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Unionida musslor (stormusslor) är en djurgrupp som kan ha positiv påverkan på ekosystemfunktioner i vattendrag och ökar den biologiska mångfalden. Musslorna absorberar föda genom filtrering av vattenmassor, vilket genererar en bättre vattenkvalitet och ger mat till andra bottendjur. Syftet med denna studie var att undersöka en av våra svenska stormusslor flodpärlmusslan (Margaritifera margaritifera), och om det fanns något samband mellan musselbankar och bottenfauna. Provtagning av bottenfauna skedde i Vasslabäcken, Örebro län. 54 prover togs totalt och 27 av dem togs vid musselbankar medan resterande 27 var kontroll utanför musselbankar. Studien visade ingen signifikant skillnad i den totala bottenfaunans täthet i musselbankar jämfört med kontrollområden utanför musselbankar. Däremot fanns en signifikant högre täthet av nattsländor i kontrollerna än i musselbankarna, medan det inte fanns några skillnader i täthet mellan kontroller och musselbankar för några andra djurordningar. Det fanns även fler ordningar utanför musselbankar än i musselbankar, men antalet individer var så få att inga långtgående slutsatser kan dras. För att vidare undersöka musslors påverkan på flora och fauna kan man undersöka fler år och säsonger under året, samt använda sig av en mer experimentell design. / Unionidea mussels (large mussels) are a group of animals that can have a positive impact on ecosystem functions in watercourses and increase biodiversity. The mussels absorb food by filtering water masses, which generates a better water quality and provides food for other benthic animals. The purpose of this study was to investigate one of our swedish large mussels the freshwater pearl mussel (Margaritifera margaritifera), and whether there was any connection between musselbanks and benthic fauna. Sampling of benthic fauna took place in Vasslabäcken, Örebro region. A total of 54 samples were taken and 27 of them were taken at musselbanks while the remaining 27 were controls outside musselbanks. The study showed no significant difference in the density of the total benthic fauna in musselbanks compared with control areas outside musselbanks. On the other hand, there was a significantly higher density of moths in the controls than in the musselbanks, while there were no differences in density between controls and musselbanks for any other animal orders. There were also more schemes outside musselbanks than in musselbanks, but the number of individuals was so small that no far-reaching conclusions can be drawn. To further investigate the impact of mussels on flora and fauna, you can investigate more years and seasons during the year, and use a more experimental design.

American Electric Power's (AEP) coal-fired Clinch River Plant, a power-generating facility in Carbo, Russell County, Virginia (USA), has impaired Clinch River biota through toxic spills in 1967 and 1970, and effluent copper (Cu) concentrations that were reported to have exceeded water quality criteria from 1985-1989. These impacts have provided impetus for many research projects addressing the absence of bivalves, including federally protected species of native mussels (Unionoidea), from sites influenced by CRP effluent. Modifications in CRP effluent during 1987 and 1993 drastically reduced Cu levels and warranted the present study, which assessed long-term biological recovery in Clinch River biota near the CRP. In 2000-2001, surveys of benthic macroinvertebrate communities and instantaneous measures of effluent toxicity did not foretell significant reductions in survivorship and growth of field-caged Asian clams (Corbicula fluminea) at sites downstream of the CRP. More importantly, these results indicated renewed toxicity in CRP effluent. Additional transplant studies using two enclosure types were conducted to isolate effects attributable to CRP effluent from the potentially confounding effects of substrate variability among study sites. While it was found that mean growth of clams was greatest in the enclosure that minimized substrate variability (p=0.0157), both enclosure types clearly distinguished significant impairment of survivorship and growth at sites downstream of the CRP discharge, and strengthened the association between impairment and CRP effluent. An intensive field investigation was undertaken to determine whether impairment observed in transplant studies extended to resident bivalves. During 2001-2002, densities and age structures of C. fluminea and distributions of mussels suggested that impairment indeed extended to resident bivalves for a distance of 0.5 to 0.6 km downstream of the CRP discharge. Impairment of bivalves was less evident below (1) a fly ash landfill and (2) coal mining activities and low-volume leachate from a bottom ash settling pond. With respect to long-term recovery, modifications in CRP effluent treatment have reduced Cu concentrations from an average of 436 mg/L in 1985-1989 to 13 mg/L in 1991-2002. Subsequently, Cu body burdens of Asian clams (Corbicula fluminea) transplanted within CRP influence have decreased from 442% of levels