2.1. Biodiversity The Sundarban world heritage is endowed with high biodiversity of aquatic and terrestrial flora and fauna. Of the 50 true mangrove plant species recorded throughout the globe, the Sundarbans alone contain 35 species, with dominance of Sundry (Heritiera fomes), followed by Gewa (Excoecaria agallocha). Several species are endemic such as Aegialitis rotundifolia, H. fomes, Sonneratia apetala, and S. griffithii. A total 334 plant species, 165 algal, 13 orchids, 17 fern, 87 monocotyledon, 230 dicotyledon, including 35 legumes, 29 grasses, 19 sedges, and 18 euphorbias, have been discovered from the Sundarban regions (Rahman and Asaduzzaman, 2008). The mangroves are also associated with flowering plants, palms, ferns, bryophytes, fungi, algae, lichens and bacteria. Bacteria are the most abundant organisms in the estuary, averaging between 106 to 107 per ml organisms in water and 108 to 1010 per gram dry weight of sediment (Lara et al., 2011). Phytoplankton community was observed to be dominated by diatoms (Bacillariophyceae) followed by Pyrrophyceae (Dinoflagellates) and Chlorophyceae. A total of 46 taxa belonging to 6 groups including the above dominants, and also Cyanophyceae, Euglenophyceae and Chrysophyceae have been recorded (Manna et al., 2010). The faunal biodiversity of Sundarbans includes 215 species of fishes, 7 species amphibian, 59 reptiles, more than 200 birds, 39 mammals, in addition to numerous species of phytoplankton or algae (>100 species), zooplankton (>100 species), and invertebrates including insects (>300 species), brachyuran crabs (26 species), polychaetes (69 species), molluscs (110 species), and micro-arthropods (44 species) (Chakraborty, 2011). The wetlands not only host large populations of economically important crabs, molluscs, shrimps (20 species), lobsters (8 species) and fishes, but also act as vital nursery grounds and natural sources for shrimp larvae meeting the demand of increasing aquacultures, and 120 species of commercially important fishes harvested by the fisherman. Among the magnificent wild animals the world famous Royal Bengal Tiger, spotted deer, barking deer, wild boars, jungle cats, civet cat, monkey, bengal fox, jackle, many beautiful birds, including heron, stork, erget, pheasant, wood pecker, bee-eater, myna, etc., lizards, dolphins and snakes are important faunal spp (Rahman and Asaduzzaman, 2008).
2.2. Diversity in habitats The major ecosystems of the Sundarban include mudflats, sand beaches, coastal dunes, estuarine networks, shallow creeks and mangrove swamps. Habitat-wise, the Sunderban ecosystems can be broadly categorized into a) mangrove swamps, b) riverine /tidal wetlands, c) agroecosystems, and d) upland/ upstream and low-land /downstream forests. The Bengal Basin deltaic regions of Sundarban have been gradually tilting towards east (Morgan and McIntire, 1959). This has probably caused the main fresh water discharge to shift gradually eastward. In accordance to the variation in the environmental conditions, like salinity, species zonations, particularly due to the succession of mangrove trees from sea towards inland as well as from east to west, are evident. Variations in rainfall and riverine input, in combination with topographic differences including tectonic uplift in the west, are causes of salinity differences, with formation of hypersaline environment in the low-lying central part, and comparatively low saline conditions of the soil in the comparatively elevated regions of the eastern and western parts (Hanebuth et al., 2013). Among the typical mangrove trees, Avicennia marina, A. alba and Bruguiera cylindrica grow in the downstream regions near the shoreline, while Bruguiera sexangula, B. gymnorhiza, Ceriops decandra and Rhizophora mucronata dominate in the central and upstream swamps. In the upstream region nearby the riverine channels certain mangrove species, which cannot withstand relatively high salinity for long time, e.g., E. agallocha, H. fomes, Sonneratia apectala, and S. caseolaris are more common, and flourishing more in the eastern part (Rahman and Asaduzzaman, 2008). Mangroves? soil consists mainly of silt, clay and fine fibrous roots, organic matter may constitute >20% of dry weight (Thong et al., 1993), and only a thin upper portion of it is aerobic. Turnover of nutrients in mangroves heavily depends on the soil type, its oxic and anoxic condition, which in turn depend on tidal inundation regime. However, Avicennia rhizospheres can change the soil environment independent of tidal influence and can translocate oxygen through roots into surrounding soils (Thibodeau and Nickerson, 1986). High soil salinity limits tree density and height, and litterfall influencing primary production and flow of carbon. The litterfall rates have high range of variability (approx. 5 t ha-1 yr-1 to 30 t ha-1 yr-1 ), depending on forest type and settings, and the rate is higher in the riverine regions (Silva et al., 1998a). Moreover, settlement of some mesophytic bioinvasive plant species has enhanced ecological instability manifold of this sensitive eco-region forcing some other plant species such as S. apetala, Avicenia alba and Acanthus ilicifolius to experience landward movement (Chakraborty et al., 2009).
3.1. Root system One of the important functions of mangroves is sediment trapping by their complex aerial root systems, and thus acting as sinks to the suspended sediments and associated organic matters, to aid in coastal land expansion (Twilley et al., 1992). Water velocity within the creek often exceed 1 m s-1 , while within swamp (large area) it rarely reach 0.1 m s-1 due to the frictions from the bed and mangrove roots. The density of prop roots and pneumatophores in a tidally driven mangrove wetland is most important in determining whether the system is eroding or accreting. Mangrove roots are also ideal places for active nutrient turnover. Nitrogen transformation (ammonification, denitrification, nitrification) or nitrogen fixation rate by bacteria are greater in mangrove soils with more plants than soils without plants, including the root system (Routray et al., 1996). Also, higher rates of bacterial sulfate reduction coincide with the presence of underground mangrove root systems (Kristensen et al., 1991). Calcium phosphate is the dominant form of phosphorus in below-ground roots, whilst litterfall turnover plays vital role in phosphorus dynamics as the nutrient is mainly stored in mangrove leaves (Silva et al., 1998b). Inorganic carbon in mangrove sediments is mostly found as FeCO3 and mangrove rhizospheres are active sites for iron precipitation (Alongi et al., 2001).
3.2. Allochthonous input and trapping of nutrients Being a river-influenced mangrove system, the Sundarban is characterized by high influx and strong out-welling of nutrients, which play a vital role on fishery productions in the adjacent coastal waters. During the monsoon, the estuarine regime of Sundarban is influenced by the interaction of the riverine discharge and the tides, which together enhance the seaward drift of the sediments, while during the dry period, with reduced freshwater input, strong tidal currents govern the estuary facilitating the upstream drift of suspended sediments in the wetlands (Rahman and Asaduzzaman, 2008). Sediments in many mangroves are highly traversed by numerous crab holes, making the mangrove soil highly porous, which also facilitates material exchange through tidal water movement. Phosphate, nitrate, and dissolved organic carbon (DOC) in groundwater may become highly concentrated compared to creek water at low tide and during dry season (Lara and Dittmar, 1999). Mixing of surface and groundwater is an important buffer mechanism for nutrient exchange between coastal and mangrove waters. Groundwater is recharged by downward infiltration, a process facilitated by high number of crab holes, during receding peak of the flood tide. Nutrient trapping in pore water can result in the reduction of tidal export, hence, groundwater flows and fluctuations can influence the total nutrient export from the mangrove (Ovalle et al., 1990).