Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea

Marine heatwaves are increasing in frequency and intensity globally against a background of long-term climate warming, yet their impacts on subsurface biogeochemistry in temperate shelf seas remain poorly constrained. Here, we present high-resolution in situ observations of chlorophyll during a Category IV marine heatwave in June 2023, collected using an autonomous ocean glider in a seasonally stratified shelf sea. Surface chlorophyll exhibited a rapid but short-lived enhancement, increasing from <1 mg m⁻³ to >5 mg m⁻³ and briefly exceeding the climatological ±2σ range by up to 40%. In contrast, subsurface chlorophyll showed a sustained enhancement, with concentrations within the subsurface chlorophyll maximum increasing from typical post-bloom values of 1–2 mg m⁻³ to >3 mg m⁻³ and episodic peaks exceeding 9 mg m⁻³. This intensification was accompanied by an increase in depth-integrated chlorophyll, reaching 140 mg m⁻². Accelerated bottom mixed-layer warming and strengthened horizontal temperature gradients are consistent with a possible influence of horizontal transport processes, while elevated surface irradiance may also have contributed. These findings highlight the importance of sustained, vertically resolved observations for detecting ecosystem responses to climate extremes.