| As an important component of the hydrosphere on land, lakes link the atmosphere, biosphere and lithosphere. Due to their continuous and rapid deposition and long temporal range that contains biological and geological information, lake sediments are principal archives for global climate and environmental changes. Lake sediments spanning long time interval record dynamic processes of climate and environment and can act as proxies to trace the uplift of Tibetan Plateau and the shift of monsoon system in Asia. On the other hand, short sequences of lake sediments can be used to reflect the effect of human activities on lake system, such as natural and cultural eutrophication and environmental pollution. In the middle reach of the Yangtze River, fragile lake ecosystems encounter quite lots of environmental problems, such as eutrophication and pollution. Restoring these lakes to their natural conditions is a vital mission as one of the two major governmentally led research projects.In lake sediments, lipids may be derived from aquatic and land plants as well as microbes living in the lakes and the surrounded watershed. These compounds, possessing high diversity in both structures and classes, are taxonomic indicators of their precursors, and thus can be used to reflect the evolution history of the lake system. Algae and bacteria are rarely preserved as fossils in lake sediments; however, their lipids can compensate this shortcoming by leaving a molecular record through the evolution of lake ecosystems.In this dissertation, three cores were recovered from the East Lake system (ELI from the Guozheng Lake,64 cm; EL2 from the Tangling Lake,45cm) and the Liangzi Lake (LFM from East Liangzi Lake,38 cm), which developed quite a different degree of eutrophication. Totally 147 sediment samples were selected for lipid analysis. Combing with a high precise 210Pb chronology, lipid data and previously published results in the literature (on studies of pigments, organic carbon stable isotopes and remains from aquatic plant, ostracods, gastropods and amoebae), the evolutionary history of these two lakes has been reconstructed. The aim of this study was to examine the ecological response of lake systems to eutrophication, and to investigate how human activities affect lake systems with the attempt to use the well documented history as a reliable reference for environmental remediation. Through comparison with other geobiological events in the deep earth history, we can also better understand how human activities may cause biotic extinction.The major findings in this study include:1.Lipids are very abundant in the sediments of these two lakes, particularly with some taxonomic specific biomarkers (e.g. highly branched isoprenes (HBI) and 2-methyl hopanol). These compounds are comprised of n-alkanes, HBI, fatty acids, fatty alcohols, n-alkan-2-ones, wax esters, steroids, and hopanoids. Alcohol compounds (fatty alcohols, sterols, and hopanols) dominate the recovered sediment samples. Wax esters are characterized by their high abundance and low-molecular-weight homologues. Among these lipids, some compounds are specific biomarkers for their biological precursors. For example, low-molecular-weight iso-and anteiso-fatty acids, alcohols and wax esters are derived from Gram positive bacteria; long-chain and strong odd-over-even predominance n-alkanes, fatty alcohols, and n-alkan-2-ones are biomarkers of higher plants. C25 HBI and C28:2â–³5,22 sterol originated from diatoms living within lakes; 2-Methyl hopanol and 4,23,24-trimethylsterol can indicate cyanobacteria and dinoflagellate, respectively. In addition, C27:1â–³5 sterol can indicate zooplanktons or phytoplanktons. Hopanoids (hopanols, hopenes and hopanes) may originate from bacteria living in the watershed.2.Lipids show highly temporal and spacial variations among these three sedimentary cores. The total concentration of lipids in ELI and EL2 cores are higher than that in the Liangzi Lake core. In addition, the two cores in the East Lake System possess more bacteria and algae derived lipids, such as short-chain n-alkanes, fatty acids, fatty alcohols, wax esters and hopanoids. The Liangzi Lake core is dominated by lipids from aquatic and land higher plants. All three cores show low abundance in lipids before 1960 AD, with low quantity of low-molecular-weight branched fatty acids, fatty alcohols and hopanoids and absence of wax esters, and C25 HBI. On the contrary, medium and long-chain n-alkanes, fatty acids, fatty alcohols and dinosterol show predominance in the lower sections of these sedimentary cores. After 1960AD, the total concentration of lipids increases sharply, especially those from bacteria and algae:C25 HBI and wax esters emerge with high abundance. In addition, short-chain n-alkanes, fatty acids, fatty alcohols, and n-alkan-2-ones relatively increase. The concentration of cyanobacterial derived 2-methyl hopanols increases by an order of magnitude. Among sterols, dinosterol, C27 and C28 sterols increase more than C29 sterols, indicating the elevation of plankton biomass (dinoflagellate, plankton animal and diatom) during this interval.3.The temporal variations of lipids in the three drill cores