Citation: Sen, H.S. and Ghorai Dipankar (2011). Whither  Coastal Ecosystem Research: Management of Salt Affected Soils sans Factors Threatening the Ecosystem Loses Significance. Indian Society of Soil Science, Bulletin 28, 49-65.

Whither Coastal Ecosystem Research: Management of Salt Affected Soils sans Factors Threatening the Ecosystem Loses Significance

H.S.Sen¹ and Dipankar Ghorai²

¹Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), Barrackpore, West Bengal, PIN 700 120; Present address – 2/74 Naktala, Kolkata 700 047, West Bengal

²Subject Matter Specialist (Ag), Krishi Vikash Kendra (CRIJAF), Burdwan 713 212, West Bengal

According to World Resources Institute (2006) coastal areas may be commonly defined as the interface or transition areas between land and sea, including large inland lakes. Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global or national scale.  According to them, the world coastline extends from 350,000-1,000,000 km in length, depending upon how finely the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Soil salinity per se in the coastal ecosystem does not have much significance as far as productivity of crops on these soils is concerned, unlike any other ecosystem, unless it is considered in association with other relevant ecological factors threatening its very stability. According to an estimate by Dirk et al. (1998), 51 percent of the world’s coastal ecosystems appear to be at significant risk of degradation from development related activities.

Different coastal ecosystems in the world

The ‘main’ components of the coastal ecosystem, besides taking into account generally about 50-100 km area landward to be designated as ‘coastal plain’ and utilized mostly by agriculture and allied activities as well as for domicile and a few other occupational purposes, are classified into components, like estuaries (1.4), macrophyte communities (2.0), mangroves (0.2), coral reefs (0.6), salt marshes (0.4) and the remaining continental shelves (~21), totaling approximately 26 x10⁶ km² area (Encyclopedia of Earth, 2007). ___________________________________________________________________________________

Delivered on 15 November 2010 at the National Symposium on “Salt-affected Soils” held during the 75^th Annual Convention of the Indian Society of Soil Science at Bhopal. ¹Email: hssen.india@gmail.com, hssen2000@hotmail.com

Mangrove swamps, as just an example, having significant role towards stability of the ecosystem, are found in tropical and sub-tropical tidal areas worldwide, like Africa, Americas (including Caribbean Islands), South America, Asia, Australasia, and Pacific Islands. A list of 15 countries having significant areas under mangrove swamps are given in Table 1. In the last 50 years, as much as 85 percent of the mangroves have been lost in Thailand, the Philippines, Pakistan, Panama and Mexico. Globally, about 50 percent of mangrove forests have been lost. An estimated 35% of mangroves have been removed due to shrimp and fish aquaculture, deforestation, and freshwater diversion. In Indonesia alone over 10,000 square kilometers of mangrove forests have been converted into brackish water ponds (called tambaks) for the cultivation of prawns and fish. Valuation of intact tropical mangroves estimated at US$ 1000 per ha drops to US$ 200 per ha due to clearance by shrimp farming (Poyya and Balachandran, 2008). Although some successful restoration efforts have taken place, these are not keeping pace with mangrove destruction.

Country	Mangroves (‘000 ha)	Global % area
Indonesia	4250	30
Brazil	1376	10
Australia	1150	8
Nigeria	970	7
Malaysia	641	5
Bangladesh	611	4
Myanmar	570	4
Vietnam	540	4
Cuba	530	4
Mexico	525	4
Senegal	440	3
India	360	3
Colombia	358	3
Cameron	350	2
Madagascar	327	2

Table 1. Estimated coverage with largest mangrove areas (Source: ITTO/ ISME, l993)

Coastal plain: Major Features and Management Issues

Coastal plain, within the ecosystem, is the landward extension of the continental shelf or the sea and used for agriculture and allied activities as well as for few other occupational purposes but is not always distinctly differentiated from the other ‘main’ components referred earlier.

Characteristics and Distribution

Of the two coastlines in India length of the East coast is higher than that of the West. The continental shelf is more stable than the coast. The continental shelf of 0-50 m depth spreads over 1,91,972 sq km and between 0-200 depth over 4,52,060 sq km. The shelf is wide (50-340 m) along the East coast. The Exclusive Economic Zone is estimated at 2.02 million sq km.

