In Situ Two-Dimensional High-Resolution Profiling of Sulfide in

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Environ. Sci. Technol. 2003, 37, 792-797

In Situ Two-Dimensional High-Resolution Profiling of Sulfide in Sediment Interstitial Waters CHRISTOPHER R. DEVRIES AND FEIYUE WANG* Environmental Science Program and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

A recently developed technique of diffusive gradients in thin films (DGT)-computer-imaging densitometry (CID) was improved to study in situ two-dimensional distributions of sulfide in sediment interstitial waters adjacent to the DGT device. The in situ profiler accumulates sulfide from the sediment interstitial water through a diffusive gel onto a AgI binding gel to form black Ag2S. The amount of sulfide bound in the binding gel is then determined from the gray scale density of the dried binding gel. New gel-making procedures were employed to produce stable, evenly dispersed AgI binding gels and to minimize the darkening of the AgI gel upon exposure to light. The improved DGTCID technique was used to profile the distribution of sulfide in sediment interstitial waters at Delta Marsh, a highly productive Prairie wetland on the south shore of Lake Manitoba, at a vertical and lateral resolution of e0.4 mm. The in situ high-resolution microprofiles revealed unprecedented two-dimensional heterogeneity in sulfide concentrations in the sediment interstitial waters adjacent to the DGT device. The mosaic distribution of oxic and sulfidic microenvironments suggested not only the complexity and heterogeneity of the biogeochemistry of sulfur species and sulfide-binding metals (e.g., Cd, Cu, Pb, Hg, Zn) in sediments but also the capability of aquatic organisms for coping with the sulfidic environment.

Introduction The sulfidic nature of most natural sediments has long been recognized, but its profound implications for a variety of geochemical and biological processes in natural waters remain to be uncovered (1). A strong complexing ligand with “Class B” metals (2), sulfide affects the speciation, cycling, and bioavailability of a variety of metals, most of which are of high environmental concern (e.g., Cd, Cu, Pb, Hg, Zn). Sulfide is also a potential toxicant to all aerobic organisms and affects the distribution of both aerobic and anaerobic organisms (1). With the development of electrochemical, optical, and biological sensors for in situ analysis of sulfide in aquatic sediments (3), it is now evident that significant vertical gradients (with respect to the sediment-water interface) of sulfide can occur within a layer as thin as micrometers to millimeters (4-8). The lateral distribution of sulfide, on the other hand, is poorly studied. Although there is a generally held view that sulfide distribution in sediment interstitial water also varies laterally, particularly in shallow * Corresponding author telephone: (204)474-6250; fax: (204)4747608; e-mail: [email protected]. 792

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waters, few data are available on how significant this lateral heterogeneity can be. Without such information, the representation of a specific vertical profile of sulfide cannot be assessed. Moreover, the roles of bioturbation and bioirrigation in affecting the biogeochemistry of sulfide and sulfide-binding metals cannot be fully addressed solely by vertical profiles without taking into account lateral variations. Our understanding of two-dimensional (i.e., vertical and lateral) distributions of sulfide (and other chemicals) is challenged by the spatial and temporal resolutions of the analytical techniques. Dialyzers (peepers), for example, collect interstitial water samples by equilibrating ultrapure water contained in their discrete compartments with the surrounding interstitial waters (9-11); the result is hence the averaged concentration throughout an area of the same size as the compartment (typically 0.3-0.6 cm by 4-8 cm) over the entire equilibrating period (typically 2-3 weeks). Electrochemical sulfide microsensors (4-8) can achieve much higher spatial (sub-millimeters to micrometers) and temporal (seconds) resolutions, but their applications have so far been limited to vertical profiling. Since each measurement is discrete, at a resolution of 0.1 mm it would take 3.75 × 105 measurements to complete a two-dimensional profiling of an area of 15 cm × 2.5 cm. Teasdale et al. (12) developed a simple yet promising technique for measuring the in situ flux of sulfide in sediment interstitial waters. The method is built on the diffusive gradients in thin films (DGT) technique (13, 14) coupled with computer-imaging densitometry (CID). It involves a DGT sampler comprising a polyacrylamide diffusive gel and a binding gel incorporated with fine AgI(s) particles. Once deployed in the sediment, sulfide species from the interstitial waters diffuse through the diffusive gel and react with pale yellow AgI(s) in the binding gel to form black Ag2S(s). The accumulated sulfide on the binding gel is then measured from the gray scale density of the binding gel obtained with computer-imaging software, which allows for the calculation of the sulfide flux from the sediment interstitial water to the DGT device. When interpreted appropriately (15), the technique can also be used to measure the sulfide concentration in the interstitial waters (12-15). One of the advantages of the DGT-CID technique is that it can, by a single measurement, profile two-dimensional distribution of sulfide in a relatively large area (can be adjusted by the size of the gel) at sub-millimeter resolutions. However, several key technical problems remained unsolved in the original study (12; see also Results and Discussion). In this paper, we report several improvements to the DGTCID technique. By employing a new gel-making procedure, we were able to produce stable, evenly dispersed, AgI binding gels. We also significantly reduced the darkening of the AgI gel upon exposure to light. The improved technique was then used to profile the distribution of sulfide in a highly productive Prairie wetland and revealed unprecedented twodimensional heterogeneity in sulfide distribution in the sediment interstitial waters adjacent to the DGT device.