accumulated at reference sites in 1986, to 163% of these levels in 2002. The reduction in effluent Cu largely explains recovery of most benthic macroinvertebrate community parameters (e.g., richness, diversity) at influenced sites from levels that were typically less than 70% of reference levels, to levels that frequently range from 80 to greater than 100% of reference levels. Nevertheless, bivalves remain impaired downstream of the CRP; survivorship and growth of C. fluminea transplanted to CRP-influenced sites have typically been less than 40 and 20% of reference values, respectively. Furthermore, C. fluminea has seldom been encountered within CRP influence for nearly two decades. Likewise, native mussels remain absent within CRP influence, but recent surveys suggest their downstream distributions are more proximate to the CRP discharge than has been reported previously. A preliminary assessment of factors potentially contributing to toxicity revealed that (1) water reclaimed from settling basins for discharge with CRP effluent significantly impaired fecundity of ceriodaphnids at concentrations of 50%, (2) LC50 values for industrial treatment chemicals were misrepresented on Material Safety Data Sheets and consequently, were subject to misapplication by operators, (3) Cu concentrations of 96 mg/L significantly impaired growth of Asian clams in artificial stream testing, and (4) effluent Al exceeded acute and chronic water quality criteria, suggesting this ion should receive further consideration in future studies. / Master of Science

Pink heelsplitter (Potamilus alatus) mussels collected from Kentucky Lake, TN were held at two bottom locations (0.6 m, 2.5 m) and suspended in pocket nets (at depth about 1.0 - 1.5 m) in a pond at the Freshwater Mollusk Conservation Center (FMCC), Virginia Tech, for 1 yr. Survival of mussels after 1 yr was significantly different, with poorest survival (30 %) in the bottom of the deep end; and no difference between the shallow end (83.3 %) and the suspended pocket nets (63.3 %). Survival of mussels was inversely related to water temperature (r = - 0.72); lowest monthly survival occurred in summer, resulting in a significant difference among the three locations with a similar trend after 1 yr. The glycogen reserves of mussels in captivity for 1 yr differed by pond location, higher in mussels at the shallow end than those in suspended pocket nets and at the deep end. Therefore, the shallow end of pond was more suitable for holding mussels long-term, while the suspended pocket nets are an alternative site for holding captive mussels. Additionally, dissolved oxygen was very low at the deep end (1.9 mg/L) in summer, while it was adequate (range from 5.7 - 6.4 mg/L) at the location of suspended pocket nets, and 5.0 mg/L at the shallow end (24.7 °C). Data for 40 specimens indicated that sexual dimorphism in valve shape occurred in P. alatus. Female mussels had a significantly (p < 0.0001) greater ratio of height (H) to length (L) (52.3 %) and width (W) to length (31.8 %) than males (H/L: 48.4 %; W/L: 28.8 %), respectively. The posterior ends were somewhat round to oval in males and bluntly squared or truncated in females. Female mussels were more inflated than males. These morphological differences can be used to distinguish females from males during field collections. The red drum (Sciaenops ocellatus) was identified as a new fish host for P. alatus, as 48 active juveniles were transformed by this species, which is not a natural host. Four glochidia were observed on the fins versus 2,307 on the gills of five red drum. Freshwater drum also was verified as a suitable host fish, but black crappie (Pomoxis nigromaculatus), banded sculpin (Cottus carolinae), yellow perch (Perca flavescens) and nile tilapia (Oreochromis nilotica) did not support transformation of glochidia to juveniles. Survival and growth of propagated juveniles of P. alatus were assessed regarding the effects of algal diets (Nannochloropsis oculata and Neochloris oleoabundans) and substrate type (fine sediment and sand). Overall, survival of juveniles after 17 d ranged from 23.8 to 66.8 %, with mean of 48.5 %; however, survival dramatically declined during the next 2 wk period to only 5.8 % (range of 1.8 to 7.8 %). Survival rate of juveniles was significantly different (p = 0.027) between substrates, but not in diets (p = 0.520), with the lowest survival rate of 23.8 % in sand substrate and fed N. oculata. Juveniles grew faster in fine sediment (23.0 % increase in shell length) than in sand substrates (10.5 % increase) (p = 0.002). Moreover, mean growth rate of juveniles was 4.9 &#956;m/d during the first 2 wk, but decreased to 0.2 &#956;m/d in the remaining 2 wk. Therefore, fine sediments seemed more appropriate for juvenile culture compared to sands. Both species of algae, N. oculata and N. oleoabundans, can be used to feed juveniles in the laboratory. Adult pink heelsplitters were used to study feasibility of pearl production by using two surgical implants (non-nucleated implant = NNI, and image pearl implant = IPI) in two ponds of different nutrient levels (FMCC pond and Duck pond). NNI and IPI pearls with purple or purplish luster were successfully produced in P. alatus. Pearl weight was not significantly different (p = 0.562) between two ponds. No differences in monthly survival rates of mussels were observed in either pond (p = 0.051), or among mussels with surgical implants and the no-surgery control mussels (p = 0.881). Consequently, P. alatus can be considered a potential species for producing purple pearls in pearl culture. Additionally, mussels in the Duck pond had higher (p < 0.0001) glycogen levels, similar to those in wild collected mussels, than those in the FMCC pond, indicating that this pond environment may be more suited for holding implanted mussels in captivity. / Master of Science

