Keywords

1 1 Mangroves: Forests or Wetlands?

Mangroves have been the subject of numerous studies worldwide and in recent years have attracted greater attention in the context of tsunamis and climate change. There is unanimity over the fact that the system includes trees and shrubs, and that they occur in intertidal areas in the tropics and subtropics. However, there continues to be confusion over their basic nature, or ‘what are mangroves?’ Whereas some prefer to restrict the use of the term only to woody plants (trees and bushes; FAO 1952), and call the ground ferns and other plants as ‘mangrove associates’ (Kathiresan and Bingham 2001 ), others apply it to the entire forest community, often described also as ‘coastal woodland’ or ’tidal forest‘—terms which basically reflect their location (FAO 1994; Melana et al. 2000 ).

Mangroves however, differ considerably from other forests of the humid tropics in several of their ecosystem attributes. FAO (1994) clearly acknowledged some differences when stating that “mangroves depend on terrestrial and tidal waters for their nourishment, and silt deposits from upland erosion as substrate for support.” It also identified the importance of the “substrate that is ever changing and dynamic over time.” Studies during the 1960s and 1970s highlighted the distinctive nature of several tropical forests such as those of the Amazon floodplain, many riverine forests, Southeast Asian peat swamps and mangroves which were variously named and classified with other forest types. During the same period, a wide range of habitats, hitherto known as bogs, fens, marshes and swamps, were brought together under the term ‘wetlands’, which were characterised by prolonged or permanent flooding of their substrates, and a vegetation adapted to such flooding and associated depleted oxygen levels in the root zones (Cowardin et al. 1979 ). Further studies demonstrated the overriding role of flooding regimes in determining the community structure and ecosystem functions of wetlands (Gopal 1990; Gopal et al. 1990 ; Keddy 2010 ). Numerous studies during the past four decades have elaborated upon the wetland characteristics of mangroves, and these are summarised in several recent publications (Alongi 2009 ; Twilley and Day 2012 ). Like most other wetlands, mangroves also have, in general, a very low diversity of plants (often with only one or two tree species being dominant), but are quite rich in their faunal and microbial diversity (see Alongi 2009 ). The tree species present have several morphological, anatomical and physiological adaptations against hypoxic to anoxic conditions in the root zone, as well as to other stress factors (e.g., dynamic substrate and salinity). The primary production in mangroves is comparable to, or more than, that in humid tropical forests. Further, this organic matter produced contributes greatly to the faunal diversity and secondary production of the adjacent open waters (Alongi 2009 ). Interestingly, despite acknowledging the importance of hydrology to the mangrove community (Kjerfve l990; Kjerfve et al. l999 ), it has received little attention from mangrove managers. In the following pages, I discuss briefly the hydrology of mangroves, particularly those in Asia, and emphasise that the management of mangroves, whether for utilization, conservation or restoration, must be based on specific hydrological considerations, consistent with the objectives.

2 Hydrological Features of Mangroves

Hydrological processes of a region are influenced by climate and geomorphology and, at the same time in a feedback loop, regulate them to a great extent . The interplay of geomorphology, climate and hydrology becomes more complex in coastal environments because of the continuous movement of water from two opposite sides—tides from the sea and freshwater runoff or river discharge from the inland side. In coastal regions, diverse landforms are created by the interactions between rainfall, river discharge, tidal frequency and amplitude, wave power, and inputs of sediments derived from terrestrial erosion (Twilley et al. 1996 ). Also, coastal biota often act as ‘engineers’, to modify physiographic features. Thus, a wide range of hydrological regimes are obtained across a hierarchy of spatial and temporal scales, that in conjunction with other major variables such as nutrients, salinity and substrate characteristics, influence the mangrove community and functions .

