Marine heatwaves and marine cold spells represent opposing thermal extremes, yet whether phytoplankton responds symmetrically to these opposing thermal extremes remains unresolved. Although both events are known to reshape phytoplankton dynamics, their relative ecological consequences remain poorly quantified within a unified analytical framework. Here we quantify asymmetric phytoplankton responses to marine heatwaves and cold spells in the Taiwan Strait, a monsoon-dominated shelf passage shaped by Kuroshio intrusion, coastal exchange and upwelling. Using chlorophyll-a and primary productivity anomalies as ecological indicators, we quantify ecological asymmetry across three complementary dimensions: response magnitude, response sensitivity and post-event recovery. Phytoplankton responds asymmetrically to marine heatwaves and marine cold spells across all three ecological dimensions, Chlorophyll-a and primary productivity decline more strongly, respond more sensitively to thermal anomalies and recover more slowly during heatwaves because intensified upper-ocean stratification exacerbates nutrient limitation. During marine cold spells, enhanced winter monsoon-driven mixing, upwelling and nutrient replenishment instead support weaker, shorter-lived and spatially heterogeneous positive or mixed ecological responses. Ecological response functions are consistently steeper for warm than for cold anomalies, and recovery following heat stress is substantially slower than following episodic cooling. These findings demonstrate that opposing thermal anomalies do not translate into opposing ecological responses, identifying ecological asymmetry as a fundamental characteristic of phytoplankton responses to marine thermal extremes. This study provides a mechanistic framework for predicting changes in marine productivity and ecosystem vulnerability under continued ocean warming.

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