| Seagrass bed is one of the most productive marine ecosystems with high ecological and economic value.But under the dual pressure of human activities and climate change,the area of seagrass bed is declining.Food web research is the basis for the study of consurmers nutrient structure and energy flow pathways in ecosystems.It plays an important role in ecosystem,which provide scientific basis managers formulate relevant environmental protection policies.Take the seagrass bed ecosystem in Wenchang city,Hainan province as an example,carbon and nitrogen stable isotope characteristics of organic carbon sources(Particulate organic matter,Sediment organic matter,seagrass and epiphytes)and consumers(25 species of fishes,6 species of crustaceans,10 species of gastropods,12 species of bivalves and 1 species of porifera)that in seagrass bed were analyzed by Stable Isotope Analysis(SIA).The trophic levels of different consumers were calculated based on the nitrogen stable isotope characteristics in the organism.And the continuous trophic spectrum was constructed preliminarily.Then IsoSource was used to calculate the contribution range of different organic carbon sources to cons umers.Combined with research results of diet of relevant consumers,a simplified diagram of food web and trophic structure in seagrass of Wenchang was constructed.By analyzing the characteristics of the food web,scientific data and theoretical basis were provided for the protection and ecological restoration of the local seagrass bed.The results indicate significant carbon stable isotopic differences among different organic carbon sources,where ?13C of particulate organic matter(POM)is the most depleted(-23.35±1.69 ‰),whereas that of seagrass is the most enriched(-9.56:±0.68 ‰).For macrobenthos,the values of ?13C and ?15 of crustaceans,gastropods and bivalves are relatively concentrated,which implies that they have relatively fixed ecological niches.The distribution range of carbon and nitrogen stable isotopes of fishes is large and irregular,which indicates that fishes have a wide range of organic carbon sources and trophic levels.For macrobenthos,the mean trophic level of bivalves is 1.98,that of gastropods is 2.13,and that of crustaceans is 2.89.Mollusks are the primary and secondary consumers of the food web,and crustaceans are higher-level consumers.In mollusks,gastropods have a wider range of trophic level and a more complex nutritional structure than bivalves because of their wider feeding habits.Trophic level of the main fishes in the seagrass bed of Wenchang city range from 2.01 to 3.91,with an average value of 2.95.Hence,fish has a complex nutritional structure.Most of the fish in seagrass ecosystem are j uveniles,which leads to the subnormal trophic level of fish.Moreover,most of them are carnivorous fish,suggesting that the seagrass bed may have more significant nursery function for the carnivorous fish in the adjacent marine ecosystem.Based on the contribution of different organic carbon sources to fishes and macrobenthos,fishes are divided into four groups by cluster analysis,and macrobenthos are divided into three groups.The results show that seagrass and epiphytes are the important organic carbon sources of 18 species of fishes and 24 species of macrobenthos.This indicates that seagrass and epiphytes are the most important organic carbon sources in seagrass bed ecosystem food web.Based on the diagram of simplified food web and nutrient structure of the seagrass bed in Wenchang,it is found that the detritivore and herbivore benthos are key intermediate l ink that drive the organic carbon of seagrass into the food web. |
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