Biochemical and stable carbon isotope records of mangrove derived organic matter in the sediment cores

Abstract

Core sediments from five mangrove ecosystems along northern Kerala Coast were collected to evaluate the nature and sources of organic matter in these ecosystems. General sedimentary parameters (pH, Eh, grain size and total organic carbon) and biochemical constituents (carbohydrate, lipid and protein) were analysed. The protein-to-carbohydrate ratio and lipid-to-carbohydrate ratio were calculated to assess the quality of organic matter in core sediments. Higher concentrations of biochemical components were recorded in surface sediments, with a dominance of carbohydrates followed by lipids and proteins. Protein/carbohydrate ratio was found to be <1 in the entire study region indicating a large content of aged and/or non-living organic matter in mangrove sediments. This also confirms the involvement of heterotrophic microorganisms in the organic carbon dynamics of the study area. The bulk elemental ratio (total organic carbon/total nitrogen) varied between 11.39 and 24.14 in the study region, recording minimum value at Kunjimangalam and maximum at Pappinissery. Samples from Kadalundi recorded low total organic carbon/total nitrogen ratio throughout the core, indicated a marine signature. Stable carbon isotopic ratio (−29.19 to −23.87 ‰) in the sediments suggested the dominance of terrestrially derived organic matter. Principal component analysis revealed that mangrove litter addition, diagenesis and accumulation of organic matter on fine grained sediments are the major processes controlling the distribution of the parameters in the study area.