Spatial and temporal dynamics of N2 fixation in the Gulf of Aqaba

The ocean is a significant sink of anthropogenic CO2, in large part because organic matter is exported to oceanic depths driving the biological sequestration of carbon in the ocean’s interior. Organic matter export depends on the supply of external nutrients to the euphotic zone (by processes such as deep mixing and biological fixation of atmospheric dinitrogen (N2)) and the subsequent production of organic matter by photosynthesis (defined as “new” production). Currently, there are no reliable estimates of “new” and “export” production in the Gulf of Aqaba, Red Sea, but deep mixing in winter and N2 fixation in summer are thought to be important contributors. Our overarching objective is to assess the annual contribution of N2 fixation and deep mixing to new production in the Gulf of Aqaba. Here we present results from a bi-monthly sampling program at a representative oligotrophic station (Station A – 750 m depth) from December 2015 to March 2017 characterizing the contribution of N2 fixation (diazotrophic composition and rates) to primary and bacterial production in the northern Gulf of Aqaba. These results will be discussed in relation to predictions from a 1-dimensional physical model of Station A, coupled with 4 biogeochemical model versions that examine N2 fixation by different diazotrophs and suggest an important contribution of heterotrophic diazotrophs to new production in the Gulf of Aqaba.