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Instream Habitat Associations Among Three Federally Threatened and a Common Freshwater Mussel Species in a Southeastern Watershed
Bijay B. Niraula, Jonathan M. Miller, J. Murray Hyde, and Paul M. Stewart

Southeastern Naturalist, Volume 14, Issue 2 (2015): 221–230

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Southeastern Naturalist 221 B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 22001155 SOUTHEASTERN NATURALIST 1V4o(2l.) :1242,1 N–2o3. 02 Instream Habitat Associations Among Three Federally Threatened and a Common Freshwater Mussel Species in a Southeastern Watershed Bijay B. Niraula1, Jonathan M. Miller1, J. Murray Hyde1, and Paul M. Stewart1,* Abstract - The current study examined instream habitat associations among 1 common and 3 federally threatened mussel species at 3 sites in a southeastern watershed (Choctawhatchee River). We documented instream habitat structures (e.g., leaf pack, root mat, root wad, woody debris, and log) in the immediate vicinity (~5 cm) of 94 Elliptio pullata (Gulf Spike), 263 Pleurobema strodeanum (Fuzzy Pigtoe), 117 Fusconaia burkei (Tapered Pigtoe), and 25 Hamiota australis (Southern Sandshell). Results of chi-square tests showed significant differences in use of instream habitat types among species (P < 0.05). At all 3 sites, the threatened species were almost exclusively associated with either woody debris or logs. When we found the mussels with leaf pack, we also recorded a majority of the individuals with at least one additional instream habitat (usually woody debris). In contrast, we observed the majority of the common species, E. pullata, either exclusively with leaf pack or, at 1 site, equally with leaf pack and woody debris. Our results suggested that instream habitats associated with threatened mussel species were relatively different from those of the common species. Introduction Freshwater mussels are one of the most imperiled groups of organisms in North America. For example, one-third of the mussel species found in Alabama are listed as federally threatened or endangered (USFWS 2014). Habitat-related factors have been identified as one of the major threats to these assemblages (Wilcove and Master 2005). Instream habitat structures influence many assemblages, including fish (Langford et al. 2012), crayfish (Jowett et al. 2008, Stewart et al. 2010), mussels (Golladay et al. 2004, Harriger et al. 2009), and aquatic macroinvertebrates (Stewart et al. 2012). However, studies examining whether or not instream habitat use varies by species within an assemblage are rare. The importance of some instream habitat structures to freshwater mussels is well established. For instance, Golladay et al. (2004) found that woody debris formed habitat for mussels and that mussel abundance in the Flint River Basin of southwestern Georgia was low where woody debris was sparse. Log-jams in Muddy Creek (Erie National Wildlife Refuge, PA) were thought to create favorable habitat and flow conditions for freshwater mussels (Mohler et al. 2006). Harriger et al. (2009) found a significant positive correlation between mussel abundance and woody debris in a first-order stream in the Upper Peninsula of Michigan, and 1Department of Biological and Environmental Sciences, Troy University, Troy, AL 36082. *Corresponding author - mstewart@troy.edu. Manuscript Editor: R. Eugene Turner Southeastern Naturalist B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 222 suggested that woody debris helped to stabilize habitat for mussel species. The results of these studies and others support the general conclusion that there is a strong relationship between instream habitats and mussel-species richness and abundance. We know of no study, however, that has examined if different mussel species tend to associate with different types of instream habitat. A large number of studies examining microhabitat preferences (hydrological and substrate variables) of unionids have been inconclusive. Haag (2012) suggested that sampling in smaller streams with dense mussel populations and low habitat diversity may be a reason for failure to identify microhabitat segregation. Habitat data for individuals of a species is needed to maximize the sample size, especially for rare species, and to provide insight into the association between a mussel species and a particular instream habitat type (McRae et al. 2004). Pleurobema strodeanum Wright (Fuzzy Pigtoe), Fusconaia burkei Walker (Tapered Pigtoe), and Hamiota australis Simpson (Southern Sandshell) were recently listed as threatened species in the Choctawhatchee River watershed under the Endangered Species Act (US Fish and Wildlife Service 2012). Elliptio pullata Lea (Gulf Spike) is one of the most common