Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 101 2018 NORTHEASTERN NATURALIST 25(1):101–116 Changes in Freshwater Mussel Communities Linked to Legacy Pollution in the Lower Delaware River Carrie J. Blakeslee1,*, Erik L. Silldorff 2,3, and Heather S. Galbraith1 Abstract - Freshwater mussels are among the most-imperiled organisms worldwide, although they provide a variety of important functions in the streams and rivers they inhabit. Among Atlantic-slope rivers, the Delaware River is known for its freshwater mussel diversity and biomass; however, limited data are available on the freshwater mussel fauna in the lower, non-tidal portion of the river. This section of the Delaware River has experienced decades of water-quality degradation from both industrial and municipal sources, primarily as a function of one of its major tributaries, the Lehigh River. We completed semi-quantitative snorkel surveys in 53.5 of the 121 km of the river to document mussel community composition and the continued impacts from pollution (particularly inputs from the Lehigh River) on mussel fauna. We detected changes in mussel catch per unit effort (CPUE) below the confluence of the Lehigh River, with significant declines in the dominant species Elliptio complanata (Eastern Elliptio) as we moved downstream from its confluence—CPUE dropped from 179 to 21 mussels/h. Patterns in mussel distribution around the Lehigh confluence matched chemical signatures of Lehigh water input. Specifically, Eastern Elliptio CPUE declined more quickly moving downstream on the Pennsylvania bank, where Lehigh River water input was more concentrated compared to the New Jersey bank. A definitive causal link remains to be established between the Lehigh River and the dramatic shifts in mussel community composition, warranting continued investigation as it relates to mussel conservation and restoration in the basin. Introduction North American lakes and rivers are home to the greatest diversity of freshwater mussels (Bivalvia, Unionoida) in the world (Lydeard et al. 2004). These organisms are beneficial to the systems they inhabit through their role in filtration, biodeposition, and nutrient cycling (Spooner and Vaughn 2006, Vaughn 2010). Mussels, however, are a highly imperiled group of freshwater organisms, exhibiting declines worldwide due to a variety of anthropogenic factors including habitat alteration, invasive species, and contaminants (Lydeard et al. 2004, Strayer et al. 2004). Due to both their ecological importance and their current status, mussel conservation has become a priority for many state and federal agencies. Successful management of these species relies on a comprehensive understanding of the factors that drive mussel distribution and abundance, data that are lacking for many populations. 1US Geological Survey, Leetown Science Center, Northern Appalachian Research Laboratory, Wellsboro, PA 16901. 2Delaware River Basin Commission, West Trenton, NJ 08628. 3Current address - Delaware Riverkeeper Network, Bristol, PA 19007. *Corresponding author - cblakeslee@usgs.gov. Manuscript Editor: Thomas Maier Northeastern Naturalist 102 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 The Delaware River is a stronghold for northern Atlantic-slope mussel diversity and biomass while at the same time functioning as the water supply source for nearly 15 million people (~5% of the US population; watershed depicted in Fig. 1a). Lellis and others (Cole 2007) conducted comprehensive semi-quantitative freshwater surveys of the upper (Upper Delaware Scenic and Recreational River) and middle (Delaware Watergap National Recreation Area) Delaware River in the early 2000s, which yielded a continuous assessment of over 230 km of the upper river (highlighted sections A and B in Fig. 1b). Those workers found a total 9 species of freshwater mussels, including previously undocumented populations of the federally endangered Alasmidonta heterodon (Lea) (Dwarf Wedgemussel). Surveys in the tidal Delaware River (Kreeger et al. 2011) expanded the species list from 9 to 12, and encompassed nearly the entire complement of northern Atlantic-slope mussel diversity. The largest tributary to the non-tidal Delaware River is the Lehigh River (~3520-km2 drainage area) located in the lower section of the Delaware (Fig. 1c). Figure 1. (a) Map of Delaware River basin. (b) Regions within the basin include (A) the Upper Delaware Scenic and Recreational River, (B) the Delaware Water Gap National Recreation Area, and (C) the Lower Delaware Scenic and Wild River. (c) Semi-quantitative snorkel surveys were conducted in the lower Delaware River basin during the summer of 2013 in 12 random and 4 targeted river reaches (dark circles; size of circle proportional to length of surveyed reach). AL refers to the reach immediately above the Lehigh River confluence; BL refers to reaches immediately below the Lehigh River confluence . Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 103 The Lehigh was historically severely polluted from acid mine-drainage and domestic and industrial waste, containing 12 Superfund sites within its watershed and impacting resident aquatic species (PAFBC 2007 and references therein; Pollison and Craighead 1968). Today, the Lehigh confluence continues to serve as an important change-point for water quality, with elevated nutrients and other contaminants, such as metals and sulfates, found in the Delaware mainstem below this site (Arcadis 2007, Diamond et al. 1997, DRBC 2010). Although modest degradation of the benthic macroinvertebrate community has been documented within the Delaware River below the Lehigh confluence relative to reference sites (Silldorff and Limbeck 2009), the extent of the aquatic community changes within the Delaware River at and below the Lehigh confluence have not been extensively evaluated, particularly for biota such as freshwater mussels. In comparison with the upper, middle, and tidal sections of the Delaware River, much less is known about the mussel fauna of the lower Delaware River above the head-of-tide; yet, some of the earliest documentation of Dwarf Wedgemussel occurred in the lower Delaware River Basin, suggesting the potential for undocumented populations (USFWS 1993). Equally important, however, is whether the freshwater mussel fauna reflects the current and historic changes in water quality. In an effort to evaluate the potential ecological effects of Lehigh River water quality on Delaware River mussel fauna, we conducted semi-quantitative catchper- unit-effort (CPUE) snorkel surveys in the lower Delaware during the summer of 2013 to determine if there were shifts in mussel species composition around the historically polluted Lehigh confluence. Methods Semi-quantitative snorkel surveys We conducted point–transect surveys at 12 randomly selected reaches to spread the survey effort throughout the entire length of the lower Delaware River (Fig. 1). Distance along the Delaware River is described using the river-mileage system, as explained on the Delaware River Basin Commission website (www.nj.gov/drbc/ basin/river/); where river miles (RM) start from the Atlantic Ocean and increase with increasing upstream distance. We randomly selected initial starting points near the upper limit of the survey area (RM 208) and near the end of the Lehigh River targeted surveys (Fig. 1). We designated evenly spaced reaches, beginning about 9 km below the 2 randomized starting points, with 4 reaches delineated above the Lehigh confluence and 8 reaches below the confluence. We supplemented surveys of the randomly selected reaches with 3 targeted-survey reaches that continuously covered the area around the Lehigh River confluence with the Delaware, beginning 4 km above and extending 6 km below the confluence. In an effort to minimize systematic survey bias, we sampled the 12 randomized survey reaches by alternating between surveys below and above the Lehigh confluence on successive survey days in a haphazard manner so that position downstream along the river was not associated with survey date. Only the 3 Lehigh confluence reaches were surveyed Northeastern Naturalist 104 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 successively in an upstream-to-downstream direction to provide continuous coverage of the river in this section. We added 1 additional reach (Reach 4x) post hoc during the 2013 season to more carefully evaluate unusual patterns in mussel abundance near the Martins Creek tributary confluence (RM 190.5). Surveys for Reach 4 began precisely at the confluence of a slate-bearing tributary, which altered the substrate within the mainstem Delaware River near the creek’s confluence; thus, we added an additional and contiguous reach (Reach 4x) upstream from the Martins Creek confluence so that we could evaluate whether the low counts in Reach 4 might be linked to the Martins Creek confluence. We divided each reach into successive segments ~200 m in length. A team of 5–7 people conducted semi-quantitative timed surveys. Teams split into 2 groups—1 on each side of the river (or island, as relevant) following methods described in Galbraith et al. (2016). We used snorkel gear to conduct visual searches of the stream bottom at a maximum depth of about 3 m, but at depths typically less than 1.5 m. In general, the teams snorkeled segments in transects from the upstream to the downstream border, but also