Sources And Spatial-temporal Variations Of Organic Matter In Surface Sediments In The Changjiang Estuary And Its Adjacent Sea Areas

The rapid economic development and intensification of human activities aggravated the eutrophication in offshore areas of China.Large amount of nutrients and terrestrial organic matter had an important influence on the ecosystem health of the the Changjiang Estuary and its adjacent sea,resulting in the water eutrophication,frequent outbreak of harmful algal blooms and expansion of oxygen-depleted waters.In this study,surface sediment samples at 55 stations from the Changjiang Estuary and its adjacent sea in summer of 2016 were collected.Total organic carbon?TOC?,total nitrogen?TN?,carbon and nitrogen stable isotopes(?¹³C and?¹⁵N)together with grain sizes composition were analyzed to study the spatial distribution,sources and environmental indication of organic matter.TOC,TN,and their stable isotopes of the surface sediments in the study area in the last 30 years were compared to study the time scale changes and dominant factors related to the distribution and source of organic matter.The results were as follows:?1?There were five types of sediment including silt sand,muddy sand,sandy silt,silt and sandy mud.The sediment became coarser from land to sea,and the sediment in the north of Changjiang Estuary was coarser than which in the south of it.The southern part of the sea area was mainly with silt sand,and there was a muddy area in the coastal area off Fujian and Zhejiang.The ranges of TOC and TN values were 0.22⁰.82%and 0.03⁰.11%,respectively.The content of TOC and TN showed a positive correlation with the percentage content of clay,indicated that finer sediment was easier to adsorpt organic matter better.The spatial distribution trend of TOC and TN was consistent.In a comparison,higher TOC?>0.5%?and TN?>0.08%?values occurred in the south and center of the study area,lower TOC?<0.4%?and TN?<0.05%?values occurred in the north and east of the study area.The offshore of Changjiang Estuary was located in the frontal area of the Changjiang Diluted Water.The Changjiang Diluted Water brings high terrestrial organic matter input,at the same time,marine organic matter content increased as a resulte of increasing nutrient loadings.Impacted by the Taiwan Warm Current,rich nutrient,suitable water temperature and salinity resulted in the high phytoplankton biomass and high marine organic matter content in the offshore of Zhejiang.Moreover,both offshore of Changjiang Estuary and Zhejiang were located in the muddy sedimentary area,the high preservation of organic matter also explainedTOC and TN values in these area.The nothern area of our study was far away from land with low terrestrial input and lower primary production content,also the coarse sediment was not conducive to the preservation of organic matter,as a result,low TOC and TN content was found in these area.The range of TOC/TN was 6.45¹1.16 with higher TOC/TN value occurred in the coastal area off Fujian and Zhejiang and lower TOC/TN value occurred in the north and east of the study area.The range of?¹³C was-23.94^-20.49‰.It decreased from east to west of the study area.Both ranges and spatial distributions of?¹³C and TOC/TN indicated a mixture of terrestrial and marine derived organic matter.The terrestrial-derived organic carbon percentage?f?ranged from 13.00%to 58.45%,the marine organic carbon percentage?TOCmarine?ranged from 0.12%to 0.55%.?¹³C,TOCmarine,f and TOC/TN indicated that the proportion of marine and terrestrial input related to the distance from coastal to offshore,and the pathway of the Changjiang Diluted Water.In the north of the study area,the lower terrestrial organic matter was caused by the block of the Changjiang Diluted Water to continue northward by Subei Coastal Current.The contribution rate of the terrestrial organic carbon in the north of the Changjiang Estuary and the Hangzhou Bay was higher,because it was near shore,and the Changjiang River injected a large number of terrestrial organic matter.The seawater of Hangzhou Bay and the Changjiang River mouth had low transparency and phytoplankton was limited by the light intensity,which lead to lower marine organic matter content in these two areas.There were higher Chl-a values and phytoplankton biomass in the frontal area outside the Changjiang Estuary cauesed higher marine organic matter content.The outside of frontal area of the Changjiang River got lower terrestrial-derived organic carbon because of the frontal resistance,and lower marine-derived organic carbon because of nutrient limitation.The muddy area in the coastal area off Zhejiang was affected by upwelling,and the primary productivity was higher,with frequent outbreak of red tide.Therefore,high TOC_marine was found and TOC_marine was positive to TOC and TN.The value of?'showed that marine devrieved organice matter accounted for more than 60%of all organic matter at 35 stations of the study area with only three stations account for 40%-50%,indicated that sedimentary organic matter was mainly marine-derived organic matter,and marine devried organic matter was the main factor affecting the contents and distributions of TOC and TN.The range of?¹⁵N was 3.66⁵.38‰,with sources of nitrogen pollution by human activities in the study area.The spatial distribution of?¹⁵N was different from which of the?¹³C and TOC/TN,and it did not show a zonal distribution trend of increasing weight from nearshore to offshore,but was characterized by plaque distribution.This may be related to the fractionation of nitrogen isotopes,the source of pollutants,the distance of transmission,and the absorption of algae.Higher value areas were in the center,northeast and south,and lower value areas were in the northwest and southwest of the study area.The central area may be affected by the effluent with high value of?¹⁵N from the Changjiang Diluted Water,and it was linked together with the area with high value of?¹⁵N in the offshore.The northwestern area may be affected by a large number of chemical fertilizers with low value of?¹⁵N from the Jiangsu Province,forming a low value area.The terrestrial organic matter in the southwest area was mainly terrestrial plants with a low value of?¹⁵N,thus forming a relatively low value area.?2?Compared with the data of surface sediment in recent 30 years,the input of sediment was greatly reduced influenced by the Three Gorges Dam,which lead to the grain size of the surface sediments coarser in the south of 31°N.The high TOC and TN areas were in the offshore of the Changjiang Estuary and the coastal area of Zhejiang.Before 2011,the relative area in the Changjiang Estuary was larger than which inthe coastal area of Zhejiang,but the high value area in the coastal area of Zhejiang was larger than which in the area of the Changjiang Estuary after 2011.This may be related to the decrease of terrestrial organic matter input in the Changjiang Estuary and the frequent outbreak of red tides in the coastal area of Zhejiang.Light value area of?¹³C was mainly located in the coastal area of Changjiang Estuary and Hangzhou Bay,and it has a trend toward to the coastal area of Zhejiang.The above results showed that the human activities aggravated the terrestrial and marine organic matter input to the surface sediments in the Changjiang Estuary and its adjacent sea.Except for a few sites in the coastal area,the source of organic matter in the surface sediments of the study area was still dominated by marine organic matter.