| Water and sediment co-create habitat structure and dynamics within aquatic systems,playing an important role in maintaining the diversity and integrity of aquatic ecosystems.However,the hydrologic and sediment regime of many river and lake systems has been modified.These alterations constantly change aquatic habitats and increase physiological stresses,further causing biodiversity loss in rivers and lakes.Linking the response of aquatic communities to naturally occurring or anthropogenically-induced hydrologic and sediment alterations and exploring how to make effective and reasonable conservation and management measures are the key issues of aquatic ecology research at present.Dongting Lake is one of international importance wetlands with important ecological value and resource value.As the first large-scale river-going lake downstream of the Three Gorges Dam,its hydrologic and sediment regime has undergone significant changes under the influence of river-lake interaction and climate variability.This paper chose Dongting Lake as study area,taking Eco-hydrology,Wetland ecology and Quantitative ecology as theoretical basis,using statistic and remote sensing,analyze the inter and intra annual variation characteristics of hydrologic and sediment regime in detail.Then database of paired water environment(habitat landscape pattern,water quality factors)and aquatic organisms(Benthic invertebrates,which are widely used in freshwater ecology,were the target groups)were used to study the ecological effects of hydrologic-sediment variation and exploring the key hydrological and sediment variables and their threshold range that should be concerned in ecological protection.The results can provide a theoretical basis for lake water resources utilization,aquatic ecological conservation and management.The main contents and conclusions are as follows:(1)Man-Kendall test(trend detection and mutation detection)were applied to reveal the inter-annual trend of the flow and sediment concentration in Dongting Lake.The result was used to divide the study period as three parts:1960-1980,1981-2002 and 2003-2014.The whole process of flow and sediment change was divided into 45 indicators referenced to Indicators of Hydrologic Alteration(IHA),and the Range of Variability Approach was used to evaluate the characteristics and overall trends of the hydrologic and sediment regime in Dongting Lake.Compared with 1960-1980,most of the characteristic variables of hydrology and sediment in Dongting Lake after 2003 changed moderately or above(change degree>0.25),and the overall change degree was 0.57(large degree of change).Compared with 1981-2002,the change of flow and sediment after 2003 is mainly reflected in the extreme high flow/sand volume on a small time scale,the extreme low flow/sand volume on the seasonal scale,the flow increase process and the high and low The frequency and duration of sand volume,the overall change is 0.32(medium change).The flow exports/inputs ratio,the dried up days of the three channels and the sediment exports/inputs ratio have all changed moderately or above compared with that in the first two periods.The river-lake interactions and lake basin stability is still continuously changing.In addition,the sediment regime is more variable than the hydrologic regime.Principal Component Analysis(PCA)was used to extract the main components of hydrology and sediment variables to characterize the overall hydrologic and sediment regime of Dongting Lake.(2)Water body edge detection and overlay analysis of multi-temporal remote sensing images were used to discriminate the coverage and partition of Dongting Lake hydro-fluctuation belt,and then analyze the landscape pattern change of frequent flooding areas closely related to aquatic organisms.The overall hydrologic and sediment regime characteristic variables in the previous study were combined to reveal the relationship between hydrologic and sediment regime and habitat landscape patterns.The results showed that the degree of fragmentation of landscape pattern in the overall region changed from rising gradually to declining gradually during 1989-2014.The complexity of the landscape increased,and has been more stable after 2003,and the landscape heterogeneity increased to some extent.For the four main classes of landscapes,the changes of sedge bottomland and reed bottomland were more significant.The plaques of sedge bottomland were more scattered and fragmentized,and the shape tended to be complicated.The degree of fragmentation and complexity of reed bottomland declined.Correlation analysis results showed that the hydrologic and sediment regime has a certain effect on the landscape pattern of the overall region,especially the extreme low flow within the year,and various classes of landscapes were also affected with various degrees by the hydrologic and sediment regime.