The North American spiny-cheek crayfish, Orconectes limosus (Rafinesque, 1817), a widespread invader in Europe, seems to have been introduced there successfully only once. According to available literature, 90 individuals of unclear origin were released in Poland in 1890. Despite this apparent bottleneck, the species has successfully colonized various aquatic habitats and has displaced native crayfish species in many places. To test whether different European populations were likely to have come from a single source and to identify their possible origin, we analyzed the diversity of the mitochondrial gene for cytochrome c oxidase subunit I (COI) of O. limosus individuals from Europe and from its original range in North America, including the presumed source region of European populations, the Delaware River watershed (eastern USA). Two haplotypes were found in European populations. One haplotype was widespread; the other was present in a single population. In contrast, 18 haplotypes were detected in North America. This result supports the hypothesis of a single overseas introduction of O. limosus and suggests that the high invasion success of this species was not limited by an introduction bottleneck. Two divergent clades were detected in North American O. limosus populations. One, which includes the dominant haplotype in Europe, was found in a large part of the species' present range. The 2^nd (diverging by >1%) was mostly restricted to a limited area in southeastern Pennsylvania. Orconectes limosus populations in the northern part of the species' North American range, at least some of which are nonindigenous themselves, may share the source area with European O. limosus. The endangered status of O. limosus populations in southeastern Pennsylvania and northeastern Maryland, where much of the species' genetic diversity resides, should be considered in conservation management.

Detrital inputs into ecosystems vary in quantity and quality (e.g., plant litter vs carrion). Variability in detrital quantity and quality potentially affects consumer biomass and rates of organic matter (OM) breakdown. We used cave streams to test 2 linked hypotheses regarding the influence of total detrital inputs on consumer biomass and the breakdown of high-quality carrion detritus. First, we hypothesized that cave systems with higher total OM availability would support a higher biomass of consumers. Second, we predicted that higher consumer biomass would cause faster carrion breakdown rates. To test these hypotheses, we quantified macroinvertebrate biomass and breakdown rates (k, d⁻¹) of carrion (mouse carcasses, Mus musculus) in 4 cave streams in northeastern Alabama and southeastern Tennessee (USA) that varied in total OM storage. We estimated carrion breakdown rates in coarse- and fine-mesh packs (10-mm and 500-µm mesh size) to isolate the influence of scavenging by crayfish. Macroinvertebrate biomass (excluding crayfish) in carrion packs was positively correlated with OM storage, but neither macroinvertebrate biomass (excluding crayfish) nor OM storage were strong predictors of carrion breakdown rates. Crayfish biomass was not correlated with total OM storage but was positively correlated with coarse-mesh breakdown rates. Our study illustrates the influence of community structure and consumer biomass on detrital breakdown rates in cave ecosystems. However, determining how detrital inputs structure cave communities will require further study.

Pesticides play a critical role in maximizing yields of economically important crops and minimizing the human health threats of disease-carrying pests, but they often have collateral effects on nontarget species. We used a mesocosm study to address how the most commonly used insecticide in the USA, malathion, applied at low, ecologically relevant concentrations (20 and 110 µg/L) affects species interactions in aquatic communities. Unlike many community ecotoxicology studies, our study assessed how malathion affects both consumptive and nonconsumptive effects of predators. We also considered how the vertical distribution of predator cues and malathion (caused by potential stratification) affects species interactions. We found no evidence for vertical stratification of malathion, a result suggesting that exposure to the pesticide was uniform throughout the water column. Malathion was lethal to some primary consumers (cladocerans) at both concentrations and to top predators (dragonflies) at the highest concentration (110 µg/L). These lethal effects initiated density-mediated indirect effects in both cases. Malathion also may have decreased dragonfly foraging efficiency, resulting in increased tadpole survival (trait-mediated indirect effect), which decreased the resources used by tadpoles (periphyton). Collectively, our results show that malathion alters species interactions. However, we suggest that the degree to which pesticides affect aquatic communities will depend strongly on the species composition of communities. Therefore, the community-level consequences of pesticide exposure are likely to vary across the ecological landscape.

