Nutrient colimitation and an emergent "ligand-iron-microbe" feedback in global ocean biogeochemistry and ecosystem models.

Primary production is limited by iron over about half of the global ocean surface. Standing stocks of dissolved iron are maintained by association with organic ligands which, in turn, are produced by microbes. This suggests a reinforcing cycle of microbial production, ligand generation, and iron stabilization only arrested when another resource, such as light or macronutrients, becomes limiting. We will show that the feedback emerges in idealized numerical simulations of the coupled marine cycles of macronutrients and iron in global biogeochemistry and ecosystem models that resolve dynamic microbial production and loss of iron-chelating ligands. The "ligand-iron-microbe" feedback drives the system towards macro- and micronutrient co-limitation on a global scale. Incorporating the feedback between oceanic microbial activity and ligand concentration allows us to reassess the climatic effects of changing iron input to the ocean on atmospheric CO₂.