MERCURY AND OTHER TREND MONITORING OF GREAT SLAVE LAKE UNDER CHANGING CLIMATE AND GLOBAL EMISSIONS
Under the Northern Contaminants Program, we are investigating mercury (Hg) in Great Slave Lake (GSL) which is located in Canada’s Northwest Territories. While North American Hg emissions have decreased, Asian emissions are increasing with their expanding economies. Climate change, including warming trends, may enhance productivity resulting in more fish prey and increased Hg methylation rates. Hg concentrations in food webs may increase in top piscivores although this may be counteracted by growth dilution.
Our study focuses on Lake Trout (Salvelinus namaycush), a cold-water pelagic species and Burbot (Lota lota), a cool-water benthic species. Both are omnivores, consuming invertebrate prey in addition to fish. Since 1998, Burbot and Lake Trout have been provided annually from East Arm (EA) by Lutsel K'e and the West Basin (WB) by Fort Resolution and Hay River with earlier mercury records available. The EA is cold, deep and oligotrophic; the WB is shallower, warmer, and more productive, largely due to the Slave River, a substantial source of water, suspended sediments, organic matter and plant nutrients.
Over 1993-2016, EA Lake Trout were slightly smaller (600±69 vs. 634±76 mm) than WB fish but older (15.5±4.8 vs. 9.7±4.0 yr) indicating lower growth rates; Hg concentrations were lower (0.17±0.08 vs. 0.20±0.08 µg/g) possibly because of a less fish-rich diet and/or lower Hg inputs and methylation rates in the EA than WB. WB Lake Trout Hg concentrations increased from the late 1970s to 2016. In the EA, Hg concentrations decreased in small (495-600 mm) fish but increased in large (600-800 mm) fish. The presence of many old fish (>20 yr), with different growth rates, presents challenges in trend analyses, particularly in the EA.
EA Burbot were smaller (539±62 vs. 663±74 mm), younger (10.4±2.4 vs. 13.0±3.2 yr) and with lower Hg concentrations (0.14±0.06 vs 0.16±0.07 µg/g) than WB fish. Hg concentrations increased over 1995-2016 in both regions.
Distinct warming trends in air temperatures have occurred over the past century; sedimentation rates and algal carbon concentrations have increased and diatom assemblages have changed; there is less evidence of recent increases in Hg concentrations and sediment flux. Trends of Hg increase in Lake Trout and Burbot were more strongly related to cooler years than warming trends. There was little evidence of changes in trophic feeding over the period of record. With low Hg concentrations, GSL continues to be an important source of fish to northerners.
GASEOUS ELEMENTAL MERCURY OBSERVATIONS OVER THE SOUTHERN OCEAN FROM THE CAPE GRIM BASELINE AIR POLLUTION STATION
The southern hemisphere (SH) and in particular Australia are not well represented, in contrast to the northern hemisphere, in terms of long time series of atmospheric mercury measurements. Recent studies have highlighted that the mean background of gaseous elemental mercury (GEM) for the SH is substantially lower than previously thought (i.e., based on spare data). The Cape Grim Baseline Air Pollution Station (CGBAPS), a Global Atmospheric Watch (GAW) station, is situated on the north-west peninsula of Tasmania, Australia (40.68° S, 144.68° E). GEM measurements have been undertaken at CGBAPS since September 2011. This location and the predominantly westerly winds in the region give CGBAPS access to air masses that have equilibrated with the Southern Ocean. Radon measurements at the site were used to determine air masses of oceanic fetch. For other wind sectors air masses are influenced by terrestrial surfaces. The six year time series from the Cape Grim station has been evaluated to determine mean background as a function of wind sector, as well as seasonal and inter-annual trends. An overall background GEM concentration at Cape Grim of 0.85 ng m-3 was determined with very little inter-annual variability with low seasonality.
FOUR YEARS (2011-2015) OF TOTAL GASEOUS MERCURY AND OTHER KEY MEASUREMENTS FROM THE CAPE VERDE ATMOSPHERIC OBSERVATORY (CVAO)
Here we will report four years (Dec 2011 – Dec 2015) of Total Gaseous Mercury (TGM) measurements at the Cape Verde Observatory (CVO), a global WMO-GAW station located in the sub-tropical remote marine boundary layer (16.85°N, 24.87°W). The measurements presented here are part of the EU Global Mercury Observation System (GMOS) network. The GMOS network of sites was established in 2011 with the aim of addressing known gaps in the spatial and temporal measurement of mercury, as well as improving knowledge of Hg speciation.
Observed TGM concentrations were between 1.03 and 1.33 ng m-3 (10th, 90th percentiles), close to the expectations based on previous interhemispheric gradient measurements. We observe a decreasing trend in TGM (0.04 ± 0.03 ng m-3 yr-1, -3.4% ± 2.4% yr-1) over the four years consistent with the reported decrease of mercury concentrations in North Atlantic surface waters and reductions in anthropogenic emissions. The trend was more visible in the summer (Jul-Sep) than in the winter (Dec-Feb), when measurements were impacted by air from the African continent and Sahara/Sahel regions. These African air masses were also associated with the highest and most variable TGM concentrations. We attribute the smaller decreasing trend to the influence of large anthropogenic sources in West Africa such as from artisanal and small-scale gold mining (ASGM).
In addition to the TGM measurements, we will show time series and interpretation of other climatically significant trace gases and greenhouse gases, for which we have up to ten year records from the Observatory. We will discuss the importance of these measurements and outline some of the plans for the future of the CVAO.
ONGOING MONITORING OF FOREST SOIL MERCURY IN VERMONT, USA
Continued monitoring of total mercury (THg) concentration in soils is essential for detecting, predicting and addressing environmental change. In cooperation with the Vermont Monitoring Cooperative (now the Forest Ecosystem Monitoring Cooperative), we have established a long-term soil monitoring study in forested sites in the northeastern USA where annual wet deposition of Hg has been in the range of 6-10 µg/m2. Five 50 x 50 m plots were located in protected areas, three on Mt. Mansfield in north-central Vermont and two in the Lye Brook Wilderness Area in southwestern Vermont. Elevation ranges from 590 to 1140 m with forest type changing from typical northern hardwood (Acer saccharum, Betula alleghaniensis and Fagus grandifolia) to high-elevation spruce-fir (Picea rubens and Abies balsamea). Each plot contains 100 5 x 5 m subplots with sampling date assigned randomly (10 per date). The initial sampling of these plots took place in the summer of 2002 and resampling occurred in 2007 and 2012. Small pits were dug in the center of each plot and the soils were sampled both by genetic horizon and depth increments. These samples have been analyzed for a suite of chemical parameters, including exchangeable cations, carbon and nitrogen. Separate samples of the uppermost humified horizon (Oa or A) were taken for total Hg (THg) analysis using appropriate protocols. Average THg concentration at each site ranged from ~200-500 ng/g. The average carbon concentration in these horizons varied between ~100-420 g/kg. There was a positive elevational gradient of THg concentration, consistent with greater deposition and with other studies. The THg:C ratio (ng/g) ranged from 0.6-1.8, with the highest ratio at the highest elevation. Continued sampling at 5-year intervals will allow detection of environmental change in response to both a changing climate and changing Hg deposition.