Experimental Section Preparation of the DGT Samplers. The diffusive gels were prepared in a way similar to the general procedures described elsewhere (12-14), with modifications to enhance the strength of the gels and to avoid setting of the gels at elevated temperature. In brief, a gel stock solution was prepared by dissolving 142.5 g of acrylamide (Merck, Omnipur Grade) and 7.5 g of N,N′-methylene bisacrylamide (Fisher, electrophoresis grade) in deionized water (>18 MΩ‚cm; Barnstead) 10.1021/es026109j CCC: $25.00

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and diluting to 500 mL. The diffusive gel was prepared by adding 250 µL of freshly prepared 10% (w/w) ammonium persulfate (Fisher) solution and 10 µL of N,N,N′,N′-tetramethylethylenediamine (TEMED; Fisher) into a deoxygenated mixture of 12.5 mL of the gel stock solution and 12.25 mL of deionized water. The solution was mixed thoroughly and pipetted immediately into a gel mould, which comprised two slightly offset, clean glass plates (20 cm × 20 cm), a thin inert vinyl spacer, and clips to hold the mould firmly together. The polymerization occurred rapidly at room temperature. Once it was completely set (no liquid remained), the gel was gently removed from the mould, cut to desired shapes, and soaked in deionized water for at least 24 h before use. The hydrated diffusive gels had a thickness of 0.4 mm. The binding gel was prepared by adding 70 µL of ammonium persulfate and 4 µL of TEMED to a mixture of 12.5 mL of the gel stock solution, 12.25 mL of deionized water, and 3 mL of 1 M AgNO3. The solution was then mixed thoroughly and immediately pipetted into a gel mould. The mould was the same as that used for the preparation of the diffusive gels, except that two vinyl spacers were used so that the thickness of the gel was doubled. After polymerization at room temperature, the set gel was cut to desired shapes and soaked in a 0.2 M KI solution in the dark. After the gels turned completely opaque (typically in 5 min), they were removed from the KI solution, rinsed, and kept in deionized water in the dark for at least 24 h before use. The hydrated binding gels had a thickness of 0.8 mm. Two different DGT samplers were used: piston holders for deployment in the laboratory calibration solutions and flat rectangular probes for in situ deployment in sediments. The piston holder (DGT Research Ltd, U.K.) was of the design of Zhang and Davison (13) and consisted of a backing cylinder and a front cap with a 2.0-cm diameter window. The in situ probe consisted of two black Plexiglas plates (19 cm × 6.5 cm × 0.2 cm): a backing plate and a front plate with a rectangle window of 15 cm × 2.5 cm. In each case, the binding gel was overlaid with the diffusive gel and a 0.2-µm hydrophilic polysulfone membrane (HT-200; Gelman). Black membranes of 0.45-µm pore size (Supor-450 Black; Gelman) were also used in some applications for comparison. They were held together within the plastic assembly with the membrane exposed through the window. The samplers were kept in deionized water in the dark before use. Calibration. Thirty DGT samplers of the piston type were exposed to sulfide solutions ranging from 0 to 200 µM for different exposure periods (1, 4, and 8 h). At each concentration, three replicate systems were used. Sulfide standard solutions were prepared from a 50 mM Na2S (99.99+%; Aldrich) stock solution with deoxygenated deionized water. The actual concentration of the sulfide stock solution was determined freshly by standardizing against a standard Na2S2O3 solution using an iodometric method. The sulfide standard solution was added into 125-mL amber glass jars with Teflon-lined lids. A DGT sampler of the piston type was placed in each jar. The tightly sealed jars were placed in an orbital shaker for 1, 4, or 8 h at a very low shaking speed (60 rpm). The DGT samplers were then retrieved from the jars. The binding gels were carefully removed from the samplers, covered by a drying cellophane, and dried on a gel dryer (model 583; Bio-Rad) at 60 °C for 2 h under vacuum. Field Profiling. The field profiling was conducted in July 2001 at Delta Marsh (98°23′ W, 50°11′ N) on the south shore of Lake Manitoba, Manitoba, Canada. Underlain by the Jurassic Amaranth and Reston Formations, the area is enriched in gypsum and anhydrite (16, 17); sulfate concentrations of up to 8.6 mM have been measured in the overlying waters of the marsh (18). Three in situ DGT samplers were deployed across the sediment-water interface at a station in Forster’s Bay in July

