Phosphate availability as a key factor controlling nitrogen fixation and the BCP in the iron rich Western Tropical South Pacific

Thierry Moutin, Mediterranean Institute of Oceanography, Aix Marseille University, Marseille, France
... and the OUTPACE team

In the context of global warming, the ocean is changing quickly, and initiating large scale oceanographic research programmes is of major importance if we wish to be able to predict the likely future patterns of change in carbon sequestration. The OUTPACE project was the culmination of a long process that has allowed scientists from different fields (from physicists specialising in Lagrangian navigation to biologists specialising in genes and biodiversity/function at the single-cell level) to work together to study the biological carbon pump (BCP) in the Western Tropical South Pacific (WTSP).

As main results, we found (i) a significant biological “soft tissue” carbon pump in the WTSP despite no winter replenishment of surface waters by nitrate, (ii) a major role of dinitrogen fixation and (iii) that the necessary excess P is mainly brought to the upper surface by local deep winter convection at an annual scale rather than by surface circulation. We confirmed a geographical trend of limitation on N2 fixation, from a P limitation in the west to an iron limitation in the central and east S Pacific Ocean, and suggest that the modifications of the BCP linked to N input by N2 fixation may be over time scales shorter than previously thought.

The specific results are detailed in 27 papers of a Biogeosciences special issue 'Interactions between planktonic organisms and biogeochemical cycles across trophic and N2 fixation gradients in the western tropical South Pacific Ocean: a multidisciplinary approach (OUTPACE experiment)' Editors: T. Moutin, S. Bonnet, K. Richards, D.G. Capone, E. Marañón, and L. Memery :