Algal Records In Sediment Of Lugu Lake And Its Response To Climate Change Over The Last200Years

Global climate change is one of the important environmental problems in the world currently. According to the IPCC (2007) report, the mean global annual temperature has risen about0.74℃during the past100years, from1906to2005AD. The annual mean temperature of inland surfaces in China increased by1.38℃from1951AD to2009AD, and has been predicted to rise by2.5～4.6℃by the end of this century. Global climate change affects not only the air temperature, but also the rainfall and sunlight intensity, and results in increase of extreme weather. The effect of global climate change on ecosystems has become an important research topic in ecology and environmental science. Lake ecosystem is an important part of the biosphere, which is directly related to human life. However, it is still scarce to study response of aquatic ecosystem to climate change in China currently.Yunnan-Guizhou Plateau is one of the five lacustrine districts in China. The lakes in this area act as important water resource to the region. The lakes are mostly fault lakes geologically. The every aquatic ecosystem shows unique features, and with high value of biodiversity preservation. The climate in this region is significantly affected by the monsoon, and the significant trend of climate warming in the rejoin over the past60years has been reported. Some lakes with little anthropogenic disturbance in this region became ideal targets for studying the impact of climate change on lake ecosystems. But due to the lack of long-term, regular ecological monitoring data of the lake ecosystems, it is not possible to understand the impact of regional climate change on the lake ecosystems.Lugu Lake is a remote, alpines, semi-closed, deep and oligotrophic freshwater lake. Due to the low population density there is relatively low anthropogenic disturbance on the lake ecosystem. Together with high forest coverage and small, closed watershed, it is an ideal study location for ecosystem responses to recent climate change. In this study the geochemical index such as TOC (Total Organic Carbon), LOI550(Loss on Ignition at550℃), TN (Total Nitrogen), TP (Total Phosphorus), C/N (TOC/TN) and sediment grain size and biological records such as sediment pigments, biogenic silica (BSi) and diatoms fossil were extracted by paleolimnology methods from sediment cores collected from Lugu Lake. Under the age-depth model of210Pb/137Cs dating, we studied the responses of algal community evolution (diatom community as important representative) in Lugu Lake over the past200years, by using correlation analysis and multivariate ordination analysis related to regional meteorological data during1951～2010AD. The main results of this study are summarized as following:1. According to the CRS model of210Pb dating, the average sediment accumulation rate of dry matter in Lugu Lake is0.14～0.15g/cm2· yr. The sediment accumulation rate (SAR) of every deposition depth was different, and SAR slightly increased firstly, and then decreased gradually from top to down of the core. The results of deposition show compaction phenomenon in bottom of the cores. The age-depth model suggests that LGH-7core (27.5cm) spans a period of approximately250years, dating back to ca.1760AD, and LGH-6core (32.5cm) spans a period of approximately210years, dating back to ca.1800AD.2. The average sediment grain size in the sediment LGH-7core ranges in15.73～32.10μm, and the median grain size was about12.85～17.82μm. The sediment was silty clay. The sediment particle show excellent sorting and good distribution of the symmetry, and bias to fine silt. The "fine-coarse-fine" pattern of sediment grain size in Lugu Lake was a response to the results of the combined effects of the different periods of rainfall change and land use in the region.3. The diatoms valves in the sediment were well preserved in sediment LGH-7core by diatom fossil analysis. In whole55sediment subsamples, we identified diatoms including classes3, orders11, families16, genera24and diatom taxa145,28of which reached a maximum abundance of>1%, and16of which had≥2%in at least one sample. The sum relative abundance of28diatom species is about90.69～98.04%of the total abundance of all diatoms. By detrended canonical correspondence analysis (DCCA) of28diatom species data, the DCCA axis1scores is1.54SD units (>1SD: Standard Deviation), and the results implied that Lugu Lake was a very sensitive system that has had substantial ecological response to recent environmental change over the last200years, and also showed that the diatom community in Lugu Lake occurred the median species changes in the past250years. And the significance of diatom-based biostratigraphic zones was calculated by using the constrained incremental sum of squares (CONISS) of16diatom species data by constrained cluster analysis, three diatom zones (ca.1760～1934AD,1934-1991AD and1991～2010AD) and five sub-zones were identified based on changes in major species abundance. Three diatom zones revealed three diatom species composition shifts in different scale. And the species shifts were characterized as planktonic—no-planktonic—planktonic taxa diatoms. Diatom assemblages were characterized as (1) planktonic diatoms were dominated by Cst. dubius and C. rhomboideo-elliptica or C. ocellata, Asterionella formosaand.