investigated unique habitats, channels, and eddies. The teams searched each segment for ~15 minutes, although total survey time and segment length varied depending on the number of surveyors and complexity of habitat. Survey crew members rotated positions during the survey to eliminate surveyor bias. Individual mussels were removed from the sediment when necessary, identified to species level, and returned to their original location. Surveyors counted only mussels visible at the sediment surface; no excavation of the substrate was completed. Difficult-to-identify species were brought to the surface for group consensus. We logged individual species counts at the end of each segment. Distances surveyed differed among reaches because of logistics, variable sampling conditions, and weather constraints; the shortest and longest reaches surveyed were 2.3 km and 4.3 km, respectively (Table 1; Fig. 1). When islands were present, we determined the best course of surveying based on logistics, potential for finding mussel populations, and field-worker safety. Delineation of Lehigh mixing zone The blending of the waters of 2 rivers at their confluence occurs gradually through a mixing zone, the characteristics of which depend on the specific features of the rivers and the river channel (Bridge 2009). No quantitative assessment of this mixing zone has been modeled for the Delaware and Lehigh confluence (RM 183.7). There are differences in water quality between these 2 rivers (DRBC 2010), and we expected a gradient in water quality in the mixing zone below their confluence; thus, we assessed this mixing zone at 1 time-point in August 2013. The specific conductance of the Lehigh River typically approaches or exceeds 200% of the specific conductance in the Delaware immediately upstream from their confluence (DRBC 2010). As a conservative water-quality parameter largely unaffected by internal biological activity, specific conductance serves as an excellent inert tracer of the differential mixing of the 2 water sources. Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 105 Table 1. Total individual mussel count and catch per unit effort (CPUE) of 7 species identified in sample reaches in semi-quantitative snorkel surveys of the lower Delaware River basin during the summer of 2013. Reach = sample reaches surveyed (4x, extra reach surveyed; AL, close to but above the Lehigh River confluence; BL1 and BL2, shortly below the Lehigh River confluence). General details for each reach include average river mile (RM) and time and distance surveyed. Random = reaches randomly selected for survey, targeted = additional reaches selected for survey. Random or RM Miles Hours Yellow Alewife Eastern Tidewater Triangle Reach targeted average surveyed surveyed Eastern Elliptio Lampmussel Floater Floater Mucket Creeper Floater 1 Random 206.5 2.3 24.00 3366 (140.3) 2 (0.1) 22 (0.9) 1 (0.0) 0 (0.0) 1 (0.0) 0 (0.0) 2 Random 201.1 1.8 17.03 6864 (403.0) 1 (0.1) 47 (2.8) 79 (4.6) 0 (0.0) 0 (0.0) 5 (0.3) 3 Random 194.9 2.2 21.42 2800 (130.7) 3 (0.1) 52 (2.4) 2 (0.1) 0 (0.0) 0 (0.0) 0 (0.0) 4x Targeted 191.3 1.6 15.50 798 (51.5) 4 (0.3) 6 (0.4) 3 (0.2) 0 (0.0) 3 (0.2) 0 (0.0) 4 Random 189.4 2.3 28.25 1171 (41.5) 6 (0.2) 18 (0.6) 7 (0.2) 0 (0.0) 0 (0.0) 0 (0.0) Lehigh River confluence 183.7 --------------------------------------------------------------------------------------------------------------------------------------------- AL Targeted 184.9 2.7 26.37 3826 (145.1) 1 (0.0) 53 (2.0) 28 (1.1) 0 (0.0) 0 (0.0) 0 (0.0) BL1 Targeted 182.5 2.2 21.25 945 (44.5) 1 (0.0) 7 (0.3) 5 (0.2) 0 (0.0) 0 (0.0) 0 (0.0) BL2 Targeted 180.7 1.4 13.58 205 (15.1) 1 (0.1) 4 (0.3) 0 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 5 Random 177.2 2.5 27.75 821 (29.6) 10 (0.4) 6 (0.2) 0 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 6 Random 171.5 2.0 31.65 405 (12.8) 34 (1.1) 5 (0.2) 2 (0.1) 0 (0.0) 0 (0.0) 0 (0.0) 7 Random 165.9 2.2 27.00 365 (13.5) 49 (1.8) 9 (0.3) 1 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 8 Random 160.4 1.8 11.90 259 (21.8) 23 (1.9) 0 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 9 Random 154.4 2.5 27.17 1507 (55.5) 84 (3.1) 86 (3.2) 30 (1.1) 0 (0.0) 1 (0.0) 0 (0.0) 10 Random 149.3 1.2 11.25 184 (16.4) 27 (2.4) 5 (0.4) 9 (0.8) 0 (0.0) 0 (0.0) 0 (0.0) 11 Random 143.0 2.3 27.50 356 (12.9) 456 (16.6) 36 (1.3) 9 (0.3) 0 (0.0) 0 (0.0) 0 (0.0) 12 Random 137.4 2.4 27.00 243 (9.0) 139 (5.1) 14 (0.5) 14 (0.5) 6 (0.2) 0 (0.0) 0 (0.0) Total 33.3 358.62 24,115 (67.2) 841 (2.4) 370 (1.0) 190 (0.5) 6 (0.0) 5 (0.0) 5 (0.0) Northeastern Naturalist 106 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 Using a YSI 30 meter (YSI Inc., Yellow Springs, OH), we made