Andrew Heyes

Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

The hydrology and methylmercury dynamics of a Precambrian shield headwater peatland

A hydrobiogeochemical investigation of a small headwater peatland located in the Experimental Lakes Area, northwestern Ontario, Canada, examined the surface and subsurface hydrologic pathways and their relation to the movement and spatial variability of methylmercury (CH3Hg+). The hydrology of the peatland controls the mass flux of CH3Hg+ to a downstream pond from the terrestrial ecosystems, and influences the production and/or accumulation of CH3Hg+ in the peatland. Distinct zones of groundwater recharge and discharge were observed within the peatland, and these corresponded, to low and high CH3Hg+ concentrations, respectively, in pore water. The magnitude and flux of CH3Hg+ from the peatland was governed by the area of the peatland and surrounding uplands contributing runoff. There was a threefold increase in CH3Hg+ concentration as stream base flow passed from the origin of the stream at the hillslope-peatland margin to the stream outlet at the peatland-lake interface. This relative increase in concentration was maintained during storm flow conditions even though the discharge was up to 10 times greater, indicating that the peatland is a large source of CH3Hg+. Methylmercury-laden peat pore water found near the surface of the peatland in areas of groundwater discharge moves to the stream as the water table rises to the peat surface. Of a total of 12.6 mg CH3Hg+ leaving the peatland during the 112-day study period, less than 1% was transported directly by groundwater, 41% was transported in stream base flow, and the remaining 58% was transported by storm flow. However, storm flow conditions occurred only 16% of the time, indicating the significance of episodic CH3Hg+ flux from headwater catchments on the Precambrian Shield.

Methane emissions from wetlands, southern Hudson Bay lowland

Methane emissions were measured by a static chamber technique at 39 sites along a transect from the James Bay coast at the southeastern tip of Hudson Bay to Kinosheo Lake, northwest of Moosonee, Ontario, Canada. These sites represented five major wetland ecosystems along a successional gradient from the coast inland. Measurements were made at ≈ 10-day intervals from early June to mid-August, and once in mid-September and mid-October 1990. Seasonal CH4 fluxes were small (<2 g m−2) at the recently emerged coastal marsh, coastal fen, tamarack fen, and interior fen ecosystems, except where there were shallow ponds and pools, which emitted 2–5 g CH4 m−2. At the more complex bog ecosystem locations, CH4 fluxes were small (0.3–2.0 g m−2) from hummock/hollow microtopography in the raised bogs and from the forested margin. The largest CH4 fluxes were recorded from the degrading peat sections forming shallow pools and the moss/sedge mats which were always close to saturation (1.8–16.6 g m−2). A deeper (1-m water depth) pool emitted less CH4 (1.4 g m−2). In terms of ecological succession along the transect, covering emergence over ≈ 4000 yr, CH4 emission rates increase from marsh to fen and bog, primarily through the development of peat degradation and the formation of moss/sedge lawns and pools. There were very weak statistical relationships at each site between the daily CH4 flux and peat temperature and water table. However, there was a significant (r2 = 0.44, p < 0.001) correlation between the seasonal CH4 flux and the mean position of the water table over the complete range of sites, emphasizing the overall importance of hydrology in determining CH4 flux. Laboratory incubation experiments were conducted to determine the capacity of the surface (0–20 cm depth) peat samples to produce CH4 anaerobically and consume CH4 aerobically. They revealed that many samples exhibited high CH4 consumption rates, suggesting that although CH4 production in the subsurface peat is high, CH4 emissions from these wetlands to the atmosphere are limited to a large extent by CH4 oxidation in the surface layers of the peat. Trophic status of the peat appeared to have little influence on emission rates, with the highest fluxes in the most acid (pH < 3.5) samples.