However, until recently greater emphasis has been laid on the tidal component of the hydrological regimes. On the basis of ‘local patterns of tides and terrestrial surface drainage’ in North American tropics (where only four species of mangroves occur), Lugo and Snedaker (1974) differentiated five physiognomic types of mangroves. These are: (1) fringing mangroves: along the protected shorelines and islands, influenced by daily tides, (2) riverine mangroves: along rivers and creeks, flooded by daily tides and influenced by freshwater and nutrients from the rivers, (3) mangroves on small islands in shallow bays overwashed by high tides, (4) basin mangroves which are dwarf stands along drainage depressions, forming hammocks on elevated sites, and (5) scrub mangroves comprising of dwarf shrubs along flat coastal fringes. It is widely recognised that the species-rich mangroves of South and Southeast Asia differ greatly from the species-poor neotropical mangroves and this classification is not applicable to them (see Kjerfve l990 ).

Thom (1982) and Galloway (1982) considered the role of the substrate and sedimentation, besides the tidal range, in recognising six broad categories of mangroves which covered most of those in Asia. These are:

  1. 1.

    Large deltaic systems (occurring in low tidal range, very fine allochtonous sediments, e.g. mangroves of Borneo, Sundarbans)

  2. 2.

    Tidal plains (where alluvial sediments are reworked by the tides, and there is the presence of large mudflats for the growth of mangroves)

  3. 3.

    Composite plains, under the influence of both tidal and alluvial conditions (e.g. lagoons formed behind wave-built barriers where mangroves grow)

  4. 4.

    Fringing barriers with lagoons (high wave energy conditions with autochthonous sediments of fine sand and mud, e.g. mangroves of the Philippines)

  5. 5.

    Drowned bedrock valleys (e.g. mangroves of Northern Vietnam, Eastern Malaysia or Andaman-Nicobar Islands)

  6. 6.

    Coral coasts (mangroves growing at the bottom of coral sand or on platform reefs, e.g. the mangroves of Indonesia and Singapore)

The role of river discharges was highlighted in the simpler classification scheme proposed by Woodroffe (1992) who grouped mangroves, on the basis of their hydrology, into three major types: (a) river-dominated, (b) tide-dominated and (c) interior mangroves (less influence of river/or tides). The river dominated mangroves are characterized by strong outwelling whereas the tide dominated mangroves experience bidirectional flux of water, energy and materials. Mangroves in the interior typically form sinks for sediment and nutrients .

Within tropical Asia, a large variability occurs within tidal regimes , river flow regimes and coastal geomorphology and hence, various mangrove sites experience a large variation in hydrological regimes. According to their frequency, tides are categorised as diurnal, semidiurnal (twice daily) and mixed. All three types of tides occur in the Indian Ocean although semidiurnal ones are the most widespread. Semidiurnal tides prevail on the coast along the Bay of Bengal whereas along the Arabian Sea, the tides are mixed. Tidal amplitudes are far more variable. Mauritius experiences a spring tidal range of only 0.5 m whereas the tides in the Arabian Sea reach their highest range at more than 11 m at Bhavanagar in the Gulf of Khambat and up to 7.8 m in the Gulf of Kachchh. Further northwest in Karachi (Pakistan), the tidal range is only 2.3 m. The tidal range is only 1 m at Cochin (southwest coast of India), 1.3 m at Chennai (Madras) on the eastern coast, and only 0.7 m at Colombo (Sri Lanka). Sagar Island, at the head of the Bay of Bengal, has a tidal range of 5.3 m and slightly higher tides are received at Diamond Harbor (5.78 m) and Yangon (Myanmar) (5.6 m). In the southeast Asian region tidal ranges vary similarly from > 1.5 m in Manila, about 2 m in Hong Kong and 2.5 m along Sulawesi coast to 5.8 m in New Guinea .

Freshwater flows into the mangroves come either from surface runoff from precipitation in nearby areas, or through rivers with large catchment areas. Most of the large rivers of South and Southeast Asia (Indus, Ganga, Brahmaputra, Irrawady, Mekong and Yangtse) originate in the Himalayan ranges and are fed by glaciers during the dry season and precipitation runoff during the monsoon season. Their low flows during the dry season and peak flows during the monsoons oscillate between extremes. The large spatial and temporal variability of monsoonal precipitation (which declines from east to west) results in further differences in freshwater flows in their deltas at different times of the year. Similar seasonal variations occur in river discharges in Indonesia , Malaysia and other countries.

