2a-1: Sources and cycling of mercury in coastal ecosystems

Climate change is expected to drive increases in terrestrial organic matter (OM) export to northern aquatic systems, including Norwegian coastal waters. The coastal environment is also influenced by inputs of contaminants that can pose human and ecosystem health risks. There is an increasing need to gain an understanding of how changes in terrestrial inputs to coastal waters can drive changes in ecology and contaminant dynamics in order to predict future impacts and develop effective management approaches.

Here, we characterize physicochemical conditions, lower food web ecology and mercury (Hg) bioaccumulation along two Norwegian river-fjord gradients, one in southern Norway (Storelva-Sandnesfjord) and one in subarctic Norway (Mlselv-Mlselvfjord). Briefly, seasonal water and zooplankton samples were collected along both river-fjord gradients in 2015-2016, for characterisation of physicochemical conditions, lower food web structure (using dietary markers) and Hg trophodynamics.

Physicochemical conditions along the study transects were primarily driven by conservative mixing of the fresh and marine water masses, and the rivers were an important source of silicate, Hg, and terrestrial OM to the coastal waters. Based on dietary marker analysis, near-shore zooplankton utilized terrestrial inputs as a food source, with decreasing importance of terrestrial energy sources with increasing distance from the river outflows. Hg concentrations in zooplankton were typically highest for inner fjord sites and decreased along the study transects. Estimated Hg bioaccumulation factors for zooplankton also tended to decrease along the study transects, suggesting that the elevated Hg concentrations in zooplankton from the inner fjord stations were attributable not only to the elevated Hg concentrations in inner fjord water, but also to more efficient bioaccumulation of Hg at the sites with higher freshwater influence.

These results suggest that riverine inputs to Norwegian fjords can have strong impacts on physicochemical conditions, ecology and Hg dynamics, highlighting a need for more detailed understanding of the seasonality and spatial extent of this influence, as well as the implications of future climate change driven increases in riverine inputs to Norwegian coastal waters.

Legacy Hg burial in aquatic sediments represents one of the largest reservoirs for anthropogenic Hg, which can potentially be reemitted back to the atmosphere. However, the rates and controlling factors for emission of Hg from sediments, especially Hg-contaminated sediments, remain unclear. We determined emission rates of gaseous elemental mercury (Hg0) from contaminated fiber-rich water-sediment microcosms under a wide range of redox conditions using geochemical relevant isotopic-enriched divalent Hg (HgII) tracers: 201Hg(NO3)2, 202Hg-NOM (HgII bound to natural organic matter) and β-198HgS(s) (microcrystalline metacinnabar). Up to one order of magnitude higher Hg0 emission rates were observed in anaerobic surface sediments (0-2cm) dominated by terrestrial organic matter, as compared to organic fiber dominated deeper sediment (0-10cm). β-198HgS(s) showed 15 times lower Hg0 emission rate than the other two tracers due to its low solubility. We suggest that the reductive power of aromatic functional groups (e.g., quinone and semiquinone) of terrestrial organic matter drives rates of HgII reduction and emission. We further conducted separate incubation experiments where the chemically reduced quinone compound AQDS or terrestrial organic matter (Suwanee River NOM) were added to the water-sediment microcosms. Elevated Hg0 emission from the HgII isotope tracers and ambient Hg were observed. Our results highlight that terrestrial organic matter discharged into the estuarine sediments act as electron shuttles, enhancing legacy Hg reduction and subsequent re-emission to the atmosphere. Further results on HgII reduction and emission in the presence of marine autochthonous organic matter (i.e. algae and cyanobacteria derived organic matter) will also be presented and discussed.

Methylmercury (MeHg) levels in aquatic ecosystems are strongly driven by organic matter (OM), however, we have a poor understanding of how spatial and interannual variability in the composition and concentration of OM within ecosystems control MeHg levels. To address this we determined MeHg concentrations and methylation/demethylation rate constants in water samples during late summer/early fall (2014-2016) in a subarctic coastal sea (Northern Baltic Sea). We furthermore compiled 11 years of data (2006-2016) for dissolved organic carbon (DOC) and humic content. We find that the average MeHg concentrations in offshore waters have large interannual variability (2014: 80±25 fM; 2015: 11±11 fM; 2016: 21±9 fM). We create a statistical multiple linear regression model using MeHg concentration, DOC and humic content. We show that 60% of the spatial and interannual variability in MeHg concentration in the Northern Baltic Sea can be explained by changes in DOC and humic content. The seasonal variable DOC reservoir (mostly marine DOC in offshore waters) caused an increase in MeHg concentrations. This is consistent with the prevailing hypothesis that MeHg is produced in situ in the water column during OM remineralization. Contrary to this, an increase in humic content lowered the MeHg concentration, indicating less bioavailability of HgII in the presence of humic substances. We apply the model to examine the extent of interannual variability driven by DOC composition and concentration for summer/fall MeHg concentrations between 2006 and 2016 and find a range from below detection limit (2007-2008: 13 fM) to 150 fM (2012) in offshore waters. We will present results from the study and discuss implications for food webs. We will furthermore reflect on how anticipated changes in river exports of nutrient and OM due to climate change will influence MeHg concentrations in similar systems.