species in the Choctawhatchee River watershed (Pilarczyk et al. 2006). The purpose of this study was to determine if differences in instream-habitat associations existed among these 4 mussel species. Thus the Ho is that all mussel species under study share similar instream habitat associations; the Ha is that mussel species possess different instream habitat associations. We performed the study in small streams and recorded instream habitat data for individual mussels by species in order to detect differences in instream habitat use among the species. Field-site Description The Choctawhatchee River watershed is located in southeastern Alabama and the Florida panhandle and drains a total of 12,297 km2 (Mettee et al. 1996, USDA 1995). The first site, Blue Springs (BS, 31°39'49.6''N, 85°30'18.8''W) is located on the West Fork of the Choctawhatchee River (a 4th-order stream). It was 100 m in length with an average width of 11.8 m, and began immediately upstream of the State Highway 10 bridge crossing, Barbour County, AL. The second and third sites were located on Eightmile Creek (a 2nd-order stream), Walton County, FL. Eightmile Creek is a tributary of Flat Creek ,which drains into the Pea River. The second site (8M1; 30°58'50.3''N, 86°10'45.5''W) started immediately up stream of a bridge crossing County Road 181 and was 68 m in length with an average width of 6.3 m. The third site (8M2; 30°58'46.7''N, 86°10'45.4''W) began about 73 m upstream of the end of 8M1 and had a length of 40 m and an average width of 6.3 m. All sites were characterized by a mature Quercus (oak)-Pinus (pine) overstory and a shrubdominated understory, and were comprised predominantly of sand bottoms. These sites were known to possess recently listed federally threatened and endangered mussel species (Pilarczyk et al. 2006). Southeastern Naturalist 223 B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 Methods Instream habitat-data collection We conducted 8 surveys using visual and tactile methods at each site from June to October 2012, before the official federal listing date of these species. During the first sampling event for each site, we marked the site’s extent and searched the wadeable portion of the reach for 5 person hours, and we performed subsequent samplings in the same reach. When we identified individuals as a species of interest, we inserted a species-specific color-coded flag into the sediment at the mussel’s exact location. After sampling the stream reach, we noted all instream habitats within 5 cm of each flag. The instream habitat structures recorded included leaf pack (LP), woody debris (WD), root mat (RM), root wad (RW), log (L), or none present. Elliptio pullata is one of the most common species in the Choctawhatchee watershed and was abundant at all of our sites (Pilarczyk et al. 2006); therefore, we selected E. pullata for comparison purposes, and recorded habitat data for about 30 individuals at each site. To prevent bias due to removal, relocation, and disturbance, we tagged mussels with Hallprint glue-on shellfish tags (Hallprint, South Australia, Australia) and did not record instream habitat data for tagged individuals that were recaptured during future sampling events; i.e., we recorded the habitat for each individual mussel only once throughout the study. Statistical analyses We determined the percentages of mussels found near each instream habitat and employed chi-square tests (α = 0.05) to determine if there were differences in percent frequency within each instream habitat type associated with the mussel species at each site. We employed bar graphs to illustrate differences in instream habitat use among mussel species. Because we frequently found more than one instream habitat near individual mussels, the sum of percentages of all individuals found near all instream habitats for a particular species exceeded 100% in many cases for the statistical analysis. Results We collected instream habitat data for a total of 94 E. pullata, 263 P. strodeanum, 117 F. burkei, and 25 H. australis during the current study. Blue Springs (BS) We found no leaf pack near any species at site BS. However, due to the presence of good riparian vegetation and canopy cover, the absence of leaf pack at the site was probably an anomaly. We found all other instream habitat structures near E. pullata and encountered root mat habitat most often; frequency = 45% (Fig. 1). We did not find P. strodeanum with root mats or with root wads; woody debris was the most frequently encountered instream habitat structure for that species, with a frequency of 87.5% (Fig. 1). Root mats and root wads were not present at any location where we collected H. australis individuals, whereas a log was found with 63.6% and woody debris with 33.3% of