MERCURY CYCLING IN LAKE GORDON AND LAKE PEDDER, TASMANIA (AUSTRALIA). I: IN-LAKE PROCESSES KARL C. BOWLES1,2, SIMON C. APTE1∗, WILLIAM A. MAHER2 and DAVID R. BLÜHDORN3 1 Centre for Advanced Analytical Chemistry, CSIRO Energy Technology, Bangor, Australia; 2 Ecochemistry Laboratory, Applied Ecology Research Group, CRC for Freshwater Ecology, University of Canberra, University Drive, Bruce, Australia; 3 Inland Fisheries Commission, Hobart, Tasmania (currently at Hydro Tasmania, Elizabeth St., Hobart, Tasmania) (∗ author for correspondence, e-mail: [email protected], Fax: +61 2 9710 6837) (Received 26 September 2002; accepted 2 December 2002) Abstract. The processes affecting the concentrations of total mercury (total Hg) and methylmercury (MeHg) in a freshwater system comprising two connected reservoirs in southwest Tasmania were in- vestigated. Surface concentrations of total mercury (total Hg) were temporally and spatially uniform − − in both Lake Gordon (2.3±0.4 ng L 1, n = 27) and Lake Pedder (2.3±0.3 ng L 1, n = 11). The − surface concentrations of MeHg in Lake Gordon (0.35±0.39 ng L 1, n = 25) were more variable than total Hg and MeHg typically comprised 10–20% of total Hg. The relatively high amount of total mercury present as MeHg in Lake Gordon was attributed to the high proportion of wetlands in the upper catchment (50% of total area) and in-lake contributions (ca. 40% of total MeHg). Despite the close proximity of the two lakes, MeHg concentrations in Lake Pedder were consistently lower than in Lake Gordon. This phenomenon can be explained in part by the greater contribution of direct rainfall to Lake Pedder leading to the dilution of MeHg. Water column MeHg concentrations were higher in warmer months in both lakes, reflecting increased net methylation of inorganic mercury. Unlike previous studies of seasonally anoxic lakes, depth profiles of total mercury and MeHg in Lake Gordon were uniform and were not affected by water column stratification occurring in the summer months, and oxygen depletion with depth. This suggests that redox cycling and accumulation of MeHg in the hypolimnion following seasonally-induced anoxia is not a significant part of the mer- cury cycle in Lake Gordon. The primary location of MeHg production within the lake’s water column is not conspicuous. Mercury speciation measurements made above and below the lake system over a period of 19 months indicates that after 20 yr of impoundment, the reservoirs are not significantly affecting MeHg concentrations in the downstream riverine environment. Keywords: Lake Gordon, Lake Pedder, mercury, methylation, methylmercury, natural water, reser- voir, sediment 1. Introduction Understanding mercury cycling and methylmercury (MeHg) production in aquatic environments is important as MeHg can biomagnify to levels of concern in aquatic food chains. Most lake studies have been conducted on relatively small lakes in regions where mercury bioaccumulation by fish is an issue of local importance. These studies suggest that inputs from catchment areas, as well as in-lake methyl- Water, Air, and Soil Pollution 147: 3–23, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 4 K. C. BOWLES ET AL. ation are both important sources of MeHg (Rudd, 1995). The relative importance of these two sources varies between lake type; catchment supply is important for drainage lakes, whereas most of the MeHg in seepage lakes may be the result of in situ production (Watras and Bloom, 1994; Watras et al., 1995). Methylation of inorganic mercury can occur in both sediments and waters (Rudd, 1995). Generally the boundary between oxic and anoxic zones is considered to be the region of most active methylation, whether in the water column or in the sediment (Watras et al., 1995). This is thought to reflect the occurrence of sulfate-reducing bacteria and the speciation of inorganic mercury in sub-oxic environments which both favour methylation (Benoit et al., 1999). Within lake catchments, it is acknowledged that wetlands, particularly areas of peat accumulation, are important zones of MeHg production (Branfireun et al., 1996; Hurley et al., 1995). There have been relatively few studies of mercury cycling in large lake systems (Bloom and Effler, 1990; Mason and Sullivan, 1997; Vandal et al., 1998; Kotnik et al., 2000; Bowles et al., 2001, 2002). Some large reservoirs have been studied, for example in northern Quebec, Canada (Louchouarn et al., 1993), mainly to elucidate the processes involved in the ‘reservoir effect’. Impoundment of dry land to form reservoirs has been shown to cause an increase in MeHg production, which lasts typically about 10–15 yr after impoundment (Verta et al., 1986; Scruton et al., 1994). During this time the concentrations of mercury in aquatic biota have been shown to be substantially elevated above pre-impoundment levels. There is a paucity of studies concerning the speciation and