Historical Sedimentary Records In Typical Stations In Changjiang (Yangtze) River Estuary And Its Adjacent East China Sea And Their Indications Of Eutrophication

The rapid development of economy in coastal cities in China has greatly changedthe environment and ecological system and led to serious eutrophication in theestuaries and coasts. As a result, the occurrence and intensity of algae blooms haveincreased due to the high loadings of nutrients brought by eutrophication especially inthe areas by more anthropogenic effects. The Changjiang （Yangtze） River and itsadjacent East China Sea （ECS） is the most serious eutrophied space in China. Thehistorical records which embedded in the sediments can be used to provide keyinformation on the timing and magnitude of ecological changes in estuaries andcoastal and help us to understand the mechanism and develop trends in eutrophicationand red tide. Hence, in this study, two sediment cores were selected from sitesaffected by eutrophication and being the high frequency of red tides in the Changjiang（Yangtze） River estuary and its adjacent ECS. On the basis of core chronologies, weused multiproxy approach both in biological and geochemical analyses revealedchanges in diatom assemblages, total organic carbon （TOC）, total nitrogen （TN） andbiogenic silica （BSi）, as well as historical monitoring data. We gave an indication ofnutrient in status and trends in Changjiang River estuary and its adjacent East ChinaSea and we also discussed the relationship between the anthropogenic activities anddevelop trends in eutrophication. The main study results are follows:The²¹⁰Pb dating of the core ZC13showed that the mean sedimentation rate was0.89cm/yr, in corresponding to the sedimentation duration of54years （1955–2009）. The mean sedimentation rate of core ZA3was0.71cm/yr in the upper25cm and1.78cm/yr in the rest of the core, in corresponding to the sedimentation duration of139years （1871–2010）.The main part of the sediments of core ZC13belong to the sandy silt. Inlithology, the core is of coarse grain and have fluctuations in the upper10cm. Themain part of the sediments of core ZA3belong to the clayey silt and the trends ofcomponent and gain size parameters stay stable in the upper100cm. The core is ofcoarse sand during the100cm to the bottom in lithology and we selected the upper100cm for geochemistry and diatom analyses. The sandy silt and clayey silt aresuitable for geochemical and diatom information preservation.The total organic carbon （TOC）, total nitrogen （TN） and biogenic silica （BSi）were determined in the two cores. The results showed the trends in profiles of TOC,TN and BSi were relative low and stable before the1970s, suggesting a low andstable primary production in the water column compared to recent years. The profiesof the three proxies were in increasing trends during the1970to the mid-1990s,indicating increasing nutrient loads and more anthropogenic influence on the ECSduring this period. There were fluctuations of the three proxies from the mid-1990s tothe present, but with a rising trend overall, indicating the surplus nutrients loadings inthe ECS and there were remarkable changes in structure of nutrients which was slightsilica limitation and led to more dinoflagellates blooms.The succession of sediment diatom in the two cores and significance onenvironmental change were analysed. The results indicated that the diatom cellabundance, diversity, species richness and dominant species occurred an markedsuccession in the last century. The profiles of diatom cell abundance showed increasetendency since1970s and accelerated since min-1990s and this change was driven byincrease of the planktonic diatoms. Simultaneously, the content of red tide specieswere in the increase trend since1970s, indicating the serious eutrophication occurred.The profiles of diatom cell abundance, diversity and species richness were influctuations since2000, coincides with the trends of geochemical indexes in thisperiod and support the interpretation of these indexes. Using the multiproxy approach both in geochemical and diatom, we have shownthe history of eutrophication of two sites in the Changjiang River estuary and itsadjacent ECS. Both sites were not eutrophic in the past and have been under eutrophicconditions since the1970s and the eutrophication has accelerated since the1990s,linked with anthropogenic activities.