Optimizing Mytilus edulis and Gracilaria lemaneiformis ratios for maintaining water environment stability in IMTA systems

Abstract

This study investigates the optimal integration of shellfish (Mytilus edulis) and algae (Gracilaria lemaneiformis) aquaculture in semi-enclosed bay of Sanggou Bay, China, to enhance bioremediation and water quality. An enclosure experiment spanning five days was carried out to assess the impact of six mussel-to-algae wet weight ratios (3:5, 3:10, 6:5, 6:10, 9:5, 9:10) on dissolved oxygen (DO), pH, nutrient levels, inorganic carbon dynamics, plankton abundance, and chlorophyll-a (Chl-a) concentration in the integrated multi-trophic aquaculture (IMTA) system. The in-situ enclosure experiment provided realistic insights into species-environment interactions, overcoming laboratory limitations of oversimplified systems. Results revealed diurnal DO and pH fluctuations driven by mussel respiration and algal photosynthesis. Higher G. lemaneiformis proportions (3:10) elevated DO and pH (>8.7), reduced dissolved inorganic carbon (DIC) and pCO₂, but induced phosphorus limitation (PO₄^3--P < 0.2 μmol/L), triggering pico-phytoplankton proliferation. Conversely, higher mussel ratios (9:5) lowered pH (7.7) and increased DIC, risking acidification. The ratios of 3:5, 6:5, 6:10, and 9:10 balanced ecological impacts, maintaining pH 7.8-8.5 (China's seawater standards), reducing nutrients (PO₄^3--P removal: 51.3-62.0 %), and suppressing phytoplankton miniaturization. These ratios optimized nutrient levels, minimized eutrophication risks, and stabilized carbonate system, demonstrating their efficacy in sustainable IMTA. These findings highlight the importance of balancing mussel and algae proportions in IMTA systems to achieve sustainable aquaculture practices with minimal environmental impact. Moreover, this study optimizes the ratio of M. edulis and G. lemaneiformis to balance the stability of aquaculture water environment parameters, offering actionable strategies for coastal management to mitigate ecological degradation in semi-enclosed bays.

Keywords: Carbonate system; Gracilaria lemaneiformis; IMTA; Mytilus edulis; Nutrient levels; Plankton abundance.