Two-decadal shifts in phytoplankton functional taxa (2002–2022) in response to extreme marine heatwaves

Phytoplankton play a crucial role in the primary productivity of ocean ecosystems, rendering changes in their populations particularly consequential. The effects of extreme sea surface temperature (SST) on phytoplankton are noteworthy. However, the direct characterization of total biomass by chlorophyll for analysis is inaccurate for phytoplankton functional taxa (PFTs) with different sensitivity to SST. Through the method of Diagnostic pigment analysis (DPA), these organisms can be classified into six primary functional taxa based on their distinctive pigments, which play a vital role in elucidating their ecological functions and dynamics. This study employed optical remote sensing to reconstruct daily concentrations and occupancy of six PFTs in the Eastern China Marginal Sea (ECMS) over the past 20 years. Causal analyses were conducted to identify regions with varying sensitivity to SST changes and further explore the corresponding impacts on phytoplankton concentrations. Results revealed that high-causality areas in the Bohai Sea (BS) and Yellow Sea (YS) were primarily located in nearshore waters. In contrast, high-causality regions in the East China Sea (ECS) were farther offshore, while nearshore areas exhibited lower causality. The high-causality subregions experienced significant fluctuations in phytoplankton concentrations, particularly during marine heatwave (MHW) events, with a prevailing trend of decline in taxon concentrations driven by elevated SST. In comparison, low-causality subregions displayed weaker synchronization with SST changes, as other factors influenced taxa concentrations more prominently. Notably, while phytoplankton concentrations fluctuated significantly during extreme MHW events, they eventually reverted to baseline levels. However, with the increasing frequency and intensity of MHWs due to global warming, the capacity for phytoplankton to recover under such repeated stress remains uncertain. This study emphasizes the necessity of further investigating the mechanisms driving phytoplankton responses to MHWs and highlights the importance of long-term monitoring for assessing the potential risks of ecosystem collapse resulting from ongoing climate change.