Practically no systematic study was earlier made in India to demarcate the coastal soils based on well-defined scientific indices. Notable among the past works, however, was that of Yadav et al. (1983) who suggested 3.1 million hectare area (including mangrove forests), while Szabolcs (1979) suggested 23.8 million hectare under coastal salinity in India. The coastal saline soil has been referred by various workers almost synonymously with coastal soil which is not correct since all coastal soils are not saline in nature. None of the above estimates appears to have been made on sound scientific basis. However, the latest compilation made by Velayutham et al. (1998) on the soil resources and their potentials for different Agro-ecological Sub Regions (AESR) in coastal tracts of India show total 10.78 million hectare area under this ecosystem (including the islands), which was the first scientific approach for delineation of the coastal ecosystem. Different factors limiting agricultural productivity in the coastal plains are listed as (1) Excess accumulation of soluble salts and alkalinity in soil, (2) Pre-dominance of acid sulphate soils, (3) Toxicity and deficiency of nutrients in soils, (4) Intrusion of seawater into coastal aquifers, (5) Shallow depth to underground water table rich in salts, (6) Periodic inundation of soil surface by the tidal water vis-à-vis climatic disaster and their influence on soil properties, (7) Heavy soil texture and poor infiltrability of soil, (8) Eutrophication, hypoxia and nutrient imbalance, (9) Erosion and sedimentation of soil, and (10) High population density, etc.

Soil Salinity

Soil salinity in coastal soils acts in much the same way as in inland soils except for different salt compositions in the soil solution and specific toxicity of individual ions and their interacting effects observed in case of the former. Three major types of salt affected soils exist in the coastal plains, viz. saline soil or solanchak, alkaline or sodic soil or solonetz, and of particular interest for the coastal ecosystem, the acid sulphate soils.

Saline Soil

Characteristics: Soils contain excess soluble salts (ECe > 4 dSm^-1) with pH below 8.5 and ESP lower than 15. High osmotic stress as well as specific ion toxicities cause adverse effect on plant growth due to poor uptake of water and nutrients. Salts are composed mainly of sodium, calcium, magnesium among the cations, and chloride, sulphate, carbonate, bicarbonate among the anions. In majority of the situations salt concentration along with its composition at the crop root zone varies not only spatially but also temporarily depending upon soil type, salt-rich ground water characteristics and its rate of recharge into the root zone, and nature and distribution of rainfall along with other relevant climatic parameters.

Drainage and desalinization: Efforts have been made to develop models to desalinize the salty soil through drainage under specified conditions. Different agro-hydro-salinity models, viz. ‘SALTMOD’, ‘DRAINMOD-S’ or ‘SAHYSMOD’ (Oosterbaan, 2002, 2005), developed based on sound principles of moisture and solute transport, for unconfined (phreatic) and semi-confined aquifer, have been tested in the field mostly under arid or semi-arid conditions in order to predict the water distribution and salt balance in the soil profile following different practices of drainage and their response on crop function. Singh and Singh (2006) compared different models suggesting design of the most appropriate location-specific drainage system under varying water management scenarios covering salt water intrusion, runoff, soil erosion, backwater flow, waterlogging and salinity in the coastal plains in India.

Alkaline or Sodic Soil

Characteristics: These soils contain exchangeable sodium in a quantity sufficient (ESP > 15) to interfere with the growth of most plants. In such soils ECe is generally < 4 dSm^-1 and the pH higher than 8.5. The soil colloids are usually in a state of deflocculation. The dispersive effect of exchangeable sodium will be observed, however, only if the electrolyte concentration in the soil solution is smaller than that required to flocculate the clay particles. High concentration of Mg in relation to Ca, observed in some coastal salt affected soil solution, behaves differently in terms of physico-chemical properties. The alkaline or sodic clay colloids in a dispersed state render poor physical properties primarily in respect of moisture and solute transport, aeration and thermal flux, thereby adversely affecting the plant growth. In these soils when pH of the soil solution exceeds 8.5, availability of some nutrients may be restricted resulting in nutrient imbalances. Bicarbonate toxicities occur primarily from reduced iron and other micronutrient availabilities at high pH while high Na⁺ may lead to Ca²⁺ and Mg²⁺ deficiencies (Arshad, 2008).

