BIOMARKER RECORD OF ORGANIC MATTER INPUT AND ECOLOGICAL CHANGES IN THE PICHAVARAM MANGROVE COMPLEX, SOUTHEASTERN INDIA

The nature and distribution of lipid biomarkers (alkanes, alkanols, sterols and triterpenols) were investigated in five dated sediment cores from the Pichavaram mangroves in order to 1) identify the organic matter (OM) sources and its preservation, and 2) trace recent ecological changes associated with coastal processes and anthropogenic activities. The mangrove sediments have higher biomarker concentration than the estuarine extracts. Triterpenols are the dominant biomarker in both estuarine and mangrove sediments, and constitute >50% of the total lipid extracts. The presence of n alkanols (n-C26,28,30), and abundance of phytosterols (β-sitosterol and stigmasterol), and triterpenols (taraxerol, β-amyrin, germanicol, and lupeol) indicate that mangrove vegetation is the primary source of sedimentary OM. The high abundance and unimodal distribution of the long-chain n-alkanes (n-C25,27,29), and high values of Carbon Preference Index (CPI >8) and Terrestrial Aquatic Ratio (TAR 2.4 to 41) indicate dominance (and better preservation) of plant derived OM. In contrast, bimodal distribution of n-alkanes, low CPI (0.75 to 0.90) and TAR (1.9 to 5.7) values, and the ternary plot of sterols indicate that phytoplankton/ algal derived OM is more pronounced in estuarine sediments. Diagenetic changes in OM indicate that some of the lipid fractions are reactive, and they degrade more rapidly than bulk OM. The distribution of these compounds follows first order decay kinetics, and concurs with the down core diagenetic changes. Resistance to degradation among the different lipid classes show the trend: n-alkane > sterol > triterpenol > n-alkanol. Finally, the study indicates that less freshwater discharge from the Coleroon River is causing a gradual change in mangrove ecology. There is less input of OM derived from mangrove vegetation. Biomarker trends also indicate that less salt tolerant Rhizophora spp. are gradually replaced by more salt tolerant Sueda spp. and Aviccenia spp.