With increasing concern over the aquatic environment in marine culture, the integrated multi-trophic aquaculture (IMTA) has received extensive attention in recent years. A three-dimensional numerical ocean model is developed to explore the negative impacts of aquaculture wastes and assess the bio-mitigation effect of IMTA systems on marine environments. Numerical results showed that the concentration of surface phytoplankton could be controlled by planting seaweed (a maximum reduction of 30%), and the percentage change in the improvement of bottom dissolved oxygen concentration increased to 35% at maximum due to the ingestion of organic wastes by sea cucumbers. Numerical simulations indicate that seaweeds need to be harvested in a timely manner for maximal absorption of nutrients, and the initial stocking density of sea cucumbers >3.9 individuals m−2 is preferred to further eliminate the organic wastes sinking down to the sea bottom.

Reclamation along coastal zones is a method that has been used to relieve the problems of strained resources and land. Aquaculture, as one of the major man-made activities in reclamation areas, has an environmental impact on coastal waters. The effluents from aquaculture ponds are known to enrich the levels of nutrients such as nitrogen and phosphate. The goals of the present study are to evaluate the environmental impact of mariculture on coastal waters in the east coast of Laizhou Bay, China, and to identify the nitrate sources. Monitoring the concentrations of dissolved nitrogen and phosphate was used to assess their impact on the water quality of coastal waters. A dual isotope (δ¹⁵N-NO₃ ⁻ and δ¹⁸O-NO₃ ⁻) approach was used to identify the nitrate sources. Higher dissolved nitrogen concentrations (NH₄ ⁺ and NO₃ ⁻) than PO₄ ³⁻ concentrations associated with enriched δ¹⁵N-NO₃ ⁻ values were observed in the drainage channels, sea cucumber ponds, and groundwater, which indicated that aquaculture activity has more influence on nitrogen nutrients than on phosphate nutrients. In this coastal area with seawater intrusion, nitrogen released from sea cucumber ponds accumulated in nearshore water and migrated in the offshore direction in groundwater currents. This behavior results in nitrogen enrichment in groundwater within the hinterland. Isotopic data indicate that mixing of multiple nitrate sources exists in groundwater, and nitrogen from mariculture is the main source.