Methyl mercury distributions in relation to the presence of nano- and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)

Recent findings on the distribution of methylated mercury (MeHg_T) in waters have highlighted the importance of organic carbon remineralization on the production of these compounds in the open ocean. Here, we present the first time-series (20 monthly samplings between July 2007 and May 2009) of high-resolution vertical profiles (10-12 depths in a 2350 m water column) of MeHg_T distributions in an open ocean environment, the Ligurian Sea (North-western Mediterranean Sea). Concentrations varied within the sub-picomolar range (general mean: 0.30 ± 0.17 pmol L⁻¹, n = 214) with the lowest values at the surface, increasing with depth up to the oxygen minimum zone, and decreasing slowly at greater depth. Concentrations in the surface waters never exceeded 0.15 pmol L⁻¹, while the highest concentrations (up to 0.82 pmol L⁻¹) were associated to the hypoxycline during the autumn bloom. A detailed vertical MeHg_T profile reveals a “double-peak” pattern, coincidental with the two microbial layers described by Tanaka and Rassoulzadegan (2002), the so-called “microbial food web” in the euphotic zone (<100 m) and the “microbial loop” in the aphotic zone (>100 m). Temporal variations in the MeHg_T abundance and distribution in the water column were linked to seasonality. The highest MeHg_T concentrations were found in the oxygen minimum zone during the period of stratification, and coincide with the greatest abundance of nano- and picophytoplankton (cyanobacteria, nanoflagellates, etc.) in the euphotic layer. None of our deep MeHg_T measurements (∼100 m above the sea bottom) revealed a significant sedimentary source of MeHg_T. We explored the correlation between MeHg_T concentrations and the apparent oxygen utilization, a proxy of organic matter remineralization, over the study period. Results of this study strengthen the hypothesis that net mercury methylation in the open ocean occurs in the water column, is linked to organic matter regeneration, and is promoted by the presence of small-sized nano- and picophytoplankton, that dominate under oligotrophic conditions.