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Organic matter (OM) such as organic nitrogen plays a substantial role in the global biogeochemical cycling of bio‐reactive components—amino acids (AA) in aquatic environments. Spatial and temporal variations in source, diagenesis, and fate of organic nitrogen such as AA in sediments of small tropical rivers and the role of oxbow/meandering loops under changing climatic conditions are poorly investigated. This study assessed the spatial and seasonal variations in OM composition, source, and diagenesis of a tropical small mountainous river—Netravati River, India, for 1 year. Water samples were determined for suspended particulate matter, and surface sediments were examined for bulk parameters, surface area (SA), and the L‐ and D‐enantiomers of AA. The L‐ and D‐enantiomers of AA displayed subtle seasonal variations in composition and depicted varying degrees of diagenesis. The concentration of D‐enantiomer of AA was high and showed substantial contributions from bacteria, terrestrial source, and in situ production. The D‐arginine was the most abundant D‐enantiomer of AA in the study area, possibly due to extracellular secretion by bacterial species and adsorption onto sediments, and thus, it was protected from degradation. Degradation index was more negative at the oxbow and meandering loop stations during the dry season suggesting that local geomorphologic settings steer the diagenesis of OM within the river. A negative relationship between gamma‐aminobutyric acid and organic carbon:surface area (OC:SA) ratio and a positive correlation between tyrosine and OC:SA ratio suggested accelerated loss of OM. Furthermore, the concentrations of most bulk parameters were higher in the lower reaches during monsoon and premonsoon seasons. Taken together, changes in seasons have an operational control in distinguishing the composition, source, and diagenesis of spatial OM distribution. Moreover, oxbows and river meandering loops influence the diagenetic processes in small tropical river systems. [ABSTRACT FROM AUTHOR]

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]