We examined how substrate and complex hydraulic variables limit the distribution of freshwater mussels. We sampled mussels and measured substrate and hydraulic variables (at low and high flows) at 6 sites in the Little River, Oklahoma. To test which variables were most limiting to mussel species richness and abundance, we evaluated univariate and multiple 95^th-, 90^th-, and 85^th-quantile regression models using a model selection approach. Across all 3 quantiles analyzed, hydraulic variables related to substrate stability (relative shear stress ratio [RSS] and shear stress) at high flows most limited mussel species richness and abundance. High-flow substrate stability models performed the best, but models that used substrate variables (substrate size and heterogeneity) also performed relatively well. Models that used complex hydraulic variables estimated at low flows performed poorly compared to those using the same variables estimated at high flows, a result suggesting that hydraulic conditions at low flows do not limit mussel habitat in our system. Our results demonstrate that substrate stability at high flows is an important factor governing mussel distributions. Last, our quantile regression approach successfully quantified the limiting-factor relationships of substrate and hydraulic characteristics on mussel habitat, and this approach could be used in other studies investigating habitat requirements of aquatic organisms.

Prevailing notions of foodweb structure and trophic relations in fresh waters are based on research undertaken in a limited range of latitudes or habitat types. This limitation had led to the general view that stream food webs are detritus-based with short food chains and simple interactions that often are dominated by a few key species. We used gut-content analyses and stable-isotope signatures to characterize feeding relationships and foodweb attributes of benthic communities in riffles in 2 forest streams in Hong Kong. We compared them with existing data on foodweb structure from pools in 1 of the streams and data from the literature. The 2 approaches to dietary analyses yielded complementary results, providing confidence that trophic relations and foodweb structure were adequately characterized. Food webs in both streams were remarkably similar regardless of habitat (riffle vs pool). Consumers in both streams depended primarily on autochthonous resources, as has been reported from some other tropical streams, and food chains were short although connectance was higher than has been recorded previously for stream food webs. Very few omnivores were found, and omnivory was even rarer than is typical of temperate streams, although it is common in other tropical streams and rivers. No evidence was found for dominance by a few common macroconsumer species, as observed elsewhere in the tropics. The apparently high levels of autochthony in tropical running waters imply that models of ecosystem functioning for northern temperate streams are inadequate for describing tropical systems. However, marked differences in the degree of omnivory and dominance of tropical stream food webs by macroconsumers is evidence that characterization of trophic interactions and stream ecosystem functioning cannot be captured by a simple tropical vs temperate dichotomy. Successful management of these systems will depend upon development of conceptual models that reflect the diversity of food webs within and between regions.

The genus Manophylax (Trichoptera:Apataniidae) contains 3 species from the Nearctic Region and 3 species from Japan. A 7^th species of Manophylax new for science is described and illustrated from Alaska. Habitats of larvae and pupae of the different Manophylax species are either mostly hygropetric or mostly terrestrial. Phylogenetic analysis infers 3 principal species groups occurring in the eastern and western Nearctic and eastern Palearctic Biogeographic Regions. The habitat of the western Nearctic M. annulatus Species Group is mostly hygropetric and that of the eastern Nearctic M. altus and eastern Palearctic M. futabae Species Groups are mostly terrestrial. The phylogeny suggests that the mostly terrestrial habitat evolved through a mostly hygropetric habitat. The phylogeny also implies that the eastern Nearctic and eastern Palearctic species share a more recent common ancestor than they share with the western Nearctic species, a result further supporting a commonly observed historical biogeographic pattern for these regions. Madeophylax Huryn and Wallace is redefined as a subgenus of Manophylax Wiggins, including the species of the M. altus and M. futabae Species Groups. Keys to adult males, females, larvae, and pupae of the 4 North American species of Manophylax are included.