H. australis (Fig. 1). Southeastern Naturalist B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 224 Eightmile Creek site 1 (8M1) We did not encounter logs near any individual mussel found at 8M1. Elliptio pullata was not found with root mats or root wads, and we found the highest proportion of this common species with leaf pack (Fig. 2). Leaf pack alone was found near 24.5% of P. strodeanum, 16.7% of F. burkei, and 72.7% of E. pullata (Table 1). When we found P. strodeanum and F. burkei with leaf pack, woody debris was also present the majority of the time. Eightmile Creek site 2 (8M2) Leaf pack and woody debris were the most frequently encountered instream habitats near E. pullata, but we found woody debris about 3 times more frequently than leaf pack near P. strodeanum and H. australis, and twice as often near F. burkei Figure 1. Percentage of each mussel species associated with each instream habitat variable examined at Blue Springs (BS). LP = leaf pack, RM = root mat, RW = root wad, WD = woody debris, L = log, and None = sand only or no instream habitat structure present; * denotes significant difference in percentage of instream habitat structure association among mussel species. Figure 2. Percentage of each mussel species associated with each instream habitat variable examined at Eightmile Creek site 1 (8M1). LP = leaf pack, RM = root mat, RW = root wad, WD = woody debris, L = log, and None = sand only or no instream habitat structure present; * denotes significant difference in percentage of instream habitat structure association among mussel species. Southeastern Naturalist 225 B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 (Table 1). We never found E. pullata with logs or H. australis with root mats (Fig. 3). We documented P. strodeanum and F. burkei in all types of instream habitat structures, but both species were most frequently associated with woody debris (Fig. 3). Table 1. Percent of individuals of each species found near all combinations of instream habitat structures at Blue Springs (BS), Eightmile Creek site 1 (8M1), and Eightmile Creek site 2 (8M2) in the Choctawhatchee River watershed, observed from June to October 2012. All combinations of instream habitat structures encountered at all sites are presented separately. Thus, categories are mutually exclusive. LP = leaf pack, RM = root mat, RW = root wad, WD = woody debris, L = log, and None = sand only or no instream habitat structure present. E. p. = Elliptio pullata, P. s. = Pleurobema strodeanum, H. a. = Hamiota australis, F. b. = Fusconaia burkei. Blue Springs Eightmile Creek site 1 Eightmile Creek site 2 Instream habitat E. p. P. s. H. a. E. p. P. s. F. b. E. p. P. s. F. b. H. a. LP 0.0 0.0 0.0 72.7 24.5 16.7 3.1 3.3 7.1 0.0 RM 5.0 0.0 0.0 0.0 0.0 3.3 12.5 4.9 3.5 0.0 RW 15.0 0.0 0.0 0.0 3.8 3.3 12.5 1.6 0.0 9.1 WD 10.0 62.5 9.1 0.0 24.5 23.3 15.6 25.7 25.9 45.5 L 5.0 8.3 27.3 0.0 0.0 0.0 0.0 8.7 3.5 9.1 LP, RM 0.0 0.0 0.0 0.0 1.9 0.0 3.1 1.1 3.5 0.0 LP, RW 0.0 0.0 0.0 0.0 1.9 0.0 6.3 0.5 0.0 9.1 LP, RM, WD 0.0 0.0 0.0 0.0 3.8 0.0 6.3 1.1 2.4 0.0 LP, WD 0.0 0.0 0.0 3.0 17.0 10.0 28.1 10.4 17.6 9.1 LP, WD, L 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 2.4 0.0 RM, RW 0.0 0.0 0.0 0.0 0.0 3.3 0.0 0.0 1.2 0.0 RM, WD 20.0 0.0 0.0 0.0 1.9 6.7 3.1 4.4 3.5 0.0 RM, L 25.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.4 0.0 RW, WD 0.0 0.0 0.0 0.0 1.9 0.0 0.0 0.5 2.4 0.0 RW,WD, L 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 WD, L 0.0 25.0 36.4 0.0 0.0 0.0 0.0 9.8 9.4 0.0 NONE 20.0 4.2 27.3 24.2 18.9 33.3 9.4 26.8 15.3 18.2 Figure 3. Percentage of each mussel species associated with each instream habitat variable examined at Eightmile Creek site 2 (8M2). LP = leaf pack, RM = root mat, RW = root wad, WD = woody debris, L = log, and None = sand only or no instream habitat structure present; * denotes significant difference in percentage of instream habitat structure association among mussel species. Southeastern Naturalist B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 226 Instream habitat associations We found a majority of the individuals near at least one type of instream habitat structure, but no instream structure was present at 33.3% of the locations where we recorded F. burkei at site 8M1. We encountered instream habitat structures most frequently near P. strodeanum at site BS; 4.2% of the individuals were not near instream habitat structures (Table 1). The proportions of E. pullata, P. strodeanum, and H. australis found near woody debris (P < 0.001) and logs (P < 0.001) were significantly different from each other at site BS. Pleurobema strodeanum had the highest percent of individuals found with woody debris, H. australis had the highest percent of individuals found with logs, and E. pullata