Asian rivers, particularly those rising in the Himalayas, are also characterised by their high sediment loads, which have resulted in extensive delta and mangrove formations. The topography of the coastal belt also greatly influences the direction and rate of the flows of water into the system, and also the dispersion of sediments. In the lower part of the Ganga-Brahmaputra delta, an average gradient of only 5–10 cm per km results in meandering creeks and streams, temporary islands and pools, and the influence of flooding tides extends up to 50 km inland. A major part of the Sundarban thus oscillates seasonally between a river-dominated and tide-dominated system. Oag (1939) recognised three distinct seasonal phases in the tidal regime . During the period of the southwest monsoon, freshwater flows totally nullify the effect of the flood tides, leaving the ebb tides to strongly dominate the system. During the northeast monsoon (November to February), the effects of the flood tide are only slightly greater than that of the ebb tide. Later during the dry summer (May and June), before the southwest monsoon, the effect of the flood tides is much stronger than the ebb tides, and the estuary reaches maximum salinity (Chandra and Sagar 2003 ) .

Interactions between changing sediment loads, freshwater flows and tidal patterns and other factors cause local variations in land forms and hydrology (Dijksma et al. 2011). Sedimentation has resulted in the loss of connect between river Ganga and the major rivers in the eastern part of the Sundarban, the Saptamukhi, Thakuran, Matla, Gosaba and Harinbhanga. These rivers are now largely tidal in nature (Mitra et al. 2009 ). Similarly, during ebb tides the receding waters cause scouring of the top soil, creating innumerable tiny creeks originating from the centre of the moving islands. The receding waters carry huge volumes of silt deposited along the banks of rivers and creeks during high tides. This increases the height of the river banks, as compared to the interiors of the islands. Lands on the sea faces are both continually denuded by tidal waves, as well as built up by wave action depositing silt back onto the shores (Gopal and Chauhan 2006 ). Such a dynamic deltaic environment under the dominant influence of freshwater flows and sediments from one side and high tides from the other comprises a mosaic of hydrological conditions which strongly influence the characteristics of the mangroves.

It may be worth mentioning here that groundwater (sub-surface flows) also makes a significant contribution to mangrove hydrology in many areas (e.g. Wolanski and Gardiner 1981 ; Semeniuk 1983 ; Drexler and Ewel 2001 ; Drexler and DeCarlo 2002; Mazda and Ikeda 2006 ) but has received practically negligible attention .

3 Mangrove Management

Humans have used the mangroves in Asia for more than 1000 years. In the Ganga-Brahmaputra delta, besides using the trees for timber and fuel, mangrove vegetation was cleared to create rice-cum-fish farms. Historically, mangroves were common pool resources which were gradually taken over at different times by the rulers who increasingly controlled their management. During the 14th century in India, the clearing of trees and shrubs for cultivation of rice was actively promoted by the then Turk sultan rulers of Bengal, and followed by Moghul rulers until the area was taken over by the East India Company in the middle 18th century (see Eaton 1991 ) . Available evidence suggests that the Portuguese learned the traditional Indian technique of rice-fish farming in mangrove areas and during the 14th century and transferred this technology to Angola and Mozambique (Vannucci 1997 ; Kathiresan and Bingham 2001 ). In Indonesia and the Philippines the conversion of mangroves for rice and fish dates back to the early 15th century (Hora and Pillay 1962 ; Primavera 1995 ). Later during the British rule transformation of the Sundarban into agriculture fields was promoted actively and deliberately (Richards 1990 ; Richards and Flint 1991 ). The Sundarban mangrove was declared as a reserve forest by the British in the mid 1800s specifically for resource exploitation. The Commissioner for the Sundarban was charged with the task of “regulating and managing the waterlogged forests and swamps of the lower delta” and “to ensure that private landowners cleared, settled and reclaimed Sundarban forests and swamps for rice cultivation” (Richards 1990 ). The colonial forest department sought to preserve large areas of the remaining Sundarban tidal forest by giving them legal status as reserved or protected forests which were then intensively managed to provide a sustainable supply of timber and firewood. After all, “the ultimate goal of forest management, economic considerations aside, is to exploit to the fullest the natural energies and resources available for any given site so as to produce maximum carrying capacity for the production of the desired products” (FAO 1994). In several reports, FAO (1984, 1985, 1992) has focused primarily on promotion of utilization of mangroves and their afforestation .

