Ammonium and nitrite oxidation in the upper euphotic zone of the oligotrophic Red Sea

Abstract. Nitrification is widely understood to be inhibited by light in the surface ocean, however, increasing evidence indicates its occurrence at low levels at many sites. The extent to which nitrification remains active in the euphotic zone could have important implications to new production calculations, yet it remains understudied. Here, we quantified ammonium and nitrite oxidation rates in the euphotic zone of the Gulf of Aqaba (Northern Red Sea) from late spring to late summer and examined environmental controls and implications for dark carbon fixation (chemoautotrophy) and new production. Both ammonium and nitrite oxidation were detectable throughout the euphotic zone (~0.1–0.8 nmol N L^-1 d^-1). Overall, rates increased with depth and were strongly suppressed in the highest irradiance surface waters. Integrated rates over the entire euphotic zone (24–56 µmol N m^-2 d^-1) were among the lowest reported for oligotrophic regions globally. This reflects extremely low substrate concentrations and intense, though not complete, photoinhibition. Ammonium and nitrite oxidation together supported <2 % of chemoautotrophic activity, suggesting other processes, not accounted for, such as anaplerosis may be important. Depth-resolved correlations with environmental parameters highlight light, temperature, and substrate availability as key regulators of both processes. Our results show that nitrification in the Gulf of Aqaba operates at the lower bounds of global euphotic zone rates and is loosely coupled to carbon cycling. These findings underscore the need to better resolve nitrification dynamics in ultra-oligotrophic, rapidly warming, seas to refine estimates of new production and chemoautotrophic carbon assimilation under future ocean conditions.