Anthropogenic iron deposition alters the ecosystem and carbon balance of the Indian Ocean over a centennial timescale

Phytoplankton growth in the Indian Ocean is generally limited by macronutrients (nitrogen: N and phosphorus: P) in the north and by micronutrient (iron: Fe) in the south. Increasing anthropogenic atmospheric deposition of N and dissolved Fe (dFe) into the ocean can thus lead to significant responses from marine ecosystems in this ocean basin. Previous modeling studies investigated the impacts of anthropogenic nutrient deposition on the ocean, but their results are uncertain due to incomplete representations of Fe cycling. We use a state-of-the-art ocean ecosystem and Fe cycling model to evaluate the transient responses of ocean productivity and carbon uptake in the Indian Ocean, focusing on the centennial time scale. The model incorporates all major external sources and represents a complicated internal cycling process of Fe, thus showing significant improvements in reproducing observations. Sensitivity simulations show that after a century of anthropogenic deposition, increased dFe stimulates diatoms productivity in the southern Indian Ocean poleward of 50⁰S and the southeastern tropics. Diatoms production weakens in the south of the Arabian Sea due to the P limitation, and diatoms are outcompeted there by coccolithophores and picoplankton, which have a lower P demand. These changes in diatoms and coccolithophores productions alter the balance between the organic and carbonate pumps in the Indian Ocean, increasing the carbon uptake in the south of 50⁰S and the southeastern tropics while decreasing it in the Arabian Sea. Our results reveal the important role of ecosystem dynamics in controlling the sensitivity of carbon fluxes in the Indian Ocean under the impact of anthropogenic nutrient deposition over a centennial timescale.