(3)Long-term variation characteristics of water quality factors were analyzed,combined with overall hydrologic and sediment regime characteristic variables,revealed the relationship between hydrologic and sediment regime and water quality factors.The typical water quality index of Dongting Lake showed a fluctuating trend from 1991 to 2014.The total nitrogen concentration,dissolved oxygen content and water temperature showed significant differences before and after 2003.The amplitude of variation of trophic level index also changed before and after 2003.Correlation analysis with the characteristic variables of hydrologic and sediment regime showed that except for the trophic level index and PH,the other indicators were affected with various degrees by flow and sediment conditions.The influence of lake sediment input and deposition/erosion regimes and degrees on water quality was more significant.The high-flow process and the connectivity of Yangtze River and lake also had a certain impact on water temperature.(4)The statistical methods such as trend analysis,differential analysis and non-metric multi-dimensional scale analysis were used to clarify the changes of community composition and species diversity of benthic invertebrates in Dongting Lake.And Variance partition analysis was used to distinguish the contribution of hydrologic and sediment regime and habitat environmental factors to the changes of benthic invertebrates.The results showed that abundance,species number and diversity showed a significant downward trend overall,but in the first two years after 2003,there was a short-term peak during 1988-2014.After 2004,they gradually decreased.The community composition of the whole lake,dominant species and the composition of functional feeding groups also existed differences between the two periods before and after 2003.Hydrologic and sediment regime and habitat environmental factors explained a total of 68.27%of the community changes,of which 31.48%and 12.52%were separately explained,and the contribution rate of the interactions was 24.27%.The alteration of hydrologic and sediment regime was the key influence factor affecting the benthic invertebrate community in Dongting Lake.While habitat environmental factors,especially water quality factors,were also important and should be considered in lake management projects.(5)BIO-ENV analysis and response surface methodology(RSM)were used to select out the key hydrological and sediment variables affecting each taxon,and determine the scope of management targets.The final hydrological and sediment management target range was determined as follows(made each taxon abundance meet mean ± standard deviation,and based on the requirements and the weights of all taxa):the average duration of high sediment concentration was range during 16?21 days/year;the frequency of flow decline was range during 33?38 times/year;sediment concentration of in the dry season was range during 0.10?0.12kg/m3;sediment exports/inputs ratio was range during 155.68%?278.67%;maximum average sediment concentration at seasonal scale was range during 0.10?0.15kg/m3 and the occurrence time of daily maximum flow was range during 153?216 days.(6)According to the quantitative relationship between hydrologic-sediment characteristic variables and environmental factors,key indicator species community structure and diversity,the ecological effects of hydrologic-sediment changes in Dongting Lake were summarized.The results showed as followed:the fewer high flow pulse and its shorter duration,the lower peak flow and the more drastic flow changes,causes the landscape fragmentation degree in the hydro-fluctuation belt to increase,affecting the water temperature and reducing the abundance of Aquatic Insects.The extremely low flow variables of small time scales were increased,which has increased the complexity and heterogeneity of the landscape in the hydro-fluctuation belt;the extremely low flow at the seasonal scale was reduced due to the prolonged dry period,which promoted the expansion of shallow water species.The river-lake interaction was weakened,which promoted the increase of water temperature and was not conducive to the growth of Other Taxa of benthic invertebrates.The reduction of sediment concentration in each period and scale could improve the fragmentation development of the landscape pattern in the hydro-fluctuation belt,aggravating the water pollution and reducing species diversity,but was conducive to the survival of the Scraper.As the lake basin turned from deposit dominant to erosion dominant,water quality was obviously influenced.And the stability of the lake basin changed from rising gradually to declining significantly,the more drastic erosion had a significant negative effect on aquatic organisms.In conclusion,in recent years,the alteration of hydrologic and sediment regime in Dongting Lake increased the fragmentation degree and heterogeneity of the habitat landscape patterns to a certain extent,aggravated the degree of water pollution and had more significant negative effects on benthic invertebrates.(7)Comparing the degree of satisfaction of the hydrologic and sediment regime with the management target range between the period after the operation of the Three Gorges Dam and the "Non-Three Gorges Dam" state,the results showed that the effect of the Three Gorges Dam on benthic invertebrates was mainly reflected in the changes of abundance of Mollusca.The Three Gorges Dam was not the main reason for the decrease in the diversity of benthic invertebrates in Dongting Lake. |
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