Similar exploitation of mangroves for timber and fuel wood , and conversion to paddy fields and aquaculture farms has occurred throughout Asia over centuries . Expansion of shrimp cultivation and salt pans decimated the large Chokoria Sundarbans in the delta of the Matamuhury River (Bangladesh) to a small patch of few individuals of Heritiera fomes (Biswas and Choudhury 2007 ). Economic factors driven by developmental pressures play a decisive role in the formulation of management strategies, and most South and Southeast Asian countries are among the least developed. However, it must be noted that the recent exploitation and conversion to shrimp farms in Southeast Asia are driven by global demands (Gopal 2005 ).

3.1 Rehabilitation of Degraded Mangroves

After considerable areas of mangroves were lost, converted and degraded by exploitation and aquaculture (see Kairo et al. 2001 ; Dijksma et al. 2010 ), thereby affecting the availability of resources, people turned to rehabilitation through afforestation activities based on silvicultural practices. In Malaysia, the Matang Mangrove forest reserve of the state of Perak, has been systematically managed for fuel wood and poles since 1908 (Chong 2006 ) . The silviculture system which initially followed a variable rotation age of 20–40 years and maintained some seed trees in the logged-over areas for regeneration, has now been changed to 30 years rotation with clear felling without retaining seed plants. Also, the mangroves in the Klang Islands (Selangor) are managed solely for the production of poles, charcoal, woodchips and fishing stakes, and therefore, Rhizophora apiculata and R. mucronata are the preferred species for plantation (Chong 2006 ). Extensive plantation of Sonneratia apetala and Avicennia officinalis have been undertaken in Bangladesh since 1966 (Saenger and Siddiqi 1993 ), whereas Rhizophora apiculata has been planted over more 1300 km2 in the Mekong delta in Vietnam (Blasco et al. 2001 ). In the Philippines, mangrove replantation started as community initiatives during the 1930s and government-sponsored projects were taken up in the 1970s which turned in the 1980s into large-scale international development assistance programs (Primavera and Esteban 2008 ). In Indonesia, the management of mangroves is regulated by the silvicultural practices in their harvesting and by leasing arrangements for allocating the mangroves (Kusmana 2012 ). In Java, mangrove rehabilitation by replanting abandoned shrimp ponds has been linked with poverty reduction and livelihood development (http://www.wetlands.org/?TabId=2291). In China, monocultures of Kandelia obovata , Sonneratia caseolaris and Rhizophora stylosa are promoted in mangrove reforestation despite known consequences for potential insect outbreaks (Chen et al. 2009) . In India, mangrove plantation was started as ‘restoration’ activity in Tamil Nadu by the M.S. Swaminathan Research Foundation. After the Asian tsunami, mangrove plantations have been made on a large scale, especially in Gujarat which has the second largest area of mangroves in the country (Vishwanathan 2011) .

These afforestations, most often creating monospecific stands of highly salt-tolerant species, cannot be considered as rehabilitation because they fail to restore (or even simulate) the high ecological values of the original forests (Sanyal 1998; De Leon and White 1999; Lewis 2005) . According to Walters (2004) , mangrove plantations are an efficient alternative to harvesting from unplanted, natural mangroves and their spread may reduce harvesting pressures on existing forests. However, mangrove plantations are very different in their structure and composition from natural forests which are gradually being replaced. Furthermore, plantations are not typically viewed by the planters for their environmental conservation value and are, hence, frequently cut and cleared to make space for alternative uses, especially fish farming and residential settlements.