Carbon dioxide and methane fluxes from drained peat soils, southern Quebec

Fluxes of CO2 and CH4 were determined by a static chamber technique at eight drained swamp peatland sites, with crop and forest covers. Over a 6- month period (May - October, 1991), CH4fluxes ranged from −5 to 7 mg CH4 m−2 d−1 and were not correlated with either soil temperature or water table position. Integrated seasonal emissions were −0.40 to 0.04 g CH4 m−2 over 147 days; the sites with a forest or grass cover were a small sink of CH4 whereas the sites with horticultural crops showed no significant flux. Laboratory incubations showed that the highest CH4 consumption rates (3 to 9 μg CH4 g−1 d−1) occurred in the least disturbed soils. The results, when compared with CH4 fluxes from nearby swamps which have been unaffected by drainage, suggest that drainage of temperate peatlands has reduced emissions of CH4 to the atmosphere by 0.6 - 1 × 1012g CH4 yr−1. CO2 fluxes ranged from 0 to 16 g CO2 m−2 d−1 and were correlated with the seasonal pattern of temperature in the upper part of the soil profile. Integrated seasonal fluxes for the sites in which root respiration was an unimportant contribution were 0.6 - 0.8 kg CO2 m−2 over 181 days. Aerobic laboratory incubations revealed CO2 production rates of 0.2 - 1.4 mg CO2 g−1 d−1, an average of 5 times the rate under anaerobic conditions. Using bulk density and loss-on-ignition data, we found that the seasonal CO2 fluxes translate into surface lowering of the peat of about 2 mm yr−1, whereas the commonly observed lowering in these cultivated peatlands is 20 mm yr−1. These data suggest that processes other than direct oxidation, such as shrinkage and aeolian erosion, are the major contributor to the surface lowering of the peat.

Influence of dissolved organic matter on the complexation of mercury under sulfidic conditions

The complexation of Hg under sulfidic conditions influences its bioavailability for microbial methylation. Neutral dissolved Hg-sulfide complexes are readily available to Hg-methylating bacteria in culture, and thermodynamic models predict that inorganic Hg-sulfide complexes dominate dissolved Hg speciation under natural sulfidic conditions. However, these models have not been validated in the field. To examine the complexation of Hg in natural sulfidic waters, octanol/water partitioning methods were modified for use under environmentally relevant conditions, and a centrifuge ultrafiltration technique was developed. These techniques demonstrated much lower concentrations of dissolved Hg-sulfide complexes than predicted. Furthermore, the study revealed an interaction between Hg, dissolved organic matter (DOM), and sulfide that is not captured by current thermodynamic models. Whereas Hg forms strong complexes with DOM under oxic conditions, these complexes had not been expected to form in the presence of sulfide because of the stronger affinity of Hg for sulfide relative to its affinity for DOM. The observed interaction between Hg and DOM in the presence of sulfide likely involves the formation of a DOM-Hg-sulfide complex or results from the hydrophobic partitioning of neutral Hg-sulfide complexes into the higher-molecular-weight DOM. An understanding of the mechanism of this interaction and determination of complexation coefficients for the Hg-sulfide-DOM complex are needed to adequately assess how our new finding affects Hg bioavailability, sorption, and flux.

Response of a macrotidal estuary to changes in anthropogenic mercury loading between 1850 and 2000.

Methylmercury (MeHg) bioaccumulation in marine food webs poses risks to fish-consuming populations and wildlife. Here we develop and test an estuarine mercury cycling model for a coastal embayment of the Bay of Fundy, Canada. Mass budget calculations reveal that MeHg fluxes into sediments from settling solids exceed losses from sediment-to-water diffusion and resuspension. Although measured methylation rates in benthic sediments are high, rapid demethylation results in negligible net in situ production of MeHg. These results suggest that inflowing fluvial and tidal waters, rather than coastal sediments, are the dominant MeHg sources for pelagic marine food webs in this region. Model simulations show water column MeHg concentrations peaked in the 1960s and declined by almost 40% by the year 2000. Water column MeHg concentrations respond rapidly to changes in mercury inputs, reaching 95% of steady state in approximately 2 months. Thus, MeHg concentrations in pelagic organisms can be expected to respond rapidly to mercury loading reductions achieved through regulatory controls. In contrast, MeHg concentrations in sediments have steadily increased since the onset of industrialization despite recent decreases in total mercury loading. Benthic food web MeHg concentrations are likely to continue to increase over the next several decades at present-day mercury emissions levels because the deep active sediment layer in this system contains a large amount of legacy mercury and requires hundreds of years to reach steady state with inputs.

A PRELIMINARY ASSESSMENT OF WET DEPOSITION AND EPISODIC TRANSPORT OF TOTAL AND METHYL METCURY FROM LOW ORDER BLUE RIDGE WATERSHEDS, S.E. U.S.A.

