Structure Of Fish Food Webs And Energy Flow Of Ecosystems In The East River

The East River?Dongjiang?is one of the three main tributaries of the Pearl River?Zhujiang?system,the fourth largest river system in China.It is 562 km long and has a drainage area of35,340 km²,with annual average precipitation of 1,750 mm and runoff of 32.4 billion m³.The water resources in the East River are harnessed through three major reservoirs,which are of great importance for flood control,power generation,irrigation,navigation,and water supply in the Guangdong province with more than 40 million people.The ecology and environment of the East River are of foremost importance in the sustainable development of the Pearl River Delta.The river basin has experienced rapid economic development over the past few decades,severely impacting the river ecosystem.Quantifying food web structures and dynamics is a fundamental ecological research for understanding ecosystem structure and functioning,with implications for futuremanagement and rehabilitation of rivers.We hypothesize that depending on the site-specific habitat heterogeneity,resource utilization and upstream-downstream physicochemical factors,there exists longitudinal continuum or shifts of species richness,trophic guilds,sources of organic matter and ecological niches of fish assemblages in the subtropical large East River.To test a series of these hypothesis,based on the early research results provided by our team,traditional stomach content analysis,stable isotope analysis,and ECOPATH model were used to illuminate the trophic interaction,basic carbon and nitrogen resources for fish food webs,and,trophic levels and niches of fish functional groups within food webs at six sampling sites.Finally,the circulation of materials and energy flow of six ECOPATH models were constructed along the East River.?1?83 fish species were sampled during this survey,which was 10 less than the survey in1980¹981.Compared with history results,there were 7 invasive species living in the lower reaches of the East River now,including Gambusia affinis,Labeo rohita,Cirrhinus cirrhosis,Clarias gariepinus,Hypostomus plecostomus,Oreochromis niloticus and Tilapia zillii.From upstream to downstream,the species richness increase increased from site 1 to site 5.However,the highest richness at site 5 was caused by the supplementation of 7 invasive species.The decreasing species richness from site 5 to site 6 could be attributed to:1)increasing the urban and industrial land use,the modification of river basin and the degraded and homogenized habitat limited the living space;2)the deterioration of water quality reduced the survival and reproductive capacity of sensitive fish.Influenced by human disturbance,the quality of fish assemblages is declining,indicating the aquatic ecosystems of the East River are under the interference of external stress.The correspondence analysis between the distribution of fish species at family level and sampling sites showed that the small-sized rheophilous fish species,such as Homalopteridae,Synbranchidae,Cobitidae,Gobiidae,Poeciliidae,Belontiidae and Sisoridae,mainly distributed in upper steams;large-sized and carnivorous Bagridae,Serranidae,Mastacembelidae,Channidae,Siluridae and Clariidae mainly distributed in the middle reaches;Loricariidae,Cichlidae,Anabantidae,Engraulidae,Anguillidae mainly distributed in lower reaches;wide-distributed Cyprinidae could be sampled along the whole river.The relationships between the main physical factors of river basin and the relative number of individuals?Rn?as well as the relative biomass?Rb?of fish assemblages at family level showed that elevation and precipitation were two most significant factors influencing the fish distribution and structure.Besides these two factors,discharge and water depth were more significant in the association with Rn,while the river width was more significant in the association with Rb.The highest biodiversity values were found at middle reaches sites 3⁴,where were the transition area between upstream tributaries and main channels that providing living conditions and resources availability.Moreover,the intact land-water ecotones protected by the riparian bamboo forest simulated the high habitat complexity.Compared with upper stream to middle reaches,the degradation of water quality were more serious for the fish assemblages in the lower reaches.Compared with history results,the ratio of migratory species decreased from 10.53%to3.53%;conversely,the ratio of sedentary species increased from 64.89%to 85.89%?The structure and composition of traditional feeding groups at each sampling site showed that the small-sized carnivorous fish species,which have higher requirements on water quality and habitat conditions and preferred to living in gravel substrate,are mostly influenced by environmental changes.The proportion of omnivorous fish species obviously increased,and the predatory fish species declined,the changing proportion of richness and composition of fish species with different feeding types proved the degradation of fish assemblage quality in the East River.?2?Along the East River,there existed a longitudinal continuum of resource utilization by fish consumers.The main exploited resources changed from small aquatic insect larvae and periphyton in the upper stream,to macrophytes,bivalves,shrimps and odonates in the middle river,and to detritus and