Establishment of dreissenid mussels in aquatic systems is often accompanied by major changes in the abundance, diversity, and community composition of benthic invertebrates. However, few studies have been published that address the effects of long-term dreissenid presence on the littoral benthos inhabiting hard substrata in lakes. We present the results of a depth-stratified, quantitative survey of littoral benthos conducted at 4 sites in 1993, just before dreissenid invasion, and in 2008, 14 y after the establishment of dreissenids in Lake Simcoe, Ontario. Average densities of nondreissenid invertebrates were 45× greater in 2008 than in 1993. Amphipods, isopods, chironomids and oligochaetes underwent the largest increases in absolute abundance. The taxonomic diversity of the benthic invertebrate community (α diversity) increased significantly. Community structure was more similar within and between depths and sites in 2008 than in 1993, a result implying lower β diversity of the postdreissenid littoral benthos. In addition, fewer differences were found in the densities of organisms between sampling locations in 2008 than in 1993. We suggest that dreissenids increase resource availability to benthic organisms and homogenize the littoral benthos by increasing the evenness of the distribution of food and physical-habitat resources across sites and depths. The transformation of the littoral benthic community of Lake Simcoe reflects a major change in the distribution of energy in the lake and is consistent with a dreissenid-mediated redirection of production from the pelagic to the littoral zone.

Freshwater mussels are one of the most imperiled groups of animals in North America. Effective resource management and conservation efforts require reliable estimates of species diversity and their distributions. However, sampling protocols for qualitative mussel surveys have not been well established and tested. Furthermore, the sampling effort required for a given data-quality goal (90% of species captured) often varies substantially among sites, and application of a standard effort probably would introduce bias into assemblage comparisons. We tested the sampling adequacy of time-based hand searches. We conducted 16-man-hour (mh) searches at each of 18 wadeable stream sites in Illinois that differed widely in environmental factors and historical species diversity and collected 27 to 942 individuals and 5 to 18 species per site. We accounted for imperfect species detectability by using the Chao-1 richness estimator and measured sampling adequacy as the % of the estimated number of species sampled. Four-mh searches, a frequently used effort, captured 15 to 100% of all species with an average of 61%, and yielded estimates of richness that were not significantly correlated with the estimated total richness (Pearson's r  =  0.39, p > 0.05). Ten-mh searches captured >70% of all species at >70% of sites and resulted in a significant correlation between observed and estimated richness (Pearson's r ≥ 0.78, p < 0.01). A Random Forests (RF) model based on watershed and habitat characteristics (e.g., stream size and dominant substrate types) accounted for 45% of the variance in sampling adequacy of 4-mh searches. Sampling adequacy decreased with increasing stream size and substrate size but increased with % forest in the riparian zone and logs in the stream. A 2^nd RF model was developed to predict the number of man-hours required to capture 70 ± 3% of all species, and it accounted for 37% of the variance. Our findings should serve as a guide for setting standard sampling efforts for mussel surveys in Illinois and probably other midwestern states and should provide a baseline for setting site-specific efforts. Our modeling approach is of general applicability for addressing sampling-adequacy issues in studies of any assemblage.

The main purpose of our work was to elucidate factors responsible for the geographical differences in leaf-litter decomposition rates in Spanish oligotrophic headwater streams. Decomposition experiments with alder (Alnus glutinosa) leaf litter were carried out in 22 headwater streams in 4 different climatic regions across the Iberian Peninsula (Cornisa Cantábrica, Cordillera Litoral Catalana, Sierra de Guadarrama, and Sierra Nevada). Streams that were similar in size, flowed mainly over siliceous substrate in catchments with scarce human settlements and activities, and fell within a range of low nutrient concentrations were chosen in each region. Breakdown rates were regionally variable and were low (0.109–0.198% ash-free dry mass [AFDM]/degree day [dd]) in the Cornisa Cantábrica, the most mesic and Atlantic region, and high (0.302–0.639% AFDM/dd) in Sierra de Guadarrama, one of the coldest and most inland areas. Temperature was not the determining factor affecting differences in breakdown rates among regions, and breakdown rates were not related to concentrations of dissolved nutrients. However, microbial reproductive activity (sporulation rates) was significantly correlated with dissolved P concentration. Breakdown rates were explained better by presence and feeding activities of detritivores than by decomposer activity. Incorporation of breakdown rates in assessment schemes of stream ecological status will be difficult because leaf processing does not respond unequivocally to environmental factors when climatic regions are considered. Thus, regional adjustments of baseline standards in reference conditions will be required.