surface measurements of specific conductance across the Delaware River channel at 20 stations above and below its confluence with the Lehigh River. We took measurements at each transect near both the Pennsylvania and New Jersey banks, and then at 3 positions roughly evenly spaced across the river channel, perpendicular to flow; we recorded the positions using a handheld GPS. During these measurements, water levels and discharge for both the Lehigh (Gage #01454700) and Delaware rivers (Gage #01446500) were slightly elevated, but were along the receding limb of the preceding storm’s hydrograph (peak flows occurred on 11 August 2013, whereas specific conductance measurements were taken on 15 August 2013). Data analysis We standardized mussel counts (summed across all surveyors and banks) to a CPUE ( i.e., the number of mussels found per hour of search time) for each reach. For parametric statistical analysis, we ln+1-transformed all data prior to analyses to meet assumptions of normality and equal variance. We conducted all analyses in SPSS Statistics v. 20.0.0.2 (IBM® Corporation, Pittsburgh, PA). We ran a one-way ANOVA with reach as the unit of replication to compare mussel CPUE for individual species above and below the Lehigh River using data from only the 12 randomly selected reaches (i.e., excluding all targeted reaches above and below the Lehigh because these were not randomly selected). We made this assessment for the 3 most-abundant species found in the survey, each of which contributed at least 1% of total mussel abundance—Elliptio complanata (Lightfoot) (Eastern Elliptio), Lampsilis cariosa (Say) (Yellow Lampmussel), and Anodonta implicata (Say) (Alewife Floater). We also conducted breakpoint analysis for the most abundant species, Eastern Elliptio, to identify upstream-to-downstream patterns in mussel CPUE along each of the PA and NJ banks using the segmented package in R (R Core Team 2016). Results Semi-quantitative snorkel surveys We counted a total of 25,532 mussels during nearly 360 survey-hours across 53.5 km of the lower Delaware River resulting in a total CPUE of 71 mussels per hour (Table 1). We documented 7 freshwater mussel species. The most abundant species was Eastern Elliptio (94.4% of the total), followed by Yellow Lampmussel (3.3%), and Alewife Floater (1.4%). Pyganodon cataracta (Say) (Eastern Floater), Strophitus undulatus (Say) (Creeper), Alasmidonta undulata (Say) (Triangle Floater), and Leptodea ochracea (Say) (Tidewater Mucket) comprised the remaining 0.8% of mussels found. Statistical tests on the distribution of the 3 most common mussels revealed marked shifts in the absolute and relative abundance of mussels in the lower Delaware River. The dominant Delaware River mussel, Eastern Elliptio, declined significantly (nearly 8-fold) below the Lehigh River (relative to upstream) (F(1,10) = 21.9, P = 0.001; Fig. 2), with a mean CPUE of 179 and 21 mussels/h in randomly selected segments above and below the confluence with the Lehigh Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 107 River, respectively. We observed a similar pattern when including non-randomized reaches: CPUE above the Lehigh averaged 152 individuals/h and below the Lehigh averaged 23 individuals/h (Table 1). Of all of the reaches below the Lehigh, Reach Figure 2. Mean (±SE) catch per unit effort (CPUE; number of mussels/h) for each of 12 randomized semiquantitative sampling reaches located in the lower Delaware River basin collected during the summer of 2013. Data are presented for the 3 most common species detected in our surveys: Elliptio complanata (Eastern Elliptio), Lampsilis cariosa (Yellow Lampmussel), and Anodonta implicata (Alewife Floater). Location of the Lehigh River confluence is indicated with a dashed line. Note differences in y-axis scales. Northeastern Naturalist 108 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 Figure 3. Eastern Elliptio catch per unit effort (CPUE; number of mussels/h) for surveys completed in the Upper Delaware Scenic and Recreational River (UPDE) in 2000 and in the Delaware Water Gap National Recreation Area (DEWA) in 2001 (St. John White et al., in press), as well as in the lower Delaware River above (Lower above) and below (Lower below) the confluence of the Lehigh River in 2013 (this study). 