The reference-condition approach to bioassessment often uses the observed/expected (O/E) ratio to indicate anthropogenic alteration of aquatic macroinvertebrates, fish, or periphyton assemblages. Given a list of taxa found at ≥1 minimally disturbed reference sites, E is the number of those taxa that would expected in a sampled assemblage if the sampled stream were in reference condition, and O is the number of those taxa observed in the sample. An O/E value significantly <1.0 indicates that a stream has lost taxa relative to its reference-condition expectation, possibly because of anthropogenic stress. However, the O/E index can be insensitive to stress-induced shifts in taxonomic composition that leave assemblage richness unchanged. As an alternative to O/E, I propose using BC, an adaptation of Bray–Curtis distance, to measure the compositional dissimilarity between an observed and expected assemblage directly. I compared the performance of BC and O/E at 5685 stream and lake sites throughout the contiguous 48 states of the US using 1 of 10 River Invertebrate Prediction and Classification system (RIVPACS)-type models to predict expected assemblages. Percentages of independently determined nonreference sites that were declared to be nonreference by BC exceeded the percentage declared to be nonreference by O/E by an average of 6 to 16 percentage points, depending on whether the 2 indices included low-probability taxa, whether a 1-sided or 2-sided O/E criterion was used to declare nonreference, and whether predictive or null models were used to predict expected assemblages. Correlations between BC scores and anthropogenic stressor variables were stronger than correlations between O/E scores and anthropogenic stressor variables in 18 of 25 cases. In contrast to O/E, BC can include low-probability taxa without reducing its power to detect nonreference conditions.

Zebra mussels might enhance denitrification rates by altering 3 primary controls: O₂, NO₃^–, and labile C availability. We stocked mesocosms with stream sediments and either no zebra mussels (−ZM) or 10,000 mussels/m² ( ZM) to examine these potential mechanisms. We measured sediment nitrification (nitrapyrin-inhibition technique), denitrification (chloramphenicol-amended acetylene block technique), sediment O₂ profiles (microelectrodes), and a suite of water and sediment characteristics weekly for 3 wk. Nitrification (2-way analysis of variance [ANOVA], p < 0.001) and denitrification (2-way ANOVA, p < 0.001) rates were significantly higher in ZM than in −ZM mesocosms. High-NH₄ waste from zebra mussels increased sediment nitrification rates, which increased NO₃^– availability for denitrification. Furthermore, coupled nitrification–denitrification was enhanced by a reduction in the sediment depth to anoxia in the presence of zebra mussels, and this reduction allowed both processes to occur in close spatial proximity. Despite increased denitrification rates, we did not measure an increased proportion of N lost via denitrification from the ZM mesocosms. The presence of zebra mussels can increase NO₃^– availability through increased nitrification rates, potentially exacerbating eutrophication of invaded waters.

Most methods for assessing the ecological status of streams focus on structural characteristics (water quality, community composition, riparian vegetation) but neglect functional properties of the ecosystem because routine methods to assess stream function are scarce. Metabolism, one of the most integrative ecosystem functions, can be a good indicator of stream function because it is relevant across all sizes and types of streams, is sensitive to stressors, such as eutrophication or changes in riparian cover, and can be measured continuously. Environmental controls on whole-ecosystem metabolism were measured at 19 contrasting stream reaches in the Basque Country (northern Spain). Discharge, temperature, and O₂ were monitored continuously for 15 mo, reaeration rate was calculated with the nighttime regression method, and whole-stream metabolism was calculated by the single-station open-channel method. The effect of discharge on reaeration coefficients was highly site-specific. Average gross primary production (GPP) ranged from 2.7 to 11.0 g O₂ m⁻² d⁻¹, was highest at eutrophic sites, and showed no relationship with periphyton biomass. Ecosystem respiration (ER) ranged from 6.3 to 42.6 g O₂ m⁻² d⁻¹ and was highest at polluted sites. Differences among sites increased in summer. All sites were heterotrophic on an annual basis, but 3 were autotrophic during summer. Turbidity was the main controller of primary production during summer and explained 20% and 39% of the spatial variation in GPP and net ecosystem production, respectively. Biological O₂ demand of water explained 40% of ER variance. Catchment activities also controlled GPP, which decreased as population density increased. To our knowledge, our study is the first report of continuous monitoring of whole-stream metabolism at many reaches simultaneously, and it shows the potential of this technique for routine monitoring of stream function.