Reclamation and management: The basic principle underlying reclamation of these soils is to adopt those ameliorative measures by which the exchangeable sodium will be replaced by calcium and the exchangeable sodium thus released as sodium salt is leached out of the root zone. Use of amendments and adequate leaching are the prerequisites for any reclamation measures. Because of low cost and easy availability, gypsum and sulphur have been used widely and intensively as an amendment for reclamation. Gypsum converts sodium soil into calcium soil, results in lowering of pH and improvement in soil physical conditions. On an average, for every one milli-equivalent of sodium to be replaced, 1.7 tons of gypsum or 0.32 tons of sulphur is required. Besides, iron pyrites, which is abundantly available, is also an economical amendment for sodic soils. The use of molasses along with pressmud and basic slag has also been found effective in some areas. Further, bulky organic manures, green manures, crop residues and other biomass materials may even be used for reclamation of sodic soils.

Acid Sulphate Soils

Characteristics: These soils either contain sulphuric acid or have the potential to form sulphuric acid when exposed to oxygen in the air. These soils occur naturally in both coastal (tidal) and inland or upland (freshwater) settings, as a consequence of the deposition of large amounts of organic matter, such as decaying vegetation in a waterlogged setting. These waterlogged wetlands and mangroves or highly reducing environments are ideal for the formation of sulphide-containing minerals, predominantly iron pyrite (FeS₂) in sulphidic material, which can react with the oxygen in the air to form sulphuric acid (sulphuric materials). It is generally believed that the H₂S is formed by sulphate reducing bacteria acting on sulphate from seawater, rather than the introduction of sulphide with the dredge sediments. Their most important characteristics are a field pH of below 4.

Most acid sulphate soils occur in the tropics in low lying coastal land formerly occupied by mangrove swamps. The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil (Beek et al., 1980).

The growth of most dry land crops on acid sulphate soils is hampered by the toxic levels of aluminium and the low availability of phosphorus. Toxic levels of dissolved iron plus low phosphorus are the most important adverse factors for wetland rice. In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents), high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts) in which the pyritic substratum occurs near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths (Beek et al., 1980).

Leaching and management: The older, deeply developed acid sulphate soils require no specific reclamation measures, and can be greatly improved by good fertilizer application, moderate dressings of lime (1-5 t ha^-1) and, probably most important, through good water management. In reclaiming or improving potential and young acid sulphate soils following approaches are possible: (i) pyrite and soil acidity can be removed by leaching after drying and aeration, and (ii) pyrite oxidation can be limited or stopped and existing acidity inactivated by maintaining a high water table, with or without (iii) additional liming and fertilization with phosphorus, though liming may be often uneconomic in practical use. The reclamation method cited at (ii) above, i.e. maintaining a high water table to stop pyrite oxidation and inactivate existing soil acidity, has the advantage that its effects are usually noticeable much quicker. This is especially true in young acid sulphate soils that are generally high in organic matter. Upon waterlogging, soil reduction caused by microbial decomposition of organic matter lowers acidity and may cause the pH to rise rapidly to near-neutral values. The crucial factor is, of course, the availability of fresh water for irrigation. In another study at Australia (O’Sullivan et al., 2005) the reclamation works served to lower the acid sulphate potential of the sediments by increasing the height of the water table, thereby ensuring that sulphidic sediments remain anaerobic, and by introducing carbonate containing sediments in a slurry of seawater, both of which provide buffering capacity with the ability to neutralize any acid formation. In the Muda irrigation project in Malaysia, where patches of Sulfaquepts occur among better soils, improved water management and intensive irrigation have dramatically increased the productivity of these highly acid soils (Beek et al., 1980). However, they maintain that unless sufficient fresh water is available and other prerequisites for good water management exist, the potential acid sulphate soils and young, strongly acid sulphate soils should not be reclaimed, but are better left for other types of land use, say conservation, forestry, fisheries and, sometimes, salt pans, etc..

Seawater Intrusion

Salinity build-up in soil due to salinity ingress of ground water aquifers takes place through the following major processes: (1) excessive and heavy withdrawals of ground water from coastal plain aquifers, (2) seawater ingress, (3) tidal water ingress, (4) relatively less recharge, and (5) poor land and water management.

