Eutrophication mitigation and climate warming coincide with divergent shifts in harmful algal bloom dynamics in...

How eutrophication mitigation reshapes harmful algal bloom (HAB) dynamics under climate warming remains a critical global knowledge gap, particularly whether nutrient reduction can still suppress blooms when compound climate extremes intervene. The conventional paradigm assumes that nutrient reduction alone ensures long-term HAB control. Using 20-year records from Shandong coastal waters and 2012 as the regulatory boundary, we show that nutrient reduction effectively suppressed routine blooms: excluding the 2021 large -bloom, annual HAB frequency fell by 72.2% and mean event area by 37.6%. However, the extreme 2021 event, coinciding with record autumn precipitation and anomalous warming, generated 84.4% of the total post-regulation affected area, revealing a paradoxical pattern of "fewer but more extreme" blooms. Concurrently, HAB season shifted from summer to autumn-winter, distribution expanded northward to the Yellow River Estuary, and dinoflagellate-dominated HABs increased from 43.6% to 86.7%. We postulate a "nutrient mitigation-climate warming" synergy: nutrient regulation makes the system less prone to routine blooms, while compound climate extremes (extreme precipitation plus autumn warming) act as nonlinear forcing that may trigger exceptional large-scale blooms that deviate from the post-regulation pattern. This postulate challenges the traditional single-nutrient management paradigm and implies that effective nutrient control remains essential but is insufficient in a warming climate, providing a predictive basis for climate-nutrient integrated adaptive strategies for mid-latitude coastal ecosystems worldwide.