Phytoplankton are the primary route of entry for monomethylmercury (CH3Hg) into the food web of the upper ocean (<1000 m) leading to elevated concentrations in pelagic migratory fish, a significant route of human exposure. Laboratory experiments were conducted in which the accumulation rates of intracellular CH3Hg were measured in two species of temperate marine diatoms, Thalassiosira weissflogii (Tw) and Thalassiosira psuedonana (Tp), and one species of polar diatom, Chaetoceros brevis (Cb), along with initial dissolved organic carbon (DOC) in synthetic ocean water (SOW), and DOC excreted by phytoplankton cells into SOW following CH3Hg exposures. At typical initial background DOC concentrations (~2 mg/L), short-term (<1 hr) uptake rate constants of CH3Hg normalized to cell biomass and CH3Hg concentrations were higher in Tp (3.80 L g-1 h-1) than in either Tw (0.36 L g-1 h-1) or Cb (0.16 L g-1 h-1). However, higher rate constants were observed in Tw at lower (<1 mg/L) DOC concentrations (1.02, 2.96 L g-1 h-1). A nonlinear relationship between CH3Hg uptake rates and initial DOC concentration was observed across multiple experiments for Tp (r2=0.99) and Tw (r2=0.87). DOC concentrations in incubations of Tw with CH3Hg increased over time to values that were 3 to 40 times higher than those in control experiments without CH3Hg suggesting that exposure to CH3Hg stimulated DOC production by this diatom. The observed decrease in CH3Hg uptake rates from 1 to 24 h of exposure indicates that DOC excreted by diatoms in response to CH3Hg exposure may influence CH3Hg speciation and bioavailability. Small differences in seawater DOC concentrations in the oligotrophic open ocean and larger changes in DOC in productive coastal waters may influence the speciation of CH3Hg therefore affecting its enrichment at the base of the marine food web and subsequent human exposure from higher trophic level fish.

The Yellow Sea (YS) and Bohai Sea (BS), economically important regions of the western Pacific Ocean, have been facing a variety of environmental problems, including mercury (Hg) pollution. Although methylmercury (MeHg) has been recognized as the most toxic Hg species in the environment, there is still a lack of knowledge on its cycling in the BS and YS, limiting a sound understanding of Hg cycling in both regions. To address these needs, we investigated the distribution and methylation/demethylation of Hg in both regions during two marine science cruises. A decreasing trend from inshore to offshore was observed for both total Hg (THg) and MeHg, suggesting the importance of terrestrial discharge. Methylation in the sediment and photodemethylation in the water were identified as the two most important processes controlling MeHg levels, while SO42-, THg, and dissolved organic matter were found to be the most influential environmental factors. By quantifying the in situ production/degradation, along with river input and exchange with nearby seas, sediment was found to be the most important source of MeHg; meanwhile, the water serves as the largest sink in both regions. In comparison with other marine systems, a relatively low ecosystem conversion efficiency of inorganic Hg to MeHg, i.e., low MeHg/THg ratios in the water, was observed in the BS and YS. This may result from the low efficiency of transporting THg from water to the sediment, slow methylation in the sediment, and quick photodegradation in the water. The low conversion efficiency of inorganic Hg to MeHg may be one of the convincible reasons for the low Hg levels detected in the YS and BS organisms, in comparison to the high THg concentrations in the water.

Measures to lower Hg emissions in the US have been ongoing over the past 2 decades. Observing and quantifying the impact of these reductions in the environment is difficult given the changing mixture of local and global sources and the complex cycling and recycling of mercury in the ecosystem. In this presentation we summarize Hg monitoring and research in the Chesapeake Bay watershed, with the aim of detecting changes in fluxes or risk. Studies include long-term wet-deposition measurements, including MDN network measurements from the 3 Maryland stations (since 2005); small-watersheds fluxes (since 2008), and entry into biota (Young of the Year fish (YOY), since 2008). These studies have been specifically designed to identify changes to Hg cycling in the Chesapeake Bay watershed.

Year to year variability in wet deposition is substantial. Importantly however, Hg wet deposition loadings to the Mid-Atlantic appear not to have declined over the last decade or so in concert with regional estimated emission reductions. Although there have been some decreases in the average Hg concentration in rain and loadings in urban areas.

Year to year variability in Hg export from small streams is substantial (varying on the order of 5x for both T-Hg and MeHg) and largely dependent on hydrologic variables. Needless to say and we cannot detect a temporal pattern to date.