bivalves in the lower reaches.The basic structure and distribution of fish trophic guilds in the East River corresponded to the prediction in other temperate and tropical rivers,such as the dominant insectivores in headwaters,increasing herbivores,omnivores and piscivores downstream,and detritivores in lower river.However,different from previous predictions,the utilization of allochtonous resources in headwaters,increase in planktivores and decrease in invertivores downstream were not significant in this study.The longitudinal diet shifts of eight fish species indicated that fish trophic guilds responded to spatial alternation of resource utilization and individual ontogenic needs.The utilization of limited insects,gastropods and periphyton,and increasing bivalves,macrophytes and detritus along the East River were main causes for the diet shifts of insectivores and ominivores.In addition,the downstream distribution of juveniles and adults also determined the spatial difference in resources utilized.There is a possibility that the changes in composition of trophic guilds can be influenced by the diet shifts of some crucial species with high density or biomass.Further,the spatial heterogeneity of resource distribution and availability might be the principal mechanism behind the longitudinal variations in fish trophic guilds.The high habitat heterogeneity and pristine riparian bamboo forest in the middle river might lead to the highest fish species richness and trophic guild diversity in the middle river.However,the flow regulation,high N-P nutrient concentrations and urban land use in the lower reaches,which were reflected by CCA results,significantly influenced the structure and composition of fish trophic guilds downstream.Moreover,the great contribution of exotic species to the composition of trophic guilds indicated high adaptability of tolerant species to resource utilization in deteriorated conditions.?3?At the sites seriously influenced by human activities,the import of external nutrients seriously influenced the basic signals of?¹³C and?¹⁵N of aquatic organism in the aquatic food webs.At site 3 and site 6,the?¹⁵N values of aquatic organisms were significantly higher than upper and lower reaches,respectively,indicating the nitrogen from discharged wastewater has been utilization by the primary producers and further transferred to consumers at high trophic levels.According to the results of SIAR based on both?¹³C and?¹⁵N values,periphyton was the main organic matter sources for the fish food webs in upper stream.In the transition areas between wadeable stream to non-wadeable river?site 3?,macrophyte were important sources for local fish food webs.In the middle river site 4,upstream POM and riparian organic matter were flushed into the local food webs,as a result,the?¹³C and?¹⁵N signals of POM and riparian C3 plants were more significant than other sources.In lower reaches,POM became the most important organic matter for food webs at site 5,while the contribution of periphyton increased at site 6 as local important organic matter.Moreover,as a large river that seriously influenced by human activities,the disturbed fragments of the East River were underlying the negative impacts of sewage discharge;the site-specific?¹⁵N signals indicated the imbalance and illness of the aquatic ecosystems.?4?The longitudinal variation in?¹⁵N values showed that the average of tropical levels in upper stream were much higher than that in middle-lower reaches.The higher tropical levels in upper stream were mainly caused by the large consumption of aquatic insects by insectivorous fish groups?guild I?with great contribution to the composition of upstream fish assemblages.As a result,most of the upstream fish consumers were at trophic level 3?aquatic insects were at trophic level 2?.Based on the six Layman matrix,the convex hull of?¹³C and?¹⁵N values from six wide-distributed fish species showed that the width,length,diversity,position and evenness of their ecological niche were all determined by local available organic matter sources.Combine with the SCA results,for fish consumers,the parameters?¹³C and?¹⁵N ranges,total area?TA?,mean distance to centroid?CD?,mean nearest neighbor distance?NND?,standard deviation of nearest neighbor distance?SDNND?were mainly influenced by the utilization of local food resources.Moreover,TA and CD values directly reflected the changing diversity of organic matter sources.?5?Six quite different ecosystem structures were observed along the East River.As three basic resources,periphyton,macrophyte and detritus influence the formation of trophic networks through bottom-up control.Aquatic insect collectors were important secondary production in the whole river basin,and the submerged macrophyte in the middle-down river ecosystem were limited producer resources.At the system level,the top-down control was weak as most consumers have'beneficial predation'on their prey.The invertebrate predators are keystone groups in headwater,while they were replaced by top carnivorous fish in the middle river and by zooplankton and intermediate omnivorous fish in the lower reaches of river.The attributes that estimating the system maturity indicate that stream habitats with extensive floodplain and less human disturbance were much more mature than the middle-down river ecosystems.The immaturity of was mainly caused by the eutrophication and flow regulation.