Ecological Environment Changes Recorded In Sediment Of The Changjiang Estuary And Its Adjacent Sea

The Changjiiang Estuary and its adjacent sea has high primary production andabundant marine resources. With the rapid economic development, eutrophication,anoxia and frequent redtide have become serious ecological problems. Fractions ofnitrogen and phosphorus, organic carbon （OC）, biogenic silica （BSi）, carbon andnitrogen isotope composition （δ13C, δ15N）, metals, were examined in surface and coresediments, to decipher how the environment has changed during recent years ofincreasing anthropogenic influence in the investigation region.Sediments in the Changjiang Estuary, Zhejiang and Fujiang coastal area andnortheast of study area which is near the middle part of the South Yellow Sea consistprimarily of mud and silt. Grain size shows rapid coarsening effect away from theinner shelf, and sediment in offshore area mainly consists of sand. The mixing of fineand coarse sediment results in the worst separation in the middle of study area（122.5-124.0°E）. The sedimentation rate is the highest in the Changjiang Estuary,decreasing rapidly towards offshore area.The biogeochemical characteristics and ecology environmental significance ofbiogenic elements and carbon and nitrogen isotope in the Changjiang Estuary and itsadjacent sea are presented. Various fractions of N and P, OC and BSi are high in theChangjiang Estuary, Zhejiang and Fujian coastal area and northeast part of study area,and their concentrations are low in the offshore area where sediment consists mainlyof sand. The concentrations of authigenic calcium-bound phosphorus （ACa-P）,organic phosphorus （OP）, OC, organic nitrogen （ON） and BSi are high in the innershelf and northeast part of study area, which is consistent with Chl a pattern in watercolumn; the concentrations of the loosely sorbed and iron-bound phosphorus（LSor-P+Fe-P）, detrital apatite phosphorus （DAP） and fixed ammonium (N_fix) arehigh in the inshore area and northeast part of study area, mainly caused by hugeterrigenous discharge from the Changjiang. These elements mentioned above correlate positively with Mz （except for DAP）, indicating the influence of gain size.ON and IP are the primary forms of N and P, and N_fixis an significant fraction of N,accounting for30.4%of TN. The exchangeable nitrogen (N_ex), sum of LSor-P+Fe-Pand OP are the main parts of bioavailabile nitrogen and phosphorus in sediment, andthey account for2.8%and31.8%of the total N and P, respectively. The molar ratiosof ON/BSi and OC/BSi are higher in upper layer of the core than those in lower layer,indicating different decomposition rates between ON and/or OC and BSi, ordecreasing percentage of diatom in phytoplankton composition which is induced bythe low dissolve phosphate concentration and high molar ratio of N/P in water column.δ¹³C, δ¹⁵N and molar ratio of OC/ON indicate the source of organic matter comingfrom a mixing of terrigenous input and marine production. Terrigenous input organicmatter is high in inshore area and decreases towards offshore area.The human activities impacts on ecological environment are interpreted based onthe distribution characters of metals and biogenic elements in study area. Enrichmentfactor calculations suggest metals were contaminated in inner shelf of study area, butthe contamination in offshore area was minor. Sewage discharge and nutrients fluxesfrom the Changjiang have been dramaticly increased, due to the rapid development ofeconomy since the1990s. The Concentrations of biogenic elements in core sedimentsrecord the occurrence of redtide, and metals could reflect the contamination caused byhuman activities. Sedimentation rates control the accumulation rates of biogenicelements and metals. Accumulation rates of biogenic elements and metals are highdue to high sedimentation rates in the Changjiang Estuary, and decrease towardsoffshore area.