Predation might cause the disparate distributions of fish and shrimp in Puerto Rican streams. Adult shrimp (Atya lanipes) are present only above waterfalls that pose barriers to predatory fishes. Because we have not found Atya in fish stomachs, we think that these amphidromous shrimp avoid predation by migrating to locations above waterfalls as post-larvae. We designed experiments in an artificial y-maze and in natural streams to test whether Atya avoid predatory fishes. In artificial streams, adult Atya avoided the scent of 2 fishes (Gobiomorus dormitor and Agonostomus monticola) regardless of whether the fish had recently been fed Atya, but they avoided the scent of Anguilla rostrata only when they had recently been fed Atya. Postlarval Atya avoided the scent of G. dormitor fed Atya and of crushed conspecifics. We then tested whether adult Atya in 4 natural headwater streams would redistribute themselves if exposed to the chemical scent or physical presence (visual, mechanical, and chemical cues) of G. dormitor. We expected Atya to move upstream from the manipulated pools, resulting in increased emigration from and lower Atya abundance in the pool. Atya relative abundance did not decline significantly in the pools when fish scent was added, indicating that chemical cues alone were not threatening enough to alter adult behavior in natural streams. The relative abundance of adults declined significantly in the pools when fish were added to in-stream cages, a result suggesting that addition of mechanical or visual cues to chemical cues signaled imminent danger and caused Atya to alter their behavior. Emigration increased, but lower activity levels also could have reduced abundances in pools. Our study provides mechanistic evidence that predatory fishes alter the behavior of postlarval and adult A. lanipes, and might explain why adult Atya are absent in reaches downstream of waterfalls.

The zebra mussel (Dreissena polymorpha) provides one example of successful invaders in novel environments. However, little attention has been devoted to exploring the factors regulating zebra mussel density and population size structure at the local scale. We tested effects of physicochemical factors and fish predation on the density of zebra mussels at several sites and between years in a natural lake. Water depth and roach (Rutilus rutilus) density were the most important variables affecting local zebra mussel density. Substrate was also an important factor but affected Dreissena density only at the shallowest depth examined (2 m), which also supported a large population of the mussels. Mean shell length of Dreissena increased with water depth. Our results indicate that predation pressure, intraspecific competition, and food limitation might be responsible for variation in zebra mussel density and population size structure in space and time and that fish predation might have strong top-down effects on zebra mussel populations.

Most studies of the effects of litter mixing have focused on decomposition in mixtures of senescent leaves of different plant species. In temperate forests, substantial quantities of green leaves fall in spring and summer, and previous research has shown that stream detritivores can benefit from the combined use of senescent and green leaves. We conducted a litterbag experiment to test the hypothesis that mixtures of senescent and green leaves of different species decomposed more rapidly, mediated by increased detritivore colonization, than leaves of each type in isolation. The observed abundances of 2 detritivore species, the amphipod Sternomoera rhyaca and the caddisfly Lepidostoma complicatum, were 3 to 6× higher in mixtures of senescent oak and green alder leaves than expected based on litter patches consisting of each species alone. Lepidostoma complicatum also responded positively to mixtures of senescent oak and green maple leaves, but S. rhyaca did not. These results verify that caddisfly behavior can reflect benefits gained from the combined use of senescent and green leaves. In contrast, the extent of the advantage to amphipods depends on the specific combination of senescent and green leaves. Despite the synergistic effect of mixed senescent oak and green alder leaves on detritivore colonization, observed leaf mass loss in mixtures was not higher than expected. Changes in the chemical properties of senescent oak and green alder leaves in mixtures might decrease feeding rates of detritivores because of preferential or compensatory feeding. Mixtures of senescent and green leaves did not alter leaf decomposition of each component in spring and summer, but might enhance leaf decomposition in autumn and winter by augmenting detritivore populations.

Across aquatic ecosystems, benthic secondary production generally is thought to be limited by autochthonous primary production. We estimated secondary production in 8 oligotrophic arctic lakes of varying morphometric characteristics for which we also had estimates of pelagic and benthic primary production during the summer of 2001. Four of the lakes supported similar communities of fish, and 4 were fishless. The size–frequency method was used to calculate cohort production interval (CPI)-corrected lake benthic secondary production and production to biomass (P:B) ratios for all taxa in all lakes. Taxa examined from sediment samples included Baetis spp., Grensia praeterita, and larval chironomids. Larval chironomids had the greatest secondary production overall. Whole-lake benthic secondary production ranged from 1.02 to 20.52 g dry mass m⁻² y⁻¹. No significant relationship or trend was found between benthic secondary production and primary production (whole-lake, benthic, or pelagic). A stronger trend (p  =  0.2) suggested higher benthic secondary production in fishless lakes than in lakes with fish, but within fishless lakes, no pattern was found that suggested autochthonous control of benthic secondary production. Our study is unique in that we examined multiple oligotrophic lakes and demonstrated that the secondary production of benthic fauna was, at most, weakly related to autochthonous primary production. This result is in contrast to the commonly accepted paradigm that autochthonous production limits secondary production. We suggest that allochthonous organic matter sources were very important for supporting secondary production in these oligotrophic arctic lakes.

