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Organic-matter Retention and Macroinvertebrate Utilization of Seasonally Inundated Bryophytes in a Mid-order Piedmont River
James Wood, Meryom Pattillo, and Mary Freeman

Southeastern Naturalist, Volume 15, Issue 3 (2016): 403–414

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Southeastern Naturalist 403 J. Wood, M. Pattillo, and M. Freeman 22001166 SOUTHEASTERN NATURALIST 1V5o(3l.) :1450,3 N–4o1. 43 Organic-matter Retention and Macroinvertebrate Utilization of Seasonally Inundated Bryophytes in a Mid-order Piedmont River James Wood1,*, Meryom Pattillo1, and Mary Freeman2 Abstract - There is increased understanding of the role of bryophytes in supporting invertebrate biomass and for their influence on nutrient cycling and carbon balance in aquatic systems, but the structural and functional role of bryophytes growing in seasonally inundated habitats is substantially less studied. We conducted a study on the Middle Oconee River, near Athens, GA, to assess invertebrate abundance and organic-matter retention in seasonally inundated patches of the liverwort Porella pinnata, a species that tends to be submerged only when water levels in rivers are substantially above base flow. Aquatic invertebrate utilization of these seasonally inundated habitats has rarely been investigated. Macroinvertebrate biomass, insect density, and organic-matter content were significantly greater in patches of P. pinnata than on adjacent bare rock. Bryophyte biomass explained additional variation in organic matter, insect biomass, and density. The most abundant insects in P. pinnata patches were Dipterans and Plecopterans. Our results suggest an important structural role of seasonally inundated bryophyte habitats in riverine ecosystems. Introduction Macrophytes (aquatic vascular plants, bryophytes, and large algae) play important roles in lotic ecosystems. Macrophytes influence the abundance of aquatic invertebrates by providing protection from predators and increasing resource availability (Glime 1994, Grubaugh and Wallace 1995, Lodge 1991, Nelson and Scott 1962, Suren 1992). However, macrophytes are more often represented as components of the floodplain rather than structural components within the channel. Stream macrophytes influence stream metabolism directly via photosynthesis and respiration, and these plants sequester and cycle nutrients from the water column, trap organic material, and provide habitat for epiphytic algae (Arscott et al. 1998, McWilliam-Hughes et al. 2009, Mulholland et al. 2000). The roles of bryophytes in streams are best summarized by the Stream Bryophyte Group (1999) and Glime (2015). Bryophytes have been shown to support higher abundances and biomass of aquatic invertebrates than periphyton-covered rocks (Heino and Korsu 2008, Parker and Huryn 2006). For example Lee and Hershey (2000) report higher abundances of Natarsia (Chironomidae, chironomids), Ephemerella (mayflies) and Brachycentrus and Rhyacophila (caddisflies) in bryophytes compared to bryophyte-free areas in a long-term study of Alaska’s Kuparuk River. High macroinvertebrate-biomass and abundance in bryophytes is likely 1University of Georgia River Basin Center, Athens, GA 30602. 2 USGS Patuxent Wildlife Research Center, Athens, GA 30602. *Corresponding author - Manuscript Editor:Nathan Dorn Southeastern Naturalist J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 404 due to several factors. Bryophytes are often chemically defended, resulting in low herbivory pressure and low rates of incidental ingestion of invertebrates by herbivores (Parker et al. 2007, Suren and Winterbourn 1992). These non-vascular plant taxa are hypothesized to protect invertebrates from predators such as fish and large predatory invertebrates, and to facilitate niche partitioning of submerged habitats (Niesiołowski 1980). Bryophytes can also retain organic matter, which can increase the resources available to invertebrates (Harvey et al. 1997, Suren 1991). Most studies of riverine bryophytes have been conducted in far-northern latitudes (Englund et al. 1997, Slavik et al. 2004) or southern latitudes (Suren 1996), while mid-latitudes, e.g., the piedmont region of North America, have received comparatively little attention. Piedmont rivers in the southeastern US are often sand-bottomed, interspersed with rock outcroppings, shoals, and stable