With a new focus on flow regulation by the Tennessee Valley Authority (TVA) in reservoir tailwaters, it is now possible to recover many mussel species that once occurred in these reaches. Before flows can be modified to create habitat for freshwater mussels, suitable microhabitat conditions must be defined. In this study, I used multiple approaches to define suitable microhabitats for species in the free-flowing upper Clinch River, Virginia and Tennessee, where reproducing mussel populations persist. During summer low flows in 2003 and 2004, I measured flow and substrate conditions in over 1000 microhabitat patches (0.25 m² quadrat samples) across five river reaches. Flow characteristics and embeddedness were significantly different between microhabitats occupied and unoccupied by the most abundant species (MRPP, p < 0.05). Comparison of simple and multiple logistic regression models with Akaike's Information Criteria (AIC) demonstrated that increasing Fleisswasserstammtisch (FST) hemisphere number (a measure of shear stress), decreasing degree of embeddedness, and increasing mean column velocity best explained species occurrences in a microhabitat patch. Subtle differentiation in habitat use among species was observed; however, most species appeared to be microhabitat generalists. Species were grouped into three habitat guilds using corresponding canonical analysis and cluster analysis: fast-flow specialists (FFS), fast-flow generalists (FFG), and slow-flow tolerant (SFT). I used the same data set to develop and test transferability of Habitat Suitability Criteria (HSC) for three habitat guilds and seven species of adult freshwater mussels. Nonparametric tolerance limits were used to define the range of suitable and optimal habitat during summer low flows. Optimal habitat was defined as those ranges of FST hemisphere number, mean column velocity, and embeddedness occupied by the central 50% of independent observations for a species or guild, whereas suitable habitat was defined by those ranges occupied by the central 90% of observations. The transferability of criteria to other reaches of the Clinch River was assessed using one-sided Chi-square tests. Criteria developed for the fast-flow specialist (FFS) and fast-flow generalist (FFG) guilds, as well as most criteria for species in those guilds, transferred to destination reaches. In contrast, criteria developed for the slow-flow tolerant (SFT) guild and individual constituent species consistently failed to transfer. Criteria for FFS and FFG guilds and their constituent species should be incorporated into flow simulation models such as PHABSIM to gauge the effect of minimum flows on mussel habitat quality and quantity. These criteria could also be used to determine suitable sites for mussel translocations. However, my criteria require further testing in other rivers before they can be transferred beyond the Clinch River. Behavior and physiological responses to laboratory manipulations of flow velocity and substrate particle size were used to elucidate microhabitat preferences of Actinonaias pectorosa, Potamilus alatus, and Ptychobranchus subtentum. These species appeared less stressed in the fastest flow treatment, demonstrating significantly higher oxygen consumption and oxygen-to-nitrogen (O:N) ratios than in slower flow treatments. Only P. alatus demonstrated a preference for substrate particle size, and consistently selected finer particle sizes. Actinonaias pectorosa and P. subtentum demonstrated preference for fast-flow microhabitats by readily burrowing in those conditions, while abandoning slow-flow conditions. The lack of preference for substrate particle size demonstrated by A. pectorosa and P. subtentum supports conclusions of previous studies that substrate particle size is of little or secondary importance for explaining mussel microhabitat use. These results, along with previous studies in the Clinch River, demonstrate that the stable habitats of riffles and runs; characterized by fast flows during summer low flows, low percent bedrock, and low embeddedness, are the most suitable habitats for mussel assemblages. To create and maintain suitable habitat conditions in tailwaters, releases should maintain flow over riffles at a minimum depth of no less than 30 cm in riffles that provide higher shear stress conditions (FST number > 7) and velocities (> 0.70 m/s). Periodic releases that are sufficient to transport silt and sand, but not high enough to transport larger substrate should be adequate to maintain substrates with a low degree of embeddedness. Doing so would create suitable habitat for all mussels, from the most to least specialized. Additionally, HSC developed for FFS and FFG guilds can be used to determine suitable destination sites for translocations of species belonging to these guilds. / Master of Science

