Response of surface nutrient inventories and nitrogen fixation to a tropical cyclone in the southwest Pacific

Abstract

To examine the controls of new nitrogen supply in oligotrophic waters, nitrogen fixation and nutrients were monitored during a quasi‐Lagrangian study in surface waters of the north Tasman Sea during March–April 2006. The study was initiated coincident with a tropical cyclone, with high rainfall and winds that eroded surface layer density structure and increased surface phosphate and silicate concentrations by 50%. However, there was no corresponding increase in nitrate, as the enhanced vertical mixing perturbed the phosphacline but not the deeper nitracline. Dissolved iron was highest in the upper 20 m, and declined to a minimum at 100–150 m, indicating wet deposition as the source, as also inferred from the salinity decrease. Nitrogen fixation increased significantly by an order of magnitude over the following 9 d. In deckboard perturbation experiments immediately after the cyclone, phosphate and iron addition did not elicit a response in nitrogen fixation, whereas 8 d later, when in situ dissolved iron had declined, the addition of iron stimulated nitrogen fixation by 1.5‐fold. Conversely, the addition of dust from Australia and the Gobi desert immediately after the cyclone increased nitrogen fixation by 2‐ and 4.5‐fold, respectively. The observed 10‐fold increase in nitrogen fixation in situ exceeded that reported in other nutrient perturbation experiments despite comparatively low in situ dissolved iron, indicating that iron bioavailability may be a critical factor. Although stimulation by a cyclone was unexpected, it promoted favorable conditions for diazotrophy by enhancing phosphate availability in the absence of nitrate, and increased dissolved iron supply and availability via wet deposition. This suggests that future increases in tropical cyclones, combined with other climate‐driven trends, may increase nitrogen fixation in the north Tasman Sea.