infer an evolutionary history for the lake ecosystems. Before 1966 AD, the three cores are dominated by C21-C33 n-alkanes, C22-C32 n-fatty alcohols and C25-C33 n-alkan-2-ones, together with traces of C27 sterols, dinosterol, hopanoids and branched fatty acids and alcohols. All these lipid features indicate that the East Lake System and the Liangzi Lake belong to grass-type lakes, which are dominated by aquatic plants. The total amount of lipids, TN and TP are rather low, while wax esters and C25 HBI are totally absent. The values of Pr/Ph ratio are relatively higher in this interval, revealing that the water column is full of oxygen with good water quality. After 1966 AD, the lipid profiles changed. The total concentration of lipids, TN, TP, and TOC increase sharply. Diatom (C25 HBI and C28:2â–³5,22),dinoflagellate (4,23,24-trimethylsterol), cyanobacteria (2-Methyl hopanol) and common bacteria (hopanoids and short-chain alkanes, fatty acids, fatty alcohols and wax esters, and branched fatty acids and fatty alcohols) are abundant, whilst lipids from aquatic and land plants are relatively low. During this interval, the East Lake System and the Liangzi Lake have changed to algae-type lakes, with low oxygen content in the water column and the water quality decrease sharply. My interpretations based upon lipid distributions are consistent with previous reports in the literature. In the 1960s, human activities brought great effects to these two lakes: more nutrition was introduced into these lakes, causing serious eutrophication and pollution that destroyed native aquatic plant ecosystems.4. The temporal and spacial variations of lipid compounds in these three cores can be used to indicate different degrees of eutrophication conditions in these two lakes. In the East Lake System, sedimentary samples contain much higher total lipid concentrations. In addition, bacteria and algae derived lipids take predominance during recent 30 years, revealing that severe eutrophication occurred in the East Lake System. On the other hand, bacteria and algae derived lipids are composed of a small proportion in Liangzi Lake samples, corresponding to the middle degree of eutrophication in the Liangzi Lake. In addition, lipids in East Lake samples can record algae bloom events. The cyanobacteria and bacteria lipids (2-methyl hopanol, diploptene, hopanoids, iC15 and iC17 fatty acids) increase greatly in the late 1950s, then reached a spike during 1973-1984. A second spike occurs in the 1990s, followed by the third spike in the surficial horizon. The above shift of cyanobacteria and bacteria lipids is consistent with the historical records of eutrophication and algae blooms in the East Lake System. Since the 1960s, population around the East Lake grows rapidly. More and more industrial and municipal waste water was drained into the East Lake System, causing server eutrophication. During the middle 1970s and the 1980s, cyanobacteria bloom (Microcystis, Anabaena and Aphanizomenon) happened every summer. Since 1985,large number of silver carp and bighead carp has been introduced into the East Lake. Then cyanobacteria blooms disappeared. Recently, server cyanobacteria blooms reoccur in some part of the East Lake. Furthermore, the content of dinosterol (4,23,24-trimethylsterol) reaches its peak value during the mid-and-the late 1990s, which is consistent with the dinoflagellate bloom event recorded in recent years.5.Lipids also record industrial pollution around the East Lake. From the late 1930s, charcoal and polycyclic aromatic hydrocarbons (PAHs) increased in large quantity. Both charcoal and PAHs correlate with the combustion of fossil fuels. The enrichment of charcoal and PAHs probably results from the increasing human activities around the lake from the late 1930s. From the 1950s, population increases greatly and several large-scale iron and steel enterprises built in these areas surrounding the East Lake. Accordingly, more phenanthrene was deposited in lake sediments.6. In principle, the lipid recorded geobiological events caused by human activities can also be compared to those during the Earth history. The anthropogenic influence began from the mid 1930s and increases from the 1960s. The native aquatic ecosystem was destroyed completely. A number of higher plants and animals disappeared and the biodiversity decreased sharply. Consequently, minimized and disaster species became predominance and algae blooms occurred frequently. These processes are similar to biotic events such as Tr/P crisis in principle. If the history is a mirror to the present, then humanbeings have to take great effects to protect our Earth from the sixth mass extinction.In summary, lipids in the three recovered sedimentary cores from the East Lake and the Liangzi Lake record the evolutionary history, especially the eutrophication process in these lakes. These molecular results demonstrated chronologically how human may affect lake ecosystems, and revealed the current natural conditions of these lake systems. The data provide an important foundation for environmental remediation of these lake systems in the lower Yangtze River reach.
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