Eurylophella oviruptis new species is described and illustrated from female larvae and reared subimagos and imagos collected from swamp streams in North Carolina, USA. Several key morphological characters in the larva place this new species in the Eurylophella temporalis group. Genetic comparisons with other eastern North American Eurylophella revealed a fixation of alleles unique to E. oviruptis at 4 of 19 allozyme loci. Eurylophella oviruptis appears to be obligately parthenogenetic because no males were observed in the field or laboratory and eggs taken from subimagos and imagos hatched parthenogenetically (mean hatch rate = 79%). Abdomens of 60% of subimagos reared in the laboratory burst along the ecdysial line of the first 3 tergites immediately after transformation to the subimago at the water surface. Abdominal bursting ruptured the oviducts, released most of the eggs into the water, and left the subimago trapped on the water surface with a large, inflated midgut protruding from the split in the tergites. Subimagos that did not burst (40%) flew from the water surface, molted to the imago on the 2^nd day following emergence, and then, without having mated, extruded a ball of eggs that was released into the water. Dissections of E. oviruptis and 5 other species of mayflies showed that inflation of the mayfly gut normally occurs at 3 stages: emerging larvae inflate prior to and during the molt to the subimago, subimagos (particularly males) inflate further at the imaginal molt, and female imagos inflate even further at oviposition. Gastrointestinal inflation in mayflies maintains full abdominal distention that might facilitate flight. Abdominal bursting in E. oviruptis subimagos appears to be the result of gut inflation well beyond the amount normally associated with this stage. The evolutionary significance (predation, dispersal, demographics) of oviposition by abdominal bursting is discussed.

The concept of landscape legacies has been examined extensively in terrestrial ecosystems and has led to a greater understanding of contemporary ecosystem processes. However, although stream ecosystems are tightly coupled with their catchments and, thus, probably are affected strongly by historical catchment conditions, few studies have directly examined the importance of landuse legacies on streams. We examined relationships between historical land use (1944) and contemporary (2000–2003) stream physical, chemical, and biological conditions after accounting for the influences of contemporary land use (1999) and natural landscape (catchment size) variation in 12 small streams at Fort Benning, Georgia, USA. Most stream variables showed strong relationships with contemporary land use and catchment size; however, after accounting for these factors, residual variation in many variables remained significantly related to historical land use. Residual variation in benthic particulate organic matter, diatom density, % of diatoms in Eunotia spp., fish density in runs, and whole-stream gross primary productivity correlated negatively, whereas streamwater pH correlated positively, with residual variation in fraction of disturbed land in catchments in 1944 (i.e., bare ground and unpaved road cover). Residual variation in % recovering land (i.e., early successional vegetation) in 1944 was correlated positively with residual variation in streambed instability, a macroinvertebrate biotic index, and fish richness, but correlated negatively with residual variation in most benthic macroinvertebrate metrics examined (e.g., Chironomidae and total richness, Shannon diversity). In contrast, residual variation in whole-stream respiration rates was not explained by historical land use. Our results suggest that historical land use continues to influence important physical and chemical variables in these streams, and in turn, probably influences associated biota. Beyond providing insight into biotic interactions and their associations with environmental conditions, identification of landuse legacies also will improve understanding of stream impairment in contemporary minimally disturbed catchments, enabling more accurate assessment of reference conditions in studies of biotic integrity and restoration.

We examined the relationships between the distribution of dominant herbivorous insect grazers (Glossosoma larvae), environmental factors (current velocity, water depth, periphyton biomass), and grazer–periphyton interactions at multiple spatial scales (microhabitat, riffle, reach) in a stream. We used multiple regression models to explain densities of Glossosoma larvae at each spatial scale in terms of the environmental factors. All r²-values were significantly higher at the riffle than at the microhabitat or reach scales. Thus, the riffle scale provided better predictions of Glossosoma larval density than did the microhabitat (smaller) and reach (larger) scales. The r²-values of exponential regressions between grazer densities and periphyton biomass were lower at the microhabitat than at the riffle or reach scales. These results indicate that the patterns of relationships between the insect grazers and periphyton were detected more clearly at larger than at smaller scales.

