Under global climate change, marine heatwaves (MHWs) have become an important driver affecting polar marine ecosystems. The Barents Sea is one of the first Arctic regions to experience seasonally ice-free conditions, supporting highly productive phytoplankton communities that play a vital role in the Arctic ecosystem and the global carbon cycle. However, investigations into how MHWs influence primary productivity and whether these impacts vary with season and different phases of MHW evolution remain limited. This study aims to investigate the seasonal responses of primary productivity to MHWs in the Barents Sea during 1998–2024, examine the differences between the warming and cooling phases of MHWs, and explore the underlying mechanisms. Using satellite and in situ observations, chlorophyll-a was employed as a proxy for primary productivity. Variations in chlorophyll anomalies during MHWs were analyzed in conjunction with mixed layer depth (MLD), photosynthetically active radiation (PAR), and nutrient availability. The results reveal a pronounced seasonal cycle in the chlorophyll response to MHWs. Positive chlorophyll anomalies were observed during March–April, followed by significant negative anomalies from May to July, while the negative anomalies weakened with a slight recovery in August. Both the warming and cooling stages were characterized predominantly by negative chlorophyll anomalies, whereas positive anomalies were mainly concentrated southeast of the Svalbard and in the central and southeastern Barents Sea, suggesting that regional ocean circulation may modulate phytoplankton responses to MHWs. Mechanism analyses further indicate that seasonal variations in mixed layer dynamics, nutrient supply, and light availability jointly determine the direction of primary productivity responses to MHWs. Our findings demonstrate that the response of primary productivity to MHWs in high-latitude oceans cannot be simply characterized as either positive or negative, but instead exhibits pronounced seasonal and stage-dependent variability, considering regional nutrient conditions, phytoplankton community structure, and oceanographic processes. This study provides new observational evidence for understanding the ecological impacts of Arctic marine heatwaves and their potential future changes.
 

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