Mercury contamination of the North Fork Holston River below Saltville, Virginia has nearly extirpated most mussel populations. Because natural recovery of these populations has not occurred, this research combined field and laboratory assessments to determine the extent of ecological impairment in the river. In situ 60-day Asian clam (Corbicula fluminea) growth studies in 2005 showed a positive correlation (p=0.03) between low clam growth and sediment mercury levels. Because of severe low flow conditions of the NFHR in late 2005 conductivity dissipation from a point source brine discharge downstream rarely reached background level (~345 µS/cm) and was observed as high as 690 µS/cm 640 m below the discharge site. In addition, conductivity doubled in the river section adjacent to the remediated Ponds 5 and 6 (rm 81.6 and 80.4). Such low flow conditions (mean flow < 50 ft3/sec) occur in the NFHR approximately every five years. This low flow situation also evidenced a thick white flocculent or floc observed to accumulate at the base of the two remediated ponds. Analysis of the flocculent determined it to be high in aluminum (1.9-38 mg/L) and iron (2.0-51.0 mg/L), well above US Environmental Protection Agency Water Quality Criteria limits (0.0087 and 1.0 mg/L, respectively); riverine sediments collected below the accumulated floc also had high levels of calcium (240,000-380,000 mg/kg) and mercury(0.62-1.7 mg/kg). Acute tests with juveniles of Villosa iris and <24-hr old Ceriodaphnia dubia were used to measure the toxicity of the brine discharge, which had a conductivity of ~ 14,000 µS/cm. Results of these tests indicated C. dubia to be more sensitive than V. iris; however, chronic toxicity test results were similar for V. iris and C. dubia. The Lowest Observed Adverse Effect Concentration (LOAEC) for mussel survivorship after 28 days was 10,000 µS/cm, while the LOAEC for growth was 5,000 µS/cm. LOAECs for the C. dubia 7-day chronic were 25 % (survivorship) and 12.5 % (reproduction), while mean conductivity at these two concentrations was 4,054 and 2,211 µS/cm, respectively. Toxicity tests conducted with Pond 6 dyke cut discharges resulted in similar lethal concentrations for C. dubia and V. iris. Forty-eight hour LC50s of these discharges ranged from 12.07-15.95 % for C. dubia, and 17.36-18.95 % for V. iris. Dyke cut discharges also exhibited exceedingly high alkaline pH (11.5-12.2), which caused 100 % mortality to C. dubia in 15 min. The Pond 5 and 6 dyke discharges are the likely source for the flocculent accumulation at the base of the two remediated pond areas. The combined effect of mercury, aluminum and iron, along with periodic fluxes of high conductivity and alkaline pH during low flow conditions may contribute to low mussel recruitment downstream of Saltville, VA. / Master of Science

The decline in mussel fauna of the Powell River has been attributed to extensive mining activities in headwater streams of Wise and Lee counties, Virginia. Surface mining causes erosion, sedimentation, and contamination of water with toxic substances from coal washing and waste piles. Historical water quality data of the Powell River have documented concentrations of metals at levels determined to be toxic to molluscs, specifically zinc (Zn) and copper (Cu). Acute toxicity tests with Zn, Cu, and mixtures of these two metals were conducted with glochidia and juvenile freshwater mussels. Effects of varying conditions such as water source, temperature, length of exposure, species, and lifestage were determined. Additionally, the effects of Powell River sediment on survival and growth of juvenile mussels were evaluated. The Cumberland moccasin shell (Wedionidus conradicus) was the most sensitive species tested, with 24-hr and 48-hr LC50 values for glochidia ranging from 423 to 725 μg Zn/L. Glochidia of the pheasantshell (Actinonaias pectorosa) exhibited LC50 values from 274 to 2886 μg Zn/L, depending on test conditions. Similar results were obtained for glochidia of the rainbow (Villosa iris), with LC50 values ranging from 577 to 4123 μg Zn/L. Juveniles were more sensitive, with 48-hr LC50 values ranging from 360 to 1885 μg Zn/L for A. pectorosa, and 339 to 1122 μg Zn/L for V. iris, depending on test conditions. Juvenile mussels were affected by Zn at lower concentrations as evidenced by valve gaping and a swollen and nonresponsive foot. Copper was 5 to 15 times more toxic than Zn, with 48-hr LC50 values ranging from 52 to 156 μg Cu/L, and ECS5O values ranging from 25 to 115 μg Cu/L for juveniles of A. pectorosa. Copper appeared to exert a different toxic mode of action, as evidenced by closed valves and reduced siphoning. In general, sensitivities