Dual pollution risks of phosphorus and heavy metals in cage aquaculture: Implications for Larimichthys crocea farming

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Intensive cage aquaculture poses considerable environmental threats due to the concurrent release of heavy metals and nutrients. Metals such as cadmium (Cd) and mercury (Hg), which are toxic, persistent, and capable of bioaccumulation, compromise ecosystem health and stability. Simultaneously, excessive phosphorus (P) input drives eutrophication, leading to algal blooms, hypoxia, and elevated mortality in aquatic organisms. Although these stressors are interrelated and often co-occur, their combined impacts on aquatic ecosystems remain insufficiently understood. Here, we investigated the distribution and correlations of P and heavy metals (Cd, Lead (Pb), chromium (Cr), copper (Cu), zinc (Zn), Hg, arsenic (As)) in water and sediments under varying cage densities (small to large) at the world's largest large yellow croaker aquaculture base in Sansha Bay, using non-cage areas as reference. The results showed that (1) P concentrations in both overlying water and sediments with higher cage culture densities. Specifically, the total P (TP) and PO43--P in overlying water ranged from 1.23 to 4.91 mg·L−1 and 0.05–4.59 mg·L−1, respectively, while TP and available P in sediment ranged from 643.42 to 1879.12 mg·kg−1 and 210.37–769.35 mg·kg−1, respectively, with active P increasing and the residual P decreasing. (2) Heavy metal (Cu, Cd, Pb, Cr, Zn, As, Hg) exhibited similar trends to total P, accumulating more in higher-density cage culture areas. Variations were observed in active heavy metals among densities were minor. (3) Strong correlations between P and heavy metals were observed, particularly in medium-to-large and large density cage cultures. A cage density of 250–400 was identified as a threshold to avoid dual pollution risks. This study provides theoretical guidance for sustainable breeding strategies for large yellow croaker in temperate nearshore waters.