| Oceans are the largest carbon pool on earth, and marine food webs play an important role in the carbon cycle. Research on marine food webs has revealed many key processes in ecosystems, such as nutrient cycling and energy flow. However, the complexity of basic carbon sources complicates the study of trophic structure and carbon cycling in the food web. Many studies have focused on the contribution of food web carbon sources in rivers, lakes, estuaries, and other aquatic ecosystems, but few have considered the contribution of food web carbon sources in the sea adjacent to coastal islands. The adjacent sea area of nearshore islands is a transition area between ocean and land and plays an important role in the carbon cycle and ocean carbon sink. The Miaodao Archipelago, located in the Bohai Strait in northern China, is at the intersection of the Bohai Sea and the Yellow Sea. Its ecosystem includes sea, intertidal zones, and island land and is extremely rich in biological resources. It is an ideal location to study the carbon source and sink dynamics of the food web in seas adjacent to islands. Isotope ecology provides a unique perspective for the study of carbon sources and sinks. The isotopic composition of an animal´s tissues depends on its food and reflects the overarching environmental conditions assimilated over time. An animal´s carbon isotopic composition can reveal its food sources. Because stable nitrogen isotopes are typically enriched in the consumer´s body, the amount of 15N increases with higher trophic levels. To investigate the contribution of different carbon sources to the food web and the mechanism of carbon sequestration, we studied the basic food resources and food web in the area adjacent to the Miaodao Archipelago in autumn 2020. The stable isotope analysis in the R (SIAR) package in R 4.0.5 was used to estimate the contribution of multiple food sources to each consumer species; this model incorporates stable isotope values (δ13C and δ15N) in the siarmcmcdirichletv4 command to estimate the relative energy contributions of different base energy sources (base food sources or primary producers). Based on the stable isotope method of carbon and nitrogen, the Bayesian mixing model SIAR analyzes the relative contributions of different carbon sources (phytoplankton, macroalgae, and attached algae, suspended particulate organic matter (POM), and substrate organic matter (SOM)) to the main consumer groups (aquatic benthic invertebrates, omnivores, benthivores, and piscivores). Based on this, we discuss marine carbon sinks and carbon sink fisheries. The δ13C isotope ranges from –26.54‰ to –16.92‰, δ15N ranges from 0.38‰ to 8.58‰, and δ13C ranges from –25.43‰ to –16.74‰, δ15N ranges from 2.84‰ to 13.06 ‰ for benthic invertebrates. Fish δ13C ranges from –22.26‰ to –15.10‰, and δ15N ranges from 7.35‰ to 14.54‰. The results showed that algae (phytoplankton, macroalgae, and ancillary algae) and SOM were the main carbon sources in the autumn food web of the waters adjacent to the Miaodao Archipelago, and the contribution of algae was the highest, while the contribution of POM was relatively low. The carbon source was mainly endogenous. Macroalgae and attached algae contribute more to aquatic benthic invertebrates. With regards to carbon sink fisheries, increasing algal cultivation and mixed cultivation of shellfish or shellfish feeding fish can increase carbon sink capacity and promote the development of carbon sink fisheries. The contributions of large algae, adherent algae, and benthic algae to the marine food web are controversial. In the present isotope study, large algae and adherent algae were the main carbon source of benthic invertebrates and fish, and their contribution to the carbon source was relatively high. Although food webs play a very important role in the ocean carbon sink and carbon cycle, the specific carbon sink process is still unclear. This was a preliminary study on the ocean carbon sink from the perspective of stable isotopes, and the results contribute to our understanding of the ocean carbon sink process. Through research on the contribution of carbon sources to the food web, the carbon intake of fish can be calculated more accurately, which helps us further understand the pathways and processes of carbon sequestration in marine organisms, the carbon cycle at the sea-land interface and promotes the development of a more reasonable carbon sequestration fishery management strategy. However, the food web is a dynamic and complex process, and many uncertainties underlie the carbon sink mechanisms of marine organisms. The carbon source contribution is only a relatively simplified model, which cannot fully interpret complex carbon source mechanisms. In future studies, the selection of carbon sources should be optimized under the guidance of the carbon sink fishery concept. The biological role of food webs in marine carbon sinks should be further studied. |
