Phytoplankton responses of an semi-enclosed sea to SST perturbation

Marine heatwaves (MHWs) have become more frequent and intense under ongoing climate warming, posing increasing risks to coastal ecosystems. This study investigated the characteristics of summer MHWs in a semi-enclose sea (i.e., the Bohai Sea) from 2010 to 2019 and evaluated nutrient and phytoplankton responses under varying sea surface temperature (SST) changes through numerical sensitivity experiments. The higher SSTs (25–28 °C) and longer-lasting MHWs were consistently detected in shallow nearshore regions (<10 m) of Laizhou, Bohai, and Liaodong Bays. Maximum MHW intensities reached 6 °C above climatology within the nearshore regions. Nutrients and phytoplankton biomass displayed pronounced spatial heterogeneity, with elevated Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP), and phytoplankton biomass in nearshore zones (< 10 m) and substantially lower concentrations offshore. Sensitivity experiments demonstrate an asymmetric ecological response to SST changes. Warming SST consistently elevated DIN and strongly suppressed phytoplankton biomass across the basin, particularly in shallow coastal regions, reflecting that summer SST already approaches or exceeds the thermal optimum for phytoplankton. In contrast, cooling produced weaker and more heterogeneous effects. Initial low temperatures under SST reduction suppressed phytoplankton growth and nutrient uptake, but biomass gradually recovered as temperatures moved toward optimal levels, leading to moderate DIN declines later in summer. Results suggest that continued warming and intensified MHWs promote nutrient accumulation and suppress phytoplankton biomass in this semi-enclosed shallow sea, potentially disrupting biogeochemical cycles. These findings provide valuable insights for assessing ecosystem vulnerability under future climate change.