Environ. Sci. Technol. 2009, 43, 5605–5611
Beaver Ponds Increase Methylmercury Concentrations in Canadian Shield Streams along Vegetation and Pond-Age Gradients V I R G I N I E R O Y , † M A R C A M Y O T , * ,† A N D RICHARD CARIGNAN† Universite´ de Montre´al, Groupe de Recherche Interuniversitaire en Limnologie (GRIL), De´partement des sciences biologiques, C.P. 6128, Succursale Centre-Ville, Pavillon Marie-Victorin, Montre´al, Que´bec, Canada H3C 3J7
Received July 29, 2008. Revised manuscript received June 16, 2009. Accepted June 17, 2009.
Beaver impoundments flood forested areas and may be important production sites for methylmercury (MeHg) because of the resulting enhanced microbial activity and oxygen depletion. The influence of 17 beaver impoundments on streamwater chemistry [total mercury (THg), MeHg, nutrients, cations, and anions)] was investigated by sampling sites located along vegetation and pond-age gradients in southwestern Quebec (Canada). Recently inundated beaver ponds (20 years old) may be due to the degradation of less labile organic matter as ponds age. Beavers actively alter watersheds by building impoundments, and our findings indicate that this landscape disturbance may be a significant source of MeHg to downstream water bodies.
Experimental Section
Introduction Beavers (Castor canadensis) create ponds on streams to ensure their survival during the winter (1). Beaver impoundments are habitats for some fish species and for wildlife favoring ponds and marshes, but they have also negative effects related to human use such as flooding of cottages on small lakes and blocking of culverts (1). Flooding kills most woody species and creates wetlands, sites with environmental conditions that may be favorable for high methylmercury (MeHg) production (2). MeHg, a potent neurotoxin, is subject to preferential biomagnification along aquatic food webs, and consumption of contaminated fish puts humans at risk (3). Most governmental regulations focus on industrial emissions because atmospheric mercury depositions have been linked to fish * Corresponding author phone: 514-343-7496; e-mail: m.amyot@ umontreal.ca. † Universite´ de Montre´al. 10.1021/es901193x CCC: $40.75
Published on Web 07/01/2009
contamination (4, 5). However, local perturbations such as flooding of forested areas are also critical because they promote in situ formation of MeHg. Studies in boreal regions following the construction of hydroelectric dams (6-8) and experimental reservoirs (9, 10) and in natural wetlands (2, 11, 12) have all shown high MeHg burdens in the underlying sediments, pore water, water column, inundated vegetation, and aquatic organisms. Higher methylation rates are believed to result from increased bacterial activity promoted by the submersion of terrestrial organic matter (9, 13). Upon flooding, soils become anoxic as oxygen diffusion becomes too slow to compensate for microbial respiration, leading to the creation of a reducing environment (14). Beavers actively alter the landscape by creating impoundments that have conditions similar to man-made reservoirs, though on a much smaller scale. Forested areas flooded by beavers are currently increasing in northeastern North America in general (15, 16) and in the Laurentian region of Quebec (Canada) in particular (17). Beaver resurgence is mainly tied to limited population control by humans and wolves and to an abundance of forage and habitat (15, 17). Few studies exist on the role of beaver ponds in the movement of mercury, although some streams and lakes affected by beaver activity have been shown to receive higher concentrations of MeHg and nutrients (16, 18, 19). In particular, Driscoll et al. (16, 18) underscored important influences of a beaver pond on mercury chemistry, but they mainly focused on the annual variability of a single, longestablished beaver impoundment. It is thus difficult to extrapolate conclusions from these studies on the overall impact of beaver dams without an assessment of environmental factors controlling MeHg production in a variety of impoundments that differ in age and forest type. This study presents the first field survey of numerous beaver impoundments to determine their potential as a source of MeHg to downstream water bodies. The main objectives of this paper are to (1) determine if a large set of beaver impoundments causes an increase of mercury and nutrient concentrations in downstream waters, (2) evaluate water chemistry variability among beaver pond outlets along a vegetation gradient from mixed to coniferous forests, and (3) determine whether methylation efficiency (% of THg as MeHg) varies with the age of the impoundment, similar to what occurs in man-made reservoirs.
2009 American Chemical Society
Study Area. The study was conducted in the Laurentian region of the Canadian Shield located about 75 km north of Montreal (QC, Canada) (Figure S1 of the Supporting Information). The area is underlain by granitic or anorthositic bedrock covered by 1-5 m of glacial tills (Grenville geological province of Canada). Soils are mostly humic cryorthods (U.S. classification) or orthic ferro-humic podzols (Canadian classification) (20). Climate is humid continental with cold winters and warm, moist summers. Annual precipitation averages 1100 mm, with 30% falling as snow (20). The Laurentian region is in the Nordic temperate vegetation zone. In the text, the term “coniferous site” refers to the sites with predominantly coniferous trees and “mixed sites” to sites with coniferous and deciduous vegetation. Main deciduous and coniferous tree species are presented in the Supporting Information. Seventeen beaver impoundments of various ages (recent, intermediate, and old; see definition below) were sampled from 17 August to 1 September, 2006 in a ∼4000 km2 area VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(45°50′43.5′′-46°28′31′′ N, 74°09′39′′-74°20′08′′ W) (Figure S1 of the Supporting Information). Beaver ponds were located along an altitude gradient from 245 to 592 m. Forest composition is governed locally by altitude, with coniferous vegetation typifying the higher elevation sites (above 450 m, n ) 5) and mixed forest more prevalent in the lower elevation sites (below 450 m, n ) 12). Beaver pond inlets were firstor second-order streams; note that inlets of sites TRE2 and TRE4 (Figure S1 of the Supporting Information) were draining a riparian coniferous wetland. The average depth of the beaver ponds was 0.94 m (range, 0.30-1.60 m; n ) 13). Age of beaver impoundments was assessed using a qualitative classification based on visual clues (density of standing dead trees, surface of open water zones, dam condition, and type of macrophyte assemblages) derived from the literature (21-23) and knowledge of trappers. A recent impoundment (20 years old) is characterized by few remaining standing trees, large open water areas, and high macrophyte density. An intermediate impoundment is between 10 and 20 years old and has intermediate characteristics. Sampling was conducted during low flow conditions in late summer. High discharge events were avoided by sampling at least 48 h after rain events to avoid resuspension of particulate materials in streams. Only two rain events occurred during the sampling period on August 20 and 27 with respective total amounts of 52.8 mm and 24.2 mm. Field Sampling. Water samples were collected from running water of all inlets and outlets (both n ) 17). Samples in outlets were taken 1 m below the dam (n ) 17) and 150 m downstream (n ) 16). Field constraints limited the collection of water in one 150 m downstream station. Total mercury (THg) and methylmercury (MeHg) were measured in all stations. Water samples for THg and MeHg were collected in triplicate and stored in 250 mL amber glass bottles that had previously been acid-washed and thoroughly rinsed with ultrapure water. All aqueous mercury samples were preserved with 0.4% ultraclean hydrochloric acid (24) and kept in the dark and refrigerated (