Modeling on ground water behaviour: Salt water intrusion takes several forms. Horizontal intrusion occurs as the saline water from the coast slowly pushes the fresh inland ground water landward and upward. Its cause can be both natural (due to rising sea levels) and man induced, (say, by pumping of fresh water from coastal wells) (Fig. 1a). Pumping from coastal wells can also draw salt water downward from surface sources, such as tidal creeks, canals, embayment (Fig. 1b). This type of intrusion occurs within the zone of capture of pumping wells, which is local in nature, where significant drawdown of the water table causes induced surface infiltration. A third of intrusion is called ‘upconing’. Upconing also occurs within the zone of capture of a pumping well, with salt water drawn upward toward the well from salt water existing in deeper aquifers (Fig. 1c) (Maimone, http://cms.ce.gatech.edu/gwri/uploads/proceedings/1999/MaimoneM-99.pdf).      

Fig. 1. Different forms of salt water intrusion (a) horizontal movement towards supply well,      (b) induced downward movement from surface sources such as creeks, (c) upconing beneath a supply well; arranged  vertically downwards (Source: Maimone,  http://cms.ce.gatech.edu/gwri/uploads/proceedings/1999/MaimoneM-99.pdf)

Management models and control of seawater intrusion: Indiscriminate use of water resources, particularly under ground, thus poses a major threat to destabilize the ecosystem. Different management models at varying degrees of success have been reported in the literature by various workers to find out developing withdrawal management methodologies for determining the number of viable locations for wells and the quantities of water which can be pumped from coastal aquifers while protecting the wells from seawater intrusion in order to satisfy the demand (social dimension), maximizing the economic benefits (economical aspects), and controlling the saltwater intrusion (environmental concern). One such  optimisation model was developed for planning and managing saltwater intrusion into coastal aquifer systems (Da Silva et al., https://repositorium.sdum.uminho.pt/bitstream/1822/8682/1/Optimal%20Management%20of%20Grondwater%20Withdrawals%20in%20Coastal%20Aquifers.pdf) using the simulation/ optimisation approach for managing water resources in the areas, suggesting the best location of the wells with specific flow rate, and thereby, the best policies to maximize the present value of economic results of meeting water demands, and to keep under control the saltwater intrusion.

Various engineering methods are in use worldwide for the control of coastal seawater intrusion. In India  sporadic work has been done, as for example in Tamil Nadu (Chennai) and along Saurashtra Coast in

Gujarat state (Mangrol-Chorwad-Veraval area). Methods that may be employed for control of seawater ingress into aquifers are listed and described  (Anon, http://megphed.gov.in/knowledge/RainwaterHarvest/Chap11.pdf) as: (1) Modification of ground water pumping and extraction pattern, (2) Artificial recharge, (3)  Injection barrier, (4)  Extraction barrier, (5) Subsurface barrier, (6) Tidal regulators/ Check dam/ Reservoirs

For an effective and long term solution to the problem of seawater intrusion into ground water aquifer in the coastal plain it is necessary to develop location-specific optimization model to decide on suitable locations of the pumping wells and rates of withdrawal of the ground water from these wells after due consideration of the relevant factors. Attempts for suitable constructions either by pushing saline water front further seaward through check dams or injection barriers, and/ or allowing more surface water infiltration to recharge the ground water through creation of reservoirs behind the dams, or through subsurface barriers, etc. are mostly in experimental stage though worth consideration, and its adoption is subject to economic viability.

Integrated Water Management

If the water table, rich in salts, is present at a very shallow depth (generally not exceeding a depth of 2 m below the soil surface), it contributes salts to the root zone during the dry season through upward capillary rise in response to evapotranspiration demand of soil moisture. The net salt loading in the root zone will be positive (salinity will build up) or negative (desalinization will take place) depending upon the relative rate of recharge of salts by upward rise to rate of downward flux of salts through leaching. The relative salt loading will thus be treated generally as positive during dry season, and negative (waterlogging on the soil surface) during wet season due to high rainfall, and the process will be repeated in the each year in a seasonally cyclic mode.

