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.
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.
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.