woody material where bryophytes often proliferate. Most piedmont rivers experience seasonal fluctuations in discharge and regularly reach bank-full conditions. One common bryophyte found on rocks and wood in seasonally inundated habitats in eastern North America is the liverwort Porella pinnata L. (Breil 1977). Bryophyte studies have generally focused on mosses, and few studies have investigated the relationship between liverworts and invertebrates. Furthermore, few studies have measured aquatic invertebrate use of bryophytes in seasonally inundated habitats, including roots and rock faces that are within the channel but above base-flow conditions. Whereas seasonal inundation of the floodplain enhances resources and habitat for stream biota (Junk et al. 1989), increased discharge within the river channel temporarily inundates bryophytes that could be subsequently inhabited by aquatic organisms. We predicted that P. pinnata would provide substantially better invertebrate habitat than bare rock and would function as a trap for organic matter under high-flow conditions. To test this prediction, we quantified invertebrate biomass and abundance, and the mass of organic matter in bryophytes growing in seasonally inundated habitats in the Middle Oconee River, a mid-order river located in Athens, GA. Site Description The aim of this study was to assess invertebrate utilization and organic-matter retention by bryophytes, especially the liverwort Porella pinnata in the Middle Oconee River near Athens, GA (Fig. 1). The Middle Oconee River is a 6th-order piedmont river in the Altamaha River drainage, GA, with a late-winter median flow between 14 cms and 17 cms (~500–600 cfs). The river channel alternates between slow-flowing, sand-bottomed pools and faster-flowing bedrock shoals with cobbled riffles. Like most piedmont rivers in the southeastern US, past agricultural practices and mill dams have left the channel incised, with steep banks and large amounts of sediment deposited on the historical floodplain (Jackson et al. 2005). Deciduous trees, including Acer rubrum L. (Red Maple), Platanus occidentalis L. (Sycamore), and Ligustrum sinense Lour. (Chinese Privet), are the dominant riparian vegetation. Rock outcrops and submerged woody debris are common, especially at bends in the rivers. Porella pinnata grows abundantly on rock outcroppings and tree roots that are seasonally submerged along the river channel (Fig. 2). The primary study Southeastern Naturalist 405 J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 Figure 1. Map showing the Oconee River basin in Athens–Clarke County, GA. The study reach on the Middle Oconee River (bold line) flows southeast past the city of Athens (black star) before its confluence with the North Oconee River to form the Oconee River. Figure 2. Photograph of rock outcrop in the Middle Oconee River, Athens, GA. showing the epilithic liverwort Porella pinnata extending above and below the water line. Bryophytes cover a substantial area within the river channel, often growing attached to tree roots and rocks in seasonally inundated locations. Southeastern Naturalist J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 406 species was Porella pinnata (hereafter Porella), but we also opportunistically sampled adjacent patches of the moss Sematophyllum demissum (Wils.) Mitt. Methods We collected all samples along an 11-km section of the Middle Oconee River between Ben Burton Park and the State Botanical Garden of Georgia on 8 and 9 March 2015. We took samples from submerged rock faces and boulders within the channel approximately every 1.5 km, measured water velocity and turbidity, and collected water samples at each sampling location. We took water samples back to the lab and determined conductivity and pH. We obtained macroinvertebrate samples by placing a short section of PVC pipe (area = 58.056 cm2) attached to a polyethylene collection bag securely against the rock face between 1 and 20 cm below the water surface. Keeping pressure on the sampler to prevent loss of sample material, we forced a modified plastic putty knife between the sampler and the rock, and scraped the sample material into the bag. We paired each Porella sample with a control sample taken at similar depth from a submerged bare rock-face (i.e., rock faces without bryophyte or other macrophytes), usually within 1 m of the bryophyte-sample point and