Developing spatially explicit conservation strategies for stream fishes requires an understanding of the spatial structure of dispersal within stream networks. We explored spatial patterns of stream fish dispersal by evaluating how the size and proximity of connected streams (i.e., stream network topology) explained variation in fish assemblage structure and how this relationship varied with local stream size. We used data from the US Environmental Protection Agency's Environmental Monitoring and Assessment Program in wadeable streams of the Mid-Atlantic Highlands region (n = 308 sites). We quantified stream network topology with a continuous analysis based on the rate of downstream flow accumulation from sites and with a discrete analysis based on the presence of mainstem river confluences (i.e., basin area >250 km²) within 20 fluvial km (fkm) from sites. Continuous variation in stream network topology was related to local species richness within a distance of ∼10 fkm, suggesting an influence of fish dispersal within this spatial grain. This effect was explained largely by catostomid species, cyprinid species, and riverine species, but was not explained by zoogeographic regions, ecoregions, sampling period, or spatial autocorrelation. Sites near mainstem river confluences supported greater species richness and abundance of catostomid, cyprinid, and ictalurid fishes than did sites >20 fkm from such confluences. Assemblages at sites on the smallest streams were not related to stream network topology, consistent with the hypothesis that local stream size regulates the influence of regional dispersal. These results demonstrate that the size and proximity of connected streams influence the spatial distribution of fish and suggest that these influences can be incorporated into the designs of stream bioassessments and reserves to enhance management efficacy.

Resource availability is an important ecosystem attribute that can influence species distributions and ecosystem processes. We manipulated the quantity of leaf litter, a critical resource in streams, in a replicated field experiment to test whether: 1) greater litter quantity promotes microbial leaf decomposition (through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition by leaf-shredding invertebrates (because shredders aggregate on rare resource patches). In each of 3 streams, we identified reaches in which litter quantity was either: 1) augmented, 2) depleted, or 3) left unchanged. We determined decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- and fine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shredding macroinvertebrates. Responses to litter manipulations were complex. In 2 streams, litter quantities differed among treatments, but high quantities of litter in the control reach of the 3^rd stream produced an overall variable pattern. Microbial decomposition was similar across litter treatments. In contrast, in the 2 streams where litter manipulation was successful, decomposition in coarse-mesh bags tended to be faster where litter was scarce than where it was abundant. Abundances of total and leaf-shredding macroinvertebrates in litter bags did not differ among litter manipulations in these 2 streams. However, a litter-consuming amphipod (Gammarus fossarum) tended to be most abundant in bags placed in litter-depleted reaches in the 2 streams, indicating that this large and highly mobile shredder might have been instrumental in causing differences in decomposition in response to litter manipulations. Overall, the effects caused by alteration of litter quantities on leaf decomposition and macroinvertebrate colonization were relatively weak. Nevertheless, results from 2 of the 3 streams where litter manipulation was successful were consistent with the hypothesis that short-term changes in resource availability might influence ecosystem processes by determining the spatial distribution of key consumers.

Coupled production between algae and bacteria in stream epilithon was assessed along a nutrient-enrichment gradient in 8 Texas streams with open canopies. Photosynthesis (PS) and bacterial biomass production (BBP) were measured simultaneously using a dual-label radioassay (¹⁴C-HCO₃^– uptake and ³H-L-leucine incorporation into protein) on multiple samples within a stream reach. PS and BBP were measured after light (1200–1500 μmol m⁻² s⁻¹) and dark incubations. The degree of coupled production between algae and bacteria within a stream was estimated as the covariation (i.e., correlation or covariance) between PS and BBP derived from unshaded replicates in each stream. Streamwater nutrients ranged from 0.18 to 8.1 mg/L total N and 0.009 to 2.0 mg/L total P. Epilithon N and P content (as % dry mass) and C:N:P ratios varied widely among streams and were positively correlated with streamwater nutrient concentrations. Mean BBP measured in light incubations (BBP_L) was greater than mean BBP measured in dark incubations (BBP_D), and the difference between the 2 means (BBP_L – BBP_D) was positively correlated with mean PS among streams (R² = 0.53). Covariance between PS and BBP_L within streams (COV_PS–BBP) decreased as epilithon nutrient content increased. COV_PS–BBP was positively correlated with both epilithon C:N (R² = 0.78) and C:P (R² = 0.77) among streams. These results suggest that algal and bacterial production are decoupled by nutrient enrichment, and that algae might rely more heavily on bacterial-regenerated nutrients than on streamwater nutrients to support production in nutrient-poor streams.