of early life stages of mussels to Zn and Cu increased with higher temperature, soft water, and length of exposure. At certain concentrations, Zn seemingly had an antagonistic effect (less than additive) when mixed with Cu. This effect was evidenced by reduced mortality of juveniles in Cu solutions when Zn was added at concentrations of about 400 to 800 μg/L. However, this antagonistic effect was not reflected in the percent of juveniles affected, which increased continuously with increasing metal concentrations. Glass beads were found unsuitable as a control substratum for use in sediment testing with juvenile mussels. Results of sediment tests indicated that sediment in some areas of the Powell River may be toxic to juvenile mussels, and that toxicity may be linked to water quality. After 10 days, survival of juveniles in sediment collected downstream of a coal processing plant was significantly lower than survival in sediment upstream of the plant (p = 0.01). Further, survival in sediments with dechlorinated tap water was significantly higher than survival in sediment with river water (p = 0.0002). After 20 and 30 days, survival was similar among sites and water types. High metal concentrations in the river water appeared to contribute to toxicity, because juveniles in tap water displayed consistently better growth, and initially better survival than juveniles in river water and sediment. This toxicity was not apparent in sediments collected from the same sites less than two months later, suggesting the character of the sediments may change as new suspended sediment is deposited. The USEPA water quality criteria for Zn (adjusting for water hardness) are 174 μg/L (acute) and 158 μg/L (chronic), whereas copper criteria are set at 28 μg/L (acute) and 18 μg/L (chronic). Powell River water samples collected during 1991 contained concentrations of Zn and Cu exceeding these criteria, as well as concentrations shown to have adverse effects on mussel populations. Results suggest that some metals are introduced into the river system in runoff, whereas Cu is being introduced as an episodic event. Intensive monitoring of water quality is needed to identify specific sources of metal pollution. If levels of heavy metal concentrations remain high, then the declining mussel populations of the Powell River will not recover, and endangered species may be extirpated from Virginia. / Master of Science

The Clinch River (CR) and North Fork Holston River (NFHR) contain some of the most diverse freshwater mussel assemblages in the United States; however, both rivers are experiencing declines in mussel populations. The first component of this study used histological evaluations and water quality data to determine whether mussels were negatively impacted in the CR zone of decline (ZD) and to inform future management of freshwater mussels in the river. In the 91 kilometer (km) section from Carbo, Virginia (CRKM 431) downstream to Speers Ferry, Virginia (CRKM 340), referred to as the ZD, mussel density decreased >90% from 1979 to 2014 at key sites such as Semones Island (CRKM 378.3) and Pendleton Island (CRKM 364.2). Laboratory propagated mussels were placed in cages in the river for one year from June 2012 to May 2013 at four sites within the ZD and four sites in reaches where mussel populations remain stable or are increasing, a zone of stability (ZS). The survival, growth and histological results indicated that there are continuing impacts to mussels in the ZD. Research investigating impacts to the ZD and methods to improve water quality in this zone are needed. The laboratory component of this study examined sublethal effects of potassium (K⁺), chloride (Cl⁻), and un-ionized ammonia (NH₃-N) on mussel tissues at concentrations relevant to those found in the NFHR. Historical industrial activities at Saltville, Virginia, as well as continued pollution of the NFHR from chemical waste ponds at this location, are believed to be significant contributors to mussel declines. Contaminant seepages from the waste ponds that include Cl⁻, K⁺, and NH₃-N have been shown to be toxic to adult and juvenile mussels. A three-month laboratory study was conducted to assess impacts to organ tissues (gills, digestive glands, kidneys, and gonads) of adult Villosa iris exposed to environmentally relevant concentrations of K⁻ (4 and 8 mg/L), Cl⁻ (230 and 705 mg/L), and NH₃-N (0.014 and 0.15 mg/L) using histological evaluations. No detectable differences were observed among the histological endpoints from mussels held in treatments and control (p>0.05). The study design was modified and repeated using increased concentrations of K⁺ (8, 16, and 32 mg/L) and Cl⁻ (705, 1410, and 2820 mg/L) for a two-month exposure period. Due to issues with maintaining NH3-N in mussel holding chambers, the second study did not the second study did not include NH₃-N exposures. Control mussels in both studies had a higher abundance of lipofuscin in kidneys and degraded cytoplasm in the digestive gland diverticula compared to baseline mussels, indicating that captivity influenced mussel tissues. Future studies are needed to more thoroughly address these captivity effects. Both survival and histological data in the second test showed a significant negative effect of the increased concentrations of Cl⁻ and K⁻, which were representative of those found at some sites in the NFHR downstream of Saltville, Virginia. / Master of Science

