Species Diversity And Trophic Structure Of The Fishes In The Intertidal Salt Marsh Of Nanhui Dongtan, Changjiang River Estuary

Fish is an important part of the estuarine intertidal salt marsh ecosystem and plays an important role in the structure and function of the ecosystem.As an important ecological transition zone,intertidal salt marsh provides fish with various ecological functions such as feeding,spawning,breeding and shelter.Influenced by multiple factors,such as tides,freshwater flow,seasons,etc.,and under a large number of human activities,the environmental conditions of the ecosystem are complex and variable.It is of great significance for habitat protection and sustainable development of fish resources to investigate the temporal and spatial differences in fish species diversity and trophic structure the intertidal salt marsh of estuary and to analyze the main factors influencing the fish species diversity and trophic structure.Based on the present researches at home and abroad,the typical area of the salt marsh in the intertidal zone of Nanhui Dongtan in the Yangtze estuary was choosen to set up a fixed sampling sites in our study.The characteristics of fish assemblages and related environmental factors were surveyed seasonly.Based on these,the spatial and temporal differences of fish species diversity and trophic structure as well as its correlations with environmental factors were analyzed to explore the main influencing factors of fish assemblages and the ecological functions of intertidal salt marsh.It can provide a scientific basis for the ecological restoration of the intertidal salt marsh as well as the conservation of fish resources.Main results and conclusions were summarized as follows:1.The fishes were mainly small fishes and juveniles,with diverse ecological groups,dominated by resident estuarine species and demersal fish respectively.There were differences in fish community composition and diversity characteristics between the vegetated zone and adjacent mudflat zone.The diversity of fishes in summer and autumn was higher than that in winter and spring.The diversity of fishes in the mudflat zone was higher than that in the vegetated zone.?1?During the investigation,25 species of fishes were recorded,belonging to 8orders,16 families and 24 genera,of which Perciformes was the largest proportion observed.According to the residential type,the largest proportion was resident estuarine species.According to the inhabiting water layer,the largest proportion was demersal fish.The fishes caught were small in terms of both body length and body weight,dominated by small fishes and juveniles.?2?There were 11 species were recorded both in the vegetated zone and adjacent mudflat zone.Some species only appeared in the mudflat zone.The number of species,biomass,abundance and species diversity of the fishes recorded in the mudflat zone were higher than those in the vegetated zone.The main dominant species in the vegetated zone included Mugil cephalus,Boleophthalmus pectinirostris and Eleutheronema tetradactylum.The main dominant species in the mudflat zone included Cynoglossus joyneri,Odontamblyopus rubicundus and Lateolabrax maculatus.The superiority of dominant species in the vegetated zone was higher than that in mudflat zone.?3?There were seasonal changes in fish composition in two different habitats.Compared with those in summer and autumn,the number of species,biomass and abundance were at low levels in winter and spring.The number of species in autumn was higher than that in summer,but the abundance,biomass and diversity of fishes were less than those in summer.The indicator species in autumn were Mugil cephalus,Eleutheronema tetradactylum,Takifugu obscurus,and Johnius belangerii.The indicator species in summer were Boleophthalmus pectinirostris and Mugil cephalus.According to the cluster and nMDS analysis on the fishes in the different habitats and seasons,the influence of different seasons on fishes was greater than that of different subhabitats.2.The fishes were mainly benthic feeding.Crustaceans were important bait,and the detection rate and abundance of microplastics were high.There were spatial and temporal differences in feeding,stable isotope composition and trophic structure of the fishes.?1?The fishes had a wide range of food sources,mainly benthic feeding.Crustaceans?mainly benthic shrimps and crabs?were important bait for the fishes in the intertidal salt marsh.Microplastics were found in the main fishes,with high microplastics detection rate and abundance.The feeding of different species showed temporal and spatial differences.?2?The ?¹³C value of fishes ranged from?30.14‰to?14.01‰with a span of 16.13‰;the value of?¹⁵N ranged from 7.70‰to 16.61‰with a span of 8.91‰.The?