Black spruce from boreal forests located in northeastern Canada (Quebec) were ... in northeastern Canada, in the province of Quebec. ... the 1870s or the 1930s.
MERCURY CONCENTRATION IN TREE RINGS OF BLACK SPRUCE (PICEA M A R I A N A MILL. B.S.P.) IN BOREAL QUEBEC, CANADA LI Z H A N G , JUN-LONG QIAN and DOLORS PLANAS * Chaire de la Recherche en Environnement, Universit~ du Quebec ?t Montreal, C.P. 8888, Succ. A, Montreal, H3C 3P8 Canada (Received 4 March 1993; accepted in final form 21 December 1993)
Abstract. Black spruce from boreal forests located in northeastern Canada (Quebec) were sampled during the summer of 1990, at two stations located at 6~ latitude from one another. The objective of the study was to compare the temporal and spatial evolution of mercury in the tree rings of sites with differing degrees of mercury contamination in their soils. Mean mercury tree ring contents ranged from 13 to 37 ng/g, and were more concentrated in the southern than in the northern station. No evident relationship was found between annual growth and corresponding mercury concentrations. The difference in tree ring mercury content associated with geographic orientation of the disks indicates that daily exposure to sunlight as well as temperature may affect mercury uptake, and that the mercury observed in the tree rings must be deposited from the atmosphere onto the tree surface.
1. Introduction
Since the development of hydroelectric reservoir impoundments in North America in the 1970s, mercury concentrations in fish have dramatically increased (Meister et al., 1979; Bodaly etal., 1984; Messier and Roy, 1987; Jackson, 1988). In general, the enhancement of mercury concentrations in the aquatic and terrestrial ecosystems of northern latitudes have been attributed to anthropogenic sources (Nriagu, 1979, 1989; Evans, 1986). This pollution has steadily increased in recent years (Nriagu and Pacyna, 1988), due mainly to fossil-fuel burning and the incineration of municipal wastes (Nater and Grigal, 1992). Mercury analyses of precipitation in northwestern Quebec, Canada, for example, showed concentrations 4 to 10 times higher than those found in the lake waters of regions where fish show serious mercury contamination (Brouzes et al., 1977). In small watersheds the contribution of total mercury from the atmosphere is estimated to be several times greater than that originating from the catchment area (Lindqvist et al.., 1991; Fitzgerald et al., 1991). It has generally been assumed that atmospheric mercury deposition is dominated by global scale processes, and is thus regionally uniform. However recent papers have shown regional trends in mercury distribution both in Europe (Lindqvist et al., 1991) and in the United States (Nater and Grigal, 1992). Mercury pollution in a local area could be : lobilized and cycled in the atmosphere for long periods of time (Nriagu, 1~93). * To whom the correspondence must be addressed. Water, Air and Soil Pollution 81: 163-173, 1995. (~ 1995 Kluwer Academic Publishers. Printed in the Netherlands.
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Tree-ring analysis has been reported as a potentially useful technique in assessing the historical evolution of atmospheric pollution (Robitaille, 1981; Baes III and McLaughlin, 1984; Hornbeck and Smith, 1985; Innes and Cook, 1989). The relationship between tree growth and climatical factors may reflect an indirect change in the composition and quality of the air (Conkey, 1976; LaMarche, 1978; Payette et al., 1985). The deposition of mercury enriched aerosols can be intercepted by the surface vegetation or be accumulated in the forest soil (Nicholson et al., 1980; Natel and Grigal, 1992) and be present in the ground water (Krabbenhoft and Babiaez, 1992). Trees might thus accumulate trace metals from the soil, via roots, or from the air via leaves (Robitaille, 1981). This study was designed to investigate the historical evolution of mercury concentration in tree rings from boreal forests. To accomplish this objective two sites were chosen characterized by the same tree species and dominant wind direction, but differing in their degree of mercury contamination in the soil.
2. Material and Methods
2.1. LOCATIONAND DESCRIPTIONOF STUDYSITE Two sampling sites located near man-made reservoirs of differing age were selected in northeastern Canada, in the province of Quebec. The first site is near Laporte Lake (47~176176176 and the second, located at 6 ~ latitude to the north, is near Detcheverry Lake (53~176176176 (Figure 1). These two natural lakes serve as reference ecosystems to the two mercury-contaminated reservoirs: Cabonga reservoir, impounded in the 1920s, and La Grande-2 (LG-2) reservoir, created in 1978-79, respectively. Prevailing winds in both regions are generally from the west, the southwest or the south. 2.2. SAMPLINGSTRATEGY The tree species sampled was the black spruce (Piea mariana [Mill] BSP). This species forms nearly monospecific forests in northeastern Canada (Sirois and Payette, 1991) and is quite abundant in forests of the Cabonga region (Rowe, 1972). During July and September of 1990, disks (thin cross sections) were sampled from seven trees at station 6 (Laporte Lake) and ten at station 23 (Detcheverry Lake). These disks were removed from stumps cut at a height of about 1 meter. 2.3. ANALYTICALMETHODS Dating of tree rings was carried out by measuring ring widths (i.e., annual growth) with a Henson micrometer. The disks were then cut into 5-year sections which were considered to provide sufficient sample mass for the chemical analysis of
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Fig. 1. Insert - general map of Canada. The enlarged shaded area shows the general location of the sampling sites (A), in the province of Quebec.
