Internal nutrient loading is a potential source of eutrophication in Shenzhen Bay, China

Abstract As a highly eutrophic area, Shenzhen Bay in southern China has accumulated plant dissolved inorganic nitrogen (DIN) and phosphate (PO43--P), resulting in various environmental problems. The nutrient loading was calculated to classify runoff patterns of the bay and provide possible management solutions. However, previous studies have shown that external loading alone cannot explain the source of nutrition in Shenzhen Bay. In our study, for the first time, we proved quantitatively that internal nutrient loading was the main contributor of the total input fluxes of dissolved DIN and PO43--P in Shenzhen Bay using the LOICZ (land–ocean interactions in the coastal zone) model. Our results showed that the internal loading supplied 65 and 69% of total input fluxes of dissolved DIN and PO43--P, which were c. 4.0 × 106 and 0.3 × 106 mol/d, respectively. The estimated nutrient input flux from terrigenous loading were c. 1.38 × 106 and 0.1 × 106 mol/d of DIN and PO43--P, respectively. Notably, the internal loading from sediment resuspension supplied c. 3.5–15.7 × 106 and 0.5–1.9 × 106 mol/d of particle nitrogen and bioavailable phosphate to Shenzhen Bay, respectively. Moreover, 51 and 62% of DIN and PO43--P were exported through water exchange and residual flows from Shenzhen Bay into the Pearl River. These results demonstrate that internal loading dynamics and nutrient losses to the Pearl River jointly influence the eutrophication process in Shenzhen Bay.