Freshwater bivalves of the order Unioniformes represent the largest bivalve radiation in freshwater. The unioniform radiation is unique in the class Bivalvia because it has an obligate parasitic larval stage on the gills or fins of fish; it is divided into 6 families, 181 genera, and ∼800 species. These families are distributed across 6 of the 7 continents and represent the most endangered group of freshwater animals alive today. North American unioniform bivalves have been the subject of study and illustration since Martin Lister, 1686, and over the past 320 y, significant gains have been made in our understanding of the evolutionary history and systematics of these animals. Here, the current state of unioniform systematics and evolution is summarized, and suggestions for future research themes are proposed. Advancement in the areas of systematics and evolutionary relationships within the Unioniformes will require a resurgence of survey work and reevaluation of all taxa, especially outside of North America and Western Europe. This work will require collection of animals for shell morphology, comparative anatomy, and molecular analyses. Along with reexamination of described taxa, a renewed emphasis on the natural history, host-fish relationships, ecology, and physiology of these animals is needed. Traditional conchological and anatomical characters should be reevaluated, new character suites should be added, and new morphometric methods should be applied. The fossil record of freshwater bivalves should be carefully reviewed, and phylogenetic hypotheses including fossil taxa must be developed. We will have to expand our set of molecular tools to include or develop additional markers, such as single-copy nuclear genes and microsatellites. Examination of double uniparental inheritance of mitochondrial deoxyribonucleic acid (DNA) is providing new insights into the evolution of this order. Mitochondrial gene order differs among genera but is still to be explored. Expansion of our understanding of the evolutionary relationships and history of unioniform bivalves will provide a solid foundation to study the zoogeography of these rather sessile, obligate freshwater organisms. The unique natural history of unioniform bivalves provides a fertile area for testing and developing evolutionary theories, and, as our understanding of the systematics of these animals improves, a better understanding of the evolution of this expansive radiation in freshwater will develop.

Freshwater mussel larval parasitism of fish is unique among bivalves. The relationship is primarily phoretic rather than nutritive; only the smallest glochidia and the haustorial larva grow substantially while on the host. Growth of the smallest larvae suggests a lower functional size limit of ∼150 μm for the juvenile stage. Most Ambleminae, the most diverse North American clade, infect host gills by attracting feeding fish. Many species of Pleurobemini and some Lampsilini release conglutinates of eggs and larvae that resemble host food items. Many Lampsilini and a few Quadrulini use mantle modifications to attract host fish to the female. The mantle of some Quadrulini forms a posterior chamber that holds glochidia for immediate release in response to host fish. In many Lampsilini, mantle flap lures and a protrusible marsupium promote attack by the host fish and direct extraction of glochidia from the marsupium by the host. Host extraction of glochidia from the brooding female might have favored the evolution of long-term brooding in Lampsilini because glochidia need not be released by the female to encounter the host. A remarkable derivative of the host extraction strategy evolved in Epioblasma, which catch fish between the valves and release glochidia directly to the trapped host before releasing it. Host specificity is a critical feature of the evolutionary diversification and conservation biology of Unionoida. As temporary parasites, mussels must primarily evade the innate immune responses of the host, rather than the adaptive (acquired) responses. Evolution of host specificity is associated with selective encounter of host taxa, either because of host attraction strategies or because of dominance of particular host species in the habitat. The intricate relationships between mussels and fish are easily disrupted and, thus, contribute to the imperilment of many mussel species, yet they also fascinate us and compel conservation efforts.

