Marine Heatwaves and Iceberg Melting in Polar Areas Intensify Phytoplankton Blooms

Climate change has led to increases in the intensity and frequency of marine heatwaves (MHWs). However, the impact of MHWs on phytoplankton at the global scale remains unclear. The metaheuristic superlearner proposed in this research indicates that the occurrence of MHWs weakens the Fe limitation of phytoplankton growth, leading to intensified phytoplankton blooms. The shock transmission effect analysis further reveals the interactions among sea surface temperature (SST), iceberg melting, Fe, ammonium ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NH</mml:mi> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> <mml:annotation>$$ {\mathrm{NH}}_4^{+} $$</mml:annotation></mml:semantics> </mml:math> ) and nitrate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NO</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> <mml:annotation>$$ {\mathrm{NO}}_3^{-} $$</mml:annotation></mml:semantics> </mml:math> ); namely, the occurrence of MHWs in polar regions has led to iceberg melting, triggering a derivative shock of iceberg melting. Compared with a single MHWs event, the dual shock disrupted the effects of Fe, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NH</mml:mi> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> <mml:annotation>$$ {\mathrm{NH}}_4^{+} $$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NO</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> <mml:annotation>$$ {\mathrm{NO}}_3^{-} $$</mml:annotation></mml:semantics> </mml:math> on limiting the growth of phytoplankton, resulting in a 54.90% increase in the growth rate of phytoplankton and leading to the massive reproduction of phytoplankton in polar regions. In addition, compared with that in the low-emission scenario (SSP126), the coverage area of globally fragile marine regions with respect to intensified phytoplankton blooms will increase by 5.84% under the medium-emission scenario (SSP245) and by 9.29% under the high-emission scenario (SSP585). Specifically, the Global South and developing Pacific island countries are fragile regions that need scientific (marine protected area guidance) and financial (such as a foundation for marine protection) assistance to resist the increasing intensity and expansion of phytoplankton blooms under climate change.