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Sundarban mangroves

SCIENTIFIC RESEARCH INITIATIVE

Hydrodynamics

Each mangrove is unique due to influence of local physical factors and biological processes that dictate local mangrove ecology. While some mangroves can be more influenced by the river input, others, as in fringed forests, are more dictated by sea. This makes understanding local abiotic factors and physical environment dictated ecological processes important to understand Sundarban mangroves ecosystems.

Hydrodynamics in mangroves is controlled by – 1) its tides, 2) mangrove vegetation and 3) geometry of mangrove waterways. Friction from mangrove vegetation have been found to cause strong ebb current, asymmetric flood and ebb tide and lag in the tidal phase between upper and lower creeks in mangroves in south-east Asia(Thailand). Channel geometry and bathymetry causes lateral or vertical variations in channel velocities. These variations in water velocity cause vertical shear stress which ultimately affect mixing processes in creek water.

Residence time

Mangrove creeks are important routes for tidal exchange of dissolved and particulate nutrients between the forest environment and adjacent coastal water. Residence time(the average length of time during which a substance, a portion of material, or an object is in a given location) in creeks is one important indicator that dictate fate of nutrients in water column. Long residence time allows nutrient uptake by vegetation e.g. phytoplankton and mangrove trees whereas short residence time export materials to the ocean. Residence time in mangrove forest vary from few days to months determined by topography, size and type and thus hydrodynamics.

Lateral trapping is significant in determining residence time in the estuary. Depending on configuration of mangrove swamps, tropical dry season water can be trapped within estuaries for 2-8 weeks. Even during wet season, it is possible for floodwater to be trapped in upper reach of estuaries and creeks. Near the mouth on the other hand, mixing is rapid preventing such trapping.

Tides

Catchment size, estuarine geomorphology, tidal range and rainfall patterns are important in determining water mixing and  nutrient distribution in the estuaries and hence the biota as well. Salinity at wet season outside estuaries can be completely reversing during dry season when salinity gets high in trapped water in creeks as evaporation continues. Thus biota is exposed between 0 >– 35% salinity. Horizontal and vertical mixing is also crucial for pH and inorganic nutrient concentration and oxygen concentrations. Creeks in the upstream often have higher residence time, where literfall decomposition lower pH and deplete oxygen. Besides increased bacterial population can also lead to lower oxygen concentration.

At present, the Hooghly and Meghna rivers are the only perennial source of freshwater directly feeding Sundarbans. All other rivers indicated in the figure are tidal inlets of brackish water of the Bay of Bengal. Due to the eastward subsidence  of the Sundarbans, a substantial reduction in freshwater flowing into western part of the forest and a natural west-to-east salinity gradients across the delta. As a consequence, Heritiera, the principle species of the Sundarbans, went extinct form Indian part of Sundarbans. A recent investigation by Elission et al. 2000 found no clear zonation in vegetation in the mangroves. The study nevertheless, recorded a tidal amplitude throughout the Sundarbans is recorded as 3-4 m. Information on daily local tides are available online.

As a report from IUCN(1997) recollects:

The land is molded by tidal action, resulting in a distinctive physiography. An intricate network of inter-connetcing waterways, of which the larger channels of often a mile or more in width run in a generally north-south direction, intersects the whole area. Innumerable small khals drain the land at each ebb. Rivers tend to be long and straight, a consequence of the strong tidal forces and the clay and silt deposits which resist erosion. Easily eroded sands collect at the river mouths and form banks and chars, which are blown into dunes above the high-water mark by the strong south-west monsoon. Finer silts are washed out into the Bay of Bengal but, where they are protected from wave action, mud flats form in the lee of the dunes. These become overlain with sand from the dunes, and develop into grassy middens. This process of island building continues for as long as the area on the windward side is exposed to wave action. With the formation of the next island further out, silt begins to accumulate along the shore of the island and sand is blown or washed away (Seidensticker and Hai, 1985). Apart from Baleswar River the waterways carry little freshwater as they are cut off from the Ganges, the outflow of which has shifted from the Hooghly-Bhagirathi channels in India progressively eastwards since the 17th century. They are kept open largely by the diurnal tidal flow (Seidensticker and Hai, 1985).