Select freshwater mussels in the genera Fusconaia, Pleurobema, and Pleuronaia were collected primarily in the upper Tennessee River basin from 2012 to 2014 for phylogenetic and morphological assessments. Freshwater mussels in these genera are similar in appearance, hence the need for phylogenetic verification and morphological assessment. Phylogenetic analyses of the mitochondrial gene ND1 and the nuclear gene ITS1 revealed three unrecognized, phylogenetically distinct species. These species were separated from their closest congener by 2.85%, 3.17%, and 6.32% based on pairwise genetic distances of ND1. Gaps created from aligning ITS1 sequences were coded as fifth characters, which phylogenetically separated most closely related species. Analyses of ND1 agreed with previous literature on the phylogenetic distinctiveness of Pleuronaia species, with the exception of the DNA sequences of P. gibberum, which grouped outside this genus based on the analyses conducted in this study. Morphological variation was recorded for eight of the species to include quantitative and qualitative characters as well as geometric morphometric analyses. Three decision trees were created from quantitative and qualitative characters using classification and regression tree analyses. The best-performing tree used quantitative and qualitative characters describing shell-only scenarios and obtained 80.6% correct classification on terminal nodes. Canonical variates analysis on geometric morphometric shell data revealed large morphological overlap between species. Goodall's F-tests between pairs of species revealed significant differences (a=0.05) between all but one species pairs; however, examination of landmarks on shells concluded large overlap of landmarks between species pairs. Lack of morphologically distinct characters to readily identify these phylogenetically distinct species indicates large morphological overlap among these species. Biologists need to be cognizant that morphologically cryptic species may exist in systems often explored. Three dichotomous keys were created from classification trees to identify select individuals in the genera Fusconaia, Pleurobema, and Pleuronaia; two of these keys, one for shells and one for live mussels were tested by participants with varying mussel identification skills to represent novices and experts. Both keys used continuous (quantitative) and categorical variables to guide participants to identifications. Novices, who had no prior mussel identification experience, correctly identified mussels with a 50% accuracy using the shell key and with a 51% accuracy using the live key. Experts, who had at least three years of experience identifying mussels, correctly identified mussels with a 58% accuracy using the shell key and with a 68% accuracy using the live key; however one expert noted that they did not use the live key to correctly identify one mussel. Morphological overlap of variables between mussels likely resulted in failure to consistently identify mussels correctly. Important management decisions and project implementations require accurate assessment of species' localities and populations. Incorrect species identification could hinder species' recovery efforts or prevent projects that otherwise could have continued if species are misidentified. If a mussel collection is thought to be a new record or could affect a project, I recommend that molecular genetic identifications be used to verify the species identity. / Master of Science

Understanding factors that influence a species' distribution and abundance across their annual cycle is needed for range-wide conservation planning. Every year during spring migration, thousands of federally threatened (U.S.A.) and endangered (Canada) migratory Western Atlantic red knots (Calidris canutus rufa, 'red knot') use Virginia's barrier islands as stopover habitat to regain the fat required to continue flights to breeding grounds. Because the red knot completes one of the longest avian migrations in the world and relies on variable prey resources at its stopover grounds, the red knot exemplifies the challenges faced by long-distance migrant shorebirds. These challenges may be exacerbated by climate change, as long-distance migrants may be unable to adapt quickly to changing prey ranges and abundances, resulting in spatial and temporal mismatches between predators and prey. More specifically, as climate change causes ocean temperatures near Virginia's barrier islands to rise, organisms that live within the intertidal zone, like blue mussels (Mytilus edulis), are experiencing range shifts. Here, we 1) confirmed what prey red knots select in Virginia, 2) addressed the factors that affect red knot site selection, red knot flock size, and prey abundances across Virginia's barrier island intertidal shoreline