1b: Biogeochemical cycling of mercury in the ocean

Approximately 130 water samples were collected at the head of the Monterey Bay submarine canyon for the analysis of dimethylmercury (DMHg),monomethylmercury(MMHg), and total mercury (THg) during a one-year study. Through a bottom-moored seawater intake system, (17 meters depth) measurements of sigma-t, temperature, salinity, oxygen, turbidity, fluorescence, and pH were made concurrently with the collection of each sample. An initial, intensive 30-hour study showedthat variations in DMHg were strongly associated with the undulation of internal wavesand their associated water masses as they were sequentially entrained into the seawater intakes. To avoid aliasing of seasonal fluctuations, this finding led to periodic, twice daily sampling to capture water from above and within thethermocline, a region over which strong gradients in DMHg and MMHg are typically found. In the higher sigma-t strata DMHg and MMHg varied from 0 to 140 fM and 11 to 160 fM respectively, whereas the lower sigma-t strata variations in the species ranged from 0 to 100 and 5 to 112 fM, respectively. When fluctuations in internal wave height are filtered out, a strong relationship between methylated species and upwelling season is observed. THg varied from 373 to 4000 fM, however there was no relationship seen with respect to water mass. pH and DMHg exhibited an inverse relationship where higher pH samples had lower DMHg concentrations. In contrast, higher salinity water showed higher concentrations of DMHg. There is also a strong relationship with apparent oxygen utilization (AOU) and DMHg. In this study we will present mercury concentrations with coastal upwelling indices and examine processes responsible for the degradation of organic mater in the upper water column to understand mercury speciation over the study period and discuss these findings in the context of seasonally forced oceanographic conditions present in thenearshorecoastal environment of the California Current and Monterey Bay.

Humans are exposed to toxic methylmercury (MeHg) mostly from the consumption of seafood, particularly top-predator fish (e.g., tuna, swordfish, mackerel) that can accumulate high concentrations of MeHg. A recent investigation has shown that MeHg concentrations in predatory fish of the subtropical North Pacific Ocean increased with greater foraging depth. However, little is known about the MeHg content of smaller fish inhabiting meso- and bathypelagic zones of the ocean. We sampled dragonfish, lanternfish, bristlemouths, and hatchetfish with vertical net trawls at multiple locations in the northern Gulf of Mexico as part of the DEEPEND project. Preliminary results suggest that MeHg concentrations vary widely within and among these fish species. The three species of Myctophiformes fishes that migrate to shallower depths to feed at night have lower concentrations of MeHg than four of the five Stomiiformes species that do not migrate from depth. Moreover, mean MeHg concentration is positively correlated with average foraging depth among fish species. Differences of MeHg concentrations in prey fish as a function of vertical migration and foraging depth in the ocean provide a mechanism to explain how concentrations in predatory fish increase with greater foraging depth.

Labrador Sea is one of the main the entrance into the ocean interior, and the mercury (Hg) cycle in the ocean is heavily impacted by human emissions, it is the reasons why we report here the first high resolution Hg distribution pattern along a transect from Greenland to Labrador coasts performed after the 2014-winter convection. Total Hg concentrations in unfiltered samples ranged from 0.25 to 0.66 pM averaging 0.43 ± 0.1 pM (n = 106). Generally, Hg concentrations increased downward from surface to deep waters, and southward from Greenland to Labrador. Concentrations in filtered samples represented from 62 to 92 % of the unfiltered waters and exhibited a similar distribution. Most striking is: (i) the very low Hg concentrations in the Labrador Sea Water formed during the 2014-winter convection, and (ii) the high Hg concentrations in the waters of the Labrador Current, which receive waters from the Canadian Archipelago and the Baffin Bay.

In seawater mercury is a very reactive element. Once in the water, it can be present in different forms: elemental mercury Hg(0), Hg(II) complexes and organic Hg forms (mainly methyl mercury (MeHg) and dimethyl mercury (DMeHg)). Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during several oceanographic cruises on board the Italian research vessels Urania and Minerva since summer 2000 as part of different European (Mercyms, GMOS) and national research projects, covering regions from Strait of Gibraltar on the west, and Levantine Sea on the east, as well as Adriatic Sea with the Gulf of Trieste as Mediterranean most northern part. The study includes deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg), and with some water quality parameters in coastal and open sea deep water profiles. Recent studies in the Mediterranean Sea revealed the presence of enhanced Hg emission rates and/or more active atmospheric transformation processes in this region due to a number of climatological, meteorological, geographical and geological features that characterize the basin.