References
Ellison, A.M., Mukherjee, B.B. and Karim, A., 2000. Testing patterns of zonation in mangroves: scale dependence and environmental correlates in the Sundarbans of Bangladesh. Journal of Ecology, 88(5), pp.813-824.

IUCN. 1997. Sundarbans Wildlife Sanctuaries Bangladesh. http://ecnphlgnajanjnkcmbpancdjoidceilk/content/web/viewer.html?file=http%3A%2F%2Fwhc.unesco.org%2Farchive%2Fadvisory_body_evaluation%2F798.pdf

Holmer, M. 2009. Mangroves of South-east Asia. In Black, K. and Shimmield, G. eds. Biogeochemistry of marine systems. John Wiley & Sons. p.1-31.

Seidensticker, J. and Hai, A. 1985. The Sundarbans wildlife management plan: conservation in the Bangladesh coastal zone. World Wildlife Fund, Switzerland.


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Survival strategy : Salt exclusion at mangrove roots

Problem – extreme salinity around root zone

Mangroves prefer soils rich in clay – impermeable clay, but not too much of it! As one can understand, clays are mostly tiny particles, packed closely and tightly with each other. It makes water movement through rhizosphere(root zone) difficult which makes these soils poorly drained. Since water movement is not easy under such condition, Mangroves find it difficult to take up water for themselves? Well, that is a challenge!

The strategy to overcome the extreme:

The answer is  convection current. Red mangroves create a convection current to pull water (and salt along with it)from soil to roots. But they simply cannot take salt as well because too much salt inside would kill them! To overcome that, they leave salt outside on root surface(salt exclusion) as they continue to seep in freshwater. Mangroves have special  barrier formation that let Na+ and Cl- ions(figure below) from salts stay outside root membrane instead.

However, that leads to building up of a hypersaline condition next to mangroves roots which is bad for them as well! The saline condition is even more exaggerated by the low flow rates which is common for these impermeable soils. How that extra-salt is removed from their rootzone and role of crabs in that will be discussed in our future posts.

That is how my friend, mangroves, fixed on its position, keep manipulating living conditions (salt and water)around it and beyond, for itself and you and me and all!

regulatingsaltlevels
Mechanism of salt exlusions at roots by mangroves

 

Reference:
McGowan, Kelly T., and Jonathan B. Martin. "Chemical composition of mangrove-generated brines in Bishop Harbor, Florida: Interactions with submarine groundwater discharge." Marine chemistry 104.1 (2007): 58-68.

Soils of Sundarban mangroves

Soils of the Sundarbans mangrove forest differ from other inland soils in that they are subjected to the effects of salinity and waterlogging, which naturally affect the vegetation.

  • In places soils are semi-solid and poorly consolidated.
  • pH ranges widely from 5.3 to 8.0.
  • Texture: soil is in general medium textured; sandy loam, silt loam or clay loam. The grain size distribution is highly variable. Silt loam is the dominant textural class. However, Ray et al. 2014 reported a 72-87% silt content in 0-60cm depth for Indian part of Sundarbans.
  • Porosity: 0.7 (Ray et al. 2014)
  • Sodium and calcium contents of the soil vary from 5.7 to 29.8 meq/100g dry soil, respectively, and are generally low in the eastern region and higher towards the west.
  • Available potassium (K) content of the soil is low, 0.3-1.3 meq/100g dry soil.
  • Organic matter content varies between 4% and 10% in dry soil.
  • Soil salinity increases from 5ppt in east (slight to moderate) to 30ppt in west (highly saline), but the salinity is not uniform from north to south throughout the forest.
  • Geology: the area is the result of extensive fluvio-marine deposits of the river Ganges and the Bay of Bengal and the character of the sediment is silty clay. Rajkumar et al. (2012) reported the dominant association of kaolinite with subordinate amount of quartz, illite and chlorite in the clay minerals in the Sundarban mangrove core sediment. The adsorption capabilities result from a net negative charge on the structure of minerals, high surface area and high porosity (Alkan et al. 2004).

