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

Abstract

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 À1 , 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 À1 , while the highest concentrations (up to 0.82 pmol L À1 ) 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.

Freshwater discharges drive high levels of methylmercury in Arctic marine biota

“…Stepwise regression analysis on a suite of parameters shows that nitrate alone explains 90% of the variability in measured methylation rates (P < 0.01). We postulate that this finding reflects the association between the degradation of organic matter in surface waters and MeHg production proposed by others (7)(8)(9)(10).…”

“…Prior work in the upper ocean suggests that production of methylated Hg species is linked to heterotrophic bacterial activity responsible for the turnover of organic carbon in the marine water column, as reflected by changes in nitrate, phosphate, AOU, and OCRR (7)(8)(9)(10)(11). Lake Melville is oligotrophic, and nitrate concentrations in inflowing tributaries are low (<3 μM).…”

“…In open-ocean seawater, strong associations between methylated Hg concentrations and nutrients, apparent oxygen utilization (AOU), and organic carbon remineralization rates (OCRR) are observed across major basins (7)(8)(9)(10). These associations reflect subsurface water-column production of MeHg and coincide with peaks in heterotrophic bacterial activity (7)(8)(9)(10).…”

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[15] In other oceans, it has recently been demonstrated that MeHg can be produced during the remineralisation of algal detritus in the water column. [44,86,98,99] The vertical transport of MeHg associated with particulate flux from surface waters was found to be relatively unimportant compared with the in situ production of MeHg that occurred in association with nutrient maxima at subsurface water depths. [86] Little is known about this process in the Arctic; however, the Arctic Ocean does possess pervasive strong nutrient maxima below the polar mixed layer.…”

Methylmercury production below the mixed layer in the North Pacific Ocean

“…As DMHg is not bioaccumulated and because its Hg isotopic composition has not been measured, it is not treated in detail here. MMHg production in the oxygen minimum zone (OMZ) of the oceanic water column has been hypothesized 7,13 and recent field studies have offered new evidence supporting in situ methylation not only in OMZs (refs 3,6) but also in oxic surface waters 4,5,[15][16][17] . Vertical differences in ocean biogeochemistry and the relative contributions of different Hg sources to the overall MMHg burdens in fish have wide-ranging implications for understanding how MMHg levels in marine biota will respond to future changes in anthropogenic Hg emissions 3,18 .…”