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 (N 2 ). 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 northsouth 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 N 2 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.nitrogen fixation | atmospheric iron deposition

Diagnosing oceanic nutrient deficiency

“…Inferences from such sensitivity analyses and significant remaining caveats associated with the plasticity of phytoplankton elemental stoichiometry are revisited below. An idealized model [24,25,87] is also used to illustrate the expected qualitative outcome of interactions between three key nutrients, N, P and Fe, and two broadly defined types of organism, a non-diazotrophic primary producer and a diazotrophic primary producer. The latter organisms are assumed to have a higher requirement for Fe as a result of the high abundance of this element within the nitrogenase enzyme which catalyses N 2 fixation [88,89].…”

“…The latter organisms are assumed to have a higher requirement for Fe as a result of the high abundance of this element within the nitrogenase enzyme which catalyses N 2 fixation [88,89]. The basic model is essentially identical to that described previously [87]. In the current context, the model was run [25].…”

Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

“…Wet atmospheric deposition (rain, fog and snow) also plays an important role in supplying Al to the North Atlantic (Schlosser et al, 2014;Shelley et al, 2017) and the global ocean (Guerzoni et al, 1997;Vink and Measures, 2001). Glacial run off has been reported as a pronounced source for Arctic 15 and Antarctic surface waters (Brown et al, 2010;Schlosser et al, 2017), but its importance beyond the immediate source regions has not yet been established.…”

How well can we quantify dust deposition to the ocean?

“…Efforts to evaluate the supply of dust to the ocean are motivated in large part by the desire to assess the role of aerosols as a source of iron, an essential nutrient whose abundance is thought to influence rates and geographical patterns of photosynthesis and nitrogen fixation [1][2][3][4][5][6][7]. Ocean productivity, in turn, is a major component of the global carbon cycle [8], so there is a recognized need to include dust supply in global biogeochemical models [9].…”