A Primary Study On Sources Of The Organic Matters In Sediment Of The Red-tide Zone In The East China Sea And Its Relationship With Phytoplankton Community

Sea areas adjacent to the Changjiang River estuary is the most prominent area forcoastal eutrophication in China, where large-scale harmful algal blooms (red tides) occurfrequently and the problem of bottom-water hypoxia is getting worse. After the harmfulalgal blooms, algal cells settled into the seabed, which may be an important source fororganic matters deposited into this area and deteriorate the status of hypoxia in the bottomwater. In the current study, the Changjiang River estuary and its adjacent waters wereselected as the targeted area, and sediment collected from this area were studied to analyzethe sources of organic matters as well as its long-term variation, and to explore therelationship between the organic matter and the phytoplankton community. Meanwhile,potential linkage between algal blooms and seasonal hypoxia events in this region werealso discussed.Two sediment cores, including core2950from the “red tide zone” south to theChangjiang River estuary and core3100from the Changjiang River estuary, were collectedin2009. Based on the chronological results thtough210Pb dating, the long-term changes oforganic matter content was analyzed via organic carbon and the source of organic matterwas traced via carbon and nitrogen isotopes (δ13C, δ15N) and C/N ratio. Potential changesof the phytoplankton community was also discussed via the analysis of pigments in thesediment cores. Meanwhile, pigments in the surface sediment was adopted as indicator forsettled microalgae, and the distribution pattern of deposited phytoplankton after the algalblooms were studied to explore the relationship with the bottom water hypoxia issue.Based on210Pb activity, sediment deposition rate in core2950and3100was0.58cmyr-1and1.23cm yr-1，respectively, and core2950and3100can be traced back as far as the1920s and the1950s. Core3100which collected from the Changjiang River estuary,showed no significant changes of δ13C, δ15N, suggesting that organic matters in the corewere mainly from terrestrial origin. Although the organic matter in core3100was higherthan that in core2950, no clear variation pattern could be observed for organic matters andpigments, probably due to the highly dynamic environment in the esuary. In core2950which collected from the red tide zone, the organic carbon content showed a slight changefrom the1970s, and a significant increase after the1990s. The total nitrogen content alsoshowed significant increase after the1990s. The carbon and nitrogen isotopes analysissuggested that organic carbon after the1990s mainly come from the marine biologicalproduction. Similar as the organic carbon content variation pattern in the core, pigment content also showed a slight increasing trend from the1970s followed by a significantincrease after the1990s. Total chlorophyll a (including its degradation products, such aspheophorbide a, pheophytin a etc.) and β-carotene content showed an obvious increasingtrend after the1990s which were similar to the organic carbon content. After the1990s,diatoxanthin, zeathanthin and lutein showed increasing trends over time, but alloxanthinshowed a decreasing trend. It’s suggested that organic matter accumulated in core2950after the1990s may result from the the increased primary production driven by coastaleutrophication, intensified hypoxia, or both. Significant variation of pigments characteristicfor different microalgal groups after the1990s may reflect significant changes of thephytoplankton community.Analysis of pigments in the surface sediment detected chlorophyll a(Chl a),fucoxanthin (Fuco), peridinin (Perid),19’-butanoyloxyFucoxanthin (But-Fuco), alloxanthin(Allo), zeaxanthin (Zea) and lutein (Lut), which can be detected in all of the surfacesediment samples. After the diatom bloom in the beginning of April, content of Chl a, Fucoand But-Fuco significantly increased in the surface sediments, while there were nosignificant differences in the content and distribution for other pigments. High-values ofChl a, Fuco and But-Fuco in the surface sediments is generally consistent with thedistribution area of the diatom bloom in April, which is located outwards of the50metersisobath in the southeast part of the investigation area. Presence of But-Fuco in the surfacesediment indicated that there were blooms in waters caused by microalgae containingBut-Fuco, which have the similar settling pattern to the diatoms. The area of high-valueChl a, Fuco and But-Fuco in the surface sediment is similar to the hypoxic bottom water,which suggested that organic matters derived from algal blooms may affect the formationof summer hypoxia zone. However, the detailed processes and mechanisms need furtherstudy.Findings in the current study indicated that ecosystem in the Changjiang River estuaryadjacent waters changed significantly since the1990s and the organic matter content haveincreased significantly. Meanwhile, phytoplankton community also showed significantchanges based on the the pigment analysis over the core. The reason accounting for theincreased organic matter accumulation after the1990s remained unclear, probably due toincreased organic matter supply or increasing hypoxia, or both. Algal cells deposited afterthe bloom in spring has a potential linkage with bottom-water hypoxia in this area, and thedecomposition of the organic matter will undoubtedly exacerbate the hypoxia status. the synergetic effects of HABs and hypoxia in this region will further deteriorate theecosystem in the Changjiang River estuary and its adjacent waters, the processes, trendand potential impacts need close attention.