3.2 Management for Conservation

The process of conservation of mangroves for biodiversity protection began only after India’s independence and the partition of the Indian Sundarban between India and East Pakistan (now Bangladesh). In India, three wildlife sanctuaries were created (spread over three decades) within the Sundarban (Lothian Island, Sajnakhali Wildlife Sanctuary and Haliday Island). In 1973, an area of 2,585 km2 was declared as a Tiger Reserve of which the core area of 1,330 km2 was later designated as a National Park. These protected areas focused on characteristic wildlife such as the tiger, spotted deer, wild boar and rhesus macaque. After Bangladesh became a sovereign state, it created, in 1977, three wildlife sanctuaries on three disjunct deltaic islands in the Sundarban forest division of Khulna district. In 1987, the Sundarban National Park in India, and in 1997, parts of the Sundarban in Bangladesh, were inscribed on the World Heritage list (IUCN 1997). The entire Indian Sundarban area, including reclaimed lands, has also been designated as the Sundarban Biosphere Reserve of which the core zone comprises the national park and the Tiger reserve. Approaches to conservation, however, differ considerably between the two countries (for detailed discussion, see Seidensticker et al. 1991 ). India has designated Bhitarkanika mangroves on the eastern seacoast as a Ramsar site, while in Bangladesh most of its Sundarbans reserved forest has been designated as a Ramsar site. Some of the Ramsar sites in Sri Lanka have small patches of mangroves within them whereas almost all of the mangroves in Pakistan are covered by the Indus delta Ramsar site .

China has an extensive network of wetland nature reserves of which several are important mangrove areas and have also been designated under the Ramsar convention. Most of the countries of Southeast Asia have established protected areas comprised of important mangroves. About 20 % of the total mangroves in Southeast Asia have thus been protected (Giesen et al. 2007), although the proportion of protected areas varies greatly among countries. Indonesia has the largest area of protected mangroves followed by Papua-New Guinea, but Cambodia has the largest proportion (49 %) of its mangroves within the protected area network. The proportion of mangroves protected in Indonesia, Papua-New Guinea and Thailand is about 27 %, 25 % and 10 %, respectively. It is interesting to note that Vietnam has promoted extensive regeneration of mangroves after their near total destruction during the war. These mangroves include the 42,630 ha Can Gio nature reserve in the Mekong delta that was declared a UNESCO biosphere reserve in the beginning of 2000. Within Southeast Asia, several important mangroves have also been designated as Ramsar sites.

3.3 Management Problems

The management of natural resources has moved over the decades from exploitation (sustained utilization) to conservation and rehabilitation (or restoration) of degraded ones . We often talk of sustainable management and ecosystem-based management which sustains the composition, structure, functioning and ecosystem services . It requires proper understanding of the ecological interactions and processes operating within an ecosystem, and also the setting up of explicit goals and policies (cf. Christensen et al.1996) .

After the extensive loss and degradation of mangroves throughout Asia, some areas are now being protected, apparently for the conservation of biodiversity. Monocultures of mangrove species are also raised in degraded areas in recognition of their productive and protective function. However, these ‘management practices’ ignore the diversity of ecosystem services of mangroves and fail to address the negative changes in habitats due to various reasons, including invasive species, changing hydrology and salinity levels and deteriorating water quality. According to the FAO (1994), “Habitat protection is the ultimate goal of conservation, to which all other approaches are subsidiary.” For conservationists worldwide, mangroves present the great immediate challenge. Technically, mangroves are easier to manage compared to the species rich humid tropical forests. There are typically only a handful of mangrove species, many of which coppice or regenerate freely. However, whereas the terrestrial forester is concerned primarily with managing forests grown on stable and firm ground, in the tidal swamps he has to manage aquatic resources as well. It is the aquatic resources—the freshwater flows , tidal flows and the ecological processes in the aquatic environment—that are grossly ignored in mangrove management .

