Semi-enclosed coastal seas are traditionally regarded as highly productive regions. However, in recent decades, many coastal systems have experienced substantial nutrient reductions and even signs of oligotrophication, leading to declining fishery production and reduced aquaculture quality. To address these issues, nutrient management strategies have been implemented in several coastal seas to enhance coastal productivity. However, coastal environments are strongly influenced by variable hydrodynamic conditions, which may significantly regulate the ecosystem responses to nutrient inputs. The role of hydrodynamics in controlling the effectiveness of nutrient management therefore remains insufficiently quantified.

In this study, we focus on Hiuchi-Nada in the Seto Inland Sea, Japan, where low dissolved inorganic nitrogen (DIN) concentrations have caused discoloration of Nori aquaculture. A coupled physical-biogeochemical model was applied to evaluate the effectiveness of increasing land-based nitrogen inputs. Sensitivity experiments were conducted in which DIN loads from point sources were increased by two- to five-fold during the aquaculture season (October-March).

The simulations show that the increases in DIN concentrations were significantly attenuated during periods of enhanced boundary water transport, despite continuously elevated nitrogen inputs from the land. Enhanced bidirectional boundary transport accelerated the flushing of nitrogen from coastal waters, although net water transport remained nearly unchanged. As a result, a large proportion of the added nitrogen was exported to adjacent seas, while only a limited fraction contributed to local retention, including sedimentation. The model results further suggest that the contribution to nori aquaculture farms is limited under such highly flushed conditions. In addition, the ecosystem rapidly returned to near-baseline conditions within approximately one month after nutrient inputs were reduced, indicating the transient effect of nutrient manipulation in this highly flushed coastal system. The results also suggest that dissolved inorganic phosphorus (DIP) may become a potential limiting factor if only nitrogen inputs are increased.

These findings indicate that the effectiveness of nutrient management in semi-enclosed coastal seas highlight that hydrodynamic flushing plays a critical role. Incorporating temporal variability in water exchange processes is therefore essential for designing effective nutrient management strategies.

Seto Inland Sea,aquaculture,Biogeochemical,Nori