Trophic amplification of Southern Ocean plankton emerges from changing seasonality

Southern Ocean plankton dynamics are climate-sensitive. Using CMIP6 multi-model ensemble projections, we find that the seemingly stable Southern Ocean plankton biomass under climate change masks opposing trends across different geographic zones. Plankton in the subpolar zone is projected to remain relatively stable due to compensatory changes in bottom-up and top-down processes. Plankton in the subtropical and seasonal ice zones are projected to experience negative and positive trophic amplification, respectively, with zooplankton decreasing and increasing proportionally more than phytoplankton. The predicted trophic amplification arises from uneven seasonal changes in zooplankton grazing, driven by changes in phytoplankton. The negative trophic amplification in the subtropical zone primarily originates during the shallow mixed-layer period, where further shoaling of the mixed layer in the changing climate accentuates nutrient limitation. The consequent reduction in phytoplankton growth and concentration leads to a disproportionate decline in zooplankton grazing and biomass. The positive trophic amplification in the seasonal ice zone also occurs mainly during the shallow mixed-layer season. Improved light availability due to shoaling of the relatively deep mixed layer, sea ice retreat, and warmer temperatures boost phytoplankton growth, together with further suppressing surface phytoplankton, resulting in a disproportionate increase in zooplankton grazing and biomass. Our results underscore the importance of assessing seasonal and regional variations in plankton dynamics to unveil the nuanced effects of climate change on marine plankton ecosystems.