Constraining phytoplankton response to climate change in the Southern Ocean using observed mixed layer depth seasonality

Phytoplankton, as the base of the marine food web, has great importance for marine ecosystems and the global carbon cycle. However, Earth system models indicate considerable uncertainty of our knowledge about the underlying processes that determine phytoplankton evolution under climate change. Particularly large differences between models can be found in the Southern Ocean, a region notorious for its difficulty in modeling. The objective of this study is to analyze the potential phytoplankton response to climate change from both a "bottom-up" and a "top-down" perspective. Within the Southern Ocean, we determine a relationship between surface phytoplankton and mixed layer depth under present-day seasonality and apply it to climate change on a longer timescale. Applying this present-day constraint, we confirm the trend of increasing surface phytoplankton by the end of the 21^st century under a 'high emissions no mitigation scenario' with further reduction in phytoplankton projection uncertainty. The increase of surface phytoplankton is due to weakening bottom-up control as a result of improving light conditions with shoaling mixed layers. At the same time, due to shoaling mixed layers, total phytoplankton biomass integrated over the water column slightly decreases. Zooplankton follows the trend of surface phytoplankton and shows an increase. This is mainly caused by improved zooplankton grazing conditions with shoaling mixed layers that result in enhanced efficiency of trophic energy transfer. In comparison with the changes in bottom-up conditions, top-down control appears to become increasingly important under climate change in the Southern Ocean.