No abstract

Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0–150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 µmol N m−2 d−1) compared to those measured in the Coral Sea (2 to 109 µmol N m−2 d−1). Rates measured in the Solomon Sea were in the upper range (100–1000 µmol N m−2 d−1) or higher than rates compiled in the global MARine Ecosystem biomass DATa database, indicating that this region has some of the highest N2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN‐A1 and UCYN‐A2) and the proteobacteria γ‐24774A11 dominated in the Coral Sea and were correlated with N2 fixation rates (p < 0.05), Trichodesmium and UCYN‐B dominated in the Solomon Sea and were correlated (p < 0.05) with N2 fixation rates. UCYN‐A were totally absent in the Solomon Sea. The biogeographical distribution of diazotrophs is discussed within the context of patterns in measured environmental parameters.

International audienceIron availability in the Southern Ocean controls phytoplankton growth, community composition and the uptake of atmospheric CO₂ by the biological pump. The KEOPS-2 (KErguelen Ocean and Plateau compared Study 2) "process study", took place around the Kerguelen Plateau in the Indian sector of the Southern Ocean. This is a region naturally fertilised with iron on the scale of hundreds to thousands of square kilometres, producing a mosaic of spring blooms which show distinct biological and biogeochemical responses to fertilisation. This paper presents biogeochemical iron budgets (incorporating vertical and lateral supply, internal cycling, and sinks) for three contrasting sites: an upstream high-nutrient low-chlorophyll reference, over the plateau and in the offshore plume east of the Kerguelen Islands. These budgets show that distinct regional environments driven by complex circulation and transport pathways are responsible for differences in the mode and strength of iron supply, with vertical supply dominant on the plateau and lateral supply dominant in the plume. Iron supply from "new" sources (diffusion, upwelling, entrainment, lateral advection, atmospheric dust) to the surface waters of the plume was double that above the plateau and 20 times greater than at the reference site, whilst iron demand (measured by cellular uptake) in the plume was similar to that above the plateau but 40 times greater than at the reference site. "Recycled" iron supply by bacterial regeneration and zooplankton grazing was a relatively minor component at all sites (< 8 % of new supply), in contrast to earlier findings from other biogeochemical iron budgets in the Southern Ocean. Over the plateau, a particulate iron dissolution term of 2.5 % was invoked to balance the budget; this approximately doubled the standing stock of dissolved iron in the mixed layer. The exchange of iron between dissolved, biogenic particulate and lithogenic particulate pools was highly dynamic in time and space, resulting in a decoupling of the iron supply and carbon export and, importantly, controlling the efficiency of fertilisation