The biological diversity reflected by nearly 300,000 caddisfly specimens collected throughout Minnesota since 1985 was compared with that of 25,000 specimens recorded prior to 1950 and was analyzed based on the 5 caddisfly regions of Minnesota. In the Lake Superior, Northern, and Southeastern regions, >90% of species known historically from each region were recovered and additional species were discovered. In the Northwestern and Southern regions—the most disturbed areas of Minnesota—species recovery ranged from 60 to 70%. Historical and contemporary assemblages were similar to each other in the former 3 regions and markedly different in the latter 2. Prior to 1950, species in all trophic functional groups were widespread in all regions. A similar pattern still exists in the Lake Superior, Northern, and Southeastern regions, whereas the Northwestern and Southern regions are now dominated by filtering collectors in all sizes of lakes and streams. Over 65% of species extirpated from any region were in the long-lived families Limnephilidae and Phryganeidae, and 70% of these same species were in the shredder functional group. Almost 30% of the statewide fauna has been found from <5 localities since 1950, suggesting a degree of imperilment on par with that of freshwater bivalves, gastropods, and fish. These observed losses of biodiversity and changes in trophic composition have probably occurred as a result of anthropogenic disturbance throughout most of the northcentral US.

Filter-feeding mussels historically comprised most of benthic biomass in many streams. They contribute to stream ecosystem functioning by linking the water column and benthic habitats. Both native and nonnative species coexist in many streams, but their ecological roles are not well quantified. The invasive bivalve, Corbicula fluminea, has the potential to alter profoundly organic matter dynamics and nutrient cycling in streams. We compared stable isotope ratios and tissue and biodeposit stoichiometry of the native freshwater mussel, Elliptio crassidens, and C. fluminea in a Coastal Plain stream (Ichawaynochaway Creek, a tributary to the lower Flint River, Georgia, USA) to assess their trophic niche space and potential effects on nutrient cycling. We hypothesized that C. fluminea would assimilate a larger range of materials than E. crassidens. To determine dietary overlap of C. fluminea and E. crassidens, we measured the elemental and stable isotopic compositions (δ¹³C and δ¹⁵N) of their tissue. Corbicula fluminea showed lower trophic fidelity than E. crassidens and was able to acquire and assimilate a wide range of resources, as illustrated by their wide range of δ¹³C values. Corbicula fluminea also might alter nutrient cycling in the benthic environment of streams because they retain less N than E. crassidens, as reflected by their higher tissue C∶N. In the laboratory, we measured C and N in biodeposits (feces and pseudofeces) from the 2 species. Corbicula fluminea released more N through their biodeposits relative to E. crassidens by mass, a result implying that C. fluminea might modify nutrient cycling in streams. Our results show important differences in the food resources assimilated and the nutrients deposited as feces and pseudofeces by these 2 bivalves. Furthermore, our results demonstrate how invasive species, such as C. fluminea, can alter aquatic environments through differences in species traits within a functional group.

We examined recent faunal changes in freshwater mussels (Unionidae) in northern Ohio. We assessed species diversity in 2 agricultural watersheds, the Vermilion and Huron Rivers, and compared these faunas among 7 neighboring rivers within the Lake Erie watershed that varied in levels of human impact, geological structure, and fish diversity. We found 19 mussel species in each of the Huron and Vermilion Rivers, and 89% of species overlapped between the 2 rivers. Museum records suggested that as many as 4 species might now be missing from each river, and across the 9 tributaries in the region, species loss has been common. The total number of species once present in each river was positively correlated with watershed area, but the size of watersheds no longer provides a significant estimate of mussel or fish diversity. Current mussel faunas also were weakly correlated with the fish species present, suggesting that mussel species ranges have not been limited by a lack of available fish hosts.