during 2007 – 2018, and 3) predicted the origin of juvenile blue mussels, a key prey resource for red knots in Virginia. To determine which prey are most available to red knots in Virginia, we collected sand and peat substrate core samples from Virginia's ocean intertidal zone and counted the number of prey in each sample. We compared these prey availability data to prey DNA data obtained from fecal DNA metabarcoding analyses on red knot feces (n = 100) collected on peat and sand substrates between 2017 – 2019. Red knots consumed prey from Orders Veneroida (clams), Mytiloida (mussels), Diptera (flies), and Amphipoda/Calanoida (crustaceans). While crustaceans were the most abundant prey on both sand (70.80% of total prey counted) and peat (74.88%) substrates, red knots selected crustaceans less than expected given their availability. Red knots selected clams and mussels, supporting their status as bivalve specialists in Virginia. After determining which prey red knots consumed and selected in Virginia, we predicted the number of red knots using Virginia's barrier island stopover during their migratory stopover (May 14 – 27, 2007 – 2018) annually. We used confirmed prey, tide, distance to known roosts, and red knot winter counts from Tierra del Fuego to inform zero-inflated negative binomial mixed-effects regression models of red knot site selection and flock size in Virginia. We also used generalized linear mixed-effects regression models to determine how climatic and geomorphological factors affected prey abundances. Modeled red knot peak counts were highest in 2012 (11,644) and lowest in 2014 (2,792; x̄ = 7,055, SD = 2,841); the trend over time was variable but there was no evidence of a linear increase or decrease. Red knots selected foraging locations with more prey, though red knot flock size did not consistently relate to prey abundance. Tide, substrate, and water temperature affected prey availability. While different prey responded to these covariates in variable ways, prey generally were most abundant on peat banks at low tide. Given the importance of blue mussels in the red knot's diet and distribution in Virginia, if the blue mussel's range continues to contract northward, red knots could be faced with additional fat replenishment challenges. We analyzed the variation in blue mussels from 2010 – 2018 by collecting core samples on peat banks in Virginia and counting the number of blue mussels in the cores. To approximate the origin of Virginia's juvenile blue mussels and determine how continued ocean temperature warming may further affect the blue mussel's range contraction, we conducted oxygen stable isotope (δ¹⁸O_c) analyses on 74 blue mussel shell umbos (the first portion of the shell precipitated) and shell edges (the most recently precipitated shell) to compare and predict where different portions of the shell were formed. We compared blue mussel shell compositions to δ¹⁸O_c calculated in equilibrium with regional ocean water using recorded δ¹⁸O_w data and sea surface temperature data from ocean buoys between New Hampshire and Virginia. Blue mussel abundance/core sample declined over the duration of our study (Spearman's rank correlation coefficient: ρ(rho) = -0.31, p < 0.001), with the highest abundance in 2010 (x̄ blue mussels/core sample = 537.88, SE = 85.85) and lowest in 2016 (x̄ = 34.08 blue mussels/core sample, SE = 6.96). Blue mussel umbos (x̄ δ¹⁸O_c = -0.23‰, SE = 0.12) contained more positive δ18Oc than shell edges (x̄ δ¹⁸O_c = -0.53‰, SE = 0.20), suggesting that Virginia's blue mussels originated from ocean populations in more saline and/or colder water than that within Virginia's intertidal zone. Blue mussel umbo δ¹⁸O_c were not different than δ¹⁸O_c calculated in equilibrium with regional ocean water off the Virginia and Delaware coasts, suggesting that Virginia's blue mussels originated in ocean waters between Delaware and Virginia; however, they may have originated in waters as far north as New York in some years, potentially decreasing the risk of blue mussels being completely extirpated from Virginia in the near future. While red knots currently use spring migratory stopovers across the United States' Atlantic Coast, from Florida to New Jersey, the largest spring concentrations of knots are confined to the Delaware Bay and Virginia's barrier islands. Because these stopover grounds support large proportions of the red knot's migratory population, any changes in the factors that affect red knots at these stopover sites could have lasting implications for red knots. The blue mussel's range contraction and decline over time in Virginia, for example, is concerning from a conservation perspective. Red knots require easily accessible and abundant prey resources to efficiently replenish fat-stores needed for continued migration and breeding. Additionally, because red knots breed within a narrow period, any delays on stopover grounds could prevent red knots from breeding, even if they survive migration. Our research demonstrates that red knots use prey abundance as a determinant when selecting foraging locations, and that peat banks, while