2001 (the water depth was about 1.0 m at the time of sampling). After 4 h, the probes were retrieved, and the binding gels were removed and dried on a gel dryer, as described above. For comparison, three in situ peepers of the type described by Carignan et al. (10) were also deployed at the site for 2 weeks. Each peeper consisted of a 30 × 15 × 1 cm Plexiglas plate in which 0.6 × 7.0 × 0.6 cm compartments spaced 1 cm center to center were machined, a 0.2-µm hydrophilic polysulfone membrane to cover the bottom plate, and a top 0.2-cm Plexiglas cover sheet. The Plexiglas components of the samplers were kept under a N2 atmosphere for a minimum of 15 d prior to filling the cells with deionized water and covering them with the membrane. It is critical to remove O2 from the Plexiglas to avoid its slow release into the sampler compartments during in situ equilibrium, as this can significantly alter the shape of the profiles of redox-sensitive species such as sulfide (11). The assembled peepers were replaced under a N2 atmosphere again for at least 7 d prior to placement in the marsh sediment. After 2 weeks, the samplers were retrieved, and 1.5 mL of the water sample from each compartment was collected immediately with N2purged polypropylene syringes and injected through a Teflon septum into pre-evacuated amber glass vials containing 0.2 M N,N-dimethyl-p-phenylenediamine sulfate (C8H12N2‚H2SO4; Fluka) and 0.2 M FeCl3 (60 µL of each). Samples were stored at 4 °C in the dark until return to the laboratory where sulfide was measured immediately; the analysis was typically done within 24 h after the sampling. Analyses. Sulfide accumulated in the dried binding gels of DGT probes was determined by CID as described by Teasdale et al. (12), except that the brightness of the scanned images was not adjusted. The dried gel was scanned by a flat-bed scanner (ScanJet 4300C; Hewlett-Packard) and analyzed for gray scale density with Scion Image (β 4.0.2; Scion Corporation). By using a scanning resolution of 300 dpi, the vertical and lateral resolution of the imaging process is about 85 µm. However, the actual spatial resolution of the technique is also limited by the thickness of the diffusion gel layer (0.4 mm in this study), as relaxation of sharp features occurs as a result of partial vertical diffusion of the analyte as it passes through the gel (12, 19, 20). Sulfide in the water samples collected by the peepers was determined with a UV-visible spectrophotometer (UV 2101 PC or Cary 50), using the methylene blue method (21). The color-developed samples were diluted 50 times with 0.4 M HCl before the analysis. The evenness of the AgI(s) distribution in the binding gels was analyzed with an environmental scanning electron microscope (ESEM) (XL 30; Philips) at 6.1 Torr and 7.1 KV.

