Relationship Between Phytoplankton Succession History And Human Activities In Watershed

Lake ecosystem has been impacting by directly or indirectly of human activities in the lake catchment.Over the last 100 years,the human activities(industrialisation,urbanisation and agriculture activities)have caused the deterioration of lake water quality and eutrophication which has been the main reason for degradation of lake ecosystem.The close relationship between succession of phytoplankton community and eutrophication,so it is necessary to understand the relationship between human activities and phytoplankton community succession.Since the end of 1970s,Lakes have gradually changed from the oligotrophication or meso-eutrophication to eutrophication,the number,area and eutrophication of the eutrophic lakes show an increasing trend in the Yunnan-Guizhou Plateau.However,the relationship between the process phytoplankton community succession and human activities which have not been fully elucidated in hypereutrophic lake of Yunnan-Guizhou Plateau.Qilu Lake is a typical representative of the hypereutrophic lakes in the Yunnan-Guizhou Plateau.Qilu Lake experienced a process of low—mesotropher—eutrophication—heavy eutrophication,and small lake catchment area and relatively simple disturbance of human activities,which were helpful to understand the law and driving factors of phytoplankton succession.A case study of Qilu Lake,it was clarify the phytoplankton community succession process and driving mechanism in the hypereutrophic lakes of the fragile ecosystem areas,which the results provide foundation for the restoration of lake ecosystem.By extracting and analyzing geochemical indexs,pigments and fossil diatoms from sediment core of Qilu lake,and under the age-depth model of 210Pb/137Cs dating,the phytoplankton community structure succession process of Qilu lake in recent 100 years were reconstructed.Combined with human activity related factors,the phytoplankton community succession process and driving factors in Qilu lake were further analyzed.The main research results of this paper are as follows:1)Through the survey of water quality and plankton in Qilu Lake from April 2017 to January 2018,it was found that the annual average eutrophication index(TLI)was 68.05,which was in the moderate to hyper-eutrophication level.Because the affect come from by both exogenous and endogenous pollution in the catchment,the water quality of Qilu Lake was in class V and V kinds of inferior,and the water quality was poor in summer and spring in study period.During the investigation period,163 species of phytoplankton were observed.The distinct seasonal succession of phytoplankton community,the dominant species were single,and filamentous algae was dominant in Qilu Lake.Mougeotia parvula was the dominant species in spring,and filamentous algae were the dominant species in other seasons.It is sensitive to low temperature,and the drop of water temperature prevents its growth.So Planktothrix mougeotii was the dominant species in summer,which likes to grow in eutrophic water with low transparency.In autumn,the dominance of Planktothrix mougeotii decreased significantly,and the dominant species was replaced by Planktothrix agardhii in eutrophic lake which could like low temperature and low light,high dissolved nitrogen and low dissolved phosphorus.The dominant species was still Planktothrix agardhii in winter,and Pseudanabaena limnetica replaced Planktothrix mougeotii as the second dominant species.In a word,the phosphorus,CODMn,NH3-N and water temperature were the main driving factors for the seasonal succession of phytoplankton communities in Qilu Lake.2)According to the CIC model of 210Pb dating,the average sediment accumulation rate of dry matter in Qilu Lake is 0.485 cm/a,and the accumulation rate was high.The age-depth model suggests that the sediment core of the deepest Qilu Lake spanned a period of approximately 130 years,and dates were extrapolated to approximately 1889 AD.The average grain size in the Qilu Lake sediment core ranges in 0.43?1.40 ?m,and the median grain size was about 0.46?1.11 ?m.The sediment was fine silty clay.The"fine-coarse-fine" pattern of sediment grain size in Qilu Lake.The change trend of grain size in Qilu Lake sediment does not fluctuate synchronously with the rainfall in the catchment and the accumulation rate was high,which was a related to the strong human activities in the catchment.The TN,TP,TOC and LOI550 in sediment show increasing trends in Qilu Lake in recent 100 years,which may indicate an increase in primary productivity in the lake and its catchment.The C/N ratios in the Qilu Lake sediment core ranges in 9.91?30.86,and the showed that "internal source-internal and external mixed source-internal source-internal and external mixed source" change pattern of sediment C/N ratios in Qilu Lake,which may indicate the source of organic matter was complicated.Since the end of ca.1950s,the C/N ratio in sediment show gradually decreasing trends,while the contents of TN and TOC show an increasing trend in Qilu lake sediment.This change trends may indicate that the enhancement of human activities(e.g.a large area agricultural activities,domestic sewage discharge)in Qilu Lake catchment resulted to the continuous increasing of nutrients,phytoplankton density,primary productivity,and the eutrophication has been increasing gradually in Qilu Lake.3)Metals in sediments of lake act as proxy of human activities in catchment.The variation trends of metal contents,EF and Igeo in sediments of Qilu Lake were obvious different,which was related to the human activities of change and intensity.Due to the construction of local water conservancy facilities and the expansion of urban construction land in the basin,metals(Fe,Al,Zn,Ni,Cr)mainly from natural sources have reduced the surface runoff into the lake,and the contents and accumulation degree of