Concentrations of measured Hg species were characterized by seasonal and spatial variations. Relatively high portions of DGM indicate high reactivity of Hg in open marine waters. DGM was present in surface waters mainly as Hg0 as no DMeHg was detected, while towards the bottom a noticeable, but relatively small portion of DMeHg (<5 %) is present in most of the Mediterranean. In Adriatic DMeHg was present in higher fraction and was detected in surface waters, but in very low concentrations. DGM generally increases with depth, suggesting a source of volatile Hg in deeper waters. Average DGM concentration was the highest in deep water masses (WMDW and EMDW). From certain profiles it is possible to observe that increase of DGM corresponds to a decrease in dissolved oxygen levels, suggesting that DGM is produced by biologically mediated processes in the oxygen minimum zone. The observed decrease towards the surface is a result of the balance between production and loss processes on boundary between water and atmosphere.

Monomethylmercury (MMHg) was measured in the sea surface microlayer (SML), an operationally defined 10-100μm thick film typically enriched in organics and normally associated with high concentrations of hydrophobic constituents (hydrocarbons, PCBs, carbohydrates and pesticides). In 2015, two cruises along the California Coast, SML samples were collected from a small workboat, using an acid cleaned glass plate and glass tubes. Samples were analyzed via gas chromatography (GC) coupled with a Tekran© Cold Vapor Atomic Fluorescence Spectrophotometer (CVAFS) Model 2500 Mercury Detector. SML values for MMHg ranged from 16.02 - 380.39 fM, corresponding to enrichment factors (EF) of 2.5 – 29.6 relative to underlying water from the mixed layer. These represent the higher end of other trace metal enrichments in this microenvironment and are the first such measurements to our knowledge. The lipophilic nature of MMHg likely lends to its partitioning into the hydrophobic organic material of the SML. Adsorbtion of MMHg onto buoyant bodies such as microplastics, fish eggs and larvae, also present in the SML, may serve as entry into the food web through ingestion. MMHg EF of the SML correlated strongly with temperature (n=7 r=0.7229, p = 0.0665). Higher sea surface temperatures can be indicative of stratification in areas of downwelling, which concentrate substances and objects floating at the surface. Based on calculated values, these trace metal sampling techniques likely yield enriched, yet still diluted samples (due to method associated dilution of the method itself). Aerosolization of MMHg laden SML could create more concentrated MMHg marine-derived aerosol particles, and are potentially a more accurate representation of true MMHg enrichment in the SML and its impact on floating objects and the source of aerosol-derived MMHg to marine advective fog and sea spray. Preliminary results of marine aerosol collections will be presented. Establishing and evaluating the pathway of MMHg in the SML would further inform the global cycling of mercury species.

Natural microbial processes in seawater and sediments can convert inorganic mercury into its toxic form, methylmercury (MeHg). Our research tries to identify relationship between mercury species and microbial abundance in seawater. We performed series of samplings during oceanographic cruises aboard the research vessel Bios Dva from March 2014 to December 2015 in the Central Adriatic Sea. Research was constrained in transect from the Bay of Kastela to the island of Vis. We collected non-filtered water samples for determination of total mercury (THg), dissolved gaseous mercury (DGM), methylmercury (MeHg) and microbial species in Adriatic coastal and open waters. Plankton samples for the determination of THg and MeHg were collected using vertical towing nets (53 and 200 µm). THg concentrations are the lowest in the pristine environment of the island of Vis (0.14-1.10 ng/L). The highest THg values (0.92-5.58 ng/L) are found in the Bay of Kastela which had been affected by previous contamination from chlor-alkali industrial waste waters. DGM always shows slightly higher values in contaminated areas (31.8-351 pg/L) than in the pristine environment (22.1-245 pg/L). MeHg concentrations vary, but range from 1.07-34.3 pg/L for all stations, with the highest values found in the Bay of Kastela. Percentage of THg present as MeHg is low (1.2% and 3.1% in coastal and open sea, respectively) indicating deficiency of conditions for mercury methylation or high MeHg demethylation. MeHg concentration in seawater is correlated with abundance of picoeukaryotes, indicating possible microbial methylation in the water column. Number of picoeukaryotes is the highest in the Bay of Kastela (0.44-31.8 millions/L) which has been affected by industrial effluents from surrounding cities. The lowest number (0.63-19.9 millions/L) is found near the island of Vis. The highest THg concentrations in plankton are found in the Bay of Kastela (49.8-1115 ng/g and 103-249 ng/g d.w. for 53 and 200 µm fraction, respectively), while the lowest values are found at the Split Channel station (39.5-96.5 ng/g and 5.27-96.8 ng/g d.w. for 53 and 200 µm fraction, respectively). MeHg concentrations are more variable (0.29-14.2 ng/g and 2.00-24.3 ng/g d.w. for 53 and 200 µm fraction, respectively). THg concentrations in plankton show biodilution effect which is best seen in the Bay of Kastela. On the contrary, MeHg bioaccumulation along trophic levels is observed through higher MeHg concentrations and MeHg percentage in 200 µm fraction, compared to 53 µm fraction.