Results and Discussion Evenness of AgI(s) in the Binding Gel. Since the DGT-CID technique is based on the color change of the binding gel, it is crucial that the AgI(s) be evenly and finely dispersed in the binding gel; otherwise the change in the color cannot be solely attributed to sulfide. Teasdale et al. (12) compared two procedures for the preparation of binding gels. The first method involved addition of the finely ground AgI(s) to the gel solution followed by polymerization, whereas the second method involved the successive immersion of the normal diffusive gel in solutions of AgNO3 and KI. They found that although the latter method produced a very even and finely dispersed binding gel, Ag2S was formed preferentially at the exterior surface of the gel and was consequently prone to becoming detached with handling. As a result, they used the first method, which produced a AgI(s) layer of about 50 µm at one side of the gel. Our preliminary experiments following this procedure, however, failed to produce an evenly dispersed AgI layer on VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. ESEM image of dispersed AgI(s) (white spots) in a polyacrylamide binding gel. the gel because of the insolubility of AgI(s) in the gel solution. Even if the AgI powders were ground to very fine particles and the gel solution was vigorously stirred, they settled out quickly from the gel solution during the preparation and polymerization, resulting in unpredictable and irreproducible distributions of AgI(s) in the binding gel. To solve this problem, we used a different method. Instead of adding the insoluble AgI(s), we added an aqueous AgNO3 solution into the gel solution, followed by immersion of the set gel in a KI solution of a sufficiently high concentration. The I- ions in the KI solution diffused into the hydrogel and reacted with the embedded Ag+, which produced AgI(s) in the gel in situ. Since the added Ag+ apparently accelerated the polymerization process, the amounts of ammonium persulfate and TEMED were reduced (see Experimental Section) to allow sufficient time for the gel preparation. The ESEM analysis of the AgI(s) binding gels (Figure 1) confirmed that the AgI(s) produced by this procedure was not only very fine particles (ca. 0.3 µm) but also dispersed evenly throughout the gel (not just on one side of the gel). Once reacted with sulfide, the formed Ag2S was firmly held on the gel. We observed the detachment of the formed Ag2S from the gel surface only in one case when an extremely high sulfide concentration (200 mM for 4 h) was used in the calibration. As pointed out below, at this high concentration the calibration curve reached a plateau and was thus inappropriate. Darkening of the Binding Gel upon Exposure to Light. The AgI in the binding gel was prone to darkening upon prolonged exposure to light, because of the photoreduction of AgI to Ag. Teasdale et al. (12) noted that this was mainly a problem during setting of the gel, which took place at elevated temperatures (42 °C for 45-60 min) in their gelmaking procedure and cautioned the use of the technique close to an oxic-anoxic boundary or in the presence of light. By using a different type of cross-linker and different ratios of the gel solutions to ammonium persulfate and TEMED (see Experimental Section), we were able to set both the diffusive and the binding gels at room temperature without any additional heating and therefore minimized the gel darkening in this step. To minimize the exposure of the gel to light, black instead of clear Plexiglas plates were used to assemble the samplers. To test whether the white membrane (HT-200; Gelman) used in the probe might allow light to pass through and cause darkening of the binding gel, black membranes (Supor450 Black; Gelman) were used in several field studies. No significant differences were found between the sulfide profiles obtained by the white and black membranes. 794

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FIGURE 2. Typical calibration curves at a deposition time of 4 h (A) and 1 and 8 h (B), showing the relationship between gray scale density (0-255) and sulfide concentration in the solution. The symbols are the actual measurements (mean ( SD), and the lines are the fitted calibration curves. Calibration Curve. Figure 2A shows a typical calibration curve for a deployment time of 4 h. A very significant nonlinear relationship was obtained between the sulfide concentration and the gray scale density when ∑S(-II) e 110 µM:

gray scale density ) 44.26 ln(0.66

∑S(-II) + 1.33)

R2 ) 0.997, n ) 7 (1)

A calibration curve of the similar type was also reported by Teasdale et al. (12). The logarithmic relationship indicates that the densitometric measurement is very sensitive at low sulfide concentrations and less sensitive at high sulfide concentrations. At ∑S(-II) > 110 µM, the gray scale density reached a plateau and the calibration curve was thus no longer valid. Since the DGT technique is a kinetic accumulating process (13, 14), the actual working range of the calibration curve can be optimized by changing the deployment time or the diffusive gel thickness. In sediment interstitial waters where sulfide concentrations are higher or lower than the range shown in Figure 2A, a shorter or longer period of deployment can be used (Figure 2B). In practice, the minimum deployment time should not be less than 1 h, and the maximum deployment time varies from weeks to months depending on the binding capacity of the binding gel and the intensity

