The impact of mariculture on nutrient dynamics and identification of the nitrate sources in coastal waters

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 (δ(15)N-NO3(-) and δ(18)O-NO3(-)) approach was used to identify the nitrate sources. Higher dissolved nitrogen concentrations (NH4(+) and NO3(-)) than PO4(3-) concentrations associated with enriched δ(15)N-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.