2a-2: Sources and cycling of mercury in coastal ecosystems

Sediments are a known sink for inorganic mercury (Hg) entering into coastal ecosystems, and they foster a geochemical environment conducive to high Hg methylation rates. Therefore, they have been considered a major source of methylmercury (MeHg) to estuarine water columns and coastal food webs. However, there is frequently no correlation between sediment and forage fish MeHg levels across coastal systems, suggesting the presence of a different, potentially more significant source of MeHg available to pelagic organisms. Of particular interest is the potential for net water column MeHg production relative to sediment production and flux to the water column, especially within flocculated material suspended and/or at the sediment water interface. While most studies focus on methylation within an integrated sediment section (typically, the top 2 or 4 cm), it is possible that methylation rates are higher in flocculated, unconsolidated surface sediment than in the deeper sediments; this material is also more likely to re-enter the water column via resuspension where it may be more relevant for biotic exposure. To better understand the importance of in situ methylation and exchange processes between estuarine sediments and the overlying water column, we measured methylation and demethylation potentials in estuaries of varying historic mercury contamination using isotopically labeled mercury species (Me199Hg and 200Hg(II)). Four geochemically diverse zones were investigated in 2016; surface water, bottom water, unconsolidated surface sediments, and consolidated near-surface sediments. Water samples were incubated unfiltered in the dark and in the light. Abiotic demethylation was monitored with 0.2 μm filtered water samples kept in the light. Results suggest that methylation within the water column is complex and more highly variable in space and time within estuarine systems than net MeHg production in the sediment. Our initial estimates suggest that integrated water column methylation is comparable to the potential MeHg flux from the sediments at certain locations within an estuary. Overall, it appears that the surface floc layers in the systems studied in 2016 have dissimilar methylation and demethylation rates to deeper, consolidated sediments, with slightly higher rates in the unconsolidated floc layer. The results of the 2016 studies will be discussed in detail and will be contrasted to a similar study we conducted in 2015 from a broader range of sites along the US East Coast. Our overall findings suggest that water column methylation in coastal ecosystems should not be ignored when considering sources of MeHg to coastal food webs.

Water column methylmercury (MeHg) concentrations are an important indicator for food web mercury burdens in coastal aquatic ecosystems. Several studies have observed strong correlations between forage fish MeHg body burdens and water column particulate MeHg, but less of a relationship to sediments where high rates of mercury (Hg) methylation are frequently measured. To further study this, we compared water column methyl- and total Hg dynamics along estuarine salinity gradients in systems of varying historic mercury contamination including Berrys Creek and the Hackensack River in New Jersey, the Penobscot River in Maine, and the Pawcatuck River in Connecticut/ Rhode Island. Each system was sampled twice in 2016, once in late spring and once in mid-summer, with some sites also sampled several times over a tidal cycle. To better refine our understanding of particulate methylmercury dynamics, the suspended particles were sampled in bulk and as a 20-0.45 μm size fraction using sequential filtration techniques. Auxiliary data collected to help explain the particulate trends include chlorophyll a and phaeopigments, suspended solids, carbon/nitrogen/sulfur concentrations and isotopic signatures, and dissolved methyl- and total Hg. Sediments were also collected at the time of sampling to assess background mercury contamination. Our results indicate that MeHg and total Hg are overall greater at the more contaminated sites. However, the water column particulate pools do not directly reflect the near surface sediment concentrations at the location in which they were collected, particularly for MeHg, suggesting that the suspended pool of MeHg in the water column may not be directly related to sediment production. Dissolved MeHg dynamics track the particulate MeHg in the water column. However, in the more contaminated sites the majority of the water column MeHg is associated with particles in comparison to more pristine sites where a greater portion of the MeHg is in the dissolved phase. We will discuss the implications of our results in terms of understanding the mechanisms of MeHg transfer to the food web, and its variability across coastal ecosystems.

