Sediment Diatom Records And Eutrophication Reconstruction In The Changjiang River And Its Adjacent Area

This study analyzed the temporal and spatial distribution characteristics of surface sediment diatom fossils and their relationship with environmental variables in the Changjiang River Estuary?CRE?and its adjacent waters.A Diatom-DIN?dissolved inorganic nitrogen?quantitative transfer model was developed by combining surface sediment diatom fossils data and corresponding upper water parameters.The diatom fossils in a sediment core were also investigated to reconstruct the historical changes of DIN by the Diatom-DIN quantitative transfer model with the support of core chronologies.We discussed the effect of human activities on eutrophication in the CRE and adjacent waters by DIN reconstruction and historical changes of sediment grain size and total organic carbon?TOC?,total nitrogen?TN?and biological silicon?BSi?.Major results and conclusions are as the following:1.Surface sediments in the CRE and its adjacent waters were mainly composed by clayey silt,silty sand and sand.In the investigated area,the grain size of surface sediment appeared to have an obvious characteristic:fine in south and near shore area,coarse in north and offshore area.Fine grained sediments are more favorable for the preservation of organic matter?TOC,TN and BSi?.The distribution characters of BSi had significant positive correlations with TOC and TN?P<0.01?,which means diatoms should be one of the main sources of organic matter.2.A total of 114 taxa were identified in 34 surface sediments,including 72 centric diatom taxa,41 pennate diatom taxa,and 1 silicoflagellate species?Dictyocha fibula?.The diatom fossils absolute abundance(D_abs)ranged from 0.24×10³ valves/g DW to49.32×10³ valves/g DW,with an average of 16.69×10³ valves/g DW,the D_abs did not significantly differ between the inshore and offshore areas.The Shannon-Weaver index?H'?ranged from 3.0-5.0,with an average of 4.0.The richness index?D?ranged from2.8-5.3,with an average of 4.1.Both parameters showed a downward trend from inshore to offshore areas.17 dominant species were selected,the relative abundance of Paralia sulcata ranged from 4.9%to 56.1%,with the highest average abundance of20.1%among the 17 dominant taxa.The D_abs was significantly affected by grain size,diatom fossils were better preserved in fine sediments.The diversity of diatom fossils increased with increasing nutrient concentration,grain composition had weaker influenced on species diversity,the composition of diatom fossils showed apparent coastal-offshore difference.Salinity,DIN,temperature and water depth affected the composition and distribution of diatom taxa independently?P<0.05?,which can be used to establish transfer models for paleoenvironmental reconstruction in the CRE and its adjacent waters.3.The diatom fossils data form 32 surface sediments and the corresponding upper water DIN data were used to establish a Diatom-DIN quantitative transfer model by Weighted-Averaging Partial Least Squares?WA-PLS?calibration model.The reliability of the function was tested by leave-one-out statistical test,with RMSEP=0.096 and R²=0.910 between observed and inferred values,showing its good predictive ability.Further test by 2012 field survey data?22 samples?also proved that the function had a certain range of application,the prediction results were more accurate in the CRE and its adjacent?n=18,R²=0.85?than in the Yellow Sea area?4 samples?.4.Core DH8-2 was mainly composed by clayey silt,no large fluctuations occurred in grain size.The ²¹⁰Pb and ¹³⁷Cs dating of core DH8-2 showed that the mean sedimentation rate was 0.996 cm/a,in corresponding to the sedimentation duration of50 years?1962-2012?.A total of 70 diatom taxa were identified in core DH8-2,including 72 centric taxa,41 pennate taxa.12 dominant species were selected,the relative abundance of Podosira stelliger ranged from 0.49%to 68.18%,this species had the highest average abundance of 24.15%among the 12 dominant taxa.Since 1972,the contents of TOC,TN and BSi,the D_abs and diversity all showed significant increase,benthic species decreased,phytoplankton species increased,diatom with large cell diameters have been gradually replaced by diatom of smaller diameter,diatom species prefer the eutrophic environment increased,these phenomena indicated increase in nutrient concentration and changes of nutrient structure.5.The reconstruction results by Diatom-DIN transfer model showed a significant increase of DIN content since 1962?5.82?mol/L?to 2012?9.85?mol/L?in DH8-2sampling area.During the 50 years,the concentration of nutrient in the area has increased significantly,eutrophication has increased.Further analysis found that the DIN content was at a low level?5.95?mol/L in average?during 1962 to 1972.The DIN content increased significantly?7.93?mol/L in average?during 1972-1998 period,indicating intensified human activity in this period,large amounts of nutrient input led to significant increase of DIN content,the structure of nutrients also changed.The DIN content continued to rise during 1998-2012?9.71?mol/L in average?,but the increasing rate was lower than last period.The composition and structure of diatom fossils obviously changed in this period,indicating a further variation in nutrient structure,which was confirmed by the outbreaks of red tides dominated by dinoflagellates start form 1998.