| The stable output of target species of high-trophic fishery resources in marine ranching depends on the supply level of basic food organisms.As an intermediate link in the marine food chain,crustaceans,on the one hand,act as food organisms to restrict the output of proliferating target species through the upward effect,and on the other hand,affect the stability of the basic energy flow link due to the downward nutrient cascade effect,and then determine the nutrient structure dynamics of marine ranching.Therefore,based on the data of crustacean trawl survey in the marine ranching construction area of Dachen Island,this paper analyzed the community pattern and the food source of dominant species at the local scale,so as to clarify its ecological value and potential position in the marine ranching.Bottom trawl sampling was carried out in the open sea area and rocky reef area of the planning and construction area of Marine Ranching in September and November 2020 and January and April 2021.The biodiversity was comprehensively evaluated from three dimensions: species diversity,phylogenetic diversity and functional diversity.Multivariate statistical methods were employed including hierarchical clustering and non-metric multidimensional scaling(n MDS)to analyze the community structure.Environmental factors such as temperature,salinity and depth were investigated simultaneously to resolve the influencing factors using redundancy analysis(RDA),while community stability was assessed using abundance biomass curves(ABC)and W-statistics.DNA barcoding technology was used to analyze the food sources of dominant shrimp species to reveal the key energy sources supporting the crustacean community in the sea area.The main findings are as follows:(1)Characteristics of biodiversity: On the time scale,biodiversity had seasonal heterogeneity.In autumn,the species composition was the most abundant,the genetic relationship was relatively distant,and the functional richness was the lowest.The main reason was the occupation of ecological space by predatory crustaceans consisting of Oratosquilla oratoria and Portunus trituberculatus,which made the effective resource use inefficient and leaved ecological space vacant.In summer,the number of species was the lowest,but functional richness was higher,as well as fewer ecological niches remained.Because the dominant position of predatory crustaceans was not prominent,the resources in the summer sea area were more fully utilized and the interspecific competition was intensified.The number of species was intermediate in winter and spring,with a high degree of complementary ecological niches,reduced resources of predatory crustaceans,and intense interspecific competition.At the spatial scale,the sea area showed consistency,as interregional differences in crustacean composition at the local scale were not significant.There were different degrees of correlations among indices of the same dimension,with the strongest correlation among species diversity indices.Meanwhile,There was a strong correlation between species richness and functional diversity across latitudes,for example,FRic was always constrained by species richness(S).The taxonomic diversity index(Delta)and the taxonomic difference index(Delta*)were significantly correlated with the functional dispersion index(FDis).The combined use of all dimensions of diversity in biodiversity assessment will reveal a more comprehensive and complete ecological process behind the index.The three dimensions of biodiversity were determined by different ecological factors,with functional diversity responding most strongly to local-scale environmental factors,followed by spectral diversity,and species diversity likely driven primarily by larger-scale ecological factors.(2)Spatial-temporal pattern of community structure: A total of 53 species of crustacean were collected,belonging to 29 genera,13 families,and 2 orders,with shrimps accounting for 56.06% of the total species number.There were 43 species found in the autumn,which was followed by 41 species in the winter,31 species in the spring,and 26 species in the summer.The seasonal changes of the dominant species were obvious.In summer and autumn,the dominant species were medium-and large-sized economic species,such as Parapenaeopsis hardwickii,Solenocera crassicornis,and Portunus trituberculatus,whereas small individual bait species like Charybdis bimaculata and Parapenaeopsis tenella dominated in the winter and spring.Oratosquilla oratotria was revealed as the only dominant species in common in the four seasons.The density of crustacean resources fluctuated dramatically throughout the year with an uneven spatial distribution,peaking in autumn and declining in spring,and the highly significant higher in summer and autumn than in winter and spring(P<0.001),with the highest resource density in the near rocky reef area and the smallest in the shallow water area on the estuary side.Affected by the migration and selective distribution of seasonal migratory species,as well as the regional differentiation of the hydrological environment in the reef system,the crustacean community structure had significant seasonal changes and regional differences,forming a pattern of high heterogeneity in summer and autumn,high similarity in winter and spring,and outstanding differentiation between shallow and deep water areas on the estuary side and the outer side.Environmental factors such as bottom water temperature,bottom salinity,bottom dissolved oxygen and water depth were significantly correlated to community structure heterogeneity.The ABC curve showed that crustacean community was stable and had a strong tolerance to disturbance.The study demonstrated that the island reef ecosystem can support more abundant crustacean resources.The dynamics of crustacean resource density change in the Dachen Island Reef waters were mainly derived from seasonal migratory species such as Penaeus hardwickii,P.sinensis,and P.trituberculatus.Crustacean communities still have ’ subtle ’ structural differences at the local scale.(3)Food sources of dominant shrimps: P.harveyi and P.sinensis had a wide range of food sources,involving 210 species from 37 phyla.Chordata,Arthropod,Nemertean,Dinophyta,Chlorophyta and Cercozoa composed the top ten phylums of relative abundance.The two shrimps had similar bait taxa,but different feeding preference species and divergent feeding habits.Chaeturichthys stigmatias and Acanthogobius ommaturus as the absolute dominant bait of P.harveyi,while animal bait,mainly fish and small crustaceans,contributed up to 50% of its diet.P.harveyi was closer to a highly opportunistic carnivorous and scavenging predator,preferring to obtain energy from animal foods at higher trophic levels.Sardinella zunasi,Gymnodinium,Coccomyxa were advantage baits of P.sinensis.It was an omnivorous shrimp that the proportion of plant and animal bait is equal.Nutrient niche separation between the two shrimps facilitated the full use of material and energy in the sea. |
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