Effects Of Fronts On The Spatial Distribution Of Phytoplankton, Sedimentary Organic Matter, And Sedimentary Diatoms In The Changjiang Estuary And Its Adjacent Waters

Fronts divide the water masses in the ocean and influence the spatial distribution of marine organisms.There are ten quasi-stationary fronts in the continental shelf seas of China,among which the plume front outside the Changjiang Estuary and the adjacent Zhejiang-Fujian Front have a significant effect on the pattern of the ecosystem.Studies have revealed a significant spatial response relationship between phytoplankton and the front changes in the Changjiang Estuary and its adjacent waters.However,it is unclear if this relationship is spatially related to the organic matter in the surface sediments of the seafloor.This study investigated the spatial pattern and response of chlorophyll a(Chl-a),sedimentary organic matter,and diatom assemblages in the Changjiang Estuary and its adjacent waters under the influence of fronts based on the observational data of water masses and surface sediments.The main results are as follows:(1)The study on the distribution characteristics and relationship between Chl-a and environmental factors in typical sections in summer 2019 found that the water mass is well mixed in the estuary shoreward of the sediment front.Under the influence of Changjiang runoff and the barrier effect of sediment front,the concentrations of TSM and nutrients were the highest.The high values of TSM caused a significant light limitation,resulting in a low concentration of Chl-a(1.7±0.5?g·L^-1).The water mass beyond the plume front showed vertical stratification,and its surface layer was characterized by the lowest TSM and nutrients.Due to insufficient nutrient supply,the Chl-a concentration was only 0.2?g·L^-1.A high concentration of Chl-a(7.5±4.1?g·L^-1)mainly occurred in the transition zone between sediment and plume front,where nutrients were contributed by Changjiang runoff and upwelling.Meanwhile,rapid deposition of TSM across the sediment front alleviated light limitation in this transition zone,contributing to the phytoplankton growth and accumulation.(2)The grain size,total organic carbon(TOC),total nitrogen(TN),and stable isotope of carbon and nitrogen(?¹³C and?¹⁵N)data of surface sediments from 32cruises from 1980 to 2018 were analyzed.The results show that the grain size in the study region has prominent zoning characteristics,which are correlated with the location of the Zhejiang-Fujian Front and the Changjiang Ring Front.Subregion I(Zhejiang and Fujian coastal areas),III(Subei Shoal),and IV(southwest Cheju Island)are fine-grained areas,and subregion II(East China Sea Shelf)is a coarse-grained area.The distribution pattern and numerical ranges of TOC/TN,?¹³C,and?¹⁵N reflect the characteristics of the mixing of marine and terrigenous organic matter.The fronts may account for the differences in the sources of organic matter in different subregions.Inside the plume front,the influence of terrigenous organic matter is the highest near the Changjiang Estuary in subregion I.Surrounded by the Changjiang Ring Front,the influence of terrigenous organic matter is the second highest in a small area in the south of subregion III.Influenced by the Changjiang Ring Front,the northern part of subregions II,III,and most of subregion IV exhibit similar mixed characteristics of marine-terrigenous sources.There is a spatial correspondence between the high value of marine organic matter and primary productivity and the location of fronts and upwelling.The spatial correspondence may be achieved through the control effect of fronts on deposition,which is reflected in the different spatial patterns of deposition rates and grain size between I-IV zones.(3)The spatial distribution characteristics of absolute abundance,species diversity,and species richness of sedimentary diatoms and the effect of fronts were analyzed.The results showed that the absolute abundance of diatom in the Subei Shoal area north of32°N was significantly higher than that in the Changjiang Estuary and the southern sea area.The species diversity in the north of 32.0°N was marginally lower than that in the south of 32.0°N.The absolute abundance was the highest in the Subei Shoal,but the diversity and richness index of the species was low.Absolute abundance,species diversity,and richness index of diatoms outside Changjiang Estuary are elevated.There is a spatial correlation between the fronts and diatom assemblages.The fronts affect the species distribution of sedimentary diatoms,and there was a corresponding relationship between the spatial distribution characteristics of sedimentary diatoms and sedimentary organic matter.The Changjiang plume assemblage and Zhejiang coastal assemblage are mainly distributed inside the Zhejiang-Fujian Front.Outside the Zhejiang-Fujian Front,there is a combination and interlacing phenomenon of assemblages closing to the upwelling.Due to the influence of Zhejiang-Fujian Front,the species composition of the Changjiang plume assemblage was slightly different in the northern part of 32.0°N and the southern part of 32.0°N.Given the above,the frontal structure of the Changjiang Estuary and its adjacent waters has a zoning function on the spatial distribution of phytoplankton in the upper water and the organic matter in the sediments.The results explore the spatial distribution characteristics and corresponding relationship of phytoplankton and surface sediment organic matter under the influence of fronts,extend our understanding of the biogeographic division of fronts in sediments,and have reference value for understanding the ecological effect of fronts,clarifying the ecological spatial distribution pattern and the coupling relationship of water masses and surface sediment.