 

Reference:

[Source: http://wikimapia.org/1651148/The-Great-Sundarbans-Bangladesh-and-India]


Alkan, M., Demirbas, O., Celikcapa, S., Dogan, M., 2004. Sorption of acid red 57 from aqueous solutions onto sepiolite. J. Hazard.Mater. 116, 135–145.

Rajkumar,K., Ramanathan,A.L., Behera,P.N., 2012. Characterization of clay minerals in the Sundarban Mangrove River sediments by SEM/EDS. J. Geol. Soc. India 80, 429–434.

Ray, R., Majumder, N., Das, S., Chowdhury, C., & Jana, T. K. (2014). Biogeochemical cycle of nitrogen in a tropical mangrove ecosystem, east coast of India. Marine Chemistry, 167, 33–43. http://doi.org/10.1016/j.marchem.2014.04.007

 

 

Crabs: the mangrove ecosystem engineers

More I get to know what crabs do for mangroves, the more I find it amusing, sometimes even hilarious. These little guys (gals) probably have no idea how they are helping mangrove ecosystems while “minding their own business” and that is what make them the ecosystem engineers of mangroves!

Crabs are the major macrofauna, ecologically engineering the mangroves through digging burrows. Two major species whose names keep coming up are Sesermids(Grapsidae) and fiddler crabs(Ocypodidae); the former being ubiquitous to any mangroves in the Indo-West pacific. A major part of their diet consist of leaf litter. Without considering their contribution any calculation of trophic detritus web and detritus export from mangroves will be incomplete. But what is it that these species do that make them so important?!

Neosarmatium smithi
Family: Sesarmidae

Crabs dig burrows in the sediment to seek refuge from predation and environmental extremes which the mangrove is renowned for! Basically they open up windows of opportunities through burrows for both physical and chemical processes and interaction between groundwater, substrate and nutrient dynamics.

Burrows not only are their shelters but affect sediment topography and biogeochemistry by modifying particle size distribution, drainage, redox conditions and organic matter as well as nutrient availability1,2 and food storage3,4,5. When talking about their food, these herbivores literally retain, bury, macerate and ingest litter and macroalgal mats6,7,8. 

These process not only prevent nutrient loss but further encourage decomposition. Crabs literally increase nutrient use efficiency by removing up to 75% litter from forest floor. Thus they play critical role in nutrient cycling by influencing nutrient efficiency and availability to other mangrove flora, fauna and microbes. However as ecosystem effects of crabs are caused by non-trophic activities, they have earned the reputation of the “ecosystem engineer”. They literally alter physical structure, transport condition and substance chemistry9. 

smithi_leaf_bur
Family: Sesarmidae

However, in the Indo-pacific region, predatory behavior of crabs on mangrove propagules has also been observed as a threat for mangrove flora. The problem is severe more for managed mangroves (in Malaysia); in fact, predation was found responsible for complete failure of A. marina in northeastern Australia (Smith 1992). It demands similar investigation for Sundarbans mangroves as well.

nevertheless, ecoengineering role of crabs has a lot to discuss and yet more to investigate into. I intend to do the former in near future and the later, well, am sure the scientists are not giving it any rest 😉

Ciao.