stored both samples in the lab on ice. We collected a total of 10 Porella–control pairs; 3 samples of Sematophyllum, 1 of Porella, and 1 additional unpaired control-sample from bare rock were also collected opportunistically from the same section of river. We collected these additional samples due to uncertainty about our ability to collect adequate replicates during our sampling trip because of difficulties in accessing submerged rock faces during high-flow conditions, to increase our sample size, and to increase the diversity of bryophytes sampled in the study. In the laboratory, we vigorously washed bryophyte samples under running water, and collected macroinvertebrates and sediments from the rinse water in a 60-μm-mesh sieve. We examined the washed bryophyte material under a microscope to detect any remaining invertebrates, then dried the samples at 60 °C for 48 h, and weighed them to obtain dry mass. Invertebrates and organic matter (OM) (DM) were subsequently stored in polyethylene bags, preserved in 70% ethanol, and dyed with Rose Bengal. We sorted invertebrates from OM and bryophytes under a dissecting microscope and identified insect taxa to family and non-insect taxa to Class (Oligochaeta) or Subclass (Copepoda). We used family-level length–mass relationships to estimate dry mass from published regressions for insects; Class and Subclass estimates were used for non-insect taxa (Benke et al. 1999). We expressed invertebrate biomass as total DM divided by the sampling area. We dried all OM and sediments at 60 °C for 48 h, weighed, ashed in a muffle furnace at 500 ºC for 4 h, and reweighed samples. We calculated ash-free dry mass (AFDM, mg per cm2 of sampling area) as DM minus ash mass. To test the effect of Porella on invertebrates, we conducted paired-sample analyses for invertebrate biomass, total insect biomass, Diptera biomass, and biomass of the combined orders of Ephemeroptera, Plecoptera, and Trichoptera (EPT). We separately tabulated insect (total, EPT, Diptera), Oligochaeta, and Copepoda densities for Sematophyllum, Porella, and control (rock) habitats. We Southeastern Naturalist 407 J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 employed Wilcoxon signed ranks to compare Porella and control samples because our data had a non-normal distribution and we used linear regression to compare the relationships between bryophyte biomass (Porella only, and Sematophyllum and Porella combined), invertebrate biomass, insect densities, and organic matter AFDM. To improve normality of regression residuals, we transformed values by taking either natural logarithms or square roots of predictor and response variables. In order to put our results into a larger context, we obtained insect density (abundance per unit area) and biomass estimates for the lotic macrophyte Podostemum ceratophyllum Michx. (Hornleaf Riverweed) from Grubaugh and Wallace (1995:Table 6) by subtracting estimates for non-insect taxa from total macroinvertebrate values. Grubaugh and Wallace (1995) expressed insect biomass as AFDM g m-2; thus, we converted our insect DM estimates to AFDM g m-2 using published values for % ash for insect families or orders (Diptera) (Benke et al. 1999). To facilitate comparisons, we converted our estimated insect abundances to individuals m-2. Results Discharge for the Middle Oconee River during sampling was approximately 14.5 cms (510 cfs) as measured by US Geological Survey gage 02217500. Physiochemical measurements (mean ± 1 SE) were: turbidity (NTU) = 15 ± 1.0, pH = 6.8 ± 0.01, and specific conductance at 25 ºC = 89.8 ± 0.35 μS cm-1. Water temperature was 7 ºC at the Georgia State Botanical Gardens on 8 March 2015. Water velocity at sample locations ranged from 0.01 to 0.37 m s-1 (mean = 0.11 ± 0.02 m s-1). Our samples included 7 insect families in 4 orders: Diptera (Chironomidae, Ceratopogonidae), Ephemeroptera (Heptageniidae, Baetidae, Ephemerellidae), Plecoptera (Perlodidae), and Trichoptera (Hydropsychidae); we identified Class Oligochaeta and Subclass Copepoda (Table 1). Invertebrate biomass and density, expressed as the mean ± 1 SE, varied substantially between Porella and control samples. Table 1. Mean invertebrate biomass (DM g m-2 ± 1 SE) and density (individuals m-2 ± 1 SE) from all sampled surfaces in the Middle Oconee River in March 2015. Sampled habitat included Porella pinnata (n = 11), Sematophyllum demissum (n = 3), and adjacent rock faces without bryophyte coverage (Control, n = 11). Habitat/ variable Diptera Ephemeroptera Plecoptera Trichoptera Oligochaeta Copepoda Control Biomass 0.02 ± 0.01 0.03 ± 0.03 less than 0.01 ± less than 0.01 0.00 ± 0.00 0.01 ± 0.01 less than 0.01 ± less than 0.01 Density 548.1 47.0 31.1 0.0 438.4 234.9 ± 106.0 ± 24.6 ± 21.0 ± 0.0 ± 164.8 ± 186.2 Porella Biomass 0.10 ± 0.03 0.16 ± 0.07 0.69 ± 0.21 0.08 ± 0.06 less than 0.01 ± 0.01 less than 0.01 ± less than 0.01 Density 3084.8 234.9 438.4 78.3 735.6 78.3 ± 719.4 ± 84.6 ± 94.0 ± 48.5 ± 324.4 ± 78.3 Sematophyllum Biomass 0.39 ± 0.28 less than 0.01 ± less than 0.01 0.21 ± 0.16 0.99 ± 0.98 0.01 ± less than 0.01 less than 0.01 ± less than 0.01 Density 8267.9 114.8 459.3 344.5 3789.4 57.4 ± 3287.8 ± 57.4 ± 151.9 ± 263.1 ± 1337.9 ± 30.0 Southeastern Naturalist J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 408 In comparisons of paired samples, total invertebrate biomass was 14.6 times larger in Porella samples than in the controls (Porella = 0.088 mg cm-2 ± 0.020, control = 0.006 mg cm-2 ± 0.003, V = 53, P < 0.01; Fig. 3a). Total insect biomass was almost 18 times greater in Porella compared with control samples (Porella = 0.088 mg cm-2 ± 0.020, control = 0.005 mg cm-2 ± 0.003, V = 53, P < 0.01; Fig. 3b), and the biomass of EPT taxa was nearly 16 times greater in Porella than in controls (Porella = 0.080 mg cm-2 ± 0.020, control = 0.003 mg cm-2 ± 0.003, V = 44, P = 0.01, Fig. 3c). The average dipteran biomass was 4 times greater in Porella, but the difference between Porella and the controls was not statistically significant (P ≤ Figure 3. Box and whisker plots of invertebrate biomass (dry mass, mg cm-2) and organic matter (AFDM mg cm-2) for submerged bare-rock faces (control) and mats of Porella pinnata in the Middle Oconee River, Athens, GA. Wilcoxon signed ranks analysis was conducted on 10 paired Porella and control samples, (a) invertebrate biomass (P < 0.01), (b) insect biomass (P < 0.01), (c) biomass of insect orders Ephemeroptera, Plecoptera and Trichoptera (EPT) (P = 0.01), and (d) organic matter mass (P < 0.01). Upper edge of box = 3rd quartile, dark line within the box = median, lower edge of box = 1st quartile, and the whiskers indicate the range * the interquartile range. Circles represent outliers . Southeastern Naturalist 409 J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 0.05) due to considerable variation between sites (Porella = 0.008 mg cm2 ± 0.002, control = 0.002 mg cm-2 ± 0.001, V = 45, P = 0.08; data not shown). Insect families were unevenly distributed between samples from different habitats. Diptera was the most abundant order in all sampled habitats (Table 1) occurring in 91% of the control samples and 100% of the bryophyte samples. We detected Plecopterans in 72% of all Porella samples, and Trichopterans in 36% of all the bryophyte samples and 2 out of the 3 Sematophyllum samples. We recorded Oligochaetes in 85% of all bryophyte samples (Porella and Sematophyllum combined) and 73% of all control samples. In paired samples, average insect density was 5 times higher in Porella than in controls (Porella = 0.324 cm-2 ± 0.055, control = 0.064 cm-2 ± 0.064, V = 55, P less than 0.01; Fig. 4a); EPT taxa density was nearly 10 times higher in Porella than in control samples (Porella =0.065 cm-2 ± 0.012, control = 0.007 cm-2 ± 0.004, V = 36, Figure 4. Box and whisker plots of insect density (individuals per cm-2) for submerged bare-rock faces (control) and Porella pinnata mats in the Middle Oconee River, Athens, GA. Wilcoxon signed ranks analysis was conducted on 10 paired Porella and control samples, (a) total insect (P < 0.01), (b) EPT (P < 0.01), and (c) Diptera (P < 0.01). Upper edge of box = 3rd quartile,dark line within the box = median, lower edge of box = 1st quartile, and the whiskers indicate the range * the interquartile range. The circle represents an outlier. Southeastern Naturalist J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 410 P = 0.01; Fig. 4b), and dipterans were over 4.5 times more abundant in Porella samples compared with control samples (Porella = 0.258 cm-2 ± 0.057, control = 0.057 cm-2 ± 0.011, V= 45, P < 0.01; Fig. 4c). Trapped organic matter averaged 1.82 mg AFDM cm-2 ± 0.413 in the paired Porella samples; the control samples held significantly less material (0.550 mg AFDM cm-2 ± 0.180, V = 45, P < 