Biogeochemical evidence of heterotrophic N<sub>2</sub> fixation in the Gulf of Aqaba (Israel), Red Sea

Abstract

Abstract. Recent studies demonstrate that marine N 2 fixation can be carried out without light by heterotrophic N 2 -fixers (diazotrophs). However, direct measurements of N 2 fixation in aphotic environments are relatively scarce. Heterotrophic, as well as unicellular and colonial photoautotrophic diazotrophs, are present in the oligotrophic Gulf of Aqaba (northern Red 10 Sea). This study evaluates the relative importance of these different diazotrophs by combining biogeochemical models with time series measurements at a 700m-deep monitoring station in the Gulf of Aqaba. At this location, an excess of nitrate is present throughout most of the water column, especially in deep waters during stratified conditions. An excess of phosphate occurs only at the surface during nutrient-starved conditions in summer. We show that a model without N 2 fixation can replicate the observed surface chlorophyll, but fails to accurately simulate inorganic nutrient ratios throughout the water 15 column. Models with N 2 fixation improve simulated deep nitrate by enriching sinking organic matter in nitrogen, suggesting that N 2 fixation is necessary to explain the observations. The observed vertical structure of nutrient ratios and oxygen is reproduced best with a model that includes heterotrophic, and colonial and unicellular autotrophic diazotrophs. These results suggest that heterotrophic N 2 fixation explains the observed excess nitrogen in deep water at this location. If heterotrophic diazotrophs are generally present in oligotrophic ocean regions, their consideration would increase current estimates of 20 global N 2 fixation and may require explicit representation in large-scale models.