Analysis Of Food Web’s Isotopic Baseline And Enrichment In The Macroalgae Bed Of Gouqi Island

Gouqi island is located in the northeast of the Zhoushan islands in Zhejiang province. The macroalgae beds, which is surrounded by waters of this island, attribute to one of the typical ecosystem of coastal waters. Meanwhile, it is the important habitats of fish, arthropods, mollusks, echinoderms and other marine organisms. The material recycle and energy flow of macroalgae beds food web is the key topics in the research of this ecosystem, and has important significance in examination of its complex structure and function.Stable isotope analysis is an effective tool in studying mass and energy dynamics in aquatic ecosystem. This technique is based on the fact that as predators consume prey, the change in the carbon stable isotope values [δ¹³C(C¹³/¹²C)] of the predator are conservative, relative to the prey consumed, allowing identification of basal productivity or foraging locations. While the nitrogen stable isotope values [δ¹⁵N(¹⁵N/¹⁴N)] show a stepwise enrichment from prey to predators, providing a method to quantify trophic position. Isotopic baseline species can reflect the isotopic characteristics of the original source material in the food web. Therefore, the accurate identification on isotopic baseline species of the food web is of importance when estimating the trophic levels of consumers and evaluating the trophic structure of kelp ecosystem. Primary consumers（i.e. snail and mussel） have low metabolic rate and long life history. Therefore, many aquatic ecologists have attempted to use snails and mussels as the baseline species. However, in the macroalgae beds, pelagic food chain and benthic food chain are existed simultaneously, and the variation in the δ¹³C and δ¹⁵N of the base of the food web have been found within and among ecosystems, which might make it difficult for investigating the food web structure and carbon flows using stable isotope ratios accurately. Therefore, we need to estimate the isotopic baseline in specific ecosystem. In this study, we collected the primary consumer species with large abundance and wide distribution in kelp ecosystem, the foot tissues of snail and mussels were used for the stable isotope analyses, respectively. The relationship between the morphology and isotopic characteristics was investigated. The δ¹³C and δ¹⁵N values of all species from seaweed field between sampling sites（seasons） were explored to select the isotopic baseline species. On this basis, the feeding preferences of isotopic baseline species were estimated through the indoor feeding analysis, calculated the nutrition contribution of different algae and discrimination of the ecosystem. We hope to provide basic data and important references for the research of other island waters in the East China Sea. The results can be summarized as follows:（1） Ontogenetic change in dietary was observed in organisms in different life cycle stages, and reflected by δ¹³C and δ¹⁵N values. The feeding organs of snail and mussel was changed with body growth, this may affect isotopic values of organisms, interfered the choice of baseline species. Based on the survey results of benthic species in 2012, we chose four primary consumer species with large abundance and wide distribution. Two snail species（Turbo petholatus, Turbo cornutus） and two mussel species（Septifer virgatus and Modiolus comptus） were collected in kelp ecosystem. The relationship between the morphology and isotopic characteristics was investigated. The results showed that the less influence of growth was found to δ¹³C and δ¹⁵N values of T. petholatus, and significant relationship were found between the morphology and isotopic characteristics of other primary consumer species. Therefore, in the subsequent identification on isotopic baseline species of the food web, the samples were grouped by different sizes.（2） Snails and mussels exhibited significantly different stable carbon but similar stable nitrogen signatures, suggesting they have similar trophic position but selectively ingest and assimilate different suspended particulate mixture from littoral and pelagic food chain, respectively. The results showed that δ¹³C and δ¹⁵N of macro-benthos were affected both by their diets and habitat environment. Compared with T. petholatus, T. cornutus had no significant difference in isotopic compositions were found between the two snail species in each sites（seasons）. We found that T. cornutus reflected the isotopic signature of the base of the littoral food web. The variation scales in δ¹⁵N and δ¹³C of different mussel species were different.（3） Stable isotope analysis can be used to determine the nutritional contribution rate of different food sources. In the process of feeding organisms preferentially removes heavy isotope(C¹³, ¹⁵N) and leaves light isotope(¹²C, ¹⁴N). Generally the enrichment of isotope named discrimination（Δ）. The discrimination is very important for the analysis of the contributions rate. T. cornutus was the baseline species of benthic food chain, the feeding preferences of isotopic baseline species were estimated through the indoor feeding analysis, calculated the nutrition contribution of different algae and discrimination of the ecosystem. The results showed that the Δδ¹³C and Δδ¹⁵N were 0.00‰ and 3.35‰, respectively. T. cornutus preferred to feed soft algae, however, the main source of nutrients from the hard algae.