Response of Southern Ocean Resource Stress in a Changing Climate

Phytoplankton underpin ocean net primary production (NPP) and Southern Ocean phytoplankton display different ecological-biogeochemical traits, compared to temperate species. Climate models currently forecast consistent across-model NPP increases due to climate change, yet neglect specific aspects of the Southern Ocean ecological-biogeochemical system. We conducted experiments to evaluate how key regional traits, including multiple limiting nutrients, unique photophysiology and differential resource acquisition, drive changes in the projected response of resource stress, NPP and export production under a high emissions scenario. Although Southern Ocean iron limitation is widespread, it declines in the future and is replaced by growing manganese limitation, as concentrations cannot support increasing growth rates. Distinct phytoplankton traits either amplify or dampen climate-driven changes, depending on whether they are those typical of Antarctic or temperate phytoplankton, respectively. Overall, future Southern Ocean NPP trends may be more uncertain than currently assumed and future efforts should focus on accounting for regional ecological-biogeochemical differences. In the Southern Ocean, phytoplankton net primary production (NPP) and carbon export are considered to be limited by the micronutrient iron. Climate models project increasing NPP and carbon export in response to the increased iron concentrations, with high confidence. However these simulations neglect phytoplankton that are well adapted to the unique conditions of the Southern Ocean. We used a global ocean biogeochemical model to address how regional differences in phytoplankton photophysiology, nutrient uptake, and limitation may affect NPP and carbon export in the Southern Ocean under a changing climate. From these experiments, iron limitation decreases in the future, but is replaced by growing manganese limitation. NPP and export production increase as in prior work but Southern Ocean adapted would amplify this signal, while temperate phytoplankton would dampen it. Overall, our experiments show that we may overestimate our confidence in the projected response of Southern Ocean NPP and export production. Improved confidence requires better mechanistic representations of key ecological processes informed by emerging observations and experimental work. The impact of iron and manganese stress and role of unique Southern Ocean traits in a changing climate is assessed Climate change decreases the area of the Southern Ocean limited by iron and increases the area limited by manganese Traits typical of Southern Ocean phytoplankton amplify, while temperate traits reverse, the expected climate-driven trends