The potential influence of clams on water quality improvement in mariculture ponds: a comprehensive assessment utilizing single-factor and water quality index (WQI) methods

Good water quality is a key factor to ensure the success of aquaculture. Filter-feeding clams are essential economic species in aquaculture and can produce important ecological functions through their physiological activities. To evaluate the potential of clams to improve water quality in integrated aquaculture ponds, five groups of land-based enclosure aquaculture systems were established, i.e., a swimming crab (Portunus trituberculatus)-kuruma shrimp (Marsupenaeus japonicus) bi-species culture system (referred to as PM) and four swimming crab-kuruma shrimp-razor clam (Sinonovacula constricta) tri-species culture system based on PM with different clam densities (referred to as PMS₁, PMS₂, PMS₃, and PMS₄ in order of increasing clam density). The basic physiochemical parameters, water nutrients, and heavy metals of the aquaculture water in different systems were measured during the experimental period, and the water quality was assessed and contrasted utilizing both a single-factor approach and a comprehensive water quality index (WQI) evaluation method. The results showed that water pH, contents of chlorophyll-a (Chl-a), suspended particulate matter (SPM), and total nitrogen (TN) decreased significantly with the increase of clam stocking density, while chemical oxygen demand (COD), transparency, total inorganic nitrogen (TIN), phosphate (PO₄³⁻), and total phosphorus (TP) increased significantly, and there was no significant difference in the contents of Cu²⁺ and Zn²⁺ in the water. During the aquaculture period, except for PO₄³⁻ content, the water pH, TIN, Cu²⁺, and Zn²⁺ levels in the various aquaculture systems all met the class II standard for the discharge of marine aquaculture tailwater, while COD and SPM generally fell within the class I standard. As the clam density increased, the WQI initially rose, peaking in PMS₂, but subsequently declined, attaining its lowest point in PMS₄. The above findings suggest that co-culturing razor clams at an appropriate density (50 g m⁻² under the condition of the present study) within the integrated aquaculture system combining swimming crabs and kuruma shrimp can serve as an efficient means of enhancing water quality and promoting environmental sustainability.