Climate change could irreversibly modify Southern Ocean ecosystems. Marine ecosystem model (MEM) ensembles can assist policy making by projecting future changes and allowing the evaluation and assessment of alternative management approaches. However, projected future changes in total consumer biomass from the Fisheries and Marine Ecosystem Model Intercomparison Project (FishMIP) global MEM ensemble highlight an uncertain future for the Southern Ocean, indicating the need for a region-specific ensemble. A large source of model uncertainty originates from the Earth system models (ESMs) used to force FishMIP models, particularly future changes to lower trophic level biomass and sea ice coverage. To build confidence in regional MEMs as ecosystem-based management tools in a changing climate that can better account for uncertainty, we propose the development of a Southern Ocean Marine Ecosystem Model Ensemble (SOMEME) contributing to the FishMIP 2.0 regional model intercomparison initiative. One of the challenges hampering progress of regional MEM ensembles is achieving the balance of global standardised inputs with regional relevance. As a first step, we design a SOMEME simulation protocol, that builds on and extends the existing FishMIP framework, in stages that include: detailed skill assessment of climate forcing variables for Southern Ocean regions, extension of fishing forcing data to include whaling, and new simulations that assess ecological links to sea-ice processes in an ensemble of candidate regional MEMs. These extensions will help advance assessments of urgently needed climate change impacts on Southern Ocean ecosystems.

Diatoms are prominent eukaryotic photoautotrophs in today’s oceans. While dominant in nitrate-rich conditions, they face competitive exclusion by other phytoplankton when ammonium forms the bulk of bioavailable nitrogen. The extent to which this competitive exclusion defines diatom abundance worldwide and the consequences of potential future ammonium enrichment remain unexplored and unquantified. Here, using phytoplankton abundance proxies from the Tara Oceans dataset and an ocean-biogeochemical model, we demonstrate that ammonium enrichment reduces diatom prevalence in marine ecosystems at the global-scale. Under a high emission scenario, we anticipate 98% of the euphotic zones to experience ammonium enrichment by 2081-2100 and attribute a majority (70%) of future diatom displacement to competitive exclusion by other phytoplankton as bioavailable nitrogen supply shifts from nitrate to ammonium. Overall, the form of nitrogen emerges as a significant but previously underestimated stressor affecting diatoms and ocean ecosystems globally.