3.4 Tidal Regimes and Mangrove Species

Hydrologically, the tidal regimes are comprised of, besides the tidal amplitude, frequency, duration and timing, particularly if the tide is experienced in different parts of the intertidal zone . These components are then affected by the local variation in elevation profile due to sedimentation. Various species of mangroves respond differently to different tidal regimes. For example, in the Indian part of the Sundarban, a mangrove stand that experiences total diurnal inundation is dominated by Avicennia marina and A. alba while Excoecaria agallocha Ceriops dacandra and Acanthus ilicifolius dominate at sites that are not completely inundated (Saha and Choudhury 1995) . Nypa fruticans also seems to prefer sites with low level of tidal inundation (Siddiqi 1995) . Van Loon et al. (2007) observed, in Vietnam, that in an area with an irregular tidal regime and/or an irregular elevation profile, the duration of inundation is more important, and the vegetation can be better characterised by the duration per inundation and per day. Experimental and field studies in China have shown that Bruguiera gymnorrhiza had lower tolerance to soil flooding than Kandelia obovata (Ye et al. 2003) , while the optimal growth of the latter species was obtained at 2–4 h flooding per tidal cycle (Chen et al. 2004; 2005) . Sonneratia apetala has greater tolerance to high tide (Chen et al. 2009) .

3.5 Importance of Freshwater Flows

As mentioned earlier, the majority of the Asian mangroves lie in the deltas of major Himalayan rivers which carry enormous sediment loads to the oceans. The monsoonal climate with large spatial and temporal variability adds to the variability of freshwater flows to the mangroves. Thus, unlike other mangroves, the Asian mangroves are mostly river dominated. The greater combined freshwater flows from precipitation, surface runoff and river discharges are directly correlated with higher mangrove species richness, height, and productivity (Saenger and Snedaker 1993) . Within India, there is a distinct and prominent correlation between freshwater discharge from the rivers and the mangrove species richness (see Selvam 2003) . As the total annual discharge decreases in Krishna and Cauvery rivers, the mangrove species richness declines sharply (and Avicennia marina is becoming dominant), despite the fact that the annual rainfall remains similar or slightly higher than in the River Godavari’s delta. The effect of higher rainfall seems to be nullified by the prolonged dry season. Similarly, absence of significant river discharges and low rainfall on the western coast of India are reflected in very low species richness, but heavy rainfall produces large freshwater flows in the Andaman-Nicobar Islands, resulting in species richness nearly as high as in the Sundarban. Similar relationships between river discharges and rainfall (duration and total amount) are evident in different islands of Indonesia, Malaysia and the Philippines. Although the importance of freshwater inflows to mangrove forests has been recognized for a long time (see Wolanski and Gardiner 1981) , these very flows have been usually overlooked in Asian countries while the tidal hydrodynamics and influences are discussed in detail. According to Duke et al. (1998) , although mangrove species differ in their tolerances across a wide range of salinities, none essentially requires saltwater to survive (see also Ball 2002) .

The importance of freshwater flows, particularly to river dominated mangroves, is far more significant with respect to other ecosystem services . With reference to the protective function of mangroves against cyclonic storms and tsunamis, the height and density of trees would be an important factor. Many studies on the growth of mangrove species show that high salinities have negative effects on metabolism, growth, productivity and height (see Cintr’on et al. 1978; Naidoo 2010; Feller et al. 2010). In Vietnam, recently Loi (2008) has reported on the hydrology and its effects on the mangrove community structure and functions in the Can Gio biosphere reserve. It is well known that over the past several decades, the mangrove structure in Sundarban is becoming simpler and the average height of the trees is decreasing. As a long-term consequence of decline in freshwater flows and increase in salinity, Heritierais being replaced by Excoecaria (Christensen 1984) and Nypa fruticans and Phoenix paludosaare declining rapidly. It is estimated that in the Bangladesh part of the Sundarban, 0.4 % of the forest area is replaced by dwarf species every year. This also causes a decline in the habitat for birds, monkeys and other tree-dwelling species .

The dependence of coastal fisheries on mangroves has been a major theme of discussion among mangrove researchers (see Baran 1999; Sukardjo 2004; Islam and Wahab 2005; Manson et al. 2005) . It is also pointed out that river-dominated mangroves characterized by high nutrient influx and strong out-welling from mangroves, play a significant role in maintaining the fishery production of the adjacent coastal waters (Selvam 2003) . Thus, reduction in freshwater flow affects the amount of nutrient exported to the coastal environment and thereby, the fishery production .

