Trophic structure and energy flows in the radial sand ridges 

and adjacent waters off the coast of Jiangsu Province

Abstract: The radial sand ridge group in Jiangsu coastal area, located in the southern Yellow Sea, is a unique combination of large sand bodies in the shallow waters of the continental shelf of China. It becomes a special ecological function area due to the influence of multiple factors such as land runoff, sea current and seabed topography, with unique ecological environment. However, fishery resources in this area have declined due to multiple pressures such as overfishing and environmental degradation. At present, studies on this area mainly focus on species diversity, community structure, environmental evolution and simulation, and ecosystem service, while studies on the trophic structure and energy flows in this ecosystem are few. This study analyzed the state of the ecosystem in the area by using Ecopath model, which defined the ecosystem as consisting of a series of functional groups with ecologically similar characteristics, all of which could basically cover the whole process of energy flow in the ecosystem. The model was based on the principle of nutrient balance, which described the balance of energy input and output of the system at a specific time. Subsequently, a massbalance model was constructed by Ecopath with Ecosim software based on the comprehensive survey data collected from the radial sand ridge areas of Jiangsu Province from 2018 to 2019. And the trophic structure, mixed trophic impact and overall characteristics of this ecosystem were analyzed. The ecosystem condition of this area was also evaluated to provide technical support for the protection and restoration of the ecosystem of the radial sand ridge group off Jiangsu, as well as to provide a theoretical basis for protection, management and rational use of the biological resources in this area. The results showed that trophic levels of functional groups in this ecosystem varied from 1.00 to 4.31, consisting of 27 functional groups. The food web of this marine ecosystem was mainly composed of grazing food chain and detritus food chain. The trophic levels of the main fishery species and dominant species in this area ranged from 3.0 to 4.0, and the trophic levels of top predators such as Miichthys miiuy and Harpadon nehereus were above 4.0. Mixed trophic effects analysis (MTI) showed that zooplanktivores, Erisphex potti, other shrimps, Echinodermata, other mollusks and zooplankton played key roles in energy transfer. Within this ecosystem, the energy conversion efficiency of lower trophic levels was low mainly due to the large number of terrestrial organic detritus and phytoplankton, and insufficient utilization by the primary and secondary consumers in this ecosystem. In recent years, the coastal waters of Jiangsu Province have been continuously affected by human disturbances such as reclamation projects and deepwater channel management in the Yangtze River Estuary, which have affected the habitats of filterfeeding organisms (e.g., shellfish, small shrimp and crab species) that use detritus and phytoplankton as their main food sources, resulting in lower utilization of low trophic level prey organisms. The energy conversion efficiency was significantly higher at high trophic levels, which might be due to the fact that the main fishery organisms in this ecosystem were mostly located between trophic levels Ⅲ and Ⅳ, and they played a key role in the energy conversion process of the ecosystem because of their more efficient use of lower trophic level bait organisms. The total ecological conversion efficiency of the ecosystem was 14.93 %, which was slightly higher than that in the other adjacent sea areas. This indicated that the overall utilization of energy in this ecosystem was higher, which might be due to the fact that the radial sand ridge group sea area was crisscrossed by harbors and branches, densely covered with sand hones, and was in a complex tidal dynamic environment. And it was also subject to the transport, remodeling and accumulation of strong tides, which was conducive to maintaining the diversity and high productivity of the biological community. The main nutrient sources of this ecosystem were detritus and phytoplankton, and the energy flows between trophic levels in the grazing food chain were higher than that in the detrital food chain. The total flow (TST) of this ecosystem was higher (8 950 t·km-2·a-1), indicating that the overall scale of the ecosystem was relatively large, which was conducive to maintaining the ecosystem health. The total primary production (TPP) value of the ecosystem in the radial sand ridge areas of Jiangsu Province was 4 135 t·km-2·a-1, which was higher than that in the adjacent areas, but lower than that in the Gouqi kelp bed. However, the ratio of total primary production to total respiration (TPP/TR) in this ecosystem was 6.97, which was significantly higher than that in the adjacent areas. The connection index (CI) and system omnivorous index (SOI) were 0.34 and 0.28, respectively. The connectivity index was at a high level compared with the neighboring areas, while the system omnivorous index was at an intermediate level, which indicated that this ecosystem was in an unstable state and was susceptible to external disturbances. This paper will contribute to the understanding of trophic structure and overall condition of the ecosystem in the radial sand ridge areas of Jiangsu Province, which can provide technical support for the protection and restoration of this ecosystem and theoretical guidance for the sustainable use and management of fishery resources. In the future, we should strengthen the protection and restoration of the ecosystem in this sea area, reduce the impact of anthropogenic disturbance on the ecosystem, and make full use of the high primary productivity of the ecosystem in this sea area through measures such as stocking and releasing. At the same time, it should be combined with the construction of marine protected areas, control of fishing intensity and other measures to strengthen the conservation of fishery resources and ecosystem restoration, increase the stability of the ecosystem in this sea area, and give full play to the resource and environmental advantages of the ecosystem of the radial sand ridge group, which will be of great practical significance for promoting the highquality development of marine fisheries in Jiangsu Province, and the construction of marine ecological civilization.

Keywords: radial sand ridges; Ecopath model; trophic structure; energy flow