Sen and Oosterbaan (1992) presented a practical working method on integrated water management for Sundarbans (India) through surface gravity induced drainage during summer/ wet season (through land shaping)-cum-excess rainwater storage for irrigation during dry season. They computed for the same region drainable surplus, which may be stored for irrigation during dry (deficit) period. Ambast and Sen (2006) developed a computer simulation model and a user-friendly software ‘RAINSIM’ for the same, developed primarily for Sundarbans region for small holdings, based on the hydrological processes (Fig. 2), and the same tested duly for different agro-climatic regions in India for (i) computation of soil water balance, (ii) optimal design of water storage in the ‘On-farm reservoir (OFR)’ by converting 20 % of the watershed, (iii) design of surface drainage in deep waterlogged areas to reduce water congestion in 75 % of the area, and (iv) design of a simple linear programme to propose optimal land allocation under various constraints of land, water or other critical inputs to arrive at a contingency plan for maximization of profit. They also reported use of remote sensing and GIS in mapping lowland lands, vegetation, crop yield estimation, along with performance assessment of irrigation/ drainage systems.

Irrigation Water Resources

Fig.2. Comprehensive framework of hydrological processes at different scales (Source: Ambast and Sen, 2006)

In spite of the coastal ecosystem presenting a delicate equilibrium among the different components there is however no firm strategy, as of now, for exploitation of water resources for irrigation and other purposes for long term solution in any sector. The European Commission (2007) observed, based on a study by Spanish researchers, how an inappropriately planned coastal development could lead to increasing water consumption to unsustainable levels, for which future planning for sustainable development, based particularly on water resources, should be such as not to disturb the ecosystem in the long run. The technological developments in this region should focus on the areas, viz. artificial recharge of the aquifer, recycling of water, desalinzation of seawater, weather modification, improved irrigation management practices, and use of marginally poor quality water.

It is suggested that location-specific programme on water allocation under different sources should be drawn up for each region, based on soil, climate, water, and crop parameters, as well as their spatial variations, as per appropriate strategies to be worked out, with minimal dependence on abstraction of water from the underground aquifer, but with increasing dependence on other means, like use of surface water sources by recycling of rainwater stored and fresh water available using innovative seawater desalination technology, and conjunctive use of marginally saline water available, with overall target to increase water productivity and cropping intensity phasewise, and conserve the ecosystem at the same time.

Fertility Management and Soil Quality

With regard to soil fertility, the coastal soils are usually rich in available K and micro-nutrients (except Zn), low to medium in available N and are having variable available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is lost through volatilization (Sen and Bandyopadhyay, 1987). Effect of salinity on the microbial and biochemical parameters of the salt affected soils in Sundarbans (India) was studied at nine different sites showing that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating adverse effect of soil salinity. About 59%, 50%, and 20% variation in MBC/OC, FDHA/OC, and BSR/MBC (metabolic quotient, qCO₂), respectively, which are indicators of environmental stress, could be explained by the variation in EC_e. The decrease in MBC and microbial activities with a rise in salinity was ascribed as probably one of the reasons for the poor crop growth in salt affected coastal soils (Tripathi et al., 2006). It was suggested that integrated nutrient management should be very effective for increasing its use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice. According to them, it is imperative to view the nutrient elements and their interactions with the salt components together instead of considering each of them in isolation.

The importance of improved soil quality in the coastal plains through higher SOC level of the soils was highlighted by Mandal et al. (2008). IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), which includes large areas under coastal plains (Fig. 3). They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.

Coastal Ecosystem: Ecological Factors

Carbon Sequestration

Fig. 3. Distribution of rainfed lowland rice soil quality area in Asia. Each dot is coloured to represent the assumed dominant soil class (Source: Haefele and Hijmans, 2009)

Modeling C sequestration so far indicated that coastal marsh ecosystems tend to sequester C continuously with increasing storage capacity as marsh age progresses and its area increases. Thus, C sequestration in coastal marsh ecosystems under positive accretionary balance acts as a negative feedback mechanism to global warming.  Choi and Wang (2004) were of the opinion that dynamics of carbon cycling in coastal wetlands and its response to sea level change associated with global warming is still poorly understood. However, they also observed during their study at Florida that salt marshes in this area have been and continue to be a sink for atmospheric carbon dioxide. Because of higher rates of C sequestration and lower CH₄ emissions, coastal wetlands could be more valuable C sinks per unit area than other ecosystems in a warmer world. Brigham et al. (2006) stated that the estuarine wetlands sequester carbon at a rate about 10-fold higher on an area basis than any other wetland ecosystem due to high sedimentation rates, high soil carbon content, and constant burial due to sea level rise.

