Ocean models show large-scale kelp farming for CDR depletes nutrients and cuts phytoplankton uptake by up to 8%

Researchers at the University of Bern and the UK National Oceanography Centre ran high-resolution Earth-system simulations of macroalgae deployment. Berger et al. tracked nitrogen, phosphorus and iron drawdown without artificial fertilisation; Yool et al. added iron fertilisation scenarios. Both found rapid nutrient depletion that curtailed seaweed growth by 95% after 25 years and halved surface plankton biomass when iron was supplied. Only 0.05% of the ocean surface met criteria for net-positive sequestration without ecological trade-offs. The results expose a fundamental constraint for marine carbon dioxide removal portfolios that rely on biomass sinking. Because 90% of coastal macroalgae carbon is rapidly remineralised, any net removal requires offshore transport or sinking, yet those same waters are iron-limited. Fertilising to overcome the limit simply transfers nutrients from the surface food web to depth, curtailing fish production. This mirrors documented failures of iron-fertilisation experiments that boosted blooms without sustained carbon export. Current venture-backed proposals, including Running Tide’s wood-puck arrays and Kelp Blue’s Namibian farms, assume gigatonne-scale potential without accounting for basin-wide nutrient competition. Only two small patches off Senegal and southern Australia remain viable in the unfertilised runs. Scaling even these patches cannot reach the 1 Gt CO2 yr-1 threshold required by most 1.5 °C pathways. Policy frameworks such as Article 6 of the Paris Agreement and emerging EU CDR certification schemes must therefore incorporate dynamic nutrient-tracer constraints rather than static biomass-yield assumptions. Next-generation models coupling higher-resolution coastal physics with fisheries food-web modules are required before any large-scale permitting.