Sources, Composition And Food Chain Transfer Of Organic Matter In The Estuarine Tidal Ecosystem

Estuary is the transitional zone between land and ocean.Being affected by physical,chemical and biological processes,a large quantity of organic matter originated from land,ocean and local place has accumulated in the estuarine and coastal areas.Moreover,the estuarine tidal flat is one of important regions where the biogeochemical cycling is very intense within the ecosystems on the earth's surface. The biogeochemical cycling of organic matter has the important ecological significance to sustaining higer productivity and biomass in the esturine tidal ecosystem.Therefore,to study the origin,fate and transport processes of organic matter in the complex ecosystem has recently become one of the headline scientific problems in the fields of physical geography and estuarine and coastal environmental geochemistry.This study chosed one of the largest turbid estuaries in the world—Yangtze Estuary.Through in situ information collection and laboratory analysis,the sources, biochemical composition and food chain transfer processes of organic matter in the Yangtze estuarine tidal ecosystem were studied using biogeochemical and stable isotopic technology.Based on the stable carbon isotopes in the organic constituents from the southem bank of Yangtze Estuary.It was found that the ratios of stable carbon isotope in the suspended organic matter(SPOM)were respectively -25.8‰～-23.4‰and -25.1‰～-22.9‰in February and August,while the values of carbon isotope in surface sediments were -25.0‰～-20.4‰and -24.7‰～-19.5‰respectively.The spatial change of carbon isotopes displayed the trend of more positive in salt water than in freshwater areas,suggesting the mixture of marine and riverine organic matter.The data suggested that the input of sewage and tributary water contributed certain degree to the organic matter of estuarine tidal ecosystem,espetially to SPOM.Sedimentary organic matter originated from the deposition of suspended particles.Both marsh macrophytes and benthic microalgaes contributed little to sedimentary organic matter, and their effect was more notable in winter than in summer.The organic matter sources of surface sediments were evaluated at different habitats in East Chongming tidal flat.The fraction of organic matter originated from macrophytes ranged from 4%to 27%,higher in winter and spring,which showed that the input of organic matter was controlled by discharges of the Yangtze River.The underground part of marsh plants contributed more to organic matter in the low discharge seasons.Due to the seasonal change of water discharge and marine phytoplankton blooming,terrigenous inputs exceeded marine inputs in winter and the reverse results have been found in summer.The spatial and temporal distribution of labile organic matter(carbohydrate, protein and lipid)showed that they were mainly present in fine particles,especially the size less than 16μm.However,the three labile organic matter displayed different accumulating mode:carbohydrates were good bond which integrated particles and therefore reserved themselves more easily;proteins were concerned with bacteria's activities and thus grain size indirectly influenced their transformation through biological factors;lipids were not concerned apparently with sediment grain size. Benthic algae were an important source of proteins.Phaeopigments and the labile organic matter had the similar way to accumulate,and the bacteria utilized Phaeopigments in the same degree as labile organic matter did.Due to the different environment at East Chongming tidal flat and southern bank of Yangtze Estuary,the distribution of biochemical composition differed significantly. At southern bank,the accumulation of carbohydrates was controlled by hydrodynamic condition and carbohydrates gathered in fine grains;concentration of proteins was influenced by organisms in the sediment;lipids mainly originated from benthic algae. At East Chongming tidal flat,the accumulation of labile organic matter was controlled by hydrodynamic condition.The different habitats and physiognomy influenced the speed of tide and distribution of plants,which resulted in the spatial variation of labile organic matter.Labile organic matter,utilized to estimate the food potentially available for benthic consumers,accounted for only a small percentage(14.8%～29.6%)of total organic carbon,with the character of notable spatial variation.It was found that high quantity of organic matter was linked to the increase of complicated fraction.The ratios of BPC/TOC,PRT-N/TN and PRT/CHO were low,which suggested that although there was great amount of organic detritus depositing in the estuarine tidal flat,these materials were of low nutrition and difficult for benthic consumers to utilize. Biochemical compositions were used to evaluate the trophic status of coastal areas. Based on the biochemical compositions and pigments,the Multidimentional Analysis clearly indicated that three groups of transects could be identified:the first was represented by low tidal flat of East Chongrning salt marsh,which displayed carbohydrate concentration＜1000μg g^-1;the second included LC,CY and FX,with the carbohydrate concentration ranging from 1000μg g^-1to 2000μg g^-1;a third group included LHK,SDK,WSK,BLG,DH and high,midium tidal flats of East Chongming salt marsh,with the carbohydrate concentration＞2000μg g^-1.Benthicfauna can regulate the energy flow in the ecosystem.Stable carbon and nitrogen isotopes were used to trace the food sources and trophic position of macrofauna at different living environment of East Chongming tidal flat.The macrofauna showed a wide range ofδ¹³C between -14.7‰and 23.6‰,which suggested significant difference of food sources.As a whole,sediment organic matter seemed to account for heavy proportion as the food of macrofauna,while the dominant macrophytes of salt marsh appeared to play a negligible trophic role.The nitrogen isotope could reflect the trophic position of macrobenthos.It was calculated that trophic level of macrofauna was between 2.0 and 3.7,and thus the macrobenthos of East Chongming tidal flat were primary and secondary consumers.