FIGURE 3. (A) Dried binding gel showing the two-dimensional distribution of sulfide in the sediment interstitial waters at Forster’s Bay, Delta Marsh (July 31, 2001). (B) Vertical profiles of sulfide analyzed from four randomly chosen sections of panel A. The horizontal dashed line indicates the sediment-water interface. The vertical dotted line represents the upper limit of the DGT calibration curve for sulfide. of in situ microbial attack and biofouling on the membrane (13). Two-Dimensional Microprofiles of Sulfide in Sediment Interstitial Waters at Delta Marsh. Three in situ rectangular DGT probes were deployed in situ across the sedimentwater interface at Forster’s Bay of Delta Marsh in July 2001. The binding gel of one probe was accidentally broken during the drying process; hence, only the results from the other two probes are reported here. Figure 3A shows a scanned image of the binding gel from one of the probes after a 4-h in situ deposition at the site. Since the color of the image is related to the concentration of sulfide, the image provides a direct visual impression of both vertical and lateral variation in sulfide concentration adjacent to the DGT device. In addition to the expected vertical variation, the image reveals a significant degree of lateral heterogeneity in sulfide distribution in the sediment interstitial waters adjacent to the DGT device. As a result, depending on where they are measured, vertical profiles of sulfide in the sediment interstitial waters can be very different even within a few centimeters (Figure 3B). The sulfide measured by the DGT-CID may include dissolved labile bisulfide (H2S and HS-) and polysulfides (HmSn2-m; m ) 0, 1, 2 and n g 2) that can diffuse through the diffusive gel and that can readily react with AgI(s). Colloidal sulfide species are likely excluded from the measurement because of the very small pore size of the polyacrylamide diffusive gel (typically 2-5 nm; 13). Compared to most inland waters, the sulfide concentration in the interstitial waters at Delta Marsh is extremely high; concentrations of up to 1 mM were measured at the same site in late August 2001 (F. Wang, unpublished data). The high sulfide concentrations in the interstitial waters likely resulted from the abundance of sulfate (16, 17) and organic matter in the highly productive marsh. As a result, the intensity of the images of a large portion of the dried binding gels exceeded the upper limit for the calibration curve (110

