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

<p>Ammonium (NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9e82b08d319a1c899f60c89d4a61df91"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00001.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00001.png"/></svg:svg></span></span>) 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 NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="0bd088c5a706fdd4e7871717c05b4b3f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00002.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00002.png"/></svg:svg></span></span> data that are available, we project increases in the NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="43bba5feeea5818072376b211f2a452d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00003.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00003.png"/></svg:svg></span></span> <span class="inline-formula">:</span> DIN ratio in over 98 % of the ocean by the end of the 21st century under a high-emission scenario. This relative enrichment of NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="aa378b71f34a6c23384fc0eb7c6e7621"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00004.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00004.png"/></svg:svg></span></span> 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 <i>Tara</i> Oceans, we demonstrate that shifts in the form of DIN to NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="3fe22ea21bb8c3940d1d54b092ea883d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00005.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00005.png"/></svg:svg></span></span> may impact phytoplankton communities by disadvantaging nitrate-dependent taxa like diatoms while promoting taxa better adapted to NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a896672a5a2c6d19cc4695ffc843bfde"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4865-2025-ie00006.svg" width="8pt" height="15pt" src="bg-22-4865-2025-ie00006.png"/></svg:svg></span></span>. 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.</p>