Jyoti Srivastava, Anjum Farooqui, S Hussain
2012 Online) An Online International Journal Available at   unpublished
Salinity in coastal wetland plays a vital role in the distribution of mangrove species, their productivity and growth. It is normally controlled by climate, hydrology, rainfall, topography and tidal flooding. Textural analysis of two sedimentary soil cores (2-5m deep) from Pichavaram mangrove wetland, Cauvery River delta shows the overall predominance of fine clay with intermittent phases of sand. The results show high salinity in the aqueous soil solution of clayey sediment (average 4.0 and
more » ... (average 4.0 and maximum 10.2) facilitating the accumulation of salts in the root zone or at the soil surface when the capillary water evaporates. However, the intermittent sandy zones, within a core have larger pore size which shows lower salinity (average 1.6 and maximum 2.2) perhaps due to percolation of salts through capillary action. The ultimate source of salinity in the mangrove habitat is the seawater ingression which is higher during summer than during monsoon. Restriction of inland water input to estuaries by damming water for agriculture and various other purposes coupled with weak monsoonal pattern allows excess water evaporation from exposed wetlands. This results into salt accretion and increase in salinity. Thus low energy rivers/ streams flowing in gentle relief deposits fine silty/ clayey sediments in the wetlands that act as reservoirs of high salt accumulation which is vulnerable for a mangrove forest. The paper deals with the past sediment depositional environment induced by climatic and relative sea level changes. INTRODUCTION Mangrove wetland is highly productive and occupies the intertidal zone in tropical and subtropical regions, which are characterized by small topographic gradients and large tidal amplitude. The sediment dynamics in such an ecosystem is mainly regulated by both internal (flocculation, dissolution, mixing, etc.) and external (e.g. river input, agricultural runoff, pollution) factors. Mangroves stabilize the coastal zone from erosion by acting as a buffer zone between land and sea. These forests in the sheltered boggy hydro environment make possible the deposition of fine sediments normally enriched with nutrients, metals and minerals. Thus, mangrove sediments play a pivotal role in the biogeochemical processes by behaving as both source and sink for nutrients and other materials. The interactions of mangrove plants and sediments are complex and dynamic as they cope with a harsh saline intertidal environment (Saenger, 2002) despite of degradation by human activities (Klekowski et al., 1994) as well as by natural disturbances (Seralathan et al., 2006). Salinity is one of the important stress factors in a mangrove wetland. A variation in soil salinity in the estuary is due to distance from the coast, tidal incursions and freshwater inputs. Anthropogenic pressures and sea level changes influence the mechanism of saltwater intrusion and freshwater runoff which in turn decides the fate of mangroves in the low-lying deltaic areas along the southeast coast of India. The soil is repeatedly flooded and well drained in the Pichavaram estuary which supports luxuriant growth of mangroves. The composition of mangroves is mainly determined by the tolerance of different species to substrate and saline conditions, which in turn determines the dominance of one or various species at a specific site. At present, most of the estuaries and near shore wetlands along the Southeast coast are covered by salinity tolerant back mangroves like Avicennia and Suaeda species. Most of the areas along the Southeast coast were inhabited by diverse species of mangroves in the past (Farooqui and Vaz, 2000; Farooqui and Achyuthan, 2006) and are either