Urbanization is an important ecological modifier of stream catchments and significantly alters C type and concentration and heavy metal input to aquatic systems. C and heavy metals are important determinants of microbial community structure (population dynamics) and function (physiological processes). Understanding how changes at the landscape scale affect key nutrient-transformation processes (e.g., denitrification) and C metabolism (e.g., methanogenesis) at the micro scale requires simultaneous determination of the structural and functional responses of bacterial communities. We compared total bacterial (16S ribosomal ribonucleic acid [rRNA] gene) and denitrifying bacterial (nosZ gene) community structures in sediments taken from an urban and a nonurban stream before and after C and Zn additions. Microcosms containing stream sediment were supplemented with either proteinaceous or leaf-leachate C, increasing the C content by ∼36 and 130 mg/L, respectively. C was added in the presence and absence of Zn addition (200 mg/kg). Changes in community function were determined simultaneously with additions by measuring the rates of CO₂, N₂, and CH₄ accumulation in the headspace of each mesocosm. Site had the greatest effect on total and denitrifying bacterial community structures. Added C caused a small but significant change in total bacterial community structure, but did not change denitrifying bacterial community structure. C enrichment caused a significant and rapid change in community function. Rates of methanogenesis were significantly higher at the nonurban than at the urban site and were significantly affected by C type (significant site × C interaction). Denitrification and CO₂ production were unaffected by C additions but differed between sites. Zn addition changed total bacterial community structure but not denitrifying bacterial community structure. Zn affected the rate of methanogenesis, but the effect size was less than for C type. Community structures remained different between sites at the end of the study despite exposure to Zn concentrations that reflected conditions in urbanized streams and C sources that mimicked those from urban or nonurban streams. Our study supports previous observations that microbial community structure is strongly influenced by prior sediment history. The broader implication is that landscape-scale changes like urbanization alter other physical and chemical properties of the stream sediment that influence community structure more than the Zn or C concentrations in our study.

Headwaters (stream orders 1–2) traditionally have been considered depauperate compared to mid-order streams (orders 3–4)—a conclusion that arises from a perception of streams as linear systems and emphasizes change in average α (local) diversity along streams. We hypothesized an opposite pattern for β (among-site) diversity and suggest that headwaters might account for a large degree of basin-scale biodiversity if considered within the more realistic framework of streams as branching networks. We assembled pre-existing biodiversity data from across the globe to test this hypothesis broadly at the population-genetic (mitochondrial haplotype diversity within species) and community (species/taxonomic diversity) levels, with a focus on macroinvertebrates. We standardized 18 (9 headwater and 9 mid-order) population-genetic and 16 (10 headwater and 6 mid-order) community-level ecoregional data sets from 5 global ecozones for robust comparisons of β-diversity estimates between the 2 stream-size categories. At the population-genetic level, we applied measures of among-site variation commonly used at both population-genetic (F_ST and Φ_ST) and community (Sørensen's dissimilarity with both presence/absence and abundance data) levels and developed a novel strategy to compare expected rates of loss of γ (regional) diversity as individual sites are eliminated sequentially from regions. At the community level, we limited analyses to Sørensen's presence/absence measures. We found that Sørensen's dissimilarity was significantly greater among headwaters than among mid-order streams at both population-genetic and community levels. We also showed that individual headwater reaches accounted for greater proportions of genetic γ diversity than did mid-order reaches. However, neither F_ST nor Φ_ST was significantly different between stream-size categories. These measures, which have been used traditionally for comparisons of population-genetic variation, measure proportions of total variation rather than solely among-site variation (i.e., they also are influenced by within-site variation). In contrast, Sørensen's dissimilarity measures only among-site variation and, therefore, is presumably more useful for reflecting general β diversity. Overall results suggest that, on average, headwaters probably contribute disproportionately to biodiversity at the network scale. This finding demands a shift in thinking about the biodiversity contributions of small headwaters and has strong conservation implications for imperiled headwater streams around the world.