9 had a slightly higher CPUE for Eastern Elliptio, but the remaining reaches below the Lehigh were consistently low. Counts of Eastern Elliptio were not uniformly high above the Lehigh River (Table 1, Fig. 2). In particular CPUE was substantially lower than in other random reaches above the Lehigh in Reach 4 and in the additional targeted Reach 4x. The subsequent downstream reach (above Lehigh or AL) showed increased abundances comparable to the numbers observed in Reaches 1 and 3 (Table 1). Eastern Elliptio CPUE above the Lehigh was similar to that observed in the Upper Delaware Scenic and Recreational River (UPDE) and the Delaware Water Gap National Recreation Area (DEWA) in surveys conducted in the early 2000s (Fig. 3) (St. John White et al., in press). The results for Yellow Lampmussel were the reverse of patterns seen for Eastern Elliptio. We found significant increases in CPUE for the randomly selected segments below the Lehigh River confluence compared to those randomly surveyed above (F(1,10) = 8.9, P = 0.014; Table 1; Fig. 2). Yellow Lampmussel was rarely found above the Lehigh (Table 1), but became increasingly common below the Lehigh confluence, peaking at 456 individuals (CPUE of 16.6 in Reach 11) 16 km above the head-of-tide (Trenton, NJ) and 65 km below the Lehigh confluence (Table 1, Fig. 2). Yellow Lampmussel’s CPUE was over an order of magnitude smaller than that of Eastern Elliptio (Table 1, Fig. 2): Yellow Lampmussel’s maximum CPUE (16.6 mussels/h) was over 20 times lower than the maximum CPUE for Eastern Elliptio (403 mussels/h). We detected no statistical difference for Alewife Floater CPUE above versus below the Lehigh (F(1,10) = 2.8, P = 0.126; Fig. 2). Nonetheless, Alewife Floater CPUE patterns were similar to those found for Eastern Elliptio, where CPUE ranged from 0.4 mussels/h to 2.8 mussels/h upstream of the Lehigh, but remained consistently low (0.2–0.3 mussel/h) immediately below the Lehigh confluence (Table 1, Fig. 2). Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 109 We noted some increase in Alewife Floater CPUE in the most downstream survey reaches: moderate numbers of Alewife Floaters were found in the 4 survey reaches furthest downstream with CPUEs consistent with those seen above the Lehigh (Table 1, Fig. 2). Our analyses did not discern any influence of the Lehigh River for the other 4 species detected in our surveys—Eastern Floater, Tidewater Mucket, Creeper, and Triangle Floater—due to their low numbers (Table 1). Despite historical documentation of Dwarf Wedgemussel in this part of the basin, we found none during our sampling. Delineation of Lehigh mixing zone The ratio of Delaware:Lehigh discharge (2.3:1) was lower than the drainagearea ratio (3.5:1), indicating the Lehigh flow was proportionally higher than that of the Delaware on the sample date, and thus may have mixed more quickly (i.e., further upstream) at the time of our measurements than it does under median conditions (i.e., our measurements of the mixing zone may over-estimate how quickly Table 2. Specific conductance (μS/cm) used as a tracer for Lehigh River confluence mixing into the lower Delaware River basin. Measurements were from surface water collected at 5 points across the Delaware River channel for 20 stations above and below the Lehi gh confluence on 15 August 2013. Specific conductance (μS/cm) River mile Mid-way between Mid-way between (RM) PA near shore PA and center Center NJ and center NJ near shore 183.95 233A 190 136 136 143 183.78 174 168 149 136 139 183.70 (Lehigh River confluence) ---------------------------------------------------------------------------- 183.49 263 237 166 152 138 182.85 251 230 170 137 141 182.53 253 202 153 139 178 182.07 240 230 202 154 141 181.30 237 209 184 162 155 180.36 228 207 188 164 159 179.21 199 189 186 171 166 177.84 198 189 183 176 178 176.95 185 185 183 183 183 175.82 185 184 183 183 182 174.90 190 185 183 183 184 174.00 188 184 184 201 219B 172.64 184 185 185 200 211 171.56 183 183 184 185 207 170.73 185 186 186 191 205 169.98 184 185 187 195 202 169.49 187 187 188 194 203 168.28 186 187 188 193 199 ALocalized influence of Bushkill Creek tributary with high specific conductance near the PA bank immediately upstream of the Lehigh confluence. BLocalized influence of the Musconetcong River tributary with high specific conductance near the NJ bank immediately upstream of RM174. Northeastern Naturalist 110 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 the rivers mix). Specific conductance of the Delaware River above the Lehigh confluence varied across the channel (Table 2), presumably due to the influence of a small tributary (Bushkill Creek; 207 km2 drainage area), but the primary body of water had specific conductance between 136 μS/cm and 149 μS/cm (Table 2). Specific conductance readings for the Lehigh River at Glendon, PA (Gage #01454700; ~3 km upstream within the Lehigh from its confluence with the Delaware River) ranged between 250 μS/cm and 274 μS/cm. At the first transect below the Lehigh confluence (RM 183.5), the Delaware River showed a gradient of specific