Sources and distribution of organic matter in thirty five tropical estuaries along the west coast of India-a preliminary assessment

Studies characterizing the sources of organic matter (OM) to the west coast of India (WCI) and its continental shelf are limited. This study examined sedimentary OM in 35 estuaries along the WCI using molecular biomarkers (lignin phenol), elemental ratio (C/N), and stable carbon isotope (δ13C) values. Multivariate statistical techniques, such as cluster analysis, identified similar sedimentary chemical properties among the estuaries and their distribution patterns highlight the strong control of geographical provenance on sedimentary OM composition from south to north along the WCI. Results of an end-member mixing model reveal that terrigenous sources (C3 plants, C4 plants, and soil) contribute ∼80% of estuarine OM, with the remaining 20% derived from marine sources (marine plankton and estuarine macrophytes). In the estuaries of large rivers, such as the Narmada and Sabarmati rivers, C4 plants and soil OM were found to be the dominant contributors of OM, which is likely the result of an abundance of C4 vegetation and agriculture in their catchment areas. High OC (organic carbon content) of sediments (0.5–5%) from the WCI estuaries indicates that large amounts of OM are present in the sediments. The sources of OM (plant and soil) shift substantially throughout the study area, corresponding to changes in land use patterns along the Western Ghats. Sediments with low nitrogen contents (C/N > 15−20) and degraded lignin ((Ad/Al)V = 0.4–0.6 and DHBA/V = 0.16–0.34) were observed in all estuaries, indicating humification and/or degradation of OM originating from terrestrial plants (bio-degradation) and soil (de-mineralization). The collective results of this study illustrate the benefits of using biomarkers (lignin phenols) along with C/N and δ13C values for evaluating land use changes and the impacts of land use changes on aquatic ecosystems.