Lakes can play a large role in structuring the chemical and physical templates of streams, yet they often are overlooked when stream function is evaluated. We examined how lakes within stream networks affect organic matter decomposition. We used a cotton-strip assay to evaluate cellulose decomposition potential (CDP) as loss of cotton-strip tensile strength in streams upstream and downstream of 4 mountain lakes in a lake district in the Sawtooth Mountains, Idaho. In addition, we used this assay to determine decomposition potentials along a longitudinal gradient in 1 stream–lake network. Decomposition coefficients (k) were 43% greater in lake-outflow streams than lake-inflow streams. Temperature explained 65% of the variability in decomposition rates across lake inflows and outflows, and decomposition rates in inflow streams were highly correlated with mean daily temperature (r  =  0.99). Conversely, decomposition rates in outflow stream were strongly correlated with total N/total P ratios (TN/TP; r  =  0.94). When temperature ranges were small, as was observed across 4 lake outflows and along a longitudinal gradient, decomposition rates appeared to be driven by differences in nutrient availability, mainly TP and, to a lesser extent, TN. Together, these results demonstrate that although temperature is a primary driver of decomposition, when temperature is similar across locations, nutrient availability can drive decomposition rates. Our results indicate that lakes within fluvial networks can modify stream function by altering both temperature and nutrient availability.

Changes in community structure and life-history traits of benthic invertebrates were examined along a longitudinal intermittence gradient in an alluvial river. The gradient was characterized with modeled and measured hydrologic, chemical, and physical environmental variables. The invertebrates were collected in the Selwyn River, southeastern New Zealand, at multiple sites in each of 4 river sections with distinct hydrological conditions (perennial-losing, ephemeral, intermittent, perennial-gaining). Values of hydrological metrics for each site were generated with an empirical model developed for the Selwyn River. The metrics included 4 that characterized intermittent flow (flow permanence, flow duration, drying frequency, distance to nearest perennial site). Most invertebrate richness and density metrics were significantly higher in the perennial-losing and perennial-gaining sections than in the ephemeral and intermittent sections. A principle components analysis (PCA) separated invertebrate samples from the 4 sections along 2 primary factors. Nine of 13 hydrological metrics, including the 4 intermittence metrics, were correlated with the PCA site scores. Linear regressions indicated that most taxon-richness metrics and some density metrics were related to flow permanence, flow duration, or both. Based on the regression analysis, we predicted that 1.9 taxa/m² are added with each 10% increase in flow permanence, and 0.5 taxa/m² are added with each 10-d increase in flow duration. Results from a nestedness analysis indicated that communities at ephemeral and intermittent sites were nested subsets of the communities at perennial sites, and the nesting order of sites was related to both flow permanence and flow duration. Assemblages of taxa with particular life-history traits (life span, fecundity, maximum size, and voltinism) varied linearly with flow permanence and flow duration. The variation in invertebrate communities along the Selwyn River was primarily the result of progressive removal of desiccation-sensitive taxa with increasing intermittence, not to selection for desiccation-resistant specialists. Quantitative intermittence–ecology relationships are needed to predict the consequences of future changes in flow intermittence, but such relationships are rare. The univariate relationships reported in our study contribute to a small but growing array of intermittence–ecology relationships.

Small headwater streams are abundant components of the riverine landscape where critical biochemical processes occur that provide clean water, energy, and nutrients to downstream reaches. Disturbance to these systems as a result of human land use has the potential to affect downstream health. Rates of metabolism and organic matter processing were measured in 22 small forested headwater streams in 2 regions of Tasmania, Australia, to evaluate the effects of forestry disturbance. Twelve of these streams had been subjected to recent clearfell-burn-and-sow (CBS) harvest. Benthic metabolism was measured in small in situ chambers (production ranged from <0.001 to 21.845 mg C m⁻² h⁻¹ and respiration from <0.001 to 4.976 mg C m⁻² h⁻¹), whole-system metabolism was estimated based on relative habitat abundance (gross primary production ranged from <0.001 to 0.297 g C m⁻² d⁻¹ and daily respiration from 0.003 to 0.072 g C m⁻² d⁻¹). Algal growth potential was measured on nutrient diffusing pots (chlorophyll a ranged from <1.0 to 40.1 mg/m²), and cellulose decomposition potential was assessed with a cotton-strip assay (cotton tensile strength loss ranged from 17.8% to 38.3% in 28 d). Sometimes an increase in the variability of response is a consequence of disturbance, but in our study, the difference between forested streams and clearcut streams was a significant increase in the mean values of all functional variables. The degree of response depended on the underlying geology (broad-scale spatial variability) of the streams. Current management practices for small headwater streams in Tasmania do not protect instream processes from forestry disturbance in the short-term (i.e., 2–5 y), and we suggest that an investigation of long-term response is warranted.

