Abstract:In recent years, aquaculture has rapidly developed in many countries, playing a positive role in ensuring food security and promoting economic development. However, it has also produced negative effects, such as water pollution and eutrophication. As the key species in integrated aquaculture systems, bivalves not only improve space utilization and provide economic benefits, but also regulate nutrient cycling, reduce water body eutrophication, increase the ability of blue carbon sinks to capture and hold carbon, improve system stability, and perform various ecosystem services, including nutrient removal and carbon sequestration. However, as a resource-dependent aquaculture industry, high-density and unreasonable bivalve aquaculture produces a strong downlink control effect on the phytoplankton community structure, which in turn restricts the carbon sink function of shellfish aquaculture ecosystems and negatively affects the ecosystem. China is the dominant country in terms of shellfish farming. In 2020, the total output of marine shellfish was 14.80 million tons, ranking first in the world. In China, the main target of marine shellfish farming is bivalves, which account for 95% of the total marine shellfish output. The total output of Ruditapes philippinarum is over 3 million tons, accounting for 90% of the global output. Jiaozhou Bay is an important large-scale aquaculture base for R. philippinarum in northern China, with a clam output of 325 000 tons, accounting for 91.5% of the total output from this base. In April 2017, the core purpose of the proposal for the Chinese Academy of Engineering entitled "Proposal on Promoting Green Development of Aquaculture Industry" was to call for the establishment of an aquaculture capacity management system. In this context, research on the carrying capacity of shellfish is of theoretical and applied significance. The ecosystem dynamics approach assesses carrying capacity based on different evaluation criteria by constructing ecosystem models to simulate key biogeochemical processes and their interactions with important biogenic elements. As our understanding of the concept of carrying capacity and ecosystem structure and function continues to improve, ecosystem dynamics methods that can describe in more detail the physical, biological, and chemical processes and their interactions involving culture organisms in aquaculture ecosystems have become the mainstream direction for global carrying capacity researchers. Although these methods are now widely used in several aquaculture bays around the world, they remain rare in China. We estimated the carrying capacity of R. philippinarum in Jiaozhou Bay based on the Dame indices and Herman model, although the assessment method used portrayed coarse lines of ecological processes, which mainly considered shellfish feeding on phytoplankton, lacking the depiction of other biological roles, and the core parameters were not sufficiently comprehensive. In the present study, the individual growth model for R. philippinarum and a biogeochemical model were coupled to build the nutrient–phytoplankton–zooplankton–detritus–clams (NPZD-C) dynamic ecosystem model of Jiaozhou Bay, and the carrying capacity of R. philippinarum was estimated dynamically. The individual growth model for R. philippinarum in Jiaozhou Bay was constructed based on the dynamic energy budget (DEB) theory following model parameterization and validation. The simulated results from the dynamic ecosystem model well fit the observed results. Regression analysis showed a significant (P < 0.01) linear correlation between the simulated and observed values of clam wet weight and phytoplankton concentration (R2 = 0.934 8 and 0.926 4, respectively). The results of carrying capacity estimation showed that the final clam yield was 10.5, 15.6, 18.9, 21.6, and 23.2 t/hm2 and the maximum phytoplankton (carbon) concentration was 231.3, 176.9, 147.6, 125.1, and 109.8 mg/m3 when the initial seeding density was 300, 500, 700, 1000, and 1500 clams/m2, respectively. Of note, carrying capacity assessed based on the ecosystem dynamics model varies depending on concerns regarding environmental quality, yield, and economic benefits, and there is no uniform standard. In the present study, the criteria used for the assessment of carrying capacity included the maximum stocking density to achieve the minimum size of commercial shellfish and the aquaculture density to maximize economic benefits within a limited period. Our assessment results can help farmers develop aquaculture management strategies. The seeding density should be less than 1000 clams/m2 if individuals with a wet weight of 5 g or more are harvested within the expected 10-month aquaculture period. According to the maximum economic and ecological benefits, the most suitable seeding density is 550~750 clams/m2. This study attempted to construct the NPZD-C ecosystem dynamics model for Jiaozhou Bay by coupling the individual growth model for R. philippinarum and a biogeochemical model and assess the carrying capacity of R. philippinarum in Jiaozhou Bay by considering economic and ecological benefits as the assessment criteria, proposing farming management suggestions for clam seeding density. The results are expected to provide data support and reference in decision-making for planning the development of the local R. philippinarum aquaculture industry and provide a theoretical basis and scientific guidance for managing shellfish aquaculture at the ecosystem level and exploiting the carbon sink function of shellfish.