In India, possibly, the first ever study made by Bhattacharyya et al. (2000) about a decade back showed SOC pool in two soil strata under different physiographic regions including coastal areas. The data based on soil analyses covering 43 soil series showed the SOC data varied from 2.4 Pg to 10.9 Pg from 30 cm to 150 cm soil depth. It will be prudent to concentrate on elaborate studies in future on monitoring SOC pool in different soil strata in coastal areas over a long period of time, and  relate  them  with  sea  level  rise,  extent  and  nature  of   land submergence with water, seawater quality, extent and nature of vegetative cover, relevant soil and climatic parameters, nature and amount of agricultural, industrial and city effluents discharged into the sea, and any other anthropogenic factors of the locality likely to influence SOC, etc. It should also be possible to create databank on SOC and related factors of the past using radiocarbon dating.

  

Sedimentation and Erosion

The dynamics of alluvial landscapes and natural sedimentation patterns that determine the nutrient and energy flows in coastal areas are increasingly being modified by human activities, in particular those that affect water flows (dams, increased water extraction, deviation of rivers) and erosion, especially due to deforestation. This prevents or slows down vertical accretion, thus aggravating salt water intrusion and impairing drainage conditions in riverine, delta or estuarine areas. It reduces or blocks sediment supply to the coast itself, which may give rise to the retreat of the coastline through wave erosion. Beach erosion is a growing problem and affects tourism revenue, especially in island nations. In the Caribbean, as much as 70 percent of beaches studied over a ten-year period were eroded.

Eutrophication, Hypoxia, Dead Zones and Nutrient Cycle

The urban developments are taking up fertile agricultural land and leading to pollution of rivers, estuaries and seas by sewage as well as industrial and agricultural effluents. In turn, this is posing a threat to coastal ecosystems, their biological diversity, environmental regulatory functions and role in generating employment and food. Overuse of fertilizer can result in eutrophication, and in extreme cases, the creation of ‘dead zones’. Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast. As the microscopic plants die and sink to the ocean floor, they feed on bacteria, which consume dissolved oxygen from surrounding waters. This limits oxygen availability for bottom-dwelling organisms and the fish that eat them. In dead zones, huge growths of algae reduce oxygen in the water to levels so low that nothing can live. There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to a study undertaken by the Virginia Institute of Marine Science. Those numbers were up from 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s. In the 1910s, only four dead zones were identified (Minard, 2008). Hypoxia in the Northern Gulf of Mexico, commonly named as the 'Gulf Dead Zone', has doubled in size since researchers first mapped it in 1985, leading to very large depletions of marine life in the affected regions (Portier, 2003). He studied changes in microbial communities as a result of oxygen depletion, the potential contribution of increasing hypoxia to marine production and emission of N₂O and CH₄, and the effect of hypoxic development on methyl mercury formation in bottom sediments.

Anthropogenic sources Annual release of fixed nitrogen (teragram) Fertilizer 80 Legumes and other plants 40 Fossil fuels 20 Biomass burning 40 Wetland draining 10 Land clearing 20 Total from human sources 210 Natural sources, viz. Soil bacteria, algae, lightning, etc. Total from natural sources 140 Source: World Resources Institute (2006)

Table 2. Global sources of Biologically Available (Fixed) Nitrogen

The World Resources Institute reported that driven by a massive increase in the use of fertilizer, the burning of fossil fuels, and a surge in land clearing and deforestation, the amount of nitrogen available for uptake at any given time has more than doubled since the 1940s. In other words, human activities now contribute more to the global supply of fixed nitrogen each year than natural processes do, with human-generated nitrogen totaling about 210 million metric tons per year, while natural processes contribute about 140 million metric tons (Table 2). This influx of extra nitrogen has caused serious distortions of the natural nutrient cycle. In some parts of northern Europe, for example, forests are receiving 10 times the natural levels of nitrogen from airborne deposition, while coastal rivers in the Northeastern United States and Northern Europe are receiving as much as 20 times the natural amount from both agricultural and airborne sources (Coastal Wiki, 2008).

