title: Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide.
abstract: 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.
authors.affiliation: Ocean and Earth Sciences, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom.
authors.forename: Christian
authors.initials: C
authors.lastname: Schlosser
authors.forename: Jessica K
authors.initials: JK
authors.lastname: Klar
authors.forename: Bronwyn D
authors.initials: BD
authors.lastname: Wake
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Snow
authors.forename: David J
authors.initials: DJ
authors.lastname: Honey
authors.forename: E Malcolm S
authors.initials: EM
authors.lastname: Woodward
authors.forename: Maeve C
authors.initials: MC
authors.lastname: Lohan
authors.forename: Eric P
authors.initials: EP
authors.lastname: Achterberg
authors.forename: C Mark
authors.initials: CM
authors.lastname: Moore
mesh_terms: D001267:Atlantic Ocean; D000458:Cyanobacteria / Q000502:physiology*; D009586:Nitrogen Fixation; D012621:Seasons
keywords: atmospheric iron deposition; nitrogen fixation