Stoichiometric regulation of nitrogen and carbon fluxes in Acropora coral facing short‐term stress of ammonium loading

Abstract Scleractinian coral evolved under nitrogen (N)‐limited conditions. The increase in N flux from anthropogenic activities to these otherwise N‐depleted environments is threatening coral health and coral reef ecosystem function. We tested the effect of elevated ammonium (NH 4 + ) loading on Acropora metabolism responses (respiration, gross primary production, and NH 4 + uptake) on healthy aquacultured Acropora sp. and compared responses to wild Acropora pulchra from Guam where corals are threatened by eutrophication. We quantified 15 N isotope uptake, metabolism in custom metabolism chambers, and tissue N and carbon (C) content following NH 4 + loading. For aquacultured Acropora sp., we found that NH 4 + loading stimulated primary production and respiration, but did not significantly alter rates of uptake or tissue C or N content. Conversely, the wild A. pulchra did not respond to NH 4 + loading and was generally resistant to short‐term exposures of NH 4 + loading. The change in rates of primary production within the aquacultured coral experiment suggests that sustained increases in N availability could upset stoichiometric regulation in coral. Few studies have coupled N cycling and metabolism rates, so our results serve as an important resource for understanding the biological activity of scleractinian coral, particularly a common wild coral from Guam, where extended events of increased anthropogenic N are increasing disease prevalence and decreasing coral reef diversity.