The marked reduction of the Indus river flow downstream from the Kotry barrage: can the mangrove ecosystems of Pakistan survive in the resulting hypersaline environment?

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Authors Ahmed, S.I.

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Link to Item http://hdl.handle.net/1834/31900 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 shel­ tered 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 km2 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 (south­ west 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. 146 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 com­ pete 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 Max. salinity Geographical tolerance (%) distribution

A vicennia marina 9.0 Pakistan (all areas) Lumnitsera racemosa 9.0 Southeast Asia Rhizophora mangle 8.0 American Continent Laguncularia racemosa 8.0 American Continent Ceriops tagal 6.0 Pakistan (Miani Hor, Keti Bundar) R. stylosa 5.5 Pakistan (Miani Hor) R. mucronata 5.5 Southeast Asia Sonneratia spp. 3.5 Southeast Asia R. apiculata 3.0 Southeast Asia Brnguiera parviflora 3.0 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 associ­ ated 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 maintain­ ing 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 fresh­ water flow from the Indus River that has played such a significant role in the devel­ opment 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 deter­ mined 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 condi­ tions, 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 148 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 Paki­ stan 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 comi­ culatum are also found. Salt tolerance of the latter species is less than that of Avicen­ nia 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 FRESH­ WATER 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 salin­ ities 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 70- 80 ppt were observed. Thus, high porewater salinities appear to have a major nega­ tive impact on mangrove growth in the Indus Delta. High porewater salinities com­ bined 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 vege­ tation. 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 predomi­ nant 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 Poten­ tial 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 RESO­ URCES 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 recogniz­ ing 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". 150 Pakistan Journal of Marine Sciences, Vol.1(2), 1992

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 rela­ tionships 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 ecosys­ tems. 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 man­ grove 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 man­ groves 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 recog­ nized 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 conse­ quences 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 contrib­ ute 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 Pakista­ ni mangrove ecosystems. 152 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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