had the lowest percent of individuals near both woody debris and logs at site BS (Table 1). Also, the proportions of these 3 mussel species found without associated instream habitat structures were significantly different (P < 0.001; Table 1). The proportions of E. pullata, P. strodeanum, and F. burkei found with leaf pack (P < 0.001) and woody debris (P < 0.001) at site 8M1 were significantly different (Fig. 2). Among these 3 mussel species, E. pullata had the highest percentage of individuals near leaf pack alone, while P. strodeanum had the highest percentage of individuals near woody debris plus leaf pack (Table 1). The proportions of E. pullata, P. strodeanum, F. burkei, and H. australis at site 8M2 that we found in leaf pack (P < 0.001), root mat (P = 0.032), and root wad (P < 0.001), and associated with no instream habitat structures (P = 0.021) were significantly different (Fig. 3). Elliptio pullata had the highest percentage of individuals near leaf pack, root mat, and root wad among the 4 species. Pleurobema strodeanum and F. burkei had the lowest percentage of individuals found near root wad among all instream habitat structures. Among the 3 sites, site 8M2 had the highest use of instream habitat structures by all 4 mussel species, and all 4 mussel species at site 8M2 were found most frequently near woody debris. Discussion The data clearly support rejection of the null hypothesis. Differences in mussel species’ associations with different instream habitats were evident across sites. The importance of woody debris/habitat and impacts of sedimentation are also noted. Difference among sites We found significant differences in instream habitat associations among mussel species. The patterns of mussel species associations with instream habitat structures were somewhat different among sites, probably due to the differences in availability of such structures; however, we did not quantify availability in our study. Sites 8M1 and BS started immediately upstream of a bridge; site 8M1 was a straight reach with little woody debris and logs, and sites BS and 8M2 had greater sinuosity and more woody debris and logs compared to 8M1 (P.M. Stewart, pers. observ.). Site 8M2 was the least disturbed site in the current study and may be suitable as a reference site to characterize habitat associations among the species found. Southeastern Naturalist 227 B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 Instream habitat associations Benke et al. (1985) quantified the availability of woody debris in the Satilla River, a coastal-plain river in southeastern Georgia. They found that woody debris, which only occupied 4% of available habitat surface, contained 60% of the total invertebrate biomass per linear meter of the stream. Similar to Benke et al. (1985), we found that 67–96% of the mussels collected had at least one instream habitat structure near them. The results of our study support the hypothesis that the threatened mussel species in these coastal-plain streams mostly use habitats near woody debris and logs. Our results are different than those of Stewart et al. (2010) who studied macrohabitat partitioning among 3 aquatic crayfish species in 4 southeastern coastal plain watersheds, including the Choctawhatchee River watershed. Contrary to our results, Stewart et al. (2010) did not find any differences in instream habitat use among the crayfish species despite using similar habitat variables and recording habitat data for each individual. They attributed the similarity in habitat use among the crayfish species to limited habitat and substrate diversity. Our findings of differences in instream habitat use among mussel species in small streams support the existence of species-specific habitat associations in some of these assemblages and suggest that mussels may be more habitat-selective than aquatic crayfish. Pleurobema strodeanum, F. burkei, and H. australis were more likely than E. pullata to be found near woody debris and less likely to be near leaf pack or leaf pack plus woody debris. Elliptio pullata showed a greater association with leaf pack than the other species; they were associated with root mat if no leaf pack was present. This observation may indicate that E. pullata prefers habitats associated with the stream bank, while the threatened mussel species prefer habitats towards the main channel. It is possible that the distribution of these instream habitat structures is driven by hydrological variables (e.g., depth and current velocity). Wallace and Benke (1984) suggested that most wood in rivers is located towards the main channel rather than the banks. Gagnon et al. (2006) suggested that species occupying habitats intermediate between slackwater and riffles prefer instream habitats near