0.01; Fig. 3d). Density data from the few samples of Sematophyllum we collected suggest it might support even higher densities of insects (Table 1) and organic matter (average for 3 unpaired samples = 5.03 mg AFDM cm-2) than Porella. Total invertebrate biomass and insect biomass significantly increased with increasing bryophyte biomass (invertebrate biomass: Porella and Sematophyllum combined, square-root transformed, adjusted R2 = 0.50, F1, 12 =12.61, P < 0.01; Porella only, adjusted R2 = 0.35, F1, 9 = 6.38, P = 0.03 (data not shown); insect biomass: Porella and Sematophyllum combined, square-root transformed, adjusted R2 = 0.48, F1, 12 = 13.12, P < 0.01; Porella only, untransformed, adjusted R2 = 0.35, F1, 9 = 6.84, P = 0.03; Fig. 5a). Insect density significantly increased with total bryophyte Figure 5. Simple linear regression relationships between bryophyte biomass and (a) total insect biomass (DM mg cm-2), (b) insect density (individuals cm-2), and (c) organic matter (AFDM mg cm-2). Porella pinnata is represented as circles, Sematophyllum demissum is shown as triangles. All lines represent significant relationships (P < 0.05). Correlations using all bryophyte samples are shown by a solid line (n = 14), and Porella only correlations are shown with a dashed line (n = 11). Untransformed data are shown in all graphs; statistical analyses were performed on transformed data. Southeastern Naturalist 411 J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 biomass, but the trend was not significant with Porella only (Porella and Sematophyllum combined, untransformed, adjusted R2 = 0.50, F1, 12 = 13.54, P < 0.01; Porella only, natural log-transformed, adjusted R2 = 0.16, F1, 9 = 2.91, P = 0.12; Fig. 5b). Retained organic matter significantly increased with bryophyte biomass (Porella and Sematophyllum combined, natural log-transformed, adjusted R2 = 0.37, F1, 12 = 8.55, P = 0.01; Porella only, natural log-transformed, adjusted R2 = 0.45, F1, 9 = 8.55, P = 0.01; Fig. 5c). We found an average of 1.1 AFDM g m-2 of insect biomass and 4983 individuals m-2 in our seasonally inundated bryophyte habitats (Porella and Sematophyllum samples combined). In comparison, using summaries reported by Grubaugh and Wallace (1995), we calculated insect biomass and abundance in Hornleaf Riverweed patches in the Middle Oconee River as 13.8 AFDM g m-2 and 41,800 individuals m-2. Discussion We found significantly higher invertebrate biomass, density, and organic matter in Porella than on adjacent bare rock faces. Bryophyte biomass was significantly correlated with invertebrate biomass, invertebrate density, and organic-matter mass. The results from this study support the conclusion that inundated bryophytes provide important aquatic invertebrate habitat in the Middle Oconee River. Previous studies have shown that insect biomass can be up to 8 times larger in bryophyte- patches compared to bryophyte-free patches (Brusven et al. 1990, Parker and Huryn 2006, Stream Bryophyte Group 1999); thus, our finding of almost 15 times more invertebrate biomass and nearly 18 times more insect biomass in Porella than in control samples does not appear unreasonable. Although our sampling methods may not have captured large and highly mobile taxa such as crayfish and odonates, our results indicate that the presence of bryophytes, even in small amounts (less than 5 mg cm-2), may have a substantial positive impact on the invertebrate community. Invertebrates may congregate in bryophytes near the water’s surface to prepare for emergence, access OM and epiphytic algae (Stream Bryophyte Group 1999), or gain protection from fish and other large predators (Glime 2015). Bryophytes may also provide a place for drifting insects to anchor and extract themselves from high flow-velocity areas. Additionally, bryophyte coverage that extends above and below the water line may provide a relatively homogenous, resource-rich habitat for invertebrates as the water level fluctuates. Although the organic matter that bryophytes retain is presumably of higher palatability and nutritional quality compared to the bryophytes themselves, some direct consumption of bryophytes has also been reported (Suren and Winterbourn 1991) and may be another reason that invertebrates colonize bryophytes. Our data support the conclusion that bryophytes in seasonally inundated habitats play a structural role in this piedmont