Recently, Ewel (2010) discussed the impacts of changes in freshwater inputs, such as those caused by water diversion upstream, to mangrove forests that often “lead at first, to subtle changes in function and eventually to dramatic changes in species composition.” She points out that “these changes may not become apparent for years or even decades, but they may have important consequences for coastal food chains and for the socio-economic benefits they extend to indigenous people” (Ewel 2010) . Further, impacts arise from the management of freshwaters upstream at different times of the year in relation to periods of rainfall, and consequently, the timing of freshwater flows into the mangroves. The impacts of freshwater flow diversion from river Ganga with reference to the Sundarbans have been a matter of intense discussion and dispute between India and Bangladesh. Unfortunately, neither have systematic studies of the ecosystem structure and functioning being undertaken, nor have the needs of freshwater flows into Sundarban been assessed to date. Similar changes in freshwater flow have also occurred in the Indus river, with consequence for the mangroves .

In the case of river-dominated mangroves of the Asian region, the impacts of human activities extend beyond the diversion of river flows. Various activities also impact the sediment load and nutrients in runoff. While a considerable amount of sediments are trapped behind dams, erosion is also accelerated by several land-based activities. Nutrients and pollutants invariably reach the mangroves, thereby affecting the ecosystem structure and function, even if these changes are not perceptible in the tree community.

Mangroves of some countries in Southeast Asia are not river dominated but are influenced most by freshwater runoff during the monsoonal rainfall as the dry periods are short.

The importance of the freshwater flows and the need to understand the freshwater requirement of the mangrove species used in plantation and rehabilitation also cannot be ignored. Selvam (2003) reports that the attempts to reintroduce Sonneratia apetala , Xylocarpus granatum and Bruguiera gymnorhiza in Pichavaram and Muthupet mangroves failed because of high soil salinity. He clearly emphasises that species with low tolerance to salinity cannot be reintroduced successfully without increasing the freshwater flow. Strangely, the view of the politicians and the water resource managers that ‘not a drop of water should go waste to the sea’, simply ignores the multiple ecosystem services of the mangroves that can be sustained only by freshwater flows, and are not restricted to wood production alone .

Another usually overlooked or underestimated contribution of freshwater flows is towards the mitigation of sea level rise impacts. The reduction in freshwater flows in the rivers because of withdrawal and diversion (as also due to altered precipitation regimes), is likely to have a synergistic effect on the decline of mangroves. Mangroves of low relief islands in carbonate settings that lack rivers are likely to be the most sensitive to sea-level rise, owing to their sediment-deficit environments. In the absence of sediment transport to river dominated mangroves, the balance between subsidence processes and accretion will be lost, aggravating the impacts of sea level rise. A combination of sustained erosion, subsidence and sea level rise implies that the lower areas and islands will continue to fall below sea level and will disappear with time .

4 Conclusion

The problems of mis-managing mangroves results from our failure to understand the true nature of the system, to recognise hydrology as the key driving variable, and above all to appreciate that Asian mangroves are very different from those of African tropics or the neo-tropics. Researchers, managers and policy makers must come out of the colonial mindset, and must not readily fall prey to outdated, inapplicable concepts and approaches usually developed outside the region. The geological, geomorphic, ecological, biological, and socio-cultural peculiarities of the Asian region, together with the diversity within it, cannot be and should not be ignored in any discussion on the human-nature relationship. Though mangrove ecosystems are receiving increasing attention, we are still far from understanding their dynamics. There has been little effort to synthesise relevant studies for a holistic management and implementation action plans.

The long term and sustainable conservation of mangroves for their multiple ecosystem services requires that they be treated as distinct from other forests. Unlike the evergreen or deciduous forests, mangroves experience a highly dynamic environment and are in a state of continued flux. They are governed by their specific flooding regimes which govern salinity gradients, nutrients and the supply of fine sediments. These primary drivers are directly controlled by the biophysical, climatic and anthropogenic processes in their watershed—the basins of the rivers whose delta they occupy. We should avoid transforming mangroves to plantations, and afforestation programmes that focus on particular species only, while ignoring ecosystem attributes, their relation to offshore impacts on fisheries and organic matter transport, etc., and instead focus on their management as integrated, holistic ecosystems with considerable biotic diversity and diverse ecosystem services.