Provenience,Composition And Transformation Of Organic Matter In Sediments And Suspended Particulates From The Tropical Rivers Of India And Hainan Island,China - Illustrated By Amino Acids

The transport of organic matter?OM?as organic nitrogen?ON?from the terrestrial to the marine environments through rivers is a vital component of the global biogeochemical cycles and these cycles are extremely sensitive to the local geomorphology,anthropogenic perturbations and global climate change.Additionally,these cycles play an important role in recycling the nutrients,supporting primary productivity and preservation processes taking place within the transition zones?estuaries?of the continental margins.Therefore,the information and understanding on the contributing sources of OM,its distribution and the factors affecting the ON cycle contributes to the forecasting of nutrients and ecological imbalances that will have an effect on the global biogeochemical cycles.The biogeochemical cycles of nitrogen taking place within the transition zones where the rivers meet the coastal seas are impacted by both natural and anthropogenic factors.The natural factors include physical?geomorphologic features and hydrodynamic sorting?,variability in rainfall?excess-floods,cyclones and typhoons;deficient-drought?due to climate change,and anthropogenic activities to meet the demands of an increasing population.The impacts of these factors and activities in the tropical regions are of immense importance to the scientific community.The aquatic and terrestrial ecosystems of the tropical regions are experiencing intense impacts of these global environmental changes?land use alterations and climate change?.The earlier studies have shown that the small tropical river systems are extremely dynamic-spatially and temporally and so are ideal settings for research.Therefore,the natural or anthropogenic factors influencing the distribution and preservation of OM during early stages of diagenesis could be understood by identifying the compositional changes in ON namely-amino acids?AA?.The AA are the basic building blocks of life and are ubiquitous in the environment.Therefore,the AA have been used successfully as biomarkers to indicate compositional changes and diagenesis of OM.During early diagenesis,the AA are preferentially utilized by microbes and are selectively preserved?adsorption?in natural environments.India and China are the two developing countries with colossal populations,and are experiencing the impacts of climate change?increased frequency of storms and droughts?and anthropogenic activities.The Hainan Island of China is one of the thickly populated and anthropogenically sensitive areas.Till date,our understanding of the biogeochemical processes within the fringe zones of Indian peninsula and the Hainan Island,China are limited.The studies on the ON?AA?sources,composition,their transport and transformation in?small?tropical rivers of India and China are scant.This thesis aims to contribute to the knowledge gap that exists by providing first-hand information and new thoughts in relation to the natural controls?geomorphological features,seasonal variability?and anthropogenic activities affecting the biogeochemical cycling of ON?using AA as a biomarker?taking place within the selected river basins of India and China.The focus of this research/thesis is to identify the factors and compositional changes affecting ON?AA?within the tropical rivers along the west coast of India?WCI?and Hainan Island,China.These regions are unique in characteristics as their drainage basins are heterogeneous spatially and temporally,and experience different monsoons.Furthermore,this study not only examines the short-term reserve of OM?suspended particulate matter-SPM?by providing a snap shot into the diagenetic processes,but also looks into the long-term reservoir?sediments?for process alterations.Advanced geochemical techniques?bulk elemental composition,isotopic ratios and biomarkers?together with other ancillary parameters?hydrologic?were used in this study to provide an in-depth perspective on OM?AA?composition,transformation and factors controlling their transport across the selected small river systems of India and China.This study pertains to the surface sediments collected from a small and medium tropical river along the west coast of India?WIC??Netravati River and Narmada River?.In addition,suspended particulate matter was also collected from a small tropical river along the WCI?Zuari River?and the Nandu River,one of largest river system of the Hainan Island in China.This research work will contribute greatly to the global research community by providing important information on the AA composition and transformations occurring within the particulate and sedimentary environments of different river systems.In addition,this study will also add to the existing knowledge gaps in identifying the responses?transport and transformation?of ON?AA?in small and medium rivers with respect to geomorphology and seasonal variability due to climate change.information on the sources,composition and transformation of ON?AA?in the surface sediments and particulates from the selected rivers and estuaries along the WCI and the tropical Hainan Island,China;?ii?to identify the factors controlling the ON?AA?transport and transformation spatially and temporally;and?iii?analyze the impacts of geomorphology?meandering river loops and oxbows?and changes in precipitation?seasonality and typhoon activity?on the composition and diagenesis of OM.In order to study these objectives,surface water and sediments were collected from the selected Indian rivers?Narmada,Zuari and Netravati River?during the monsoon,post-monsoon and pre-monsoon season.Surface waters were also collected from the Nandu River soon after the passing of the typhoon?August,2011?and during the non-typhoon period?October,2012?.Using advances geochemical techniques bulk parameters?elemental and isotope ratios?,along with ancillary parameters?grain size,surface area?SA??,and the biomarker?AA enantiomers-L-and D-forms?were analyzed.River were different with contributions from C₄ plants to the estuarine region and C₃plants and detritus to the riverine region in the Narmada River.Whereas C₃ plants and detrital matter contributed to the OM of the surface sediments collected from the estuarine and riverine region of the Netravati River.Additional sources of OM were in-situ production?marine and freshwater plankton?,soil,and bacteria and their remnants.The L-and D-enantiomers of AA displayed subtle seasonal variations in composition and depicted varying degrees of diagenesis in the sediments of the Netravati River,whereas the sediments of the Narmada River exhibited spatial variation in Serine,gamma-Aminobutyric acid?GABA?,and D-AA.The