conductance, measuring 263 μS/cm near the PA shore and 138 μS/cm near the NJ shore, indicating nearly pure Lehigh and Delaware River waters, respectively, along the shorelines at the beginning of the mixing zone (Table 2, Fig 4). The rivers mixed gradually for the next 5 km downstream to RM 180.4, with specific conductance measurements of 228 μS/cm and 159 μS/cm near the PA and NJ banks, respectively, demonstrating a persistent separation of the 2 water bodies below their confluence. Mixing accelerated in the next 5 km of river such that specific conductance readings across the channel were essentially uniform by RM 177, indicating complete mixing between the Lehigh River and the Delaware River about 10.5 km below their confluence (Table 2, Fig. 4). Correspondingly, Eastern Elliptio CPUE was higher along the NJ bank of the river than the PA bank of the river in the area corresponding to the mixing zone (Fig. 4). Breakpoint analysis identified a break in Eastern Elliptio CPUE located at RM 190 (SE = 3.1) on the PA shore of the river, whereas on the NJ shore, the break in CPUE was further downstream at RM 171 (SE = 7.1; Fig. 5). For both banks, the slope of the relationship between river mile and CPUE was not different than Figure 4. Eastern Elliptio catch per unit effort (CPUE; number of mussels/h) and specific water conductance for a portion of the lower Delaware River at the Lehigh River mixing zone (mixing zone designated with shaded region; see text for river-mile explanation). CPUE data (triangles) are plotted separately for the PA shore (left) and the NJ shore (right) collected during the summer of 2013. Specific conductance, collected on 15 August 2013, for the PA shore (black circles, solid line) and the NJ shore (open circles, dotted line) are presented in both panels. The Lehigh River confluence is denoted with a vertical dashed line. Note difference in y-axis scales. Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 111 0 below the breakpoint. Above the breakpoint, however, slopes were significantly different from 0, with a slope of -20.7 (SE = 5.3) on the PA side of the river and -3.4 (SE = 1.1) on the NJ side of the river. Discussion Snorkel surveys of the lower Delaware River confirmed the presence of 7 native freshwater mussel species with distinct changes in community composition below the confluence of its largest non-tidal tributary, the Lehigh River. CPUE of the dominant Eastern Elliptio mussel above the Lehigh River confluence was consistent with values observed in the upper and middle portions of the Delaware River. However, Eastern Elliptio CPUE declined 80% to 90% below the Lehigh confluence relative to sites upstream of the confluence. The shoreline-specific patterns in mussel CPUE suggest a strong link to the Lehigh River confluence; we observed a Figure 5. Breakpoint analysis of Eastern Elliptio catch per unit effort (CPUE; number of mussels/h) in the lower Delaware River along the PA (top panel) and NJ (bottom panel) shores collected during the summer of 2013. Dashed lines indicate the confluence of the Lehigh River with the mai n s tem lower Delaware; dotted lines indicate the break point with 95% confidence intervals (gray shading) identified by statistical analysis. Northeastern Naturalist 112 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 reduction in Eastern Elliptio CPUE in the vicinity of the Lehigh confluence on the PA bank (CPUE = 0–10 mussels/h within 5 km below Lehigh confluence), where mixing-zone measurements demonstrated nearly pure Lehigh River water. By contrast, CPUE for Eastern Elliptio declined more gradually and further downstream of the Lehigh confluence along the NJ bank (CPUE dropped consistently to less than 50 mussels/h about 10 km downstream of the Lehigh), a pattern that mirrored the gradual mixing of the Lehigh River over this distance. Although the reduction of Eastern Elliptio along the 80.5 km immediately downstream of the Lehigh confluence combined with the fine-scale differences in CPUE between the PA and NJ banks within the mixing zone are consistent with a direct effect from some aspect of the Lehigh River (e.g., water quality, sediment quality), another explanation for these patterns could be differences in habitat above and below the Lehigh confluence. Nonetheless, there are minimal differences in basin characteristics between the watershed upstream of the Lehigh confluence and that upstream of the head of tide at Trenton, NJ, where our surveys ended, that would translate to differences in instream habitat (USGS 2012): basin slope, and basin elevation varied by less than 11%; percent forest cover, urban development, and percent impervious area all differ by less than 