Dissolved and particulate metals (Fe, Zn, Cu, Cd, Pb) in two habitats from an active hydrothermal field on the EPR at 13°N

The distribution of Fe, Cu, Zn, Pb, Cd between the dissolved (<2 μm) and the particulate (>2 μm) fractions was measured after in-situ filtration in two hydrothermal habitats. The total metal concentration ranges exhibit a clear enrichment compared with the seawater concentration, accounting for the hydrothermal input for all the metals considered. Iron is the predominant metal (5–50 μM) followed by Zn and Cu. Cd and Pb are present at the nM level. At the scale studied, the behavior of temperature, pH and dissolved iron is semiconservative whereas the other dissolved and particulate metals are characterized by nonconservative patterns. The metal enrichment of the >2 μm fraction results from the settlement and accumulation of particulate matter close to the organisms, acting as a secondary metal source. The enrichment observed in the dissolved fraction can be related to the dissolution or oxidation of particles (mainly polymetallic sulfide) or to the presence of small particles and large colloids not retained on the 2 μm frit. SEM observations indicate that the bulk particulate observed is characteristic of crystalline particles settling rapidly from the high temperature smoker (sphalerite, wurtzite and pyrite), amorphous structures and eroded particles formed in the external zone of the chimney. Precipitation of Zn, Cu, Cd and Pb with Fe as wurtzite, sphalerite and pyrite is the main process taking place within the area studied and is semi-quantitative. The distribution of the dominant observed fauna has been related to the gradient resulting from the dilution process, with the alvinellids worms colonizing the hotter and more variable part of the mixing zone, but also to the metallic load of the mixing zone. Dissolved and particulate metal concentrations are therefore necessary abiotic factors to be studied in a multiparametric approach to understand the faunal distribution in hydrothermal ecosystems.