Organic matter, nutrients and major ions in the sediments of coral reefs and seagrass beds of Gulf of Mannar biosphere reserve, southeast coast of India

Comparative investigations have been made to study the distribution pattern of organic matter, nutrients and major ions in the Gulf of Mannar biospher e reserve ecosystem. Sediment samples have been collected during January 19 96 from six islands (Shingle, Kurusadai, Kori, Pumarichan, Manauli and Hare ) of this region covering coral reef zone, seagrass bed and adjacent areas. Although textural composition of sediment samples showed predominantly san dy, coral bed areas were found to have greater percentage of coral stones a nd shell fragments and with very low silt and clay fractions. About 0.5 to 40 % by dry weight of the sediments were made up of Ca derived from the cor al and molluscan organisms, due to weathering processes. The coral reef sed iments recorded low salinity and higher pH compared to seagrass bed and adj acent areas. Total organic carbon (TOC) content (1.38 to 9.11 mg/g) of the reef sediments were low when compared to the seagrass bed and adjacent area s. Higher concentrations of total nitrogen and total phosphorus were found in the coral reef zone and seagrass bed revealed that these areas may be vi ewed as a sink for nutrients, whereas the adjacent areas act as reservoir f or nutrients. The contribution of seagrass and reef associated organisms pl ay an important role in the recycling of nutrients in these environments. T he seagrass bed sediments recorded higher Na and K concentrations than the coral reef zone and adjacent areas could be due to the utilization and trap ping of these ions by seagrass and associated organisms.