total mercury (Hg). The subsamples were then manually shredded with a plastic knife and air dried at room temperature. About 0.4 g of homogenized material from each subsample was digested with 10 mL of concentrated ultrapure nitric acid and 1 mL of 6N ultrapure hydrochloric acid at 120-130~ for three hours. Blank solutions were also submitted to the whole procedure to test for any potential laboratory or reagent contaminant. Total mercury concentrations were analyzed by cold vapour atomic fluorescene spectrometry using a modified version of the Bloom and Fitzgerald (1988) technique, which is 10-100 times more sensitive than atomic absorption. For further details see Louchouam et al. (1993). The general principle of the methods is based on the analysis of the Hg ~ released from a strong acid solution. The mercury from the sample is first oxidized into Hg 2+, is reduced to Hg ~ vapour by tin (II) chloride, and is then quickly carried by a stream of Hg-free argon into a quartz cell where it is excited to fluorescence. Since annual tree growth can vary largely as a function of its reaction to the surrounding microenvironment, to external perturbations and/or to aging effects, it was necessary to standardize growth development by converting widths to index ratios. This index, calculated using the low degree polynomial regression method
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outlined by Fritts (1976), is a better signal of annual climatic fluctuations (Hughes et al., 1978). 2.4. STATISTICALMETHODS One-way analysis of variance or multiple comparisons test (Scheffe's test) were used to determine differences in mercury accumulation within or between sites (Snedecor and Cochran, 1967).
3. Results 3.1. GROWTHCURVES
Figure 2 illustrates the annual growth of the sampled trees which dated from either the 1870s or the 1930s. At station 6 (Figure 2a), a fraction of the trees were older (up to 120 years), while the rest are younger (about 60 years old). The two age groups showed a generally normal evolution with a higher growth rate in the early years followed by a progressive decrease through time, except for some obvious increases in radius growth within the last decades. Station 23 (Figure 2b) has a relatively simple and even growth curve with a small peak in the second half of the 1980s. 3.2. GROWTHINDEX Four high growth index peaks can be seen close to the end of the 1890s, 1920s, 1950s and 1980s (Figure 3) in station 6. These peaks are separated by periods of low growth indices. The lowest one occurred in the mid 1970s. From that point onward, a continual rate of increase followed with a maximal growth peak in the 1980s. As was the case for the growth curve, the growth index for station 23 depicts a much smoother curve. At this station the highest growth index occurred in the 1980s, coinciding with the results for station 6. The growth index pattern for the two stations was quite similar except for the segment in the 1970s. This overall similarity in annual growth between sites is probably related to similar local climatic conditions. 3.3. MERCURYCONTENTSIN TREE-RINGS Annual dry weight mercury contents in tree rings varied between 13 and 37 ng/g (Figure 4), on average. Total mercury concentrations were generally higher in the black spruce trees of station 6 than in those of station 23. At the former station the maximum mercury concentrations were found between 1910 and 1930. At station 23 the highest mercury concentrations occurred in early tree growth, around the 1940s.
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year Fig. 6. Predictionof total mercuryevolutionin tree rings based on the mercuryconcentrationand the growthindex. The mercury concentrations of the tree-rings according to different geographic orientation (Figure 5) revealed a heterogeneous uptake of mercury in trees. The higher concentrations were clearly oriented south or southeast. 3.4. TOTALMERCURYEVOLUTION In order to obtain annual mercury values, mercury contents were multiplied by the growth index. The resultant is an indirect expression of the total mercury which is probably accumulated annually in the tree rings. It does not take into account the effect of aging on the black spruce. The result does, however, illustrate the conceptual evolution of mercury based on actual concentration and growth tendency in relation to environmental variables. For example, in relation to Figure 6, 19101930 and the 1980s, two periods when total mercury contents were remarkably high in station 6, would correspond to dramatically high concentrations of mercury or to a large increment in annual growth.
4.
