Variability in the abundance of Trichodesmium and nitrogen fixation activities in the subtropical NE Atlantic

Nitrogen fixation by the diazotrophic community in the subtropical NE Atlantic, including the Canary Current region, was investigated during two oceanographic cruises, RODA I (summer 2006) and RODA II (winter 2007). The first evaluated the role of the Canary eddy fields in the growth of nitrogen-fixing Trichodesmium and their rates of nitrogen fixation and of the unscreened whole diazotrophic community. The second cruise was designed to look at the variability of these processes in the subtropical NE Atlantic study area that is influenced by atmospheric transport of African aerosols. During RODA I, significantly higher mean nitrogen fixation rates of unscreened whole surface water (P < 0.05, t-test) were observed at oligotrophic far-field stations than in the nutrient-enriched eddies, but the pattern was not consistent with nitrogen fixation rates of >50 μm samples (Trichodesmium) integrated over depth. However, correlation analyses showed decreasing rates of nitrogen fixation rates of >50 μm samples (Trichodesmium) at stations with higher NH₄⁺ concentrations, suggesting that combined N seemed to be an important regulator affecting nitrogen fixation rates. Temperature was also correlated with the abundance of Trichodesmium. Iron (from dust deposition), which has been reported in previous studies to stimulate N ₂ fixation, did not show any correlation with N₂ fixation rates or with the abundance of Trichodesmium.