We have almost a decade of young of the year fish monitoring in the Chesapeake Bay and in watershed reservoirs. Differences in concentrations of mercury in the YOY fish populations among the 13 sites cover an order of magnitude. Year to year variability among the sites also varies, suggesting watershed characteristics and/or reservoir management impact the cycling of mercury to differing degrees among the sites, but no temporal trend in the data has been observed to date.

In this study, we have observed that changes in land-use, population and climate, even over the short duration studied, adds complexity to the watershed flux and biota metrics. Our understanding of the Hg and MeHg budget to the Chesapeake Bay has also evolved. High concentrations of Hg and MeHg in export water of small streams of the coastal plain, combined with their large number, make them a source that rivals the large rivers; which have high flow but low concentrations of Hg and MeHg.

The potential effects of climate change in coastal ecosystems include increased rainfall with subsequent runoff from watersheds and increased carbon flux to sediments. These factors should increase the watershed delivery of mercury (Hg) to the estuarine water column and increase sediment anoxia, Hg content and Hg methylation. To examine these potential outcomes, we evaluated methylmercury (MeHg) sources at sites with high and low organic matter sediment in 6 small salt water creeks and wetland areas in Maryland (MD), Connecticut (CT) and Maine (ME; temperature range of 20 to 29 degC; sampled in 2013). Average dissolved MeHg (0.05 to 6.3 pM; 3 to 35% MeHg), suspended particle MeHg (1.1 to 84 pmol/g; 1 to 17% MeHg) and dissolved organic carbon (DOC; 0.1 to 3.3 mM) increased with latitude, and were highest at CT salt marsh and ME sites. Average sediment MeHg (0.04 to 3.5 pmol/g) and organic content (loss-on-ignition; 0.7 to 25%) also increased with latitude. Dissolved MeHg and total mercury (HgT) were correlated among sites along the latitudinal gradient, and this was also the case for suspended particle concentrations. However, average suspended particle MeHg concentrations were higher (factor of 3 to 550) than sediment MeHg levels at each site, particularly at sites in ME and CT, suggesting watershed sources of suspended particle MeHg to the creeks sampled. Similarly, dissolved MeHg concentrations at sites in ME were a factor of 1.5 to 30 higher than pore water MeHg levels. Sediment flux cores were used to measure sediment-water MeHg flux, and site average fluxes ranged from 10 to 360 pmol/sq m/d in MD and CT, which is similar to the range found for other US estuaries. However, large negative fluxes of -280 to -500 at the ME sites indicate that water column MeHg was partitioning into the sediment, and that the creek watersheds were likely supplying the elevated dissolved MeHg. Dissolved MeHg was strongly correlated with DOC across all sites, and both dissolved MeHg and HgT were highest at sites in ME where DOC was highest. Our research indicates that watershed sources of MeHg are important to Hg loading in shallow estuaries and that delivery and concentrations of MeHg in the water column appear to be regulated by organic matter.

Monomethylmercury (MMHg) production by anaerobic bacteria in sediments is considered to be a dominate source of MMHg to sediments and overlying surface water in the coastal environment. In this study, we measured total mercury (Hgt) and MMHg sediment and pore water concentrations and calculated sediment water exchange fluxes in samples collected on the coastal shelf in the California Current System. Sediment cores and overlying water were collected from 20 stations using a slow-entry multi-corer deployed during 4 oceanographic cruises over two years. The upper few centimeters of undisturbed cores were sectioned at the following depth increments: 0.5, 1, 1.5, 2, 3, 4, 5 cm. Pore waters were extracted via centrifugation and the MMHg gradients were used to calculate fluxes into the overlying water column based upon molecular diffusion alone. Sediment concentrations for Hgt and MMHg ranged from 50 to 2338 pmoles g-1 and 0.1 to 9 pmoles g-1 respectively. Pore water and overlying water MMHg concentrations ranged from 0.1 to 2.2 pM and 0.03 to 0.3 pM respectively. Diffusional MMHg sediment water exchange flux ranged from 0.1 to 1.7 pmoles m-2 d-1. While the gradients in MMHg showed significant and widespread flux that would indicate an input into the waters of the shelf these fluxes were very small compared to those calculated elsewhere and insufficient to sustain elevated concentrations at the sediment boundary layer, or at the depth of the shelf in general. Measurements made on the northwestern Atlantic shelf are in general an order of magnitude greater than those observed here. We suggest that the narrow eastern shelf of the California Current with little allochthonous inputs contrasts sharply with the broad shelf of the Eastern Seaboard with significant organic carbon, riverine and anthropogenic inputs. In general, the narrow shelf of the California Current seems to reflect the pelagic processes of the off shore regions for this element where water column production predominates the formation of the methylated forms.