Flood-pulsing is a key environmental factor that structures biotic communities in large-order river systems, but we focused our study on the effects of flood-pulsing in headwater systems. We used 10 mesocosm wetlands (10 m × 20 m) to test 2 treatments: a flood-pulse regimen in which natural flood events caused water levels to fluctuate and a static regimen in which water levels remained artificially stable. Abiotic characteristics, plants, and aquatic invertebrate communities were monitored from 2002 through 2005 in permanent pools, nonflooded banks in static wetlands, and intermittently flooded banks in flood-pulse wetlands. The flood-pulse treatment had minimal effects on environmental conditions of permanent pools, and submersed plant and aquatic invertebrate communities in permanent pools were similar in both treatments. This result suggested that resource subsidies from the floodplain to the pools were minimal. However, flood-pulsing caused observable changes to plant communities in the intermittently flooded zone (IFZ) above the permanently flooded pool. Overall plant diversity was higher in static wetlands, and % bare ground was higher in flood-pulse wetlands, results suggesting that the short, stochastic floods were a strong environmental stressor. In flood-pulse wetlands, the fluctuating water levels may have reduced the proportion of introduced, weedy, and upland plant taxa. Flood-pulse and static wetlands had distinctly different plant assemblage compositions, indicating that the abiotic stressors caused pronounced changes in the floodplain community. An indicator species analysis showed that taxa classified as obligate wetland plants were indicators in flood-pulse wetlands (e.g., Juncus canadensis, Ludwigia palustris, Dulichium arundinaceum, Eleocharis obtusa, Carex crinita, Carex lupulina, Carex vulpinoidea), but taxa classified as facultative wetland or upland plants were indicators of static wetlands (Cirsium arvense, Eupatoriadelphus maculatus, Plantago lanceolata, Bidens frondosus, Melilotus officinalis, Mentha arvensis, Daucus carota, Poa palustris). Many functional categories of plant species that were common in flood-pulse wetlands (e.g., obligate wetland plants, perennial, native and nonweedy species) are considered beneficial from a management perspective.

Island biogeography theory can be used to explain patterns of species richness on various types of habitat islands, including freshwater lake systems. Mollusk production in these systems also has been linked to various water-chemistry variables, such as pH, alkalinity, hardness, and specific conductance. We examined how mollusk diversity patterns were related to geographical and limnological factors in insular lakes of the Beaver and Manitou Archipelagos in Lake Michigan (Laurentian Great Lakes), USA. The strongest correlations observed were with shoreline development (r  =  0.80), specific conductance (r  =  0.87), and pH (r  =  0.87). Principal components analysis revealed that isolation by distance and PO₄³⁻ concentration also may have affected species richness and abundance. Shoreline length was a better predictor of species richness than surface area, but both measures of habitat size were unable to account for much of the variation in species richness. The data suggest that shoreline length and development represent available habitat area more accurately than lake area for primarily littoral-dwelling mollusks. The relatively weak correlations observed with lake area and isolation from Lake Michigan suggest that application of island biogeography theory to predict mollusk species richness using only lake surface area and isolation by distance is limited for freshwater mollusks.

The goal of our study was to investigate the interaction between the invasive mussel Limnoperna fortunei and benthic invertebrate communities at different spatial scales and under different environmental conditions. We analyzed the effects of L. fortunei on benthic invertebrate communities at different downstream distances (meters) from mussel beds and compared these trends in 2 rivers characterized by dissimilar chemical disturbance levels. In areas distant from L. fortunei beds, invertebrate composition at the 2 rivers differed strongly, probably in response to different levels of environmental pollution. In areas near L. fortunei beds, invertebrate composition at the 2 rivers was similar, suggesting that golden mussels have strong homogenizing effects on faunal makeup. We also found that facilitation was species specific and, in contrast to the general paradigm, weaker (rather than stronger) under more stressful conditions. Our results show that understanding the effects of L. fortunei requires accounting for scale- and species-specific effects.