concentration of the D-enantiomers of AA was high in the surface sediments of the Netravati and Narmada River suggesting substantial contributions from bacteria and its detritus.The Degradation Index?DI?calculated for the surface sediments of the Netravati River was more negative/degraded at the oxbow/meandering loop stations during the dry season suggesting that the local geomorphologic settings steer the diagenesis of OM within the river.Contrastingly,during the monsoon season,the river acts as a flushing pipe,diluting and dispersing the sediments and OM accumulated during the dry season.During the dry season,OM is trapped and held back within the river channel due to reduced water flow.Furthermore,a negative relationship between GABA and organic carbon:surface area?OC:SA?ratio and a positive correlation between tyrosine and OC:SA ratio,suggested accelerated loss of OM.Additionally,the concentrations of most bulk parameters?OC,clay,silt and SA?measured from the sediments of the Netravati River were higher in the lower reaches during monsoon and pre-monsoon seasons indicating the influence of seasonal variability in distribution of OM.The findings from the sediments of the Netravati and Narmada River contributed to our understanding of the role of geomorphological settings?oxbow/river meandering?,and temporal changes?variability in the quantity and frequency of rainfall?alters the provenience,composition and transformation of OM namely ON?AA?during the early stages of diagenesis.These processes have the potential to affect the natural nitrogen cycle taking place within the tropical rivers.The results of this study can be used as baseline data for future investigations to comprehend the impacts of natural and anthropogenic activities affecting the natural nitrogen cycle taking place within the fringe regions of?small?tropical rivers.Similarly,the suspended particulate matter?SPM?of the Zuari River and its estuary-west coast of India were examined for the sources and fate of OM.Surprisingly no significant temporal variations were observed in the parameters analyzed.Nonetheless,salinity,particulate organic carbon?POC?,?¹³C,particulate nitrogen?PN?,?¹⁵N,glutamic acid,serine,alanine,tyrosine,leucine and D-aspartic acid values exhibited significant spatial variability suggesting source differentiation.The POC content displayed weak temporal variability with low values observed during the post-monsoon season attributed to unequal inputs from mixed sources.The?¹³C values of POC samples in estuarine region were less depleted than the riverine samples suggesting contributions from marine plankton in addition to contributions from freshwater plankton and terrestrial C₃ plant detritus.The presence of D-amino acids such as D-alanine,D-aspartic acid,D-glutamic acid,D-serine and D-arginine,glycine and non-protein amino acid??-aminobutyric acid,GABA?indicates the contributions from bacterial sources to the organic carbon and nitrogen pool.During the monsoon and post-monsoon seasons,the OM was relatively fresher in the estuarine region in comparison to the riverine region.During the pre-monsoon season,the OM was more diagenized?older?in the estuarine region due to resuspension of SPM within the estuarine turbidity maximum?ETM?.In order to ascertain the sources and factors influencing OM,principal component?PC?were extracted.Principally five factors were extracted explaining 84.52%of the total variance.The first component/factor accounted for 27.10%of the variance suggesting the dominance of tidal influence whereas,the second component/factor?19.6%?accounted for heterotrophic bacteria and their remnants associated with the particulate matter,contributing primarily to the AA pool.Based on the results of this study we ascertained the role of the ETM controlling the sources,composition and transformation of POM in small tropical rivers.Thus,the temporal changes and alterations to the regional settings are more likely to affect the natural biogeochemical cycles of small tropical rivers.Likewise,the AA profile of the suspended particulates collected from the Nandu River,Hainan Island in China were analyzed and the concentrations were found to range from 0.1 to 1.1 mg L^-1.Based on the calculated DI and other degradation parameters it was concluded that the degradation states of the particulates were significantly different for the two sampling campaigns with relatively more diagenized OM as observed during the August,2011?typhoon?campaign.Based on the AA compositions,and D/?D+L?ratios,it is inferred that the bacteria and their remnants altered the POM content of the Nandu River and estuary?NRE?.The carbon yields of total hydrolyzable amino acids were relatively high showing a conflicting pattern,suggesting likely contributions from multiple sources including vascular plants,planktonic?marine and riverine?production,and soil leachates.Significant differences were observed in the AA?Ser,Thr,Ala,Val,Phe and Ile?measured during the two sampling campaigns suggesting preferential utilization of ON during early diagenesis.Interestingly,from the data?AA and ancillary parameters?the effect of the typhoon was perceptible as none of the measured parameters?bulk and biomarker?correlated with the measured SPM,whereas,during the non-typhoon season?October,2012?sampling campaign the measured SPM values correlated with the degradation products suggesting particle associated selective diagenesis.Since these river basins are heterogeneous with respect to time and space,the results of the analysis of amino acid behavior in these small river systems in China and India,were different spatial and temporal.The distribution characteristics in each river,was closely related to the geomorphic structure,temporal variability and the source composition within the watershed.Remineralization and utilization depends on the preservation and transformation capacity of rivers and estuaries.This study provides the baseline data on amino acid composition from suspended particulate matter and sedimentary environments,its transformation in different river sub-systems,and the mechanisms leading to differences due to seasonal variability?monsoon?on small and medium tropical rivers.The organic nitrogen cycle operating?transport and transformation?within the diverse landforms of small rivers in China and India are related to the response of climate change.Therefore,this research is crucial for managing the nutrient budgets,sediment and particulate inputs to the coastal regions.These regions have a potential to adversely influence the diversity,local fisheries and coral habitats.Understanding these biogeochemical cycles with respect to the long-term alteration due to climate change are pivotal in accepting the fate of OM exported and transformed under dynamic depositional conditions.