Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N2). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (~0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north-south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial-temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales.

Sampling: Trace-metal-clean surface seawater samples were collected during the UK GA06 GEOTRACES cruise (D361) on the UK research ship RRS Discovery. Surface seawater was pumped into the trace-metal-clean laboratory using a Teflon diaphragm pump connected by acid-washed braided PVC tubing to a towed "fish" positioned at 3-4 m. Samples were filtered in-line through 0.8/0.2-µm cartridge filter (AcroPak1000) into acid-washed lowdensity polyethylene bottles. The trace-metal samples were acidified with concentrated ultrapurity hydrochloric acid (HCl, Romil, UpA) to pH 1.9 (0.013 M H+). The acidified samples were allowed to equilibrate for at least 24 h prior analysis of dissolved Iron and dissolved Aluminium. Unfiltered seawater for dissolved inorganic phosphorus analysis was dispensed into acid-cleaned, aged, high-density polyethylene bottles using clean-sample-handling techniques according to the GO-SHIP nutrient protocols (Hydes et al. 2010).

Supplement to: Schlosser, Christian; Klar, Jessica K; Wake, Bronwyn D; Snow, Joseph T; Honey, David J; Woodward, E Malcolm S; Lohan, Maeve C; Achterberg, Eric Pieter; Moore, C Mark (2013): Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide. Proceedings of the National Academy of Sciences of the United States of America, direct submission