Rapid Bioassessment Protocols (RBPs) have been widely used to assess the ecological health of aquatic ecosystems. Specific aims of RBPs for wadeable streams are to indicate the ecological condition of a stream using low-cost protocols to allow long-term and widespread routine monitoring. Our study was part of an ongoing effort to test and standardize a protocol using benthic macroinvertebrates as indicators of the water quality of wadeable streams in southeast Brazil. One of the most controversial issues during RBP development is deciding the taxonomic resolution that should be used. We evaluated how well genus-, family-, and order-level taxonomic resolution detected a gradient of impairment. All 3 taxonomic resolutions statistically discriminated reference, intermediately impaired, and impaired sites based on assemblage structure, water-quality classification, and biotic index responses. Analysis at the genus level was more effective than analysis at other levels of taxonomic resolution for discriminating sites that varied in degradation conditions, especially when considering biotic index responses, but the lack of comprehensive taxonomic keys and information about the ecology of those genera hinder their widespread use in bioassessments. On the other hand, analyses at the order level had lower discriminating power to separate reference sites from intermediately impaired sites when considering biotic index responses. Analyses at the family level gave results similar to results at the genus level, and we support its use in a RBP program for this region, at least until better keys and autoecological knowledge are available.

Biological recovery of aquatic ecosystems from acidification damage is a slow process. In lakes near the massive Cu and Ni smelters in Sudbury, Canada, the delays might be caused by residual metals, habitat damage, altered predator–prey interactions, or other persistent ecological stressors. Assessments of benthic invertebrate communities in 24 Sudbury lakes were conducted to evaluate the relative importance of these delaying factors. At the time of sampling, all lakes had chemically recovered to a pH >6.0, but they varied widely in the duration of time above this threshold and in current metal concentrations, watershed contributions of organic matter, littoral habitat composition, and fish community composition. A model developed with redundancy analyses (RDA) of 4 groups of environmental variables (i.e., water chemistry, fish communities, physical lake descriptors, and littoral habitat) accounted for 74.9% of the variance in benthic invertebrate community metrics across these environmental gradients. Fish species richness, duration of pH recovery, and % boulder habitat were the most significant variables and explained 22%, 9%, and 8% of the variance in benthic invertebrate community metrics, respectively. Damaged systems clearly need sufficient time to recover from severe disturbances. However, our study suggests that remediation techniques, such as manipulation of predator–prey interactions through fish introductions, might speed the recovery of benthic invertebrate communities.

Benthic diatom assemblages respond to changes in water quality, and this response is evidenced by shifts in taxonomic composition. As a result, several taxon-based indices have been developed for monitoring purposes. Some authors have suggested that diatom body size might provide a simpler method for bioassessment than taxonomy-based approaches. Moreover, current knowledge of algal ecology suggests that the slopes and intercepts of density size distributions should vary with environmental characteristics. However, results from studies of the relationship between algal size and trophic variables including P have been mixed. Our objectives were to examine normalized density size distributions and richness size distributions of benthic diatoms in streams along a gradient of agricultural land use to determine whether the size distributions changed in relation to environmental variables. Benthic diatoms from 29 streams in eastern Canada were identified and average body size measurements were obtained for each taxon. Normalized density size distributions and richness size distributions were plotted, and slopes and intercepts were compared among sites using general linear models (GLMs). Despite taxonomic differences in the assemblages, slopes of the normalized density size distributions did not differ among the 29 sites, and distribution shape and intercepts for the richness size distributions did not differ among sites. The intercepts of the normalized density size distributions were significantly different among sites, and this variation was attributed in part to the effect of several environmental variables including N and P. However, this difference in intercepts represented a change in density across all size classes, rather than a size-specific change. Environmental variables did not explain a significant amount of the variance in the shapes of density or richness size distributions. These results suggest that the slopes of lotic benthic diatom size distributions do not differ in response to environmental variables including P. Thus, benthic diatom body size should not be used as a proxy for nutrient status in environmental monitoring.