Fe,Al,Zn,Ni,Cr decrease.Due to the long-distance atmospheric transportation from outside and the change of industrial structure and the increase of intensity in the catchment,metals(Cd,Pb,As,Cu,Hg)mainly from anthropogenic sources resulted to the accumulation degree of Cd,Pb,As,Cu,Hg increase,but the beginning time of the increase was obviously different.it is considered that the discharge of waste and fallout from industry of ferrous metal smelting and manufacturing,and application of chemical fertilizers were the main contributors of metal input in Qilu Lake.The metals that can remain in the atmosphere for a long time,so the long-distance atmospheric transportation from outside the catchment was a source that can not be ignored.The accumulation of Cd,Pb and As in the Qilu lake sediment were mainly associated with agricultural activities,while Hg is mainly related to industrial activities.The accumulation of Cu may be related to the exhaust gas emissions and dust deposition caused by the wear of machine bearings and brake linings in the process of ferrous metal smelting and manufacturing.4)Sediment pigment and fossil diatom records reflect the succession of phytoplankton community in Qilu lake.The analyses showed that phytoplankton community composition has significantly shifted several times.The diatom succession was five stages(ca.1889?1893 AD,1895?1935AD,1937?1968 AD,1970?1998 AD and 2001?2014 AD)were identified based on changes in dominant species abundance.Five diatom stages revealed five diatom succession shifts in different scale.And the diatom succession were characterized as the relative abundance of benthic diatoms(including epiphytic diatoms)decreased gradually until disappeared,and the dominant species changed from oligotrophic to eutrophic species.First stage,oligotrophic Fragilaria pinnata had the highest relative abundance;Second stage,trophic Aulacoseira ambigua had the highest relative abundance,the relative abundance of F.pinnata decreased and became the second dominant species;The third stage,the relative abundance of A.ambigua continued to increase,and the benthic diatom of oligotrophic was the second dominant species;Fourth stage,the trophic diatom of phytoplankton were absolute dominant;The last stage,the moderately eutrophic diatom were dominant,and the relative abundance of benthic diatom of oligotrophic was almost zero.The content of pigment in Qilu Lake sediment remained relatively stable before 1930s,and began to increase at the early 1970s.The content of pigment significantly increased from about 2003.The evolution of pigment content had basically consistent with the continuous increase of human activities in the catchment and the process of eutrophication in Qilu Lake.5)The history change of diatom community and sediment pigment by Change-point software indicated that the diatom community and sediment pigments had significantly shifted several times.The changes before the 1949 was mainly related to the changes of hydrodynamic conditions of lake caused by extreme climate(flood).The changes after the 1949 was mainly driven by eutrophication and hydrological transformation of lake,and which caused by activities related to agriculture,such as reclamation on the lakes,water conservancy project construction,industrial adjustment in catchment(planting large-scale fertilizer consuming economic crops),etc.6)The results of CCA and RDA analysis between the relatively abundance of fossil diatom and environmental factors showed that the meso-eutrophic species(i.e.A.ambigua?C.comta?C.meneghiniana?Synedra acus)was positively correlated with environmental factors(TN,TP,TOC)related to human disturbance,while oligotrophic species Fragilaria was negatively correlated with environmental factors related to human disturbance,indicating that human disturbance and its intensity were the main driving factors for the succession of diatom community assemblage in Qilu Lake.)The results of RDA analysis between the content of sediment pigmentation and metals in sediment showed that the content of pigment was positively correlated with the content of metals(Cd,Pb,Hg,As)related to human activities,especially the highest correlation was found between the content of pigment and Cd and Pb,which the Cd and Pb were main come from agricultural activities,the discharge of waste and fallout from industry of ferrous metal smelting and manufacturing.)In a word,the disturbance of human activities is the main driving force for the succession of phytoplankton community in Qilu Lake during the past 100 years.The multivariate analysis between the proxy of different human activities and diatoms and sediment pigments.The results showed that the succession of phytoplankton community in Qilu Lake was mainly driven by the agricultural activities(application of fertilizers and pesticides)and industrial activities.In summary,the phytoplankton community of Qilu lake has undergone significant succession over the past 100 years.Since the 1970s,phytoplankton density has increased evidently,and the relative abundance of meso-eutrophic species increased while the oligotrophic species decreased.The succession of phytoplankton community in Qilu Lake was mainly driven by the agricultural activities(application of fertilizers and pesticides),the increase of population in catchment,industrial activities and manufacturing activities.The anthropogenic regulation of lake hydrology(water resources utilization and land-use change)and climate conditions were also found to synergistically affect the community succession of diatoms and sediment pigments.Therefore,controlling and reducing the application chemical fertilizers and pesticides or increase it utilization rate,controlling and reducing the anthropogenic regulation of lake hydrology are still the main methods to control the increase of phytoplankton density,the aggravation of lake eutrophication and the degradation of ecosystem.