THE TONGA PROJECT

Shallow hydroThermal sOurces of trace elemeNts: potential impacts on biological productivity and the bioloGicAl carbon pump

PIs: Sophie Bonnet & Cécile Guieu

The Western Tropical South Pacific (WTSP) Ocean has recently been identified as a hotspot of N2 fixation i.e. harbors among the highest rates reported in the global ocean (Bonnet et al., 2017). N2-fixing organisms have high iron (Fe) quotas relative to non-diazotrophic plankton and their success in the WTSP has been attributed to the alleviation of Fe limitation in this region. However, our knowledge on Fe sources and distribution in the WTSP remains limited. During the OUTPACE cruise in 2015, the proposed team identified a shallow (<500 m) hydrothermal Fe source in the WTSP close to the Tonga volcanic Arc, which resulted in high concentrations (4-60 nM) of dissolved Fe (DFe) up to the photic (~0-150 m) layer (Guieu et al., 2018). Such inputs are suspected (together with high sea surface temperature ~27-29°C) to trigger diazotroph blooms in the WTSP. However, the potential impact of such hydrothermal input on plankton communities and biogeochemical cycles of biogenic elements (carbon (C), Nitrogen (N), Phosphorus (P)) remains to be studied. In this context, the main objectives of the TONGA project are:
 

• To accurately quantify Fe (and other biogeochemically relevant compounds) input from shallow (<500 m) submarine volcanoes and associated hydrothermal vents along the Tonga volcanic arc for the photic zone in comparison with atmospheric deposition,

• To study the fate of shallow hydrothermal plumes in the water column at the local and regional scales,

• To investigate the bioavailability and the potential impact of such hydrothermal inputs on planktonic communities and biogeochemical cycles in the WTSP

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To achieve this goal, we performed a 37-day oceanographic cruise In October-December 2019 (R/V L’Atalante) in the WTSP (see VIDEOS from the cruise HERE).