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

Area of Science:

• Marine ecology
• Oceanography
• Remote sensing

Background:

• Phytoplankton are crucial primary producers in ocean ecosystems.
• Total biomass by chlorophyll is inaccurate for assessing phytoplankton functional taxa (PFTs) due to varying sensitivities to sea surface temperature (SST).
• Diagnostic pigment analysis (DPA) allows classification of PFTs based on unique pigments, aiding ecological function analysis.

Purpose of the Study:

• To reconstruct daily concentrations and occupancy of six PFTs in the Eastern China Marginal Sea (ECMS) over 20 years using optical remote sensing.
• To conduct causal analyses to identify regions with varying sensitivity to SST changes and their impacts on phytoplankton.
• To explore the effects of extreme sea surface temperature events, specifically marine heatwaves (MHWs), on PFT concentrations.

Main Methods:

• Utilized optical remote sensing to reconstruct daily PFT concentrations and occupancy.
• Applied Diagnostic Pigment Analysis (DPA) to differentiate and analyze six primary PFTs.
• Conducted causal analyses to link SST variations and MHW events to PFT concentration changes in the ECMS.

Main Results:

• High-causality regions between SST and PFTs were identified in nearshore Bohai Sea (BS) and Yellow Sea (YS), and offshore East China Sea (ECS).
• High-causality areas showed significant phytoplankton fluctuations, especially during MHWs, with a general decline in taxon concentrations linked to elevated SST.
• Low-causality areas exhibited weaker synchronization with SST, indicating other factors influenced taxa concentrations more significantly.

Conclusions:

• Phytoplankton concentrations fluctuated during MHWs but typically reverted to baseline levels.
• Increasing frequency and intensity of MHWs due to global warming pose an uncertain recovery capacity for phytoplankton.
• Further investigation into phytoplankton responses to MHWs and long-term monitoring are crucial for assessing ecosystem risks.