Shellfish-macroalgae IMTA maintaining water environment stability: A case study of Crassostrea gigas and Gracilaria lemaneiformis IMTA in Sanggou Bay of China

Abstract

Shellfish and macroalgae are usually known as environmental remediation species in aquaculture systems. However, the environmental issues are frequently found in shellfish and macroalgae large scale monoculture areas. Although shellfish and macroalgae IMTA might mitigate adverse effects of monoculture, the relationships among cultured organisms, plankton and environmental factors are unclear. This study evaluated the ecological impacts of Crassostrea gigas and Gracilaria lemaneiformis monoculture versus integrated multi-trophic aquaculture (IMTA) systems in Sanggou Bay, China, through a 5-day in-situ enclosure experiment. We assessed the nutrient levels, inorganic carbon system, plankton abundance, and Chl-a concentration under different aquaculture modes. Results revealed that oyster monoculture increased PO₄^3--P levels, DIN concentrations and elevated CO₂ levels, exacerbating acidification risks. In contrast, IMTA systems with oyster-to-macroalgae ratios of 1:1 and 4:2 reduced PO₄^3--P levels by 75 %, optimized DIN removal, and mitigated CO₂ accumulation. Moreover, oyster monoculture promoted pico-phytoplankton dominance by suppressing competitors (micro-/nano-phytoplankton) and predators (zooplankton), a trend reversed in IMTA systems where macroalgae limited pico-phytoplankton proliferation. Besides, Chl-a declined across all treatments. The in-situ enclosure experiment provided realistic insights into species-environment interactions, overcoming laboratory limitations of oversimplified systems. These findings underscore that IMTA ratios of 1:1 and 4:2 enhance nutrient cycling, stabilize carbonate system, and reduce eutrophication risks. This study advances sustainable aquaculture practices by demonstrating how optimized species ratios balance productivity and environmental health, offering actionable strategies for coastal management to mitigate ecological degradation in semi-enclosed bays.

Keywords: Bioremediation; Inorganic carbon system; Integrated multi-trophic aquaculture; Nutrients; Plankton.