Methylmercury (MeHg) exposure is a recognized health concern, as it can cause brain and neurological damage in humans and animals. In marine food chains MeHg is first bioconcentrated in phytoplankton, then transferred up the food chain through consumption by herbivorous zooplankton, planktivorous fish, and eventually larger predators. However, there is relatively little experimental data on mercury (Hg) and MeHg in primary producers (phytoplankton) and primary consumers (zooplankton). There have been few measurements of the uptake and assimilation efficiencies (AE) for plankton at environmentally relevant levels of exposure, or that have been undertaken using native field populations, even though these assemblages represent the base of the marine food chain, and the initiation of MeHg exposure for higher trophic levels. The current study was therefore aimed at understanding the dynamics of Hg and MeHg uptake and trophic transfer at the lower trophic levels.

Feeding experiments were performed using stable isotopes of inorganic Hg (200Hg) and MeHg (CH3199Hg) in order to determine AEs for the specific Hg species, as well as to demonstrate potential effects of concentration and size of algae on their accumulation of Hg and MeHg as well as the bioaccumulation into the copepod Acartia tonsa. The average AE for ²⁰⁰Hg from copepods feeding on the smaller diatom (Thalassiosira pseudononna) was 41 +/- 15 % and 32 +/- 17 % for the larger diatom (Thalassiosira weisflogii). The AEs were greater for CH₃¹⁹⁹Hg, ranging around 71 +/- 9 % for the smaller diatom and 88 +/- 4 % for the larger diatom.

Assimilation Efficiencies were higher for Hg than reported for previous studies, suggesting that AEs may be related to exposure concentration. Stable isotope feeding experiments were additionally performed for algae species fed to size-fractioned field populations of zooplankton collected from different locations in Long Island Sound, and yielded varying results depending on the season of collection. Our results provide evidence that Hg and MeHg assimilation differs with zooplankton size and species composition. Overall, these experiments demonstrated that there was significant relationship between Hg and MeHg exposure concentration and algal uptake and trophic transfer to zooplankton. Furthermore, these experiments also provided evidence for earlier suggestions of active uptake of MeHg into algae at low (pM) concentrations. These differences in uptake and AE could impact modeling of the transfer of MeHg within ocean food chains, and therefore predictions of the impact of changing emissions on ocean fish concentrations.

The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft was used to sample cloud water with a Mohnen slotted-rod cloud water collector during 16 flights in July and August of 2016 off the coast of central California. Total mercury (HgT) and monomethylmercury (MMHg) were quantified in cloud water samples and relationships were observed with ancillary measurements of cloud physical and chemical parameters. Mean (± 1s) concentrations of HgT and MMHg from all samples were 9.17 ± 5.95 (N = 99) and 0.87 ± 0.66 ng L-1 (N = 19), respectively. The 19 composite samples that were analyzed for MMHg had a mean %MMHg compared to HgT of 9.5%, a value that is significantly higher than %MMHg found in California coastal rain water (~2%), but nearly equivalent to the %MMHg value found in California coastal fog water (~7%). Correlations with ancillary parameters suggest that major sources for both HgT and MMHg in cloud water were from the sea surface, especially over areas of strong wind-driven upwelling. Two flights in particular, that sampled between Monterey Bay in the south and Bodega Bay in the north, passed over mesoscale upwelling eddies as indicated by satellite altimetry, and produced samples with the highest HgT and MMHg concentrations (25.2 and 2.9 ng L-1, respectively). The samples from these flights revealed significant negative correlations of HgT and MMHg with the meridional component of wind speed (v), indicating a relationship with wind-driven upwelling. The samples from these flights also revealed significant negative correlations of HgT and MMHg with sea surface temperature and aircraft altitude, and significant positive correlations with the sodium ion (tracer of sea spray). These findings are consistent with our oceanographic measurements of dimethylmercury (DMHg) and dissolved gaseous elemental Hg (DGM) in this same region, which were found to be supersaturated in surface waters where mesoscale upwelling eddies were present, resulting in a sea-air flux of these relatively insoluble species.