¹³C and?¹⁵N value of different species showed different temporal and spatial differences.?3?The trophic structure of the fishes in the intertidal salt marsh showed spatial and temporal differences.Compared with those in the mudflat zone,the basic food sources of fishes were more diverse,the trophic vertical structure was more simple and the trophic redundancy was lower in the vegetated zone.Compared with summer and autumn,the fishes in winter and spring had a simpler trophic structure and higher redundancy.Compared with those in autumn in the the vegetated zone,fishes in summer had high trophic diversity,simpler vertical structure and lower trophic redundancy.Compared with those in autumn in the mudflat zone,fishes in summer had lower trophic diversity,more simple vertical structure and higher trophic redundancy.3.The significant spatial and temporal differences of multiple factors including the characteristics of Scirpus mariqueter,characteristic of macrobenthos,sedimentary physicochemical indicators and hydrological factors were the key leading to differences in fishes in estuarine intertidal salt marsh.?1?There were significant spatial and temporal differences in environmental factors in estuarine intertidal salt marsh.The plant density,plant height,total biomass and aboveground biomass of Scirpus mariqueter were significantly different in different seasons?P<0.01?.The density and species richness of macrobenthos were significantly different between different seasons?P<0.05?.The density,species richness,evenness index and diversity index were significantly different among different habitats?P<0.05?.The tidal hydrological factors such as flooding time,average flow rate,maximum flow rate,average water depth,maximum water depth,turbidity,temperature and salinity were significantly different in different seasons?P<0.05?.In addition to salinity,other hydrological factors were also significantly different in different habitats?P<0.05?.Sediment porewater temperature,pH and salinity had significant differences in different seasons and habitats?P<0.05?.There were significant?P<0.05?or extremely significant?P<0.01?spatial and temporal differences in water content,median diameter,bulk density,total nitrogen content,total carbon content at specific depths of the sediment.?2?Tidal hydrological factors,characteristic of macrobenthos and sediment physical indicators were the main factors influencing the diversity of fishes in intertidal salt marsh.The combination of 7 factors,including mean flow rate,maximum flow rate,number of macrobenthos species,species richness of macrobenthos,macrobenthos biomass,total carbon content?0–2 cm?and total nitrogen content?2–5 cm?of sediments,had significant effects on the total abundance of the fishes?P<0.01?.The combination of 8 factors,including mean flow rate,maximum flow rate,macrobenthos species diversity index,macrobenthos biomass,total nitrogen content?0–2 cm,5–10 cm?and bulk density?0–2 cm,2–5 cm?of sediments,had significant effects on the total biomass of the fishes?P<0.05?.There were differences in the correlation between fishes and environmental factors in different seasons,habitats,inhabiting water layers and residential types.?3?In addition to the above-mentioned factors,the morphological and biomass characteristics of Scirpus mariqueter,as a basic food source,had an important impact on the trophic structure of the fishes.The combination of 12 factors,including mean flow rate,maximum flow rate,mean water depth,salinity,number of macrobenthos species,species richness and evenness index of macrobenthos,plant height and aboveground biomass allocation of Scirpus mariqueter,porewater salinity and pH,total nitrogen content?0–2 cm?of sediment,had significant effects on the trophic structure of fishes?P<0.05?.Environmental factors had different effects on trophic structure of the fishes in different habitats and seasons.Overall,there were significant spatial and temporal differences in fish diversity and trophic structure,and they were influenced by the comprehensive effect of multiple environmental factors such as hydrology,sediments,vegetation,and macrobenthos.Attention should be paid to the importance of the salt marsh vegetation in the intertidal zone,the comprehensive effects of environmental factors on fishes should be emphasized and an ecosystem-based management mechanism should be established for the ecosystem restoration and sustainable development of fishery resources.Future research could be carried out expanding the coverage of environmental factors and in more timescales to further analyze the comprehensive mechanism of environmental factors and systematically study the key factors influencing fishes in the intertidal salt marsh.