(2) no-planktonic (including epiphytic and benthic) diatoms were dominated by Fragilaria taxa, such as F. brevistriata and F. elliptica and Fragilaria construens f. venter. C. rhomboideo-elliptica was the dominant species before ca.1990AD, but the endemic species on the Yunnan Plateau virtually disappeared after ca.1995AD.4. The content of BSi, LOI550and TOC in lake sediments are often as proxies of primary productivity in lakes and its basin. Since ca.1991AD, the contents of BSi, LOI550and TOC sediment show increasing trends in Lugu Lake, which may indicate an increase in primary productivity in the lake and its basin. C/N ratios gradually declined to a relatively low value (C/N ratios mean11.21) during the last two decades (f ca.1991～2010AD), and the C/N ratios are around the alga C/N ratios range4～10, which further indicated that the sedimentary organic matter was mainly a product of autochthonous production. By the correlation analysis between diatom species data and environment proxies indicators (TOC, TN, TP, LOI and grain size) in sediment and the meteorological data (annual mean temperature, seasonal mean temperature and precipitation) during1951-2010AD, it showed that statistically significant positive correlations were found between annual mean temperature (ATmean) and TOC and diatom concentration and P/nP ratios (Planktonic/no-planktonic taxa ratios)(p<0.01), and significant correlations were found between annual mean temperature and winter mean temperature (WITmean) and TN-. TP and BSi (p<0.05). In addition, the results of Redundancy Analysis (RDA) revealed a relation between shifts in diatom assemblages and climate changes from1951AD to2010AD. The ATmean, WITmean, spring mean temperature (SPTmean) and the sediment TN and TP resulting from human activities have played a vital role in diatom compositional species turnover in recent years (p<0.05, n=36). In conclusion, ATmean, WITmean and SPTmean were the important variables explaining diatom variances in Lugu Lake over the past century, and the anthropogenic interference may pose reinforced effect in recent decade.5. Diatom species diversity index and diatom species individual size change were analyzed in this study. Results showed that the diatom species diversity index were relatively low in Lugu Lake over the past250years, Shannon-wiener index (H’) fluctuated in the range of1.59to2.89(average of2.23), and Hill’s N2fluctuated in the range of2.1to11.78(average of5.79). Diatom species diversity gradually decreased since the early1970’s. In addition, there was a medium-sized and miniaturization trend in the diatom species individual size. Total abundance of medium-size (7～15μm) diatom species showed an increased trendce over the past two centuries, and the maximum abundance was70.88%especially after ca.1990AD. Total abundance of small-size diatom species (≥15μm) also increased in some scale. But, Total abundance of medium-size diatom species (7-15μm) markedly decreased, and the abundance always maintained at a low level after ca.1990AD.6. The analysis of pigment in sediment core LGH-6showed that the conditions of pigment preservation was acceptable over the past200years, and the sediment pigments can be used as proxy indicators of primary productivity in Lugu Lake. The content of chlorophyll derivatives (CD), total carotenoids (TC), Oscillaxanthin (Osc) and Myxanthophyll (Myx) pigment in sediment core increased significantly from the early1990’s, and LOI550and BSi contents were significantly increased during1997to2010AD. The results revealed that the primary productivity in the Lugu Lake increased in the last20years, and the proportion of lakes endogenous organic matter increased in sediments. The four kinds of pigments and their ratio were sensitive to regional temperature changes. CD, TC and LOI550in sediments showed a significant positive correlation with annual mean temperature in the last60years (p<0.01), and Osc, Myx, and BSi content showed a positive correlation with annual mean temperature (p<0.05). There is no direct correlation between annual precipitation with the content of four types of pigments above, and LOI550and BSi (p>0.05). The results indicated that warming was possibly one of the main factors driving the increasing of primary productivity of the lake over the past60years. The increasing of Osc and Myx contents and Osc/Myx ratios in the lake sediments reflected the increase of productivity of Cyanophyceae, and the proportion of amount of Oscillatoria in Cyanophyceae since the early1990’s. These results indicated that the regional climate change over the last20years not only lead to the increase of lake primary productivity in lake Lugu, and also the change of composition of phytoplankton community.In summary, aquatic ecosystem in Lugu Lake showed sensitive response to regional climate changes. Algal productivity in Lugu Lake showed a rising trend over the past60years, and the proportion of filamentous cyanobacteria was increasing in phytoplankton community. Diatom community diversity index decreased, and the diatom size tends miniaturization. The algal community change in Lugu Lake mainly drived by climate warming and reinforced by human disturbance in the region over the past century. The results of this study imply that the climate change may cause seriouse ecological problem in lake ecosystems on the Yunnan-Guizhou Plateau in the future.