distribution of mer- cury in southern hemispheric freshwater systems (Kim and Burggraf, 1999; Bowles et al., 2001, 2002). Understanding mercury cycling in the southern hemisphere is important since the region is less perturbed by anthropogenic mercury emissions than the more industrially developed northern hemisphere (Pirrone et al., 1996). Nevertheless, emissions of mercury in Asia are increasing (Pirrone et al., 1996) and this may have a significant effect on mercury cycling in the southern hemi- sphere in the coming decades. This paper describes the in-lake cycling of mercury in a temperate Tasmanian (Australia) lake system comprising two large reservoirs constructed for hydroelectric power generation. The study was initiated following the occurrence of high mercury concentrations (>0.5 µgg−1) in trout and eels in the reservoirs and downstream river system (Sanger et al., 1995). Mercury cyc- ling in the catchment areas of these lakes is described in a companion publication (Bowles et al., 2003). The specific objectives of this study were to (i) characterise the speciation of mercury in the lake system (ii) identify the key processes affecting the concentration of MeHg in the lake system and (iii) determine the long-term implications of reservoir impoundment on total Hg/MeHg concentrations in the downstream river environment. MERCURY IN LAKE GORDON: IN-LAKE PROCESSES 5 2. Experimental 2.1. SITE DESCRIPTION The Gordon/Pedder catchment is located within the Tasmanian Wilderness World Heritage Area. This region is sparsely populated and is characterised by rugged terrain and extensive areas of pristine, peaty wetlands and rainforest. The climate is mild with mean daily temperature ranges of ca. 20 ◦Cmax/10◦C min in summer and ca. 9 ◦Cmax/3◦C min in winter (Nunez, 1978). Precipitation is very high with typically 2–3 m rainfall per year (Nunez, 1978). The catchment areas are described more fully by Bowles et al. (2002b). Lake Gordon and Lake Pedder were formed by the impoundment of the Gordon River for hydroelectric power generation in the early 1970s. Table I lists basic morphological data of Lake Gordon and Lake Pedder. Lake Gordon consists of a number of basins connected by steep-sided narrows (Figure 1). Knob Basin (ad- jacent to the Gordon Dam) is the deepest part of the lake, and is approximately 140 m deep at full supply level (FSL). The lake surface is 308 m above sea level at FSL. Water for power generation is withdrawn from Knob Basin at about 52 m below FSL. The largest basin is the Calder Basin, which is considerably shallower and less steep-sided, although it also has drowned river valleys exceeding 80 m in depth. At the time of inundation, very little of the area had been cleared by logging and much of the submerged terrain is flooded forest. Lake Pedder was formed by the construction of dams on the Serpentine and Huon Rivers, at opposite ends of a valley, which previously contained the original Lake Pedder. Lake Pedder ‘nova’ has approximately the same surface area as Lake Gordon but comprises mainly the one shallow basin, up to about 20 m deep. Lake Pedder supplies about 42% of Lake Gordon’s water supply via the McPartlan Canal and is not otherwise used for power generation (Steane and Tyler, 1982). Apart from Lake Pedder, the single largest contribution of water to Lake Gordon is the Upper Gordon River. Other smaller rivers and creeks make up the rest of the inputs. Direct rainfall is also a major water input, especially to Lake Pedder due to its higher surface area to volume ratio. The lakes are not known to freeze over, but an annual pattern of stratification occurs in Lake Gordon (Steane and Tyler, 1982). Most of the outflow of water from the two lakes is via the Lower Gordon River. Both lakes have similar water chemistry and catchment characteristics. The waters have very low total dissolved salts, with seawater proportions of major ions (Buckney and Tyler, 1973; Steane and Tyler, 1982). The waters of the area carry very little suspended sediment but have high humic matter content, due to filtering of the water through layers of peat and from decomposition and leaching of button grass, tea tree species and other organic matter (Steane and Tyler, 1982). This results in organic-rich, tea coloured waters with Secchi disk measurements between 2 and 3 m (Steane and Tyler, 1982). This limits the extent of photosyn- thetic activity in the lakes and chlorophyll a measurements for the waters of Lake 6 K. C. BOWLES ET AL. − Figure 1. Spatial distribution of MeHg (ng L 1) in Lake Gordon and Lake Pedder surface waters, June 1996 winter survey. MERCURY IN LAKE GORDON: IN-LAKE PROCESSES 7 Gordon are typically around 1 µgL−1. The peat layers serve to isolate the water in the catchment from the underlying rocks (Buckney and Tyler, 1973). The aquatic fauna of Southwest Tasmania are not well characterised and know- ledge remains largely taxonomic.