Population viability analysis (PVA) provides a mechanism for analyzing extinction risk by considering processes, such as random fluctuation in demographic features, loss of genetic variation, environmental stochasticity, and the occurrence of catastrophes. Freshwater mussels (Unionoidea) are candidates for PVA because of elevated risk of extinction from anthropogenic activities. We designed a stage-based conceptual model summarizing demographic and genetic changes throughout the mussel life cycle that are associated with changes in population size. We discuss what is known about these stages and the processes that affect transitions between stages. Considerably more information is known about adults than other life stages. Much more work must be done on nonadult life stages because they are potentially vulnerable to disruption via environmental degradation, habitat fragmentation, and loss of vertebrate hosts. New approaches, such as development of molecular identification keys, use of microsatellite markers, and assignment tests to measure dispersal, promise to increase our understanding of nonadult life stages, breeding systems, and linkages among populations. Few studies have attempted to use theory from population biology and conservation genetics to gain insight into strategies for effective conservation. We suggest that more work must be done with species that are not yet critically imperiled because study of such species is likely to yield useful data for PVAs and insight into the mechanisms regulating freshwater mussel populations.

Freshwater mussel (Superfamily Unionoidea) communities are important components of food webs, and they link and influence multiple trophic levels. Mussels filter food from both the water column and sediment with ciliated gills. Differences in cilia structure and arrangement might allow mussel species to partition food resources. Mussels are omnivores that feed across trophic levels on bacteria, algae, detritus, zooplankton, and perhaps, dissolved organic matter. Living mussels and their spent shells provide or improve habitat for other organisms by providing physical structure, stabilizing and bioturbating sediments, and influencing food availability directly and indirectly through biodeposition of organic matter and nutrient excretion. Effects of mussel communities on nutrient translocation and cycling depend on mussel abundance, species composition, and environmental conditions. Nutrient-related mussel effects influence multiple trophic levels. Healthy mussel communities occur as multispecies assemblages in which species interactions are probably very important. Food limitation and competition among species have been documented, but so have positive species interactions, and rare species have been shown to benefit energetically from living in species-rich communities. Effects of mussel species on ecosystem services and food webs vary across spatial and temporal scales, and the relative importance of competition and facilitation might change at different scales.

Mussel populations and the environments they inhabit are heterogeneous and fragmented. We review 3 areas in which principles of landscape ecology might be applied to the scientific understanding and management of freshwater mussels. First, recent studies show that hydraulics can be used successfully to delineate patches of mussel habitat, but additional variables such as host fish, food, or predators are probably important under certain conditions. However, research on patch dynamics in freshwater mussels is in its infancy, and we do not know if existing methods to delineate patches are adequate. Second, mussel ecologists are starting to think about the importance of connectivity among habitat patches. Major challenges will be to determine whether connectivity can be estimated in the field and whether human activities that reduce connectivity (e.g., dams) have produced large extinction debts in mussel populations. Third, we need to better understand the links between events on the watershed (e.g., timing and amounts of water, nutrient, and sediment inputs) and the quality, extent, location, and connections among patches of mussel habitats. Because of its focus on patterns and processes, landscape ecology has the potential to improve scientific understanding and management of mussel populations and, in particular, to help define the best spatial scales for scientific studies and management activities.

Ecological stoichiometry is the study of the balance of multiple elements in ecological interactions and processes. We investigated the ecological stoichiometry of freshwater mussels across 3 seasons at 2 sites each in an agricultural watershed (Little Darby Creek [LD], Ohio) and a forested watershed (Upper Ouachita River [OR], Arkansas) (2 species/stream). We used nutrient-release experiments to determine C, N, and P content and elemental ratios in seston and in consumer-driven nutrient recycling (CNR) components (mussel soft tissues, shells, biodeposited material, and excreted nutrients). We focused on seasonal patterns of: 1) biodeposition and excretion rates; 2) seston and CNR component % C, % N, % P, C:N, C:P, and N:P; and 3) degree of homeostasis in mussels. Differences in mass-specific biodeposition and excretion rates were driven largely by seasonal factors. Percent P, C:N, C:P, and N:P of seston were seasonally variable in LD, and % C, % N, % P, C:N, C:P, and N:P of seston were seasonally variable in OR. Mussel tissues and biodeposited materials were variable for 3 to 5 of the 6 nutrient metrics in both LD and OR. Mussel shell and soft-tissue nutrient stoichiometry were relatively homeostatic and fell within stoichiometric ranges of other macroinvertebrates, except that mussel tissue had higher % P and lower C:P and N:P than are usually observed in other macroinvertebrates. Mussels can repackage nutrients in stream seston and fine benthic organic matter in the form of biodeposited material and excreted nutrients, thereby providing a significant source of C, N, and P for other benthic organisms.

