| In this study,a series of studies were conducted on the migration and transformation processes of biogenic elements in typical Chinese rivers,the Yangtze River estuary and its adjacent seas.Focusing on the biogeochemical processes of total dissolved carbohydrates(TCHO)in the water column,which were influenced by environmental factors such as runoff,climate,light intensity,suspended particulate matter(SPM)from the basin to the open sea.Based on field observations and in situ incubation experiments,we analyze the spatial and temporal variations of carbohydrates transported from rivers to adjacent seas.Moreover,we discussed indepth the impact of different basin environments,climate changes,and water mass mixing on the migration and transformation processes of TCHO.By conducting in situ incubation experiments,we clearly understand the changing trend of phytoplankton primary production mixing with freshwater and seawater,and elucidate the biochemical filter effect of estuaries with terrestrial materials.This paper further enriched and deepened our understanding of the processes of nutrient and organic carbon cycling in river-estuary-ocean continuum and their response to environmental events such as red tides.The findings of this paper include the following four main aspects:(1)The temporal and spatial variations of carbohydrate concentration in the mainstream of the Yangtze River from upstream to downstream from 2021 to 2022 revealed that runoff flushing effects,in situ production and climate change have significant effects on the concentrations of TCHO and their transport and transformation processes.Monosaccharides(MCHO)were the main components of TCHO in the Yangtze River basin.During the transportation process from upstream to downstream,TCHO concentration showed a trend of increasing and then decreasing.Anthropogenic pollutant input was also a major source of TCHO in the middle and lower reaches of the Yangtze River.Due to the different effects of soil organic matter erosion and in situ production of phytoplankton,TCHO in the mainstream of the Yangtze River was mainly low molecular weight,while lakes in the basin were mainly high molecular weight TCHO released in situ.Under extreme drought conditions,TCHO concentrations in the river increased instead of decreasing,and the changes of polysaccharides(PCHO)were particularly significant,confirming the phytoplankton preferential produced carbohydrate-based organic matter.(2)By conducting a comparative investigation of carbohydrate outputs in downstream areas of eight different river basins in China(including Liaohe River,Haihe River,Yellow River,Huaihe River,Yangtze River,Qiantang River,Minjiang River and Pearl River)during dry and flood seasons,significant differences in distribution and properties of TCHO were found between the downstream of southern and northern rivers in China.Influence by soil vegetation,and human activities in the basin,the area transport flux of carbohydrates was significantly higher in southern rivers than in northern rivers.In addition,southern rivers were dominated by low molecular weight TCHO,while northern rivers were dominated by high molecular weight TCHO.Unlike high molecular weight carbohydrate in lake waters,the majority of high molecular weight fractions in northern rivers originate from microbially derived organic matter and exhibited biodegradable characteristics.To further investigate the physical and chemical differences in biogenic elements between southern and northern rivers,a comparison of annual sampling between the Yangtze River and the Yellow River was conducted from 2019 to 2020.The study found that SPM concentration in surface water had an important influence on the in situ production and release of carbohydrates by phytoplankton.Compared with the Yangtze River,the source of carbohydrates in the Yellow River were more inclined to the flushing process of soil organic matter.(3)The spatial distribution of carbohydrates in the Yangtze River estuary and its adjacent seas exhibited obvious seasonal characteristics with higher concentrations during the flood season than during the dry season.Extreme flood events also have a significant effect on the source-sink transformation process of carbohydrates.The study conducted in the first two chapters reveals that there are two crucial sources of TCHO:runoff input and in situ production.Using a similarity salinity path analysis,it was shown that PCHO appears as a "source" and MCHO as a "sink" during the process of Changjiang diluted water.However,during the extreme flooding event of the Yangtze River from July to August 2020,there was a clear source-sink transformation of MCHO,indicating that the large-scale input of terrestrial materials could significantly affect the biogeochemical cycling of organic carbon such as carbohydrates in the adjacent sea.Principal component analysis revealed that MCHO and TCHO were mainly controlled by runoff input,while PCHO was mainly controlled by biological processes.Based on in situ incubation experiments,we found that both MCHO and PCHO showed a potential upward trend during the biological growth process,indicating that the in situ source of MCHO may be masked by the water masses during the mixing with freshwater and seawater.(4)Using extreme flood events as the research context,the mechanism of in situ release of organic matter from the zone of maximum turbidity and the zone of high chlorophyll-a values was clearly elucidated with the help of environmental parameters such as nutrient change rate,chlorophyll-a concentration,and SPM concentration.By effectively combining static parameters(nutrient concentration),dynamic parameters(nutrient variation rate),and chlorophyll-a concentration,the study found that high nutrient variation rate was observed in the medium salinity area,rather than in the Yangtze River freshwater end member region(high nutrient concentration).Similar to the distribution characteristics of MCHO,the effects of biological processes on nutrient concentrations were masked by water mass mixing processes.The high net nutrient uptake rates at the surface in the summer triggered bloom events later at the seaward stations,showing that high net nutrient uptake is the cause and high chlorophyll-a is the consequence of the bloom.In this study,we found that the estuarine region not only has a physical filter effect but also has a biogeochemical filter effect,which further deepens our understanding of the mechanism of the occurrence of marine red tide events and the key role of biogenic elements such as nutrients.In summary,the results of this study further improve our understanding of the distribution and transport processes of biogenic elements in the "land-river-estuary and adjacent sea" environment,and elucidate the effects of climate events on riverine and estuarine ecosystems.Through in situ incubation experiments,we understand the potential dynamics of nutrient and carbohydrates during red tide events,which is important for the study of marine organic carbon cycling and marine environmental events such as red tides. |