only sporadically available across the barrier islands at mid- to low-tides, contain higher prey abundances than sand. Thus, to continue maximizing the availability of prey in Virginia, measures should continue to be taken to allow natural island migration processes that encourage the presence of both sand and peat substrates. Beach nourishment and stabilization projects are often used on coastal beaches to prevent shoreline erosion; however, such actions prevent the formation of peat banks by blocking island migration processes. A reduction in peat banks could decrease the abundance of prey available to red knots, making weight gain during the critical stopover period more challenging for red knots. Additionally, beach nourishment through sand replenishment buries invertebrate prey, potentially causing mass prey mortality and reducing shorebirds' ability to access deeply buried prey. To prevent the loss of important peat banks on these islands, and to prevent disrupting predator-prey interactions, managers should continue their ongoing focus on allowing natural processes to occur on Virginia's barrier islands. / Doctor of Philosophy / Red knots (Calidris canutus rufa) are imperiled shorebirds that migrate thousands of kilometers each year from wintering grounds as far south as Argentina to breeding grounds in the Canadian Arctic. To migrate such long distances, red knots use stopover habitat, like Virginia's barrier islands, to regain the fat required to continue flights to breeding grounds. Climate change is causing the ranges of important red knot prey, such as blue mussels (Mytilus edulis), to shift. Red knots may be unable to quickly adapt to changes in prey abundances on stopover grounds, making it more challenging for them to gain the weight required for successful migration and breeding. Thus, understanding the factors that may affect red knots in Virginia are important for successful management of this shorebird. Here, we determined what prey red knots consume in Virginia, addressed the factors that affect red knots in Virginia, and predicted the origin of blue mussels, an important prey resource for red knots in Virginia. We collected sand and peat substrate samples from Virginia's ocean intertidal zone to determine what prey were available to red knots and collected red knot feces to determine what prey red knots consume. Red knots consumed clams, mussels, fly larvae, and crustaceans in Virginia and showed preference for clams and mussels over the more abundant crustaceans. Red knots selected foraging locations that had higher prey abundances than unused sites. The relationship between red knots and prey was affected by the tidal cycle, with the highest abundance of prey available and most birds foraging at low tide when peat banks were exposed. After confirming that red knots preferentially consume blue mussels, which is experiencing a northward range contraction due to increasing ocean temperatures, we analyzed how blue mussel numbers have changed over time in Virginia. Blue mussel numbers declined over the duration of our study. We also analyzed blue mussel shells to help determine where the blue mussels were spawned, as shells contain environmental clues that relate to the conditions within which the shell forms. The juvenile blue mussels red knots consumed on Virginia's peat banks likely originated in ocean waters between Delaware and Virginia, but potentially in waters as far north as New York, possibly using ocean currents to travel to Virginia's intertidal zone. Because migratory stopovers, like Virginia's barrier islands, support a large percentage of the total red knot migratory population, any changes in factors that affect red knots during their spring stopover, like prey availability, may negatively affect red knots. Because red knots need to quickly consume large quantities of prey to gain the weight needed for continued migration and breeding, managers should continue to prevent factors that negatively affect prey in Virginia. For example, we found that peat banks support high quantities of red knot prey; thus, continuing to ensure that peat banks are able to form on the islands is paramount. However, peat banks form through shoreline erosion and overwash, two natural processes which are often portrayed negatively. Beach nourishment and stabilization projects are commonly used in other coastal areas to prevent shoreline erosion and overwash along coastal beaches. If Virginia's barrier island beaches are nourished, natural processes like shoreline erosion may be less likely to occur. The absence of shoreline erosion on Virginia's barrier islands would preclude the formation of peat banks, likely decreasing prey availability for red knots. Beach nourishment also buries invertebrate prey, as sand is deposited along the beaches' intertidal zone. The disturbance caused by nourishment combined with the reduction in prey caused by it could negatively affect red knots in Virginia. Managers should continue to protect both peat and sand substrates by further allowing natural processes, like shoreline erosion, to occur on Virginia's barrier islands.