Fernandes, Dearlyn, Wu, Ying, Shirodkar, Prabhaker Vasant, Pradhan, Umesh Kumar, Zhang, Jing, and Limbu, Samwel Mchele
The preservation process of organic matter (OM) in estuarine environments determines the recycling and sinking of nutrients. This process requires the identification of sources, degradation states and the main processes affecting OM transformations. Unfortunately, our understandingof the sources, degradation and factors affecting OM distribution in tropical rivers experiencing strong seasonality and monsoonal influence is still limited. This study examined the sources, degradation and factors affecting OM distribution along the Narmada River and its estuary during different seasons. Surface waters and sediments were analyzed seasonally for selected physico-chemical parameters and bulk compositions of sediments, together with amino acids (AA, including the bacterial biomarker, D-AA). The sources of OM were soils containing detrital terrestrial plant material, with C 4 and C 3 plants dominating the estuarine and riverine stations, respectively. The other sources of OM were in-situ production, together with bacteria and their remnants. Strong seasonality and monsoonal conditions control the sources and distribution of OM in the river. Higher concentrations of total hydrolysable amino acids (THAA) were observed in riverine stations, suggesting the presence of relatively fresher OM. The lower OC:SA ratios recorded in the estuarine sediments indicated a limited OM preservation in the studied river. Positive degradation index (DI) values were obtained during the pre-monsoon season, suggesting seasonal changes in OM diagenesis. Physical (strong tidal currents, rainfall, reduced water flow due to seasonal variations and shallow water depth within the estuary) and geochemical (mineral surface adsorption processes) factors control the distribution and transport of OM. Taken together, the sources, preservation and diagenesis of terrestrial OM along the Narmada River was controlled differentially by the strong seasonal variability of the region. Thus, under variable temporal conditions, tropical estuaries and rivers form important realms for examining, determining, evaluating and assessing OM in order to better interpret nutrient budgets of the seas and oceans. Heat map of the extracted principal components (labels on the y-axis represent the principal components and the labels on the x-axis represents the different parameters; the values on color bar 1 - red, 0 - black, and −1 - green indicate the loadings). Unlabelled Image • The C 3 and C 4 plants and in-situ production contribute to OM in the river. • Microbes, tides and hydrodynamics influence the preservation of OM in the river. • Physical and geochemical factors control the transport and distribution of OM. • The degradation index experiences seasonal shifts during different seasons. [ABSTRACT FROM AUTHOR]