. The data strongly suggest photochemical in situ formation of RGHg with increasing concentrations in the tropical regions concomitant to a dramatic decrease in O 3 (from over 50 to less than 5 ppbv). A distinct diurnal variation in RGHg concentrations with maxima at midday suggests a photochemically driven oxidation of marine boundary layer Hg 0 . In the tropical area, enhanced RGHg formation and daily variation in O 3 could both be related to reactive halogen chemistry. Flux calculations also emphasize the role of the tropical marine boundary layer in mercury cycling over the North Pacific Ocean.

No abstract

[1] Atmospheric elemental mercury and reactive gaseous mercury (RGHg) concentrations, as well as ancillary parameters and meteorological data, were collected during a cruise in the North Atlantic Ocean between Bermuda and Barbados and at two land-based sites: the Chesapeake Biological Laboratory (CBL), a semirural, coastal site, and a site within the city of Baltimore, Maryland. There were two deployments at CBL, including a 6-month deployment in 2003/2004. Wet deposition samples were collected and analyzed for mercury where possible. A diurnal change in RGHg concentration was found at both CBL and over the North Atlantic Ocean, with maxima in the afternoon that coincided with maximum UV radiation, demonstrating the importance of in situ formation of RGHg. The maxima in RGHg concentration did not coincide with elevated Hg 0 but were often found under conditions of low wind speed, and over the ocean, under low ozone conditions. In contrast, at the urban site there was more evidence of local sources. The data collected at the sites are contrasted and compared and used to examine the factors controlling the formation of RGHg in open ocean and nearshore locales and to estimate the importance of the dry deposition of RGHg to surface waters.

The speciation and partition of mercury in a macrotidal estuary (Seine estuary, France) was studied in order to explore the role of the high turbidity zone (HTZ) in mercury transfer to the adjacent coastal waters. Water and particles were analyzed to determine the concentrations of various mercury species, including monomethylmercury and the inorganic fraction. The exchangeable particulate mercury, which varies with salinity, and the mercury fraction associated with the amorphous oxyhydroxides were evaluated. The distribution of dissolved mercury species during early mixing suggests non-conservative behavior of organically bound mercury at the head of the estuary. Mercury in the particles covaried positively with suspended particulate matter concentrations up to a threshold, which constitutes the typical mercury load of particles and deposited sediments of the HTZ. This distribution pattern is well explained by a dilution model: a slowly settling, low metal population of particle, characterized by relatively invariant turbidity, becomes admixed with a variable amount of higher metal content particles derived from the resuspension in the HTZ. In addition, in the HTZ, which acts as a degradation reactor for particulate organic matter, particulate mercury concentrations increase with increasing C:N ratios and amorphous oxyhydroxides particles. Mercury reaches the estuarine HTZ mainly associated with the riverine and marine particles, including organic matter and oxyhydroxides, which are temporarily trapped in the HTZ and mixed with autochthonous HTZ particles. The largest particles periodically settle and undergo diagenetic reactions and resuspensions, which lead to their mercury enrichment. Depending upon hydrodynamic conditions, mercury escapes seaward as fine particulate within the plume, partially associated with the oxyhydroxides. A surface complexation model reproduces most of the partitioning observed. In the dissolved phase the model simulation suggests that a very strong ligand must be present to explain the field observations. Low (summer) 1.32 ± 0.04 (n=5)1.34 ± 0.12 (n=8) 1.70 ± 0.17 (n=9) 1.59 ± 0.33 (n=13) (HgT) P (nmol.g -1 ) High (winter) nd 2.34 ± 0.67 (n=14) 1.21 ± 0.45 (n=9) 0.44 ± 0.21 (n=11)

The Mussel Watch program conducted along the French coasts for the last 20 years indicates that the highest mercury concentrations in the soft tissue of the blue mussel (Mytilus edulis) occur in the maritime Caux region (the eastern part of Seine Bay on the south coast of the English Channel). This region is characterized by the presence of intertidal and submarine groundwater discharges, and no particular mercury effluent has been reported. Two groundwater emergence systems (Etretat and Yport) in the karstic coastal zone of the Caux region were seasonally sampled to study mercury distribution, partitioning and speciation from these sources. Samples were also collected to compare speciation changes during freshwater-seawater mixing between these "subterranean" or "groundwater" estuaries and the adjacent Seine estuary. The mercury concentrations in the soft tissue of mussels from the same areas were monitored seasonally at the same time. The groundwater springs showed similar total mercury concentrations for bulk water samples (HgTD+P). The means of the dissolved (<0.45 µm) mercury concentrations (HgTD) were 0.99 ± 0.15 ng l-1 and 0.44 ± 0.17 ng l-1 at Etretat as at Yport respectively. High HgTD concentrations were associated with strong surface runoff over short water pathways during storm periods, while low concentrations were associated with long groundwater pathways. Mean particulate mercury concentrations were 0.22 ± 0.05 ng mg-1 and 0.16 ± 0.10 ng mg-1 at Etretat and Yport respectively, and decreased with increasing particle concentration as a result of dilution by soil erosion. Groundwater mercury speciation was characterized by high reactive-to-total mercury ratios in the dissolved phase (HgRD / HgTD: 44-95 %), and undetectable total monomethylmercury concentrations (MMHg <8 pg l-1). The HgTD distributions in the Yport and Etretat mixing zones were similar (overall mean concentration of 0.73 ± 0.21 ng l-1), but higher than those measured in the adjacent heavily industrialized Seine estuary (mean: 0.32 ± 0.13 ng l-1). In the waters along the Pays de Caux dissolved monomethylmercury (MMHgD) concentrations varied from 9.5 to 13.5 pg l-1 (2.4 to 7.8 % of the HgTD). Similar levels were measured in the Seine estuary (range: 12.2-21.1 pg l-1 ; 5.5-11.6 % of the HgTD). The specific biogeochemical characteristics of these underground karstic estuaries seem to promote higher dissolved mercury in coastal waters. While the high turbidity zone of the Seine estuary acts as a dissolved mercury removal system, the mixing zone of the Caux region receives the dissolved mercury inputs from the groundwater seepage with an apparent Hg transfer from the particulate phase. After normalization for minimizing the effect of the seasonal variations of the soft tissue weight, the mercury contents of mussels from Etretat and Yport exhibited significantly higher values than those found in the mussel from the mouth of the Seine estuary. We argue that this difference reflects the mercury distribution in the water and that the dissolved p...