coarse woody debris. It is also possible that these species are distributed based on both distance from the bank and instream habitat, which can be determined by examining habitat availability and relationship among hydrological and instream habitat variables. Future studies should quantify the availability and distribution of instream habitat features in relation to stream hydrology to determine if the differences in habitat use among mussel species found in the current study were based on hydrological variables. Importance of loss of woody habitats/debris Woody debris and logs are the major stable substrate in sandy, coastal-plain streams (Wallace and Benke 1984). Woody debris may also be the primary retentive structure against shifting sands in coastal-plain streams (Leff and McArthur 1988). Shields and Smith (1992) examined hydraulic roughness in 2 types of sand-bottomed reaches in the South Fork Obion River in western Tennessee where woody debris was either still abundant or had recently been removed following selective Southeastern Naturalist B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 228 removal guidelines. They found that the hydraulic roughness in the uncleared reaches was up to 400% higher than in cleared reaches during low flows, suggesting that woody habitat significantly enhances the stability of sand-bed streams. Threatened species in our study likely use habitats near woody debris because they provide greater substrate stability. Impacts of sedimentation Sedimentation has led to severe embeddedness (extent of fine sediments surrounding coarse sediments on streambeds) throughout the Choctawhatchee River watershed (Morris et al. 2003) and may cause clogging of filtration apparatus (Di Maio and Corkum 1995) and shell abrasion in mussels. Sediment input reduces substrate stability and available instream habitat, alters natural substrate composition, and homogenizes general flow characteristics among other stream features (Wood and Armitage 1997). These declines in viable habitat have likely contributed to concurrent reductions in mussel diversity as evidenced by the recent listing of 8 mussel species as either threatened or endangered. These combined effects will continue to impact the local fauna of coastal plain streams until better conservation techniques can be implemented. Conclusions We documented over two-thirds of all individuals at all sites examined herein with at least one instream habitat structure near them and observed several significant differences in instream habitat use among the mussel species. In general, the threatened species showed associations with woody debris and had less tendency to be near leaf pack even with the presence of additional woody debris. Our data-collection technique likely made it possible to detect these differences. Collection of instream-habitat data for mussels on an individual basis is more inclusive of mussel habitats compared to use of transects and quadrats, particularly in small streams with limited habitat diversity. The individual approach also increases sample size, which is often limited in studies of rare species, and is useful to demonstrate the differences in habitat use among the species. Woody debris in sand-bottomed streams is more important for mussels than it is in streams with larger sediment particles (cobbles, pebbles, etc.). Instream habitat structures form suitable microhabitats for mussels by creating favorable flow conditions and increasing habitat stability, which compensate for a low quantity of larger particles in many coastal plain river systems. Sedimentation affects mussel species by reducing available instream habitats and the stability provided by these major retentive features in sand-bed streams. Acknowledgments We thank Evelyn Reatégui-Zirena for field assistance, Kesley Gibson for review of the manuscript, and Sandy Pursifull, Adam Kaeser (US Fish and Wildlife Service, Panama City, FL, Field Office), and Jeff Garner (Alabama Department of Conservation and Natural Resources) for ideas associated with this project. Southeastern Naturalist 229 B.B. Niraula, J.M. Miller, J.M. Hyde, and P.M. Stewart 2015 Vol. 14, No. 2 Literature Cited Benke, A.C., R.L. Henry III, D.M. Gillespie, and R.J. Hunter. 1985. Importance of snag habitat for animal production in southeastern streams. Fisherie s 10:8–13. Di Maio, J., and L.D. Corkum. 1995. Relationship between the spatial distribution of freshwater mussels (Bivalvia: Unionidae) and the hydrological variability of rivers. Canadian Journal of Zoology 73:663–671. Gagnon, P., W. Michener, M. Freeman, and J. Brim Box. 2006. Unionid habitat and assemblage composition in coastal plain tributaries of Flint River (Georgia). 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