Lateral nitrogen input during summer in Sagami Sea with a deep trough enhances particulate organic carbon export

In deep bays linked to the open ocean, efficient carbon sequestration occurs through the sinking of particulate organic matter produced in the euphotic zone. Nutrient conditions in the surface layer of temperate bays, which are crucial for particle production, are influenced by the lateral inflows of oligotrophic open water and freshwater containing terrestrial nutrients, in addition to seasonal mixing from below. This study, conducted through eight repeated observations in 2007–2008, investigated variations in surface water suspended particulate nitrogen (PN) isotope delta (δ¹⁵N) to examine upper layer PN and nutrient dynamics in Sagami Sea, located south of Tokyo Bay, with a seafloor depth exceeding 1500 m. While δ¹⁵N_PN showed seasonal variations, such as low in winter and high in summer, its horizontal distribution in summer closely correlated with a significant salinity variation, alongside PN concentration. Analysis of the negative δ¹⁵N_PN-salinity relationship, using a conservative mixing model with the offshore water and the low-salinity water as endmembers, revealed that N with a high δ¹⁵N was supplied to Sagami Sea through freshwater inflow from Tokyo Bay. This contributed on average 74 % of the summer suspended PN in coastal waters in Sagami Sea. The transport efficiency of this N input increases after heavy rain events, which greatly stimulates summer productivity in the bay. Consequently, summer N inputs may play a vital role in exporting particulate organic carbon to the mesopelagic zone and absorbing atmospheric carbon dioxide.