Oceanic enrichment of ammonium and its impacts on phytoplankton community composition under a high-emissions scenario

Abstract. Ammonium (NH₄⁺) is an important component of the ocean’s dissolved inorganic nitrogen (DIN) pool, especially in stratified marine environments where intense recycling of organic matter elevates its supply over other forms. Using a global ocean biogeochemical model with good fidelity to the sparse NH₄⁺ data that is available, we project increases in the NH₄⁺:DIN ratio in over 98 % of the ocean by the end of the 21^st century under a high-emission scenario. This relative enrichment of NH₄⁺ is driven largely by circulation changes, and secondarily by warming-induced increases in microbial metabolism, as well as reduced nitrification rates due to pH decreases. Supplementing our model projections with geochemical measurements and phytoplankton abundance data from Tara Oceans, we demonstrate that shifts in the form of DIN to NH₄⁺ may impact phytoplankton communities by disadvantaging nitrate-dependent taxa like diatoms while promoting taxa better adapted to NH₄⁺. This could have cascading effects on marine food webs, carbon cycling, and fisheries productivity. Overall, the form of bioavailable nitrogen emerges as an potentially underappreciated driver of ecosystem structure and function in the changing ocean.