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 are available, we project increases in the NH4 : DIN ratio in over 98 % of the ocean by the end of the 21st 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 fishery productivity. Overall, the form of bioavailable nitrogen emerges as a potentially underappreciated driver of ecosystem structure and function in the changing ocean.