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The sources of organic matter in three typical mangrove systems of Cochin estuary were assessed for the first time with help of sediment bulk parameters and fatty acid biomarkers. The C/N and δ13C analysis showed intermediate values to that of autochthonous and terrestrial inputs of organic matter, signaling to a mixed origin. The biochemical composition of sedimentary organic matter revealed a dominance of lipids over proteins and carbohydrates, indicating the nutritive or energetic value as well as the freshness of organic matter in all the systems. Fatty acids ranging from C8 to C24 were identified in the study. C20:5n-3 was one of the major fatty acids at station 1, indicating significant benthic phytoplankton contribution. The zooplankton biomarkers C20:1 and C24:1 were also present in appreciable levels, suggesting the tidal influx. Principal fatty acids at station 2 were the zooplankton biomarkers C20:1 and C24:1 implying significant estuarine contribution through tidal influx. The major fatty acids at station 3 were diatom marker C20:5n-3 and the higher plant marker C24:0. Dominant fatty acids in mangrove leaves; C18:2n-6 and C18:3n-3 were also present at station 3. The low contributions of C20:1 and C24:1 confirms the limited tidal exchange at this station. Presence of significant amount polyunsaturated fatty acids in the mangrove sediments confirms the freshness of organic matter. Thus, fatty acid biomarker study of the three mangrove systems of Cochin estuary could substantiate the bulk parameter approaches effectively; distinguishing the multiple sources of the organic matter and the observed spatial variations. [ABSTRACT FROM AUTHOR]

Variability of nature and composition of organic matter in the surficial sediments of mangrove and estuarine systems of Cochin were investigated. Assessment of biochemical composition and elements such as carbon, nitrogen and sulphur were employed for this. Irrespective of the higher content of total organic matter, the labile organic matter was very low in both mangrove and estuarine sediments. Concentrations of biochemical compounds were comparatively higher in mangrove sediments. Total lipids were the dominant class among labile organic compounds in both mangrove and estuarine sediments contributing 51.4% and 45.3%, respectively. Protein to carbohydrate ratio was higher in estuarine sediments when compared to mangroves, indicating low dead organic matter accumulation, probably due to the strong hydrodynamic conditions in estuaries. Correlation analysis showed that sediment texture had no significant correlation with any of the sedimentary parameters in mangroves, whereas in estuaries, it showed significant correlations with most of the sedimentary parameters. Principal component analysis indicated three different dominant processes in mangroves namely mangrove litter addition, diagenesis and other geochemical process like siltation and sorption/desorption, while in estuaries the dominant process seems to be diagenesis. [ABSTRACT FROM AUTHOR]