Aquatic Food Web Structure And Dynamics In The Floodplain Of Poyang Lake

Floodplains provide a wealth of ecosystem service values.They are one of the most dynamic,richest,and most diversified ecosystems in the world,and they are also one of the most vulnerable ecosystems.Human disturbances have caused a high degree of degradation of their ecosystem functions.Poyang Lake has maintained the natural characteristics of the floodplain in the middle and lower reaches of the Yangtze River,China.Its water levels shown noticeable seasonal changes under the joint regulation of the fiver rivers'incoming water and the water level of the Yangtze River.Flood regimes is a key factor that affects the ecological functions and biodiversity pattern of floodplain systems.To study the impact of important ecological driving factors of flood regimes on the fish assemblage structure and food web of the Poyang Lake floodplain is meaningful.It has important guiding significance for understanding the dynamic mechanism of fishery resources in Poyang Lake,formulating the relevant managements and restoration measures.Our study were carried out in sub-lakes and river channels in Poyang Lake floodplain,based on four seasons'fishery resource survey data,we combined with stable isotopic analysis and Bayesian mixing model to study fish assemblage structure of sub-lakes and river channels;the sources of carbon and nitrogen organic matter for macrobenthos,crustaceans,and fishes including six trophic strategies;the seasonal changes of food web structure and trophic level of aquatic animals,niche of fish assemblage.The main results summarized as follows:(1)A total of 59 fish species from 44 genera,14 families and 7 orders were sampled in May,July,October.And the data in December was not used to analyze changes in fish assemblages.The richness of fish species were highest for 47 in July(high-water stage),the second was 43 in October(falling season),the lowest was 35 in May(rising season).The dominant fish species were Carassius auratus,Tachysurus fulvidraco,Cyprinus carpio,Pseudorasbora parva,Rhinogobius giurinus and Cultrichthys erythropterus.There was a significant difference in the number of fish species in the seasons,which was manifested in that the flood season was significantly higher than the rising and falling seasons,but there is no significant difference in the number of fish species between sub-lakes and river channels in Poyang Lake floodplain.Analysis of non-metric multidimensional scale(NMDS)showed that the fish assemblage structure in the flood season was significantly different from the flood season and the falling season.There was a obvious separation between the fish assemblages in the sub-lakes and the river channels,and the flood season and the falling season are the most obvious.The fish assemblage structure had regime shift in these two seasons.(2)The stable isotope differences of primary producers follow the transmission of the food chain are finally transferred to consumers.Fish and other aquatic animals also showed corresponding changes at different periods.The degree of change of?¹³C and?¹⁵N was different for fishes have different trophic strategies.The range of?¹³C for omnivorous fishes was wider,and Piscivorous fish generally had higher?¹⁵N.During different seasons,consumers'main carbon and nitrogen source supplies were different.The organic matter supplies during the flood season was submerged plants,and the average contribution ratios to macrobenthos,crustaceans,and fish were 75.1%,35.0%,36.9%,respectively.During the wet season,the water level was highest,creating opportunities for consumers to eat emergent plants and its fragment.The contribution of emergent plants reaches the highest in the wet season,but macrobenthos mainly use leaf litter as the main source of organic matter,with an average of contribution ratio of50.1%.During the falling season,consumers used leaf litter and sediment organic matter(SOM)as the main sources of organic matter,and their average proportion of contribution to macrobenthos,crustaceans,and fish were 37.9%and 23.7%,31.8%and24.0%,33.4%and 27.1%,respectively.During the dry season,the main carbon and nitrogen supplies of different consumption groups were quite different.Macrobenthos were mainly SOM and particulate organic matter(POM),with an average contribution ratio of 45.7%and 41.7%.Crustaceans are litter and POM is dominated,with an average contribution ratio of 38.0%and 22.6%.Fishes are dominated by litter and attached algae,with an average contribution ratio of 42.5%and 22.7%.(2)The two-dimensional map of?¹³C-?¹⁵N reflects the position of organisms in the food web.After the water felling,the living spaces of consumers such as fish were reduced sharply,resource competition had enhanced,and the degree of overlap of consumers in the middle trophic level was higher.And the trophic level of dominant fish species has increased significantly.The rising in water level promotes fishes to inhabit and forage on a larger scale.The Layman metrics showed that fish assemblage have more abundant food sources during wet season,more space resources were used,and had a higher diversity of food web structures.With the water level dropping until into the dry season,the hydrological changes were small,the fish assemblage structure tended to be stable,and the degree of trophic redundancy was high.