Abstract:Bivalve filter feeders, such as oysters, clams, and scallops, are an economically important species in China, with a total production of up to 14.8×106 tons in 2020, accounting for more than 69.3% of mariculture production. In recent years, the density and scale of shellfish mariculture have continued to increase in some areas owing to economic benefits. The crude farming strategies have a series of negative effects on the cultured organisms and marine ecosystems, such as inhibiting the individual growth rate and increasing individual mortality. The negative effects could then lead to the attenuation of phytoplankton, changing the structure of the phytoplankton community and benthic environment. To manage shellfish farms and the industry properly, it is important to understand the interaction between shellfish aquaculture and the marine environment and evaluate the ability of the marine ecosystem to support shellfish production. At the ecosystem level, the carrying capacity of shellfish is defined as the maximum mariculture density that does not significantly affect the structure and function of marine ecosystems. To evaluate the ecological carrying capacity of shellfish, a method that assesses the overall impact of cultured shellfish on the structure, function, and other functional groups of the ecosystem is required. Based on the principle of nutrition dynamics, the Ecopath model (Ecopath with Ecosim software, Version 6.5) is a scientific and effective tool for evaluating the ecological carrying capacity of shellfish from the perspective of material and energy balance, and fully considers predation, competition, and ecological transformation efficiencies between functional groups. The Ecopath model has been widely used to evaluate the ecological carrying capacity of shellfish in mariculture ecosystems. In addition to their important economic value, bivalve filter feeders play an important ecological role in carbon sequestration, storage, and water purification by ingesting particles and changing the nutrient cycle. In offshore ecosystems, the primary productivity is very high. Filter-feeding bivalves ingest a large amount of phytoplankton to convert particulate organic carbon into feces or pseudo-feces and accelerate the transportation of organic carbon to the seabed. Moreover, calcium carbonate shells are formed through absorption of inorganic carbon from seawater, which plays an important role in long-term carbon storage. Jiaozhou Bay is an important aquaculture base for the Manila clam, Ruditapes philippinarum, in North China. Studies have found that the cultivation of Manila clams caused disturbance to the benthic ecosystem of Jiaozhou Bay. To explore the ecological carrying capacity and ecological service functions of R. philippinarum in the bay, Ecopath with Ecosim models were applied, and the effects of further expansion of clam biomass on the community structure and functional characteristics of the Jiaozhou Bay ecosystem were evaluated. Furthermore, the effects of Manila clam aquaculture on carbon cycling were quantified. Results showed that: The ecological carrying capacity of R. philippinarum in Jiaozhou Bay was 239.9 t/km2; the average biomass of cultured clam has not yet reached the ecological carrying capacity, but the clam biomass in some areas has far exceeded this limit. When the clam biomass increased up to the level of the ecological carrying capacity, the total system throughput, capacity, ascendency, and Finn cycling index would also increase by 16.0%, 3.9%, 47.1%, and 103.0%, respectively, whereas the entropy would decrease by 10.4%, suggesting that the increase in clam biomass would lead to an increase in system maturity and stability; however, continued expansion to 10 times the ecological carrying capacity would adversely affect the ecosystem. At the individual level, a clam ingested approximately 3310.1 mg of carbon during a farming cycle, of which approximately 46.2% was deposited and 13.2% was removed through harvesting; at the population level (scaled to ecological carrying capacity), 15 000 tons of carbon could be deposited, and 6000 tons be harvested annually. These results provide theoretical guidance for the sustainable development of clam aquaculture and its ecological service functions.
