Tracing external sources of nutrients in the East China Sea and evaluating their contributions to primary production

Nutrients (dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP)) in the East China Sea (ECS) have four external sources, namely the Kuroshio, Taiwan Strait, rivers, and the atmosphere. In order to evaluate the contribution of each source of nutrients to the nutrient inventory and primary production over the ECS shelf, a tracking technique was applied to all the state variables in a low-trophic ecosystem model. Each source of nutrients has strong seasonal variations in the spatial distribution that depends closely on circulation, mixing, and stratification. The primary production supported by each source of nutrient is under the control of a combination of nutrients, temperature, and light. As a mean state over the entire ECS shelf, the Kuroshio contributes 72% of DIN input and 84% of DIP input, 57% of DIN inventory and 78% of DIP inventory, 50% DIN-based primary production and 61% DIP-based primary production and therefore is the dominant one among four sources. However, the contributions of four external nutrients have strong spatial dependence: the riverine nutrients and the atmospheric DIN dominate the inner shelf (0–50 m), the nutrients from the Taiwan Strait dominate the southern part of the middle shelf (50–100 m), and the nutrients from the Kuroshio dominate the outer shelf (100–200 m). The production efficiencies of the nutrients from the Kuroshio are low, while those from the atmosphere and Taiwan Strait are high. The riverine DIN is inefficient but DIP is efficient. The nutrient limitation, light and water temperature at the location of the nutrients determine the production efficiency of each specific sources.