 

References:
1.Mouton, E.C., Felder, D.L., 1996. Burrow distributions and population estimates for the fiddler crabs Uca spinicarpa and Uca longisignalis in a Gulf of Mexico salt marsh. Estuaries 19, 51–6.
2. Botto, F., Iribarne, O., 2000. Contrasting effect of two burrowing carbs (Chasmagnathus granulata and Uca uruguayensis) on sediment composition and transport in estuarine environments. Estuar. Coast. Shelf Sci. 51, 141–151.
3. Giddins, R.L., Lucas, J.S., Neilson, M.J., Richards, G.N., 1986. Feeding ecology of the mangrove crab Neosarmatium smithi(Crustacea, Decapoda, Sesarmidae). Mar. Ecol., Prog. Ser. 33, 147–155.
4. Warren, J.H., 1990. The use of open burrows to estimate abundances of intertidal estuarine crabs. Aust. J. Ecol. 15, 277–280.
5. Dittmann, S., 1996. Effects of macrobenthic burrows on infaunal communities in tropical tidal flats. Mar. Ecol., Prog. Ser. 134, 119–130.
6. Emmerson, W.D., McGwynne, L.E., 1992. Feeding and assimilation of mangrove leaves by the crab Sesarma meinerti Deman in relation to leaf-litter production in Mgazana, a warm temperate southern African mangrove swamp. J. Exp. Mar. Biol. Ecol. 157,41–53.7. Kristensen, E., Alongi, D.M., 2006. Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron and sulfur biogeochemistry in mangrove sediment. Limnol. Oceanogr.51, 1557–1571.
8. Lee, S.Y., 1997. Potential trophic importance of the faecal material of the mangrove sesarmine crab Sesarma messa. Mar. Ecol., Prog.Ser. 159, 275–284.
9. Kristensen, E., 2008. Mangrove crabs as ecosystem engineers; with emphasis on sediment processes. Journal of Sea Research, 59(1-2), 30–43. http://doi.org/10.1016/j.seares.2007.05.004

The eastward subsidence: its consequences

Sundarban mangroves stands on a still young and developing Bengal basin delta. Subsidence of the delta is not uniform. Tectonic activity and uneven rate of sediment deposition have caused eastern side of the delta subside more rapidly then the western portion. The only perennial source of freshwater to the forest has significantly experienced shifts in drainage pattern. The west-to-east tilt in the forest caused more freshwater flow through the lower eastern portion; eventually a natural west-to-east salinity gradient across the delta is observed.

 

Ref:
Deb, S.C. (1956) Palaeoclimatology and geophysics of the Gangetic Delta. Geological Reviews of India 18: 11-18.

We know less about Sundarabans than Saltmarsh

If one looks close enough into the global distribution of mangroves and salt marshes, one would not fail to notice the greater areal extent of the mangroves. Now, both these ecosystems are my favorite. So this discussion is not to set a rivalry between these two. Yet, with little digging into literature into the system, anyone will also note that more scientific studies involve salt marshes than we know our Sundarban mangroves.

One might argue that how scientific studies on Sundarbans can even be compared to studies on salt marsh since Sundarbans is only part of the global mangroves; but I can’t stop lamenting over the dearth specially due to the the greater biodiversity of the Mangrove than salt marsh.

Limited research funding could be one of the top reason, but as I always argue, good intention and passion towards this interestingly diverse system could appease much of that lack. In fact, I firmly believe, a sound-economic study on Sundarbans is not impossible.

Initiative perhaps remain the greatest hurdle!

Sundarbans in news

  • Dec, 2015:  “If operations should ever start at Rampal, it’s estimated that it will be burning 4.72 million tons of coal per year – that’s not only bad news for the climate but will also require a major spike in cargo shipping right through some very vulnerable ecosytems.” Read more…
    
    

Habitat function of Sundarban mangroves

Who wants a damp damp place for home? Specially  when that goes under water twice a day, never completely dries out leaving only muddy, uneven surfaces to step on? Doesn’t sound encouraging, right? Well, many actually a lot of species just call that home because it provides them shelter – exactly the criteria one seek in “home”!