Freshwater mussels (superfamily Unionoidea) are in serious global decline and in urgent need of protection and conservation. The declines have been attributed to a wide array of human activities resulting in pollution and water-quality degradation, and habitat destruction and alteration. Linkages among poor water quality, pollutant sources, and mussel decline in rivers and streams have been associated with results of laboratory-based tests of specific pollutants. However, uncertainties remain about the relationship of laboratory data to actual contaminant exposure routes for various mussel species, life stages, and in the habitats occupied during these exposures. We evaluated the pathways of exposure to environmental pollutants for all 4 life stages (free glochidia, encysted glochidia, juveniles, adults) of unionoidean mussels and found that each life stage has both common and unique characteristics that contribute to observed differences in exposure and sensitivity. Free glochidia typically are exposed only briefly (e.g., seconds to days) through surface water, whereas adults sustain exposure over years to decades through surface water, pore water, sediment, and diet. Juveniles live largely burrowed in the sediment for the first 0 to 4 y of life. Thus, sediment, pore water, and diet are the predominant exposure routes for this life stage, but surface water also might contribute to exposure during certain periods and environmental conditions. The obligate parasitic stage (encysted glochidia stage) on a host fish might be exposed from surface water while partially encysted or from toxicants in host-fish tissue while fully encysted. Laboratory methods for testing for acute and chronic exposures in water have advanced, and toxicant-specific information has increased in recent years. However, additional research is needed to understand interactions of life history, habitat, and long-term exposure to contaminants through water, pore water, sediment, and diet so that the risks of environmental exposures can be properly assessed and managed.

North American freshwater gastropods remain an understudied, yet critically imperiled, fauna. As part of a larger discussion on freshwater mollusks in this special issue, we review 4 specific areas of concern regarding freshwater gastropods and discuss how best to address those concerns in the context of conservation. Areas of concern include freshwater gastropod conservation strategies, taxonomy and systematics, ecological research, and conservation challenges. We illustrate how each of these topics relates to conservation efforts and discuss opportunities to improve our baseline knowledge of freshwater gastropod taxonomy, ecology, and conservation. We emphasize throughout that effective conservation strategies require the participation of as many affected and interested groups, from local communities to governmental agencies, as possible for successful implementation and management. We offer suggestions for the direction of cooperative conservation with regard to freshwater gastropods.

The roles of systematics in the field of conservation biology are well understood and accepted for many organisms. However, the role of systematics and taxonomy has not been reviewed in the context of species protection and management of freshwater gastropods. We provide a thorough review of the relevant theoretical literature in systematics and taxonomy and illustrate with recent examples of species delineation and taxonomy in North American freshwater gastropods that these fields play key roles in the practical designation of conservation management units. We summarize some aspects of the biology of freshwater gastropods that can confound taxonomic and management efforts. Based on our review, we recommend that effective conservation plans include the systematic research necessary to recognize unique organismal lineages as primary conservation management units. This strategy must be combined with consistent and rigorous nomenclature, taxonomy, and dissemination of research findings so that all parties have access to the highest quality information.

Many North American freshwater mollusks are at risk of extinction. Knowledge of basic ecology and systematics of the pleurocerid and hydrobiid gastropods is lacking. Pleurocerids are most diverse in southeastern USA, and we know that periphyton food limits their growth, and that their grazing, in turn, limits periphyton biomass. However, we know little about the effects of spates and current velocity on pleurocerid populations, and more work is needed to determine whether interspecific competition or significant risk from predation occurs. Hydrobiids are extremely diverse, but many species inhabit only a few springs (especially in arid western USA) and are at risk of extinction. More work is needed on their population and community ecology. Invasions pose a risk to native snail species. For example, the New Zealand mudsnail (Potamopyrgus antipodarum) interacts negatively with several hydrobiids in the Snake River in western USA. We suggest several research avenues that are needed if we are to maintain and restore pleurocerid and hydrobiid snail populations.