7% between the 2 regions. There are differences in percent of the basin that was glaciated (96% above the Lehigh compared to 74% above Trenton), however, given that Eastern Elliptio has been documented at fairly high densities further downstream in the Delaware mainstem in the areas surrounding Philadelphia (Kreeger et al. 2011), these factors seem unlikely to explain the patterns we observed in relation to the Lehigh River confluence. Habitat data collected in riffles for the DRBC biomonitoring program (E.L. Silldorff and R.L. Limbeck, Delaware River Basin Commission, West Trenton, NJ, unpubl. data) show no shifts in substrate composition below the Lehigh confluence. Although riffles are just one of the many habitats represented in our study area, they provide an initial indication that no distinct shift in habitat occurs at the Lehigh, although a more comprehensive evaluation across all habitat types is needed. The CPUE patterns of Alewife Floater matched that of Eastern Elliptio; however, CPUE for Alewife Floater appeared to rebound in the lower survey reaches to values seen above the Lehigh confluence. Two possible interpretations are readily apparent for the lower abundance of Eastern Elliptio and the possible recovery of Alewife Floater below the Lehigh confluence. First, Eastern Elliptio may be more sensitive to the stressors associated with the Lehigh confluence. The ubiquitous nature of Eastern Elliptio throughout the Northeast (Haag 2012) suggests that this species is hardy and tolerant of a wide range of environmental conditions. For example, abundant Eastern Elliptio populations have been reported in the tidal Delaware River near Philadelphia (Kreeger et al. 2011), where heavy pollution has only recently been ameliorated (Albert 1998). Therefore, higher sensitivity of Eastern Elliptio to common water-quality or sediment-quality stressors (likely present at the Lehigh confluence) would be surprising, especially relative to the rarer Alewife Floater. However, while adult Eastern Elliptio appears to be tolerant, its complex lifecycle should be considered, because juvenile, larval, and host-fish stages may be more sensitive to contaminants (juvenile L. siliquoidea [Barnes] Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 113 [Fatmucket]: Jorge et al. 2013; juvenile Eastern Elliptio: Strayer and Malcolm 2012; host fish: Machut et al. 2007). Alternatively, life-history differences between these 2 species may have allowed for more rapid recovery of Alewife Floater below the Lehigh confluence. Pollution in the Lehigh River was most severe prior to the Clean Water Act beginning in the 1970s, with measureable and significant effects of the Lehigh documented within the Delaware River (Pollison and Craighead 1968). With upgraded wastewater treatment and a shift away from heavy industry, Lehigh River water-quality has improved over the past 40 years, with attendant recovery in the Delaware River (DRBC 2010, Kauffman 2010). Some recovery of freshwater mussel communities has been observed following water-quality improvements in other native streams (Sietman et al. 2001). Faster growth rates generally observed in Anodontid species (Haag 2012) may have allowed for more rapid recolonization of Alewife Floater following water-quality improvements below the Lehigh. In contrast, Eastern Elliptio’s slower growth and longer life-span may yield a slower recovery, manifesting itself in suppressed population sizes below the Lehigh confluence. The increased CPUE for Eastern Elliptio in Reach 9 might suggest the beginning of such a recovery for this species; however, the continued low CPUE in the subsequent reaches suggest otherwise. In addition to the increases in the Eastern Elliptio at Reach 9, the Yellow Lampmussel also increased, and we found the largest numbers of both the Alewife Floater and Eastern Floater at this reach. Future research targeting this area may reveal particular flow or water-quality characteristics that support these populations and determine if these factors could aid in recovering other areas within the Delaware River below its confluence with the Lehigh River . In contrast to the patterns for Eastern Elliptio and Alewife Floater, relative abundance of Yellow Lampmussel showed substantial and steady increases moving downstream through the lower Delaware River (Table 1; Fig. 2). Similar distributional patterns for Yellow Lampmussel have been found in the lower Saint John River, where it was most abundant below head-of-tide in the mainstem and lower tributaries (Sabine et al. 2004). Longitudinal changes in the physical and chemical conditions along river corridors constitute a central paradigm in stream ecology and have been