A novel Framework for Assessing Plankton Responses to Compound SST-MLD extremes

Ocean extreme events, particularly those being spatially and temporally compound, pose substantial threats to ecosystem stability and remain insufficiently explored within current frameworks. Such extremes are expected to affect key biological groups, specifically plankton, which form the base of marine food webs. This study introduces a novel approach by coupling sea surface temperature extremes – Marine Heatwaves (MHWs) and Marine Cold-Spells (MCSs) – with Mixed Layer Depth (MLD) dynamics, a critical yet overlooked combination of extreme events. Focusing on the Mediterranean Sea, a "miniature ocean" and global climate change hotspot, we use an integrated multi-scale approach, covering the full water-column from surface to depth. Combining long-term (1998 - 2023) satellite observations, BGC-Argo derived datasets, numerical models, and in-situ measurements, we investigate the dynamic mechanisms driving plankton variability during the productive winter-spring bloom period. We demonstrate that MHW-shallow MLD compound events intensify vertical stratification and nutrient depletion, leading to reduced productivity. Conversely, MCS-deep MLD compounds stimulate primary production by enhancing vertical nutrient transport. Vertically integrated responses are more robust (80 - 88% consistency) than surface observations. In-situ evidence suggests a trophic cascade: MCS-deep MLD compounds favour larger phytoplankton, whereas MHW-shallow MLD extremes drive shifts toward smaller phytoplankton groups, with implications for the energy transfer efficiency to higher trophic levels. As climate-driven warming increases MHW frequency and suppresses MCSs, this framework enhances our capacity to predict ecological risks and offers a scalable tool for developing mitigation and adaptation strategies in a warming ocean.