Results from a preliminary sampling program designed to investigate total (THg) and methyl Hg (MeHg) deposition, cycling and transport at the Coweeta Hydrologic Laboratory western North Carolina are presented. Wet deposition samples were collected in June and July 1994 and throughfall, seep and streamwaters were intensively collected during and after a rainfall event in June 1994. All water samples were collected using ultra clean trace sampling protocol. Low elevation Watershed 18 streamwater THg concentrations peaked with discharge, increasing 6 fold to 9 ng L-1. High elevation Watershed 27 which received less than one half the precipitation Watershed 18 received during the event, exhibited THg concentrations only 1.3 times over base flow conditions. Methyl Hg concentrations remained near detection limits (≤ 0.025 ng L-1) in both streams. Dissolved MeHg concentrations were higher in shallow seep (0.097 ng L-1), throughfall (0.135 ng L-1) and precipitation (0.16 – 0.035 ng L-1) than streamwaters. Initial estimates of annual THg and MeHg deposition and transport indicate >90% retention of Thg and a >80% retention or demethylation of wet deposition MeHg is occurring in these low order watersheds.

Spatial and temporal dynamics of mercury in Precambrian Shield upland runoff

Methylated and total Hg, and TOC concentrations were measured in precipitation and runoff in a first order Precambrian Shield watershed, and in precipitation, throughfall, shallow groundwater and runoff in a zero Precambrian Shield watershed. Plots dominated by open lichen-covered bedrock and another containing small patches of conifer forest and thin discontinuous surficial deposits were monitored within the zero order catchment. Methyl (3–10 fold) and non-methyl (1.4–2.8 fold) Hg concentrations changed irregularly during rainfall and snowmelt runoff events in all catchments. Temporal patterns of Hg concentration in runoff included flushing and subsequent dilution as well as peak concentrations coinciding with peak or recession flow. Mercury export was highest from lichen-covered bedrock surfaces as a result of high runoff yields and minimal opportunity for physical retention and in the case of MeHg demethylation. Forest canopy and lichen/bedrock surfaces were often net sources for Hg while forest soils were mostly sinks. However, upland soils undergoing periodic reducing conditions appear to be sites for the in situ production of MeHg.

Bromination of Marine Dissolved Organic Matter following Full Scale Electrochemical Ballast Water Disinfection

An extensively diverse array of brominated disinfection byproducts (DBPs) were generated following electrochemical disinfection of natural coastal/estuarine water, which is one of the main treatment methods currently under consideration for ballast water treatment. Ultra-high-resolution mass spectrometry revealed 462 distinct brominated DBPs at a relative abundance in the mass spectra of more than 1%. A brominated DBP with a relative abundance of almost 22% was identified as 2,2,4-tribromo-5-hydroxy-4-cyclopentene-1,3-dione, which is an analogue to several previously described 2,2,4-trihalo-5-hydroxy-4-cyclopentene-1,3-diones in drinking water. Several other brominated molecular formulas matched those of other known brominated DBPs, such as dibromomethane, which could be generated by decarboxylation of dibromoacetic acid during ionization, dibromophenol, dibromopropanoic acid, dibromobutanoic acid, bromohydroxybenzoic acid, bromophenylacetic acid, bromooxopentenoic acid, and dibromopentenedioic acid. Via comparison to previously described chlorine-containing analogues, bromophenylacetic acid, dibromooxopentenoic acid, and dibromopentenedioic acid were also identified. A novel compound at a 4% relative abundance was identified as tribromoethenesulfonate. This compound has not been previously described as a DBP, and its core structure of tribromoethene has been demonstrated to show toxicological implications. Here we show that electrochemical disinfection, suggested as a candidate for successful ballast water treatment, caused considerable production of some previously characterized DBPs in addition to novel brominated DBPs, although several hundred compounds remain structurally uncharacterized. Our results clearly demonstrate that electrochemical and potentially direct chlorination of ballast water in estuarine and marine systems should be approached with caution and the concentrations, fate, and toxicity of DBP need to be further characterized.

Fate and transport of ambient mercury and applied mercury isotope in terrestrial upland soils: insights from the METAALICUS watershed.

The fate of mercury (Hg) deposited on forested upland soils depends on a wide array of biogeochemical and hydrological processes occurring in the soil landscape. In this study, Hg in soil, soilwater, and streamwater were measured across a forested upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada, where a stable Hg isotope (spike Hg) was applied to distinguish newly deposited Hg from Hg already resident in the watershed (ambient Hg). In total, we were able to account for 45% of the total mass of spike Hg applied to the subcatchment during the entire loading phase of the experiment, with approximately 22% of the total mass applied now residing in the top 15 cm of the mineral soil layer. Decreasing spike Hg/ambient Hg ratios with depth in the soil and soilwater suggest that spike Hg is less mobile than ambient Hg over shorter time scales. However, the transport of spike Hg into the mineral soil layer is enhanced in depressional areas where water table fluctuation is more extreme. While we expect that this pool of Hg is now effectively sequestered in the mineral horizon, future disturbance of the soil profile could remobilize this stored Hg in runoff.