Mangrove habitat dynamics in response to Holocene sea level and climate changes along southwest coast of India

Abstract

Pollen signatures, palaeobotanical data and offshore peat indicated that the west coast of India harbored an excellent cover of mangroves until the Neogene. However, the mangrove area has shrunken considerably since then, and it is now confined largely to river mouths and tidal creeks of Kerala–Konkan basin. Sea levels and geomorphologic modifications along with climate variability have brought significant changes in the hydrological regimes which in turn have altered the habitat of the mangroves considerably. Evolution of the coastal landforms, especially the modification of the wetlands and loss of sheltered bays, has affected the mangrove habitats to a greater extent that eventually led to displacement of certain core mangrove taxa. The palynological and chronological data revealed that the mangroves of southern west coast are essentially of Middle Holocene age (7220–3880 yrs BP). Such mangrove development seems to be of global significance, as Mid-Holocene establishment of mangroves has been well recorded elsewhere when sea level stabilized. Stability of mangrove development coincides with the Holocene climate optimum (HCO) when Monsoon Asia witnessed heavy precipitation. However, the mangroves responded to environmental changes due to freshwater influx controlled by monsoon rainfall and sea level changes, leading to change in species composition as revealed by their pollen signatures. The decline of mangroves since 3500 yrs BP and further degradation except at a few sites have been attributed to the prevailing arid climate and weakening of monsoon until 1500 yrs BP. Mangrove development and sustenance in fringe areas of West Kallada along the Kerala coast may be due to local hydrological regime, with steady flow of fresh water and gradual development of deltaic conditions. As such, this area has excellent potential for the rehabilitation of mangroves. Human activity resulted in probable environmental stress for mangrove cover along the North Konkan coast, while the positive trend in emergence of mangroves in the least disturbed areas of South Konkan may be attributed to strengthening of summer monsoon in the recent past.