The life table is a well known approach for understanding populations and has appeared in general ecology textbooks for >60 y. Secondary production is associated with energy flow, but methods rarely appear in texts. Our objectives were to: 1) demonstrate conceptual/analytical relationships between secondary production and life tables, 2) use a real example of pond-breeding salamanders to illustrate how the 2 approaches can be integrated, and 3) use publication frequency analysis to determine trends and biases for each approach within ecological (particularly aquatic) subdisciplines. The example illustrated that both approaches incorporate age- or stage-specific survivorship. The main difference is that life tables are used to calculate production of offspring with age-specific fecundity, and production analyses are used to calculate production of biomass with mass-specific increments. Publication frequency analysis over 12 y demonstrated that 78% more papers used life tables (698) than production (393) overall, but 50% more papers used production (353) than life tables (236) in basic research. Furthermore, production was studied primarily in aquatic (98%), mostly benthic (77%), environments. Life tables were used primarily in terrestrial environments (65%). Recognizing the relatedness of these concepts and usage biases may explain philosophical differences and help bridge gaps between terrestrial–aquatic and population–ecosystem ecologists.

Many state water-quality agencies use biological assessment methods based on lotic fish and macroinvertebrate communities, but relatively few states have incorporated algal multimetric indices into monitoring programs. Algae are good indicators for monitoring water quality because they are sensitive to many environmental stressors. We evaluated benthic algal community attributes along a landuse gradient affecting wadeable streams and rivers in Maine, USA, to identify potential bioassessment metrics. We collected epilithic algal samples from 193 locations across the state. We computed weighted-average optima for common taxa for total P, total N, specific conductance, % impervious cover, and % developed watershed, which included all land use that is no longer forest or wetland. We assigned Maine stream tolerance values and categories (sensitive, intermediate, tolerant) to taxa based on their optima and responses to watershed disturbance. We evaluated performance of algal community metrics used in multimetric indices from other regions and novel metrics based on Maine data. Metrics specific to Maine data, such as the relative richness of species characterized as being sensitive in Maine, were more correlated with % developed watershed than most metrics used in other regions. Few community-structure attributes (e.g., species richness) were useful metrics in Maine. Performance of algal bioassessment models would be improved if metrics were evaluated with attributes of local data before inclusion in multimetric indices or statistical models.

Heterogeneity generated by irregularities on the surface of streambed substrates is an important determinant of local species diversity of algae. However, few investigators have examined the effects of substrate roughness on the composition of algal growth forms and on patterns of species distribution. We examined the influence of substrate roughness on stream benthic algal assemblages through a field experiment with 2 treatments (smooth and rough artificial substrates for algal colonization). We assessed whether species richness, density, and assemblage composition of benthic algae (all taxa and those in 5 growth-form groups) differed between treatments and whether differences in species composition between substrates were the result of species turnover or nestedness. We also used a data subsampling procedure to investigate the effect of differences in species richness between treatments. Total species richness was higher on rough than on smooth substrates, but density did not differ between treatments. Species richness, density, and composition of the adnate/prostrate growth form did not differ between treatments. The erect/stalked growth form had higher species richness on rough substrates, but did not differ in density between treatments. All other growth forms (filamentous, motile, and metaphyton) had higher species richness and density on rough substrates and differed in species composition between substrates. The results of the subsampling analysis indicated that assemblage composition was affected by differences in species richness and by changes in species composition (i.e., turnover). Species distribution had a nested pattern, in which the assemblages on smooth substrates were a subgroup of the species occurring on rough substrates. We suggest that the differences in assemblage composition between smooth and rough substrates resulted from variability in species' capabilities to colonize substrates with or without crevices. This variability resulted in both nestedness and turnover.

Our paper describes the ecological controls on algal-mat diatom communities in the dynamic stream ecosystems of the McMurdo Dry Valleys in Antarctica. Dry Valley diatom communities are relatively diverse, and nearly ½ of the taxa found in these mats are considered endemic. Diatom community composition was examined in 5 streams in Taylor Valley during a 15-y cooling period that included a discrete flood event. Two hydrologic variables, total annual discharge and historical variation in discharge, gave the most parsimonious model of among-stream and interannual variation in diatom communities. Algal-mat biomass and chlorophyll a concentrations decreased after the flood, which occurred during the 2001/2002 summer season. Most algal-mat diatom communities recovered quickly after the flood. However, Green Creek, a relatively high-flow stream with low historical variation in discharge, appears to have experienced a persistent diatom community shift toward increased relative abundance of small, generalist species. Diatom relative biovolume, a proxy for the size of diatoms within a sample, was negatively correlated with stream flow, such that higher-discharge streams contained greater relative abundances of smaller diatoms than lower-flow streams. Therefore, diatom size may play a role in determining the distribution of a species in these streams and may be useful for monitoring environmental changes. Our study demonstrates the importance of understanding factors affecting ecosystem resilience, especially in polar regions, which are experiencing rapid climate changes.