Discussion
Mean total mercury concentrations in the tree tings of the black spruce of both study sites are relatively low, and remain around the background mercury concentrations
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(lower than 100 ng/g) generally found in previous studies (Lindqvist et al., 1991; Rasmussen et al., 1991). Mean total mercury concentrations are significantly higher in the southern than in the northern station for the period comprised between 1930 and 1990 (Figure 4). We do not have the aerosol mercury concentration data of these regions for the entire period in which the trees were growing, but we do know that in the southern site mercury concentrations are from 1.5 to 2 times higher in the top centimetres (over 10 to 16 cm) of the soils or sediments than in the northern site (Louchouarn et al., 1993). Thus the different mercury concentrations in the tree rings could be the consequence of the higher level of contamination in the southern station. Other factors unrelated to the abundance of mercury in the soils could also contribute to these differences. In fact the standardized growth curve shows different trends at the two stations between 1935 and 1950 and in the 1970s (Figure 3). It is known that in the northern latitudes there is a direct relationship between tree growth and precipitation (Blasing and Durick, 1984), temperature (Payette et al., 1985; Conkey, 1986; Bonan and Sirois, 1992), dryness (Cook and Jacoby, 1977) and summer degree-days (Jacoby et al., 1985). In our study mean temperature in the winter and during the growth period, and precipitation were closely relatedd to tree growth (Chaire de Recherche en Environnement/Hydro-Qurbec/CRSNG/UQAM, 1991). The climatological records existing in the southern station show low average monthly April temperatures between 1970 to 1974, the lowest since the 1930s. These adverse conditions could explain the low growth index found in the southern station. In contrast spring temperatures were high in the 1980s, which corresponds to the period when we observed relatively high growth indices at both stations. If tree growth is reduced as a result of low temperature and we assume that the bioavailability of mercury in the environment remains the same, we could expect a higher mercury concentration per unit dry mass (Figure 4). After normalizing the data to index growth (Figure 6) we see that in general the trend persists; the trees of station 6 tend to have more mercury accumulated in the rings than those of station 23. Thus it would seem that historically the trees of station 6 have been exposed to higher mercury concentrations than those of the northern station since at least the 1930's. Before the 1930s the tree rings of the southern station show a very distinct peak around the 1920s, with mercury concentrations between 1.5 to 2 times higher than those measured either before 1910 or after the 1930s (Figure 4). This high mercury level cannot be explained directly from climatological factors, since it is also present in the normalized data (Figure 5). This period corresponds to the highest mean temperatures measured in this century and to very low precipitation. These hot, dry condition favored natural forest fires that devastated the region for almost a decade. Wild forest fires are generally considered to be one of the natural sources of mercury and other trace metals to the atmosphere (Nriagu, 1989). The higher concentrations of mercury in the tree rings oriented towards the south and southeast seem to reflect the effect of daily exposure to sunlight and
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temperature rather than to the prevailing winds of the west, south or southwest. Metals may enter a tree from the atmosphere via needles and bark or from the soil via roots (Robitaille, 1981). During the daytime for example, gaseous mercury can enter trees through stomatal pores (in the epidermal surface of leaves) when carbon oxide, oxygen and water vapour are exchanged with the atmosphere (Linberg et al., 1991). Light intensity and temperature are two of numerous parameters which regulate the rate of pollutant uptake (Smith, 1984). Although root pathways are considered to be more direct and rapid transporters of pollutants inside plants (Robitaille, 1981), no evidence has shown that mercury is taken up into the above-ground parts of vegetation from soil, because there exists a barrier against mercury entry from the root system to the upper part of the plant (Lindqvist et al., 1991). This is in accordance with our findings. Mercury distribution in tree rings associated with geographic orientation thus cannot be obviously explained by root transport, however we cannot exclude the importance of a warmer climate, stronger sunshine, and higher temperature in stimulating local mercury accumulation in the ecosystem. Mercury concentrations in the tree rings were thus quite low and the difference in the annual mercury content of the tree rings between stations was only moderately significant (a = 0.05). This small difference makes it difficult to determine both the mercury source and its environmental impact on the ecosystems.
5. Conclusion The total mercury concentrations observed in the tree rings of the black spruce seem to originate from the local environment. Sunshine, temperature and geographic latitude seem to play a role in mercury uptake, but prevailing winds did not affect the accumulation process. This conclusion should be taken with caution however, since based on our study tree rings analysis does not seem suited to the investigation of the long range atmospheric transport of pollutants. The environmental cycling of mercury is extremely complex; it probably does not involve wood, but certainly does involve the needles, leaves and bark.
Acknowledgements The authors thank E Ferland and E Louchouarn for the sample collections, and Professors Y. Bergeron and E P i c h e t for their technical assistance in tree ring datation and mercury analysis respectively. This study was supported by HydroQuebec, the Natural Sciences and Engineering Research Council of Canada and the Fondation UQAM.
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