Distribution and sources of sedimentary organic matter in Chitrapuzha, a tropical tidal river, southwest coast of India

Abstract

ABSTRACTSurface sediment samples were collected from five stations along the Chitrapuzha River, during the monsoon, post-monsoon and pre-monsoon seasons, and analyzed for biochemical composition, total organic carbon (TOC), total nitrogen (TN), and stable isotopic ratios of carbon (δ13C) and nitrogen (δ15N) to identify the major sources of organic matter. Sediment grain size was found to be the key factor influencing the organic matter accumulation in surface sediments. The δ13C values vary between −27.94‰ and −26.05‰ and δ15N values ranges from −1.66‰ to −4.75‰ in surface sediments. The δ13C values indicate terrestrial inputs from higher plants, while δ15N values suggest decomposition processes in the sediments. The fraction of terrestrial organic matter (TrOM) to the total organic matter (TOM) pool ranges from 41 to 84% in the surface sediments as estimated by δ13C-based two end members mixing model. Intermediate values of TOC/TN ratios signal a combined input of both autochthonous and terrestrial organic matter sources, while protein-to-carbohydrate ratios show freshly deposited detritus in the majority of the stations. The threshold levels of protein, biopolymeric carbon, and algal contribution to biopolymeric carbon (BPC) indicate eutrophic conditions prevailing in the study area.