FIGURE 4. Laterally averaged vertical profiles of sulfide measured by three replicate peepers (represented by the symbols O, 4, and 0) and two replicate DGT probes (represented by the solid lines). The peepers and DGT probes were located within a radius of 1 m at the sampling site. The horizontal dashed line indicates the sediment-water interface. The vertical dotted line represents the upper limit of the DGT calibration curve for sulfide. µM; 4-h deployment); thus, the concentration profiles appeared to saturate (Figure 3B and Figure 4). This problem can be overcome by employing a shorter period of deployment (e.g., 1-2 h) or thicker diffusive gel. Sulfide Concentration at the DGT Surface versus in the Bulk Interstitial Water. It is important to note that the sulfide concentration “seen” by DGT devices may be quite different from the concentrations “seen” by peepers. Whereas peepers measure the sulfide concentration in the bulk interstitial water via an equilibrium process (9-11), DGT measures sulfide via an accumulating process (i.e., via the interfacial flux from the interstitial water to the DGT device). Therefore, the sulfide concentration measured by the DGT technique represents the time-averaged concentration of sulfide adjacent to the DGT device (15, 20). Since sediment interstitial waters are not well mixed, depending on the resupply rate from the solid phase, the sulfide concentration adjacent to the DGT device may become depleted and lower than that in the bulk interstitial waters. Figure 4 compares the laterally averaged vertical sulfide profiles by the DGT probes and those by the peepers. Such a comparison is for illustrative purposes only as the results from the two techniques differed greatly in spatial resolution and deployment time. Since each compartment of the peepers measured 7.0 cm in width, the vertical profiles obtained by this technique were averaged vertical profiles across a lateral dimension of 7.0 cm. In contrast, the vertical profiles obtained by the DGT probes as shown in Figure 4 were averaged vertical profiles across a lateral dimension of 2.5 cm (the width of the exposed binding gel). Moreover, the VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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DGT probes sampled interstitial waters less than 0.5 mm away from the probe over a 4-h deployment, whereas the peepers sampled a few centimeters away from the sampler over 14 d. Figure 4 shows that the sulfide concentrations measured by the DGT technique were lower than those measured by the peepers, but the vertical profiles obtained by the two techniques were of the similar shape. The difference in the sulfide concentration adjacent to the DGT device and in the bulk interstitial water suggests that there is a depletion of sulfide in the area adjacent to the DGT device. The sulfide concentration in the interstitial water is likely controlled by metastable iron sulfides (e.g., amorphous FeS and mackinawite), which have been shown to have moderate dissolution rates (22, 23). The resupply rate of sulfide from these solids as well as from desorption of surface-associated sulfide species appears to be not high enough to sustain the flux from the sediment interstitial water to the DGT device. Although this is one major limitation of using the DGT technique to measure the bulk interstitial water concentration (15), it is also a unique advantage of the DGT technique. The sulfide concentration measured by the DGT technique is probably more representative of the concentration to which aquatic organisms are actually exposed. Experiments were also conducted in the summer of 2002 to compare the sulfide profiles obtained by the same type of DGT devices for different deployment periods (2 and 4 h). Although the sulfide concentration profiles obtained were not identical because of the extreme heterogeneity of the sediment at the site, the shapes of the profiles and ranges of the concentrations were similar. Biogeochemical and Ecotoxicological Implications. With a very high vertical and lateral resolution (0.4 mm), the DGTCID technique can reveal much more detailed information of the sulfide distribution, which cannot be achieved by the peepers (typical resolution is 1 cm vertically and 7 cm laterally). Contrary to the study of Teasdale et al. (12), which reported no significant lateral variation of sulfide at a shallow site of the Conder River estuary, U.K., in this study we observed an unprecedented degree of heterogeneity in both vertical and lateral distribution of sulfide in the sediment interstitial waters of Delta Marsh (Figure 3A). Although the sulfide concentration increased generally from the overlying water down to deeper in the sediments, the presence of a variety of oxic and anoxic microenvironments is evident in Figure 3A. We believe that the oxic “branchy” microenvironments (light-colored) that are obvious mainly in the top 3-cm sediment layer in Figure 3A are created by the rhizospheres of pondweeds (e.g., Stuckenia pectinatus) that are abundant at the sampling site. It has been well-documented that the roots of many wetland plants can release oxygen (24, 25), catalyze the oxidation of sulfide (26), and thus create oxic rhizospheres in otherwise anoxic bulk sediments. Tubedwelling macro-invertebrates can also create oxic microenvironments by pumping the oxygenated overlying water through their tubes (e.g., refs 27 and 28). Figure 3A also reveals the presence of more sulfidic microenvironments (dark-colored spots in the lower layer of the sediment) in the already sulfidic bulk sediments. These highly sulfidic spots likely resulted from spatially discrete aggregates of sulfate-reducing bacteria and organic matter (e.g., fecal pellets) that can be used by these bacteria (K. Londry, personal communication). The presence of spatially discrete organic material and sulfate-reducing bacteria has been reported in recently deposited or highly permeable sediments (29, 30) as well as in Cretaceous rocks (31). The two-dimensional heterogeneity in sulfide distribution in sediment interstitial waters not only suggests the heterogeneous distribution of sulfate-reducing bacteria and the 796

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capability of a variety of aquatic organisms for avoiding and detoxifying sulfide, but also reveals the two-dimensional complexity of biogeochemistry of sulfur species and sulfidebinding elements (e.g., As, Cd, Cu, Hg, Pb, Sb, Zn). Since these elements form strong complexes with sulfide, a similar two-dimensional heterogeneity can be expected in their concentration and speciation in sediment interstitial waters. The roles of bioturbation and bioirrigation in controlling sulfur and metal speciation and bioavailability (e.g., refs 27 and 28) warrant further investigations. Furthermore, the appropriateness of the current practice of ecological risk assessment based on the bulk sediment or porewater measurements needs to be revisited (32, 33), as it ignores and destroys such natural heterogeneities.

Acknowledgments Financial support for this study was provided by the Natural Science and Engineering Research Council (NSERC) of Canada, the Metals in the Environment Research Network (MITE-RN), the Collaborative Mercury Research Network (COMERN), and the University of Manitoba Research Grant Program (URGP). A. Dyck and the Delta Marsh Field Station (University of Manitoba) assisted in the field study. We thank A. Tessier, H. Zhang, L. Sigg, and three anonymous reviewers for their very insightful comments that have improved the paper greatly.

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Received for review August 30, 2002. Revised manuscript received December 5, 2002. Accepted December 13, 2002. ES026109J

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