Climate Change

Destruction of habitats in coastal ecosystem is caused by natural disasters, such as cyclones, hurricanes, typhoons, volcanism, earthquakes and tsunamis causing colossal losses worldwide. Each year an estimated 46 million people risk flooding from storm surges. Ironically, the frequency of natural disasters is increasing with time, almost exponentially, due to climate change, as sea level rise also follows almost the similar trend (Sen, 2009). Coasts in many countries, therefore, increasingly face severe problems on account of sea level rise as a consequence of climate change (Fig. 4), leading to potential impacts on ecosystems including damage to reefs or move large amounts of bottom material, thus altering habitat, biological diversity, and ecosystem function. The worst scenario projects sea level rise of 95 cm by the year 2100. It is projected, as extreme cases, the majority of the people who would be affected in different countries are China (72 million),  Bangladesh (13 million people and loss of 16 percent of national rice production), and Egypt (6 million people and 12 to 15 percent loss of agricultural land), while between 0.3 percent (Venezuela) and 100 percent  (Kiribati and the Marshall Islands) of the population are likely to be affected (Pachauri, 2008a,b). In India, potential impacts on 1 m sea level rise might lead to inundation of 5,763 km² of land including Ganges-Brahmaputra delta facing flood risks from both large rivers and ocean storms.

Population Growth as the Driver

Rank Event Location Death toll 1 1931 China floods China 2,000,000-4,000,000 1887 Yellow River Flood China 900,000-2,000,000 3 1970 Bhola cyclone Bangladesh 500,000 4 1839 India cyclone India ≥ 300,000 5 2004 Indian Ocean tsunami Indian Ocean 229,866

Table 3. List of 5 deadliest natural disasters on the Coast (Source: Wikipedia, 2009)

Fig.4. Global average sea level rise  since 1961 at an average rate of 1.8 mm per year and since 1993 at 3.1 mm per year (Source: Pachauri, 2008b)

Apart from climate change population growth is possibly the single most factor, other than those directly or indirectly related to crop production, impacting livelihood in the coastal ecosystem. Around the world maximum people die of drowning by storm surge. It is just astonishing to note that in the cyclone of 1970 that struck Bangladesh more than 300000 people met a watery grave. Similar things happen in Australia too, but casualties were less because of lesser density of population on the vulnerable areas (Joshi, 2007). A list of 5 deadliest natural disasters on the coast is shown in Table 3.  It has been projected that number of people living within 100 km of coastlines will increase by about 35 percent in 2050 as compared to that in 1995. This type of migration will expose 2.75 billion people to coastal threats from global warming such as sea level rise and stronger hurricanes in addition to other natural disasters like tsunamis (Goudarzi, 2006). In another estimate (Schwartz, 2005), the expected change of the population (or population density) from 2000 to 2025 regionwise shows increase in almost each coastal area. The  estimates (population within 100 km of the coastline) show increase by 25 % in Asia (except Middle East), 52 % in Middle East and North Africa, 81 % in Sub-Saharan Africa, 20 % in North America, 31 % in Central America and Caribbeans, and 32 % in each South America and Oceanea, while there may be decrease by 2.5 % in Europe. In India, according to the Department of Ocean Development, there are 40 heavily polluted areas along the Indian coast (Dubey, 1993).

Conclusion

Although management of salt affected soil catches immediate attention of all concerned for augmenting productivity in the coastal ecosystem, the various ecological factors discussed above, to speak the least, besides a few others, like under-sea tectonic movement along with off-shore and on-shore protection measures required to be undertaken, demand that it should be mandatory to give a holistic look to their interaction matrix, and not the management of the salt affected soils alone, to ensure lasting stability of the ecosystem. 

Acknowledgement

Late Dr. J.S.P.Yadav, to whom the symposium is dedicated, has been pioneer in drawing the attention of the nation to the problems of the coastal ecosystem and guiding on formulating future research agenda on a variety of issues for augmenting the productivity in agriculture. The present paper reflects his thoughts and philosophy that the authors owe in presenting their views, and the nation will remain deeply indebted in translating his vision on this multiple-constrained area into action.         

References

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