Spatial And Temporal Variation In The Trophic Structure Of Coral Reef Food Webs

The coral reef ecosystem is one of the most diverse ecosystems in the world.It provides biological resources for society and perform significant economic functions.Nonetheless,coral reef ecosystems are suffering systematic degradation worldwide due to global climate change and human activities,with a sharp decline in live coral coverage,loss of three-dimensional spatial structure,and decreasing spatial heterogeneity.In recent years,due to the interference of climate change and human activities,many coral reefs around the world have experienced frequent coral bleaching events,decreased coral coverage and coral reef biological resources decline.Although coral transplantation and artificial reef technology have been used to carry out a series of coral reef restoration projects,the overall restoration effect is not satisfactory,which is not only related to the lack of detailed and in-depth understanding of the nutrient structure and energy flow process of degraded coral reef ecosystem.In particular,coral reef restoration is not only the restoration of habitat,but more importantly,the restoration of its structure and function.This requires a systematic understanding of energy flow and material circulation in coral reef ecosystems.In recent years,under the influence of various stress such as ocean warming,acidification,and overfishing,some coral reefs on Weizhou Island have suffered from coral bleaching,resulting in a decline in coral cover and biological resources.Although a series of coral reef restoration projects have been carried out using coral transplantation and artificial reef technology,the overall restoration effect was not satisfactory,which may be due to the lack of understanding of the species composition and energy flow of the entire coral reef ecosystem.In this study,we take the coral reefs of Weizhou Island as the research area,using carbon and nitrogen stable isotope technology(δ¹³C andδ¹⁵N)to study the food web structure in September 2021（autumn）and April 2022（spring）,estimated the temporal and spatial changes of trophic levels（TL）and main carbon sources of primary producers, zoobenthos,and nekton.Combined with the six quantitative indicators of community trophic structure,the feeding habits and stable isotope niche characteristics of zoobenthos and nekton were analyzed.The trophic structure of coral reef food network on Weizhou Island was constructed based on the contribution of organic carbon sources and the feeding information of each trophic guild,which provided basic data for the protection and restoration of coral reef.The major findings are as follows:（1）The organic carbon sources of Weizhou Island coral reef are mainly POM,SOM and macroalgae.The results show that theδ¹³C value of the organic carbon source of the coral reef of Weizhou Island ranges from-23.3±0.8‰to-11.9±0.6‰,and theδ¹⁵N value ranges from 4.5±0.3‰to 9.7±0.2‰.There are significant differences inδ¹³C andδ¹⁵N among different organic carbon sources,but there is little difference in stable isotope of the same type of organic carbon sources among different stations.At the seasonal scale,the difference ofδ¹³C values between the same organic carbon sources is not significant,but the difference ofδ¹⁵N values is significant.（2）Weizhou Island collection of benthic animals mainly include bivalves,gastropods,small shrimp,crabs,echinoderms and polychaetes,while nekton mainly include large shrimp and crabs（mainly shrimp and crab）and fish.The results show that theδ¹³C values of zoobenthos range from-20.7±0.3‰to-14.0±1.5‰and theδ¹⁵N values range from 7.5±0.6‰to 14.6±0.9‰in autumn,while theδ¹³C values of zoobenthos range from-19.8±0.4‰to-14.1±0.2‰and theδ¹⁵N values range from4.1±0.3‰to 14.4±0.1‰in spring.Overall,there was no significant difference in theδ¹³C values of the same group of zoobenthos among different stations,and theδ¹⁵N values of bivalves and gastropods were significantly different between spring and autumn.For nekton,theδ¹³C values in autumn were wider than in spring,while theδ¹⁵N values in spring were wider than in autumn.In addition,theδ¹³C andδ¹⁵N values among nekton were not representing significantly different among different stations,while there were significantly different among seasons.the trophic levels of zoobenthos ranged from2.0 to 2.8 in autumn and from 2.0 to 3.3 in spring,with a larger width of trophic levels in spring.There was no significant difference in the trophic levels of zoobenthos of the same guild among different stations,but there were significant differences in seasons.For nekton,the trophic level ranges from 2.5 to 3.4（average trophic level:3.0±0.8）in autumn and the trophic level ranges from 2.5 to 4.1（average trophic level:3.5±0.0.8）in spring.The result showed that the trophic level in spring was higher and wider than in spring,indicating that there are more carnivorous fishes in spring.（3）Weizhou Island reef zoobenthos were divided into seven trophic groups including bivalves,phytophagous gastropods,carnivorous gastropods,small crabs,small shrimps,echenoderms and polychaetes according to the morphological characteristics and Celsius habits of consumers.Nekton are divided into six trophic groups:omnivorous crustaceans,carnivorous crustaceans,omnivorous fishes,planktivorous fishes,benthic carnivorous fishes,and piscivorous fishes.The temporal and spatial changes of nutrient structure in zoobenthos and Nekton were preliminatively analyzed by quantitative indices of trophic structure（CR,NR,TA,CD,MNND and SDNND）.The results showed that the stable isotope niche of each trophic guild was basically consistent between seasons,but there were different degrees of overlap between different functional groups of zoobenthos and nekton,indicating that there was overlap between different trophic guilds of zoobenthos and nekton on Weizhou Island,and the food competition was fierce.（4）Calculate the carbon source contribution of different organic carbon sources to the zoobenthos and nekton by using the Bayesian mixture model.The study showed that the major carbon source of most of the trophic guilds in spring and autumn is macroalgae.Although POM and SOM gradually increased carbon sources to benthic communities in spring,the contribution of macroalgae remained large.For nekton,the main carbon source contributors were the macroalgae and SOM.Through the Ecopath model,the simplified food web and trophic structure revealed that herbivorous and omnivorous zoobenthos were the key drivers of organic carbon sources（such as macroalgae）flowing through the food web.