Interest in the biodiversity value of urban waters is growing. Understanding key ecological processes is essential for effective management of these aquatic ecosystems. Our paper focuses on identifying the key factors that structure chironomid assemblages, such as water quality and dredging, in urban waters strongly influenced by seepage of large rivers. Chironomid assemblages were studied in urban surface-water systems (man-made drainage ditches) in polder areas along lowland reaches of the rivers Rhine–Meuse in The Netherlands. Multivariate analysis was used to identify the key environmental factors. Taxon richness, Shannon index (H′), rareness of species, and life-history strategies at urban locations were compared with available data from similar man-made water bodies in rural areas, and the effectiveness of dredging for restoring chironomid diversity in urban waters was tested. Three different chironomid associations were distinguished by Two-Way Indicator Species Analysis. Variation within and among chironomid associations were significantly related to substrate (sludge layer and substrate type: sand vs clay), % cover of lemnids, submerged vegetation, filamentous algae, and water transparency. Chironomid taxon richness and H′ were similar in urban and rural waters, probably because of their similar hydrologic, morphologic, and water-quality conditions and their similar dredging and weed-control regimes. Rareness was slightly higher in urban than in rural waters. In urban water systems, chironomid taxon richness was negatively related to sludge layer and % cover of lemnids. Dredging changed chironomid species composition, and increased taxon richness and life-history strategies indicative of good O₂ conditions. Therefore, dredging can be regarded as an effective measure to restore diversity of chironomid communities in urban waters affected by nutrient-rich seepage or inlet of river water.

We quantified fish assemblages in 20 streams containing mill dams in various physical conditions (dams intact, partially breached, or relict with normal flows) in Alabama, USA, during the period from 2006 to 2008. We used a backpack electroshocker to sample three 150-m reaches per stream: 500 to 1000 m downstream of the dam, 0 to 100 m downstream of the dam, and 100 m upstream of the impoundment. Species- and trait-based analyses revealed slightly different, but often complementary, information about fish assemblages. Fish species richness and benthic conditions differed longitudinally among reaches in streams with dams. In streams with breached dams, species richness, but not trait richness, was lower in upstream reaches than in downstream reaches. Overall, species and trait richness were correlated with benthic-habitat variables in streams with relict dams and were significantly correlated with water physicochemical variables in streams with intact and breached dams. Nonmetric multidimensional scaling ordination failed to resolve any discernable site groupings based on species abundance data, and indicator species analysis revealed 1 indicator species, Esox americanus, upstream of relict dams. Fourth-corner trait analysis revealed more trait associations in reaches in streams with breached dams than in those with intact or relict dams. Generalist spawners (nest-guarding polyphils) increased and taxa with a preference for cobble substrates decreased upstream of breached dams. Few longitudinal differences were observed in streams with relict and intact dams. Taken together, dams, particularly those that are breached, appear to exert a strong upstream influence on fish species richness and functional composition and could alter the trophic structure of the entire stream through habitat modifications or limitation of fish movements.