demonstrated in freshwater mussel communities (Allan 1995, Haag 2012). Yellow Lampmussel counts above the Lehigh River were extremely sparse, and numbers did not begin to increase until well below the Lehigh mixing zone; thus, we could discern no effects of the Lehigh from our data. Increasing numbers with greater distance from the Lehigh confluence could imply a certain tolerance to historic Lehigh River stressors, increased suitable habitat for this species, and/or reduced competition with Eastern Elliptio. All of these possibilities warrant further investigation. Beyond the effects of the Lehigh River on common mussels, this survey also extended the known range of Tidewater Mucket beyond its reported tidal reaches of the Delaware River (Crumb 1977). This species is considered to be imperiled or critically imperiled throughout most of its range (NatureServe 2015). We did not detect 2 species which have been reported previously in the lower Delaware Northeastern Naturalist 114 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 Vol. 25, No. 1 River—Dwarf Wedgemussel and Lampsilis radiata (Gmelin) (Eastern Lampmussel) (R. Spear, PA Department of Environmental Protection, Harrisburg, PA, pers. comm.; USFWS 1993). The absence of these species from our survey does not mean they, or others, do not persist in the lower Delaware. The rapid nature of our survey methodology may not have been well-suited for the detection of rare species, although we found the rarer Tidewater Mucket in this study and methods similar to ours were used to identify previously undocumented populations of Dwarf Wedgemussel in the upper Delaware basin (Galbraith et al. 2016). Additionally, the survey was not continuous over the entire lower Delaware River because surveys were primarily conducted where individuals could snorkel (i.e., we excluded deep and fast areas) and did not quantify differences in instream habitat (e.g., flow, substrate). Nonetheless, these types of surveys are useful for developing species lists, documenting anomalous patterns in species distribution (such as below the Lehigh River confluence), and guiding future quantitative surveys and population estimates (Strayer and Smith 2003). Freshwater mussels provide a variety of important functions to the ecosystems they inhabit, many of which are biomass dependent and vary according to species (Spooner and Vaughn 2008, Vaughn 2010). Declines in overall mussel biomass and shifts in community composition below the Lehigh River confluence could have consequences for key ecological processes including nutrient cycling, biofiltration, and habitat for macroinvertebrates and biofilms (Spooner and Vaughn 2006, Spooner et al. 2013, Vaughn 2010). Increases in Yellow Lampmussel abundance below the Lehigh confluence were not comparable to the loss of Eastern Elliptio abundance or biomass. Therefore, Yellow Lampmussel functionally compensating for Eastern Elliptio is unlikely. In fact, biomass compensation among freshwater mussel species, in general, has been questioned, given their life-history characteristics (long-lived, slow growth, late maturation; Spooner and Vaughn 2008). A detailed examination of the ecological ramifications of shifts in mussel communities below the Lehigh River is needed, as well as a mechanistic understanding behind these shifts. For example, residual effects of lead and zinc mining have been linked to reduced freshwater mussel distribution and abundance in other systems (Angelo et al. 2007). Candidate stressors, including heavy metals and interstitial ammonia, should be investigated, while quantitative surveys for juvenile Eastern Elliptio and Alewife Floater may determine if populations are recolonizing. Such surveys, in conjunction with collection of water-quality data, may provide insight into waterquality thresholds necessary for conservation and restoration of native mussel populations in the lower Delaware River. Acknowledgments We thank Julie Bell, Jeffrey Cole, William Lellis, Robert Limbeck, Greg Mayer, Jessica Newbern, Amanda Schwartz, Micah Swann, and Eric Wentz for assistance with study design and field surveys. Funding for this research was provided by the National Park Service and the US Environmental Protection Agency to the Delaware River Basin Commission. The research was conducted under permits PA #467 (Scientific Collector Permit and Chapter 75.4 Special Permit for the Collection of Threatened and Endangered Species) and NJ Northeastern Naturalist Vol. 25, No. 1 C.J. Blakeslee, E.L. Silldorff, and H.S. Galbraith 2018 115 #SC2013153 (Scientific Collecting Permit). The USGS Fisheries Program contributed to this work. 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