river, providing habitat with increased resource availability compared to adjacent rock faces, and support the findings of other studies that indicate that bryophytes play important roles in riverine ecosystems (Parker et al. 2007; Suren 1991, 1992; Suren and Winterbourn 1991). Southeastern Naturalist J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 412 Interestingly, our estimates suggest that seasonally inundated bryophytes can support insect abundance and biomass on the order of 10% of that supported by Podostemum ceratophyllum, a perennially submerged benthic macrophyte noted for supporting extremely high rates of secondary production (Grubaugh and Wallace 1995, Hutchens et al. 2004, Nelson and Scott 1962). Using estimates presented in Grubaugh and Wallace (1995) from the Middle Oconee River at approximately the same time of year, we found that seasonally submerged bryophytes (Porella and Sematophyllum) contained about 8% of the insect biomass and 12% of the abundance reported in Podostemum. Whereas Podostemum has been recognized as ecologically important in eastern rivers due to its prevalence and role in structuring the benthic community (Hutchens et al. 2004), our data indicate that bryophytes can enhance insect abundance in seasonally inundated habitats, a habitat type where Podostemum does not occur. How much bryophytes actually influence habitat and resource availability in mid-order rivers depends on the extent of bryophyte coverage at different water levels and possibly on what type of bryophyte species are present. Our results suggest that Sematophyllum might contain as much or more macroinvertebrate biomass and organic matter as Porella, but our small sample-size reduces our ability to make conclusive statements. Sematophyllum often grows lower within the river channel and may be submerged for a longer period, thereby giving more time for colonization by invertebrates and accumulation of organic material. Thus, shifts in the composition of the bryophyte community, such as those encountered with stream-flow regulation (Englund et al. 1997) may shift resource availability to aquatic invertebrates and subsequently the invertebrate communities’ utilization of these habitats. Our data add to the small but growing body of evidence that bryophytes play important roles in stream and riverine ecosystems, and provides new support for the notion that seasonally inundated bryophytes are utilized by aquatic invertebrates. The loss of bryophytes during stream-channel restoration has been cited as a reason for only minimal changes in macroinvertebrate biodiversity after channel-restoration efforts (Louhi et al. 2011, Muotka and Laasonen 2002) and the use of bryophytes in stream-channel restoration efforts may be valuable. Bryophyte communities within the river channels are stratified, in part on the frequency and duration of inundation (Englund et al. 1997, Kimmerer and Allen 1982); thus, bryophytes could be considered in the development of management plans to ensure that appropriate ecological flow-requirements are established (Poff et al. 2010). Although periodic connections between the river channel and floodplain can increase resource availability to stream biota (Junk et al. 1989), the role of seasonally submerged bryophytes in providing additional resources to stream biota still remains understudied. Our data suggest that flows that provide seasonal habitat-connectivity to bryophytes growing within the channel can reasonably be considered when assessing the impacts of flow regulation. Furthermore, management actions that prevent the seasonal inundation of bryophytes could reduce macroinvertebrate habitat, thereby reducing resources available to higher trophic levels. Southeastern Naturalist 413 J. Wood, M. Pattillo, and M. Freeman 2016 Vol. 15, No. 3 Acknowledgments We thank Alan Covich and 2 anonymous reviewers for their helpful suggestions on the manuscript. We are grateful to Jon Skaggs for his help with field and laboratory work, Phillip Bumpers for assistance with R, and Kelly Peterson for her help with figures. We also appreciate The American Bryological and Lichenological Society-Anderson and Crum Award, and the Society of Freshwater Science-Boesel-Sanderson Fund, for their support of this research. Additional support was provided by the UGA Outdoor Recreation Center. 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