Jingjing Zheng

Marine heatwaves (MHWs) are extreme warm‐water events that can strongly perturb marine ecosystems. Although MHWs are known to extend into subsurface and deep waters, their vertical structures and biogeochemical consequences remain poorly quantified. Here, we use a physical‐biogeochemical model to characterize the vertical structures of MHWs in the South China Sea (SCS) and to assess their impacts on primary production (PP). Four distinct types of vertical structure of MHWs (shallow, sub‐reversed, sub‐intensified and deep MHWs) are identified. Deep MHWs are the most common, accounting for nearly 50% of events. Shallow and sub‐reversed MHWs are generally short‐lived and confined to the upper ocean, whereas sub‐intensified and deep MHWs persist longer and penetrate deeper. Heat budget analysis indicates that air–sea heat flux primarily drives the formation of shallow, sub‐reversed, and deep MHWs, while vertical processes dominate the maintenance of sub‐intensified MHWs. Distinct MHW structures modify vertical stratification and nutrient transport, producing spatially and vertically heterogeneous PP responses across the mixed layer and the euphotic zone. Over the past two decades, the occurrence area, duration, and total intensity of all four MHW types increased significantly, and associated PP anomalies exhibited a declining trend. This decline is largely attributable to strengthened mixed‐layer stratification and reduced mixed‐layer PP, with subsurface processes contributing little to the long‐term trend.

To assess the environmental impacts of tritium-containing wastewater discharge from the Fukushima Daiichi Nuclear Power Plant, we conducted a 10-year three-dimensional simulation of seabed-discharged dissolved and particulate tracers in the Pacific Ocean and its marginal seas using the Regional Ocean Modeling System (ROMS). The domain was partitioned into 11 regions to quantify dispersion pathways, particle trajectories, concentration evolution and inventory (accumulation) changes. Tracer levels are highest near the seabed discharge location (SDL) and subsequently dispersed to the deep western Pacific (DWP), the deep eastern Pacific (DEP), and ultimately into western Pacific marginal seas; subsurface and deep-layer tracers intrude into marginal seas earlier than surface tracers. Over the decade, discharge-induced concentration increments are even below the practical detection limits in most regions, with detectable maxima above natural background primarily only confined to the SDL ( >70 Bq/m3 ) and DWP (~20-30 Bq/m3). The SDL median remains near 10-4 Bq/m3. Median concentrations approach 10-4 Bq/m3 in open Ocean and some marginal seas (i.e., South China Sea), but do not reach this level elsewhere within the simulation period. Maximum concentrations exceed 10-2 Bq/m3 in most regions, except in the least and latest impacted regions (i.e., JSB). Overall, both dissolved and particulate tracers accumulate mainly in the open Pacific east of the discharge site, with limited impacts on marginal seas within 10 years. Given the planned intermittent discharge over ~30 years, longer-term simulations and sustained monitoring are needed to evaluate the longer-term cumulative risks to marginal seas.