Small dams are ubiquitous yet poorly understood features in many streams. Dam removal is being used increasingly in stream restoration projects as a means to enhance habitat connectivity and ecosystem function. However, habitat- and assemblage-level effects of small dams on stream mollusk assemblages are poorly documented. We examined associations between stream physicochemical habitat variables and mollusk assemblages at 22 small (<10 m) dams in 3^rd–5^th order Alabama streams. We sampled 66 reaches (3 reaches/dam) associated with intact, breached, and relict small dams. For each dam, we designated 3 study reaches: 1) immediately downstream from the dam, 2) 500 to 5000 m downstream, and 3) 500 to 5000 m upstream from the impounded or formerly impounded zone. We used principal components analysis (PCA) to examine variation in physical-habitat conditions across all sites. Four principal components accounted for ∼72% of the variation in physical-habitat conditions across sites. One PC score (PC₁, corresponding to increased substrate size) was negatively associated with several mollusk metrics including total mussel abundance, taxon richness, catch-per-unit-effort (CPUE), and density. We observed few significant differences between simple habitat variables at sites up- and downstream of dams. However, streams with intact dams had significantly higher mussel catch rates (CPUE) and taxon richness than did streams with breached or relict dams. We used forward-stepwise multiple regression to model the effects of habitat variables (as standardized PC scores) on mollusk assemblage metrics. PCs representing substrate composition were the strongest predictors of total mussel abundance and richness. Abundance of other mollusks including deposit-feeding snails, the exotic bivalve Corbicula fluminea, and fingernail clams was correlated with PC scores describing variability in substrate organic matter composition or stream gradient. We think these data indicate that some intact dams enhance mollusk habitat in downstream reaches. Streams with intact dams appear to be more geomorphically stable than streams with breached or relict dams and conditions in the mill reach may reflect preconstruction stream conditions. Breached dams warrant higher prioritization for removal than intact structures because habitat degradation may persist for decades and impede re-establishment of native mollusk populations.

The effects of barriers to migration by adult salmon smolt have been well studied, but effects to migration of juvenile salmon during freshwater residency have not been evaluated thoroughly. We tested for differences in the relative abundance of juvenile Pacific salmon upstream and downstream of road-crossing culverts in 10 streams, 5 with moderate (upland) and 5 with low (wetland) slope. We compared stream width, bed slope, and substrate in natural channels and culverts; measured culvert water velocity during base flow; and obtained maximum and minimum culvert velocities using the flow time of dissolved solutes. We compared flow velocities to swimming speeds of rearing juvenile salmon to evaluate potential passage of fish through culverts. In upland streams with spawning adults, catch-per-unit-trap (CPUT) of juvenile coho salmon was often 2 to 3× greater above than below culverts. In these reaches, downstream migration of salmon fry from spawning locations may be limited by culvert flow velocities that exceed burst swimming speeds of juveniles. Where 2 upland crossing structures were replaced with culverts that replicated natural channel slopes, widths, and substrate, flow velocities decreased and differences in coho salmon CPUT were eliminated. In wetland streams with adult spawning reduced or absent above the crossing, CPUT of juveniles was greater below than above culverts where the outlet was perched. Thus, culverts can influence upstream and downstream fish movement and altered the distribution of rearing fish within streams.

Habitat fragmentation and nonindigenous species are 2 of the leading causes of species loss globally. Thus, scientific information is needed to assess their interactive effects on the biota of lotic ecosystems. We tested the hypothesis that culverts (i.e., pipes) at road–stream intersections elevate flow velocities and differentially slow upstream locomotion of native relative to nonindigenous crayfishes. We: 1) mapped culvert locations and measured their flow; 2) quantified movement of Orconectes propinquus (native) in culverts with velocities of 2, 31, and 42 cm/s; 3) compared the movement of Orconectes rusticus (nonindigenous), O. propinquus, and Orconectes virilis (native) in a culvert with velocity of 30 cm/s; and 4) used a recirculating flume to determine the water velocity that impeded upstream movement (impedance velocity) of each species. Culverts had higher velocities than upstream riffles during baseflow conditions. Orconectes propinquus moved upstream in culverts with flows >30 cm/s, but individuals slipped more frequently and traveled slower than those tested at 2 cm/s. Orconectes rusticus moved upstream faster in culverts and had an impedance velocity ∼6 cm/s faster than O. propinquus (34.0 ± 1.2 cm/s, mean ± 1 SE) and ∼8 cm/s faster than O. virilis (32.0 ± 1.1 cm/s). Culverts impeded crayfish upstream movement differently among species. Nonindigenous species tolerated higher flows, so culverts may create a filter that favors the spread of invasive species. Culverts should be designed to keep water velocity <30 cm/s to mitigate flow effects on crayfish passage, thereby minimizing the possible combined effects of fragmentation and nonindigenous species introductions.