To evaluate biogeochemical impacts of shelf‐scale submarine groundwater discharge (SGD), the northern South China Sea (NSCS) and the East China Sea (ECS) shelf were chosen for this study. Based on a three end‐member mixing model and a Ra box model, SGD fluxes on NSCS (west and east of the Pearl River Estuary, WPRE and EPRE, respectively) and ECS shelf in winter were estimated to be 3.8–9.5 × 10⁹, 1.4–2.2 × 10⁹ and 0.7–2.2 ×10¹⁰ m³ d⁻¹, respectively. Our results were equivalent to the SGD flux to the entire Mediterranean Sea, and were an order of magnitude greater than fluxes to the South Atlantic Bight. SGD associated nutrient fluxes into WPRE, EPRE and ECS were almost in the same order of magnitude, 0.2–2.4 × 10³ mol m⁻¹ d⁻¹ for dissolved inorganic nitrogen, 1.2–9.8 mol m⁻¹ d⁻¹ for soluble reactive phosphorus, and 0.4–3.4 × 10³ mol m⁻¹ d⁻¹ for dissolved silicate. Moreover, SGD delivered 1.1–2.8 × 10⁴, 0.6–0.9 × 10⁴, 1.7–5.1 × 10⁴ mol m⁻¹ d⁻¹ dissolved inorganic carbon and 1.1–2.7 × 10⁴, 0.5–0.8 × 10⁴, 1.6–4.8 × 10⁴ mol m⁻¹ d⁻¹ total alkalinity to WPRE, EPRE and ECS, respectively. Shelf‐scale SGD is a significant source of nutrients and carbon, and may pose great impacts on regional marine ecosystems.

In the context of continuously increasing anthropogenic nitrogen inputs, knowledge of how ammonia oxidation (AO) in the ocean responds to warming is crucial to predicting future changes in marine nitrogen biogeochemistry. Here, we show divergent thermal response patterns for marine AO across a wide onshore/offshore trophic gradient. We find ammonia oxidizer community and ambient substrate co-regulate optimum temperatures (T_opt), generating distinct thermal response patterns with T_opt varying from ≤14 °C to ≥34 °C. Substrate addition elevates T_opt when ambient substrate is unsaturated. The thermal sensitivity of kinetic parameters allows us to predict responses of both AO rate and T_opt at varying substrate and temperature below the critical temperature. A warming ocean promotes nearshore AO, while suppressing offshore AO. Our findings reconcile field inconsistencies of temperature effects on AO, suggesting that predictive biogeochemical models need to include such differential warming mechanisms on this key nitrogen cycle process.