Mangroves is one of the most unique habitat ecosystem where a rich biodiversity of mangrove flora and fauna have thrived under environmental conditions considered challenging for most life forms on the planet and hence, unique indeed!

Mangroves are mainly the vegetation that occurs in inter-tidal zone – between land and sea – life determining factors like temperature, sediments, tidal currents are highly variable. Life as we know is uncertain here, since the prerequisite to any living form, i.e. oxygen, is limited. These tidal swamps are under water twice a day. Life would be next to impossible…if life had not adapted to the conditions. Substrata like aerial roots of mangroves take hold on the dumpy soil to support life; while mangrove fauna find shelter within these tangled roots and build up a mutual interaction which forms the fundamental of the mangrove biogeochemical cycle.

Sundarban is a unique habitat for its rich biodiversity; above the water, mangrove canopy is home to birds, mammals, reptiles while below the water, bivalves, sponges, algae overgrow its roots. In fact, Sundarbans the world’s only mangrove with tiger. The Royal Bengal Tiger have adapted to an almost amphibious life, being capable of swimming for long distances and feeding on fish, crab and water monitor lizards.

Mangroves themselves are inhabited from both inside and outsides, whereas spaces between mangrove roots are home to prawns, crabs, fishes. Plankton, microbenthos and invertebrates of Sundarbans are at the primary trophic level. Very few studies have attempted to understand their dynamics which is undoubtedly crucial to understand the nutrient fluxes and dynamics of the forest.

Considering its high food abundance yet low predation pressure, many species spend a part or all of their life cycles. Imagine, if mangroves disappeared, fingerlings that grow in its shelter will easily fall pray to predators instead of growing and reach maturity. Thus mangroves act as nursery habitats for commercial fisheries for FREE and support offshore fisheries. 1

Reference

  1. Nagelkerken, I. et al. The habitat function of mangroves for terrestrial and marine fauna: A review. Aquat. Bot. 89, 155–185 (2008).

Ecosystem services of Sundarbans

The islands in Sundarbans are of great economic importance as a storm barrier, shore stabilizer, nutrient and sediment trap, a source of timber and natural resources, and support a wide variety of aquatic, benthic and terrestrial organisms. It support livelihoods of the localities.

Provisioning service

Fishing and timber are among the major provisioning services in the Sundarbans. Wax and honey (by Apis dorsata bees), raw material for paper industry are also collected from the forest. Annual harvest of honey and wax are 185,000 kg and 44,400kg, respectively. The Sundarbans is an important source of fuel wood for the locals as well as the distant market. Ceriops decandra, Cynometra ramiflora, Amoora cuculatta, and Hibiscus tiliaceus which thrive under story of of the forest, are used as fuelwood. Fuelwood are also collected from the branches and twigs of taller trees as Heritiera fomes, Avicennia officinalis, Sonneratia apetala and barks of Excoecaria agallocha. C. decandra, are more abundant in the western part, are high in calorific value; its barks are reach in tannin which is locally extracted to  dye fishnets.

Leaves of Nypha fruticans are extensively used in thatching roofs of local households. Timber from Phoenix paludosa is used to build house posts, jetties and rafts.

Unlike many of the mangroves of the world, The Sundarbans is rich in floral and faunal biodiversity. The forest has about 70 plant species, 55% of which are true mangroves.

Regulating service

Coastal protection and habitat function are the major ecosystem services emphasizing on the need for its sustainable management. A natural belt, Sundarbans is often the first to face and minimize the rigorous winds and waves during cyclones saving both life, properties and its biodiversity. Nutrient, pollutant and sediment regulation of the mangroves is worth studying which offer benefit of millions of dollars if not billions.

Cultural service

Tourism is one of the rising economic activities in the area in recent years which is a major threat as well. Considering declining and threatened mangroves around the world, Sundarbans holds huge potential for scientific community and hence the global community.

 

References:
de Lacerda, L. D. (2002). Mangrove ecosystems: function and management. Springer Science & Business Media.

 

 

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