Presenter: Nathalie Vigier (Oceanography Laboratory of Villefranche sur Mer - CNRS - Sorbonne University -)

Description:
The Li residence time in the ocean being more than 1 Myr, this trace metal is uniformly distributed in oceanic waters, with a dissolved Li concentration of 180 ng/ml and a Li isotopic composition (d⁷Li) of 31.2±0.3‰. Marine hydrothermal fluids are a major source of Li and can be significantly enriched in Li compared to the open ocean, with low d⁷Li values (generally < 8‰).  When present at high levels in seawater, Li exerts a toxic effect on aquatic organisms. However, the impact of shallow Li-rich hydrothermal fluids on planktonic species has not been evaluated thus far. During the TONGA expedition in the subtropical South Pacific (Nov. 2019), we collected seawater samples and performed 35 μm and 200 μm plankton net tows following an East-West transect above two active shallow (>500 m) volcanoes associated with hydrothermal activity. After developing a specific methodology, we measured Li concentrations and Li isotope composition in these samples using ICP-MS and MC-ICP-MS respectively. Our results first show high Li concentrations (30-70 mg/g) in the planktonic biomass, eastward and westward the TONGA arc. Just above the hydrothermal sites, Li concentrations of the 200 mm plankton exhibit punctual but significant enrichment in Li (up to 120 mg/g). In contrast, Li concentrations of the 35 mm plankton are lower (10-20 mg/g) along the transect. Most plankton samples display d⁷Li values lower values than seawater. This difference may reflect either the contribution from low d⁷Li hydrothermal fluids, or biological and species effects. We are currently investigating these two possibilities by analysing the collected waters and by comparing our data to quantitative biodiversity indices.  Overall our study unravels the capacity of planktonic blooms to bioaccumulate Li, and a distinct behaviour of phyto- and zooplankton when facing Li-rich environments. Whether these enrichments can cause deleterious effects on their metabolism and biodiversity will be examined.