves. However, it must be emphasized that although there are a number of
environmental factors which together determine how vigorous a mangrove community
will become or remain, the availability of some freshwater (bearing nutrients) is
perhaps the most important among these and thus deserving our concerted attention.
For example, Korangi creek of Karachi receives abundant discharges and waste water
from both the city municipality as well as the industrial consortium. Despite this
factor, the mangroves in this area are thriving because wasted freshwater and sewage
(nutrients) dumped here are some of the ingredients needed for mangrove growth.
So far, the industrial pollutants which are also being dumped are either being trapped
in the soil or are being removed by the ultrafiltration activity of the mangrove root
system.
It should also be mentioned that our own measurements of litter production in the
Pakistan mangroves correspond to less than 3g dry weight (Snedaker and Brown,
1981; Snedaker et al., in preparation) which is substantially less than litter production
in many healthy mangrove ecosystems. Against this background is the reality of little
or no nutrients being supplied by the Indus River transport and land runoff and thus
the role played by in situ nutrient cycling and maximization of some limiting nutrients
(e.g. from leaves which are depleted of nitrogen before senescence) through nutrient
enrichment (e.g. N2 fixation) may become even more important than so far has been
realized.
USEFULNESS OF THE MANGROVES TO THE COASTAL FISHERIES RESOURCES AND THE ECONOMY
Before we give any thought to why we should consider saving - let alone renewing
and restoring - the mangrove ecosystems, we should consider what useful purpose
the mangroves serve as an economic resource for the region. As Tomlinson (1986)
has pointed out, a major problem in economic evaluation is the difficulty of recognizing indirect and direct benefits. Mangroves produce raw material (lignocellulose)
from seawater by utilizing renewable energy sources. Most of this energy is supplied
by sunlight. Tidal energy is also used because nutrients are replenished by tidal action
and estuarine runoff and detritus is flushed away. Mangroves provide fodder for
cattle and camels, timber and poles for a variety of uses, fuel wood and tannins and
dyes. The total number of people who obtain their livelihood from mangrove plants
may be more significant in social terms than profits for an enterprise. However, the
indirect relationship between mangroves and commercial fishing and shrimping is
very significant. Mangroves provide nutrition (via the detritus food chain) to marine
communities. They also provide a habitat for a variety of commercially exploited shell
and fin fish at critical phases of their life cycles by acting as "nursery grounds".
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Mangroves also stabilize shorelines and protect inshore fish habitats from sediment
pollution. An example of the impact of mangroves on coastal fisheries is provided in
figure 1 which shows that after the construction of Kotri Barrage in 1955, the fish
Fig.l. Annual variations of total fish catch and fish catch per boat along the Sindh
coast (from Milliman et al., 1984)
catch per boat along the Sindh coast began to decrease at an alarming rate and in
1980 stood at about 1/3 of the pre-barrage construction level. Thus the existence of a
correlation between fisheries resources and mangrove health, vigour, and productivity
appears to be a reasonable conclusion.
OTHER RECENT EVIDENCE SUGGESTING MANGROVE STRESS
Evidence that the mangroves of Pakistan are now under duress is further supported
by our studies of food-web interactions within this ecosystem using stable isotope
ratios of carbon (13-C/12-C) and nitrogen (15-N/14-N) ratios (King and Ahmed,
1992). Such studies have proven to be very useful in tracing interspecies trophic relationships in marine and freshwater ecosystems (Fry and Sherr, 1989). The limited
data available for mangroves indicates that mangrove carbon has d 13C characteristic
organisms measured so far have b13C values characteristic of marine systems i.e. the
Ahmed: Mangrove ecosystem of Pakistan
151
mangrove carbon is not readily apparent in the marine fauna of the mangrove ecosystems. Two hypotheses could account for the enigma of the fate of the mangrove
carbon: (1) the organisms responsible for the consumption of the mangrove detritus
have not yet been sampled which is highly unlikely, or (2) there is a relatively large
fractionation in the degradation of mangrove carbon such that the 13C of the mangrove consumers ends up looking much like organic carbon. A very high degree of
fractionation of carbon is possible through the interaction of bacteria which in mangroves have exhibited some of the highest growth ra,tes and turnover numbers (Azam
et al., in preparation).
The question of the fate of the mangrove carbort is important in view of the fact that
the mangrove trees account for the majority of the primary production in mangrove
ecosystems (Kristensen et at., 1988).
CONCLUSIONS AND RECOMMENDATIONS
The importance of the freshwater requirement of the mangroves has been recognized for the first time by the Pakistani government officials as reflected by the Water
Accord which has accepted the need for a minimum discharge below Kotri of 10
MAF. However, as shown by Meynell (1991) the flow below Kotri has averaged 35
MAF, although only 20 MAF actually reaches the delta at the time of peak flows
between July and September. Thus the new accord may in fact further decrease the
freshwater flow through the delta by 10 MAF as a result of which problems associated
with hyper salinities and decreased nutrient supply in the mangrove environments will
loom even larger. Further degradation in mangrove distributions and diversity are a
likely result. Prudent management practices warrant that the benefits accruing to
Pakistan as a result of its coastal fisheries harvests and exports and the potential
negative impacts upon these resources if mangroves were to further erode, should be
seriously taken into account before any final agreements on the Indus Water Accord
and overall water resource management plans are finalized. Any further erosion of
water supply under the present scenario is likely to have the most serious consequences on the sustenance and survival of the mangrove resources. One other factor
that should also be borne in mind is that mangroves trap sediments and thus contribute to land building. Although they do so only in geomorphically prograding regions,
they stabilize such prograding shores and prevent excessive shifting of coastlines.
Thus mangroves are important on coasts that may be subject to major tropical storms.
With the potential of global warming and sea level rise looming ever so large, the
importance of mangroves to Pakistani coastal protection may be further enhanced.
Thus future preservation and propagation of mangroves as a renewable resource
cannot be overemphasized. Management and protection of mangroves along with
selective transplantation of hardy species in selected areas on an experimental basis
are needed. Along with this more rigorous scientific evaluations involving satellite
remote sensing and geomorphological, physical, chemical and biological studies on
the mangrove ecosystems must also be conducted. Only with a spirit of cooperation
and collaboration between scientists, managers and the policy makers would emerge
the most enlightened approach for saving the currently duressed but valuable Pakistani mangrove ecosystems.
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Pakistan Journal of Marine Sciences, Vol.1(2), 1992
ACKNOWLEDGEMENTS
I would like to thank Professor Paul J. Harrison of UBC and Professor Samuel C.
Snedaker of University of Miami for their constructive criticisms. This work was
supported by a research grant awarded by the U.S. National Science Foundation
#INT-8818807.
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