Inputs of terrestrial plant detritus and nutrients play an important role in aquatic food webs, but the importance of terrestrial prey inputs in determining aquatic predator distribution and abundance has been appreciated only recently. I examined the numerical, biomass, and diet responses of a common predator, dragonfly larvae, to experimental reduction of terrestrial arthropod input into ponds. I distributed paired enclosures (n  =  7), one with a screen between the land and water (reduced subsidy) and one without a screen (ambient subsidy), near the shoreline of 2 small fishless ponds and sampled each month during the growing season in the southern Appalachian Mountains, Virginia (USA). Screens between water and land reduced the number of terrestrial arthropods that fell into screened enclosures relative to the number that fell into unscreened enclosures and open reference plots by 36%. The δ¹³C isotopic signatures of dragonfly larvae shifted towards those of aquatic prey in reduced-subsidy enclosures, a result suggesting that dragonflies consumed fewer terrestrial prey when fewer were available (ambient subsidy: 30%, reduced subsidy: 19% of diet). Overall abundance and biomass of dragonfly larvae did not change in response to reduced terrestrial arthropod inputs, despite the fact that enclosures permitted immigration/emigration. These results suggest that terrestrial arthropods can provide resources to aquatic predators in lentic systems, but that their effects on abundance and distribution might be subtle and confounded by in situ factors.

The patchy spatial distribution of benthic algae (periphyton) on gravel-bed rivers might be caused by physical disturbances during small frequent flow spates. During such spates, the gravel–cobble river bed is stable, but flows are often strong enough to transport large quantities of sand by a hopping motion called saltation. We tested the hypothesis that a spate-related refuge habitat exists in a transition zone (TZ) between the edge of the varial zone and the thalweg of the river channel where high hydraulic stress and saltating sand reduce biomass. We documented physical disturbance and periphyton biomass across 15 riffles after 3 summer spate periods in an oligomesotrophic river in Quebec. Periphyton perturbation thresholds were identified for near-bed water velocity during prespate growth (0.25 m/s) and for sand transport (64–180 g m⁻¹event⁻¹) and flow shear stress (15 Pa) during spates. Generalized linear models were used to examine cross-riffle trends in these 3 disturbance variables and in postspate periphyton biomass. The highest biomass occurred in the TZ. Periphyton increased away from the thalweg as sand transport rates decreased. Biomass continued to increase toward the edge of this zone unless disturbed by high rates of sand transport that were associated with a small, near-shore secondary peak in sand transport rate. Of the 3 disturbance variables, sand transport patterns controlled the spatial distribution of periphyton biomass after small spates with an average recurrence interval of 7 d. No cross-riffle refuge was found after a higher-magnitude spate (3× mean annual discharge) when disturbance thresholds were typically exceeded across the entire riffle. The intensity and distribution of physical disturbance, particularly sand abrasion, over the streambed dictated size and arrangement of periphyton refuge zones. These zones are crucial to promote stream system resilience to landuse change.

We conducted a series of stepwise NO₃⁻ additions to investigate the response of NO₃⁻ uptake to short-term (acute) changes in N concentration in 3 prairie streams. Observed NO₃⁻ uptake rates increased with short-term elevations in NO₃⁻ concentration and were consistent with linear and Michaelis–Menten kinetics models. We compiled these data with uptake rates from additional published studies to calculate robust estimates of N uptake kinetics for prairie streams. Half-saturation coefficients based on compiled data were 6.7 µg/L for NH₄ and 67 µg/L for NO₃⁻-N. This difference in half-saturation coefficients suggests that NH₄ is more efficiently assimilated than NO₃⁻, indicating a preference for NH₄ as an N source. Similarly, ambient concentrations of NH₄ and NO₃⁻ were less than their respective half-saturation coefficients, and aerial uptake rates were generally <5% of the maximum, suggesting severe limitation of N uptake at ambient conditions. The observed pattern of uptake kinetics suggests that physiological constraints limit biotic N uptake in these low-N streams and contrasts with the pattern of uptake observed in streams with chronically elevated ambient NO₃⁻ concentrations.

Individuals in most natural populations of predators vary in size, and size differences among individuals often result in cannibalism. Cannibalism is an extremely common phenomenon in the animal kingdom, particularly among generalist predators that engage in intraguild predation (IGP). However, few studies have specifically addressed the effects of cannibalism on IGP. The aim of my study was to investigate how trophic and behavioral interactions between 2 size classes of an intraguild (IG) predator influenced the survival and behavior of IG prey and a shared prey resource. I tested for these effects with larval odonates by exposing a shared prey resource (Ischnura verticalis) to the presence or absence of IG prey (Pachydiplax longipennis) and 2 