Biogeochemistry of climate driven shifts in Southern Ocean primary producers

Abstract. As a net source of nutrients fuelling global primary production, changes in Southern Ocean productivity are expected to influence biological carbon storage across the global ocean. Following a high emissions, low mitigation pathway, primary productivity in the Southern Ocean is predicted to increase by up to 40 % over the 21st century. The ecophysiological response of marine phytoplankton experiencing climate change will be a key determinant in understanding the impact of Southern Ocean productivity shifts on the carbon cycle. Yet, phytoplankton ecophysiology is poorly represented in CMIP6 climate models, leading to substantial uncertainty in the representation of their role in carbon sequestration. Here we synthesise the existing spatial and temporal projections of Southern Ocean productivity from CMIP6 models, separated by phytoplankton class and identify key processes where greater observational data coverage can help to improve future model performance. We find bidirectional changes in iron and light limitation of phytoplankton, while the greatest changes in productivity occur in the coastal zone of the Southern Ocean. Different phytoplankton groups are responsible for driving productivity increases at different latitudes, yet we observe that models disagree on the ecological mechanism behind these productivity changes. We propose that an evidence-based sampling approach targeting climate-driven changes in ocean biogeochemistry and community assemblages in the regions of rapid projected productivity changes could help to resolve the empirical principles underlying phytoplankton community structure in the Southern Ocean.