Non-conservative Behaviour Of Dissolved Organic Matter(DOM) And The Impacts Of Flocculation And Photodegradation On DOM In Yangtze River Estuary And Estuaries In Middle South Atlantic Bight And Their Adjacent Seas

Dissolved organic matter?DOM?in natural water bodies plays an important role in the biogeochemical cycle in the water ecosystem as well as the global carbon cycle.Previous studies have shown that the vast majority of terrestrial DOM has undergone significant changes both qualitatively and quantitatively during the transport from the rivers to the ocean.Estuaries and their adjacent coastal areas usually have extremely complex and active biogeochemical processes,and they are believed to be the hotspots where DOM is changed and modified.This makes the study of source and sink processes of DOM in estuarine and coastal regions as one of the most attracting and difficult scientific question globally.Based on previous studies and efforts globally,we chose Yangtze River Estuary?YRE?and the adjacent East China Sea?ECS?as well as the three estuaries and adjacent waters in the central South Atlantic Bight?SAB?of USA as the study area.Several known major components of DOM,such as dissolved organic carbon?DOC?,colored dissolved organic matter?CDOM?,fluorescent dissolved organic matter?FDOM?and dissolved black carbon?DBC?are analysed.We studied several important questions at regional scale relating to the issue of DOM sourc-sink research of estuarine and coastal water and we expected people's knowledge will be greatly enhanced on this important issue.In detail,the spatial and temporal distribution and variation of DOM,especially including CDOM and DBC are presented and non-conservative behaviour of the main components of DOM are discussed in YER and it's adjacent ECS as well as in three estuaries in the middle SAB and their coastal water.In light of this,the impacts of flocculation and photochemical degradation on the concentration and property of DOM are explored.Finally,the apparent quantum yield of dissolved black carbon in natural water was firstly measured using an innovative experimental design.Based on the above research,the following conclusions or goals were obtained:?1?Several components of DOM have significant spatial and seasonal differences in the YRE and its adjacent ECS.The concentration of DOM in the river mouth is higher than that in the coastal sea,and concentration in summer is higher than that in winter,and the degree of aromatization of DOM in the mouth and summer is also higher.In general,DOM is more conservertive in winter than in summer,and more conservative in middle and bottom layer than that in surface layer during DOM transportation.The three protein-like fluorescent component,especially the C3 and C5components,are the least conservative with high concentrations in the Zhoushan Islands and the southern coastal waters.These areas are also clearly separated via principal component analysis,probably representing the impact of human activities.Finally,the spatial distribution of dissolved black carbon in the Yangtze Estuary and its adjacent waters was reported for the first time.The distribution of DBC and CDOM in the Yangtze Estuary had a good coupling relationship.No significant non-conservative process was observed compared to the theological mixing line.?2?Although the geographical location is at a similar latitude,the concentration of DOM in the three estuaries of the SAB is significantly higher than that of the YRE,especially,the DOM concentration of the Ogeechee estuary is even close to an order of magnitude higher than the YRE.These difference may result from the differeces of the percentage of vegetation cover,water resident time and human activities between the rivers.At the same time,both the concentration and property of DOM in these three geographically close estuaries are also quite different.In addition to spatial differences,the DOM in the Ogeechee estuary varied among different months,but correlated with the season change.It also showed that in regions with salinity below 1,significant adding process happened.The use of high-frequency field sensors can effectively show this geographical area where those processes happened.In addition,we found that there is a strong correlation between DBC and CDOM and this correlation still work even include data from the YRE and Arctic rivers.This suggested that CDOM can be used as a useful indicator of DBC without regional restrictions.New technologies such as sensors,satellite could be used to retrieve CDOM data globally in a much easier way and hence will improve the DBC data availability as well.?3?Based on the result of laboratory mixing experiments and field sampling data,we found that the flocculation process didn't significantly alter the concentration and property of DOM in YRE,and the changes induced by flocculation were not sufficient to explain the non-conservative behaviour of DOM in the field samples.In addition,although the PRAFAC model accurately extracts most part of the modelled EEMs of FDOM calculated based on theoretical mixing,it does not accurately feedback the fluorescence peaks of the areas affected by the Raman and Rayleigh peaks.Further analysis of a variety of parameters which are calculated based on CDOM and FDOM data to show the nature of DOM found that the aromatization index SUVA₂₅₄ is the most stable and reliable index.?4?The photodegradation experiments of three different type of water samples show that prolonged light exposure can significantly decrease the concentration of most DOM components in natural water and reduce the aromatization of DOM.Photoproduction of protein-like fluorescent component was observed in Ogeechee river water sample.When 90%of CDOM was removed,the removal percentage of DBC in the three water samples from the Ogeechee Estuary,the Yangtze Estuary,and the SAB were 72.6%,46.8%,and 88.3%,respectively,showing a large difference.The degradation ratio of DBC in the two estuary water samples was significantly higher than that in the offshore water samples of the SAB,indicating that the response of DBC to photodegradation was also affected by the structure or composition of dissolved black carbon.By comparing the parameters that showing the DOM properties changing with the increasing irradiation time,it is found that the aromatization index SUVA₂₅₄ is a more reliable index to indicate DOM change affected by light.During the photo-degradation process,the proportion of DBC in DOC?DBC/DOC?and the ratio the two major carboxylic acid groups?B6CA/B5CA?chemically converted from DBC also decrease exponentially with the increasing irradiation time.The changes of DBC coupled a lot with DOC,CDOM and FDOM.Besides,the strong correlation between SUVA₂₅₄ and the ratio of B6CA/B5CA also showed that the SUVA₂₅₄ can be used to indicate the degree of aromatization of DOM as well as DBC.?5?Photodegradation will selectively degrade the more aromatic components in the DOM.This selectivity resulted in the enrichment of stable isotope(?¹³C)of the solid phased extracted DOM?SPE-DOC?with increasing irradiation time,but the stable isotopes(?¹³C)of B5CA and B6CA did not change significantly with the irradiation time.The latest research found that there is a significant discrepancy of about-6‰between the stable isotope(?¹³C)of dissolved black carbon in global major rivers and marine waters.Our results showed that this significant difference cannot be explained by the photodegradation process.?6?Apparent quantum yield?AQY?is the most crucial parameter to estimate the changes?production or degradation?of substances in natural water due to the photochemical process.We deployed an innovative method to measure the apparent quantum yield of DBC in water using the Ogeechee River water and the coastal waters of the SAB.The results showed that the AQY of DBC obtained from the two water samples is not significantly different,and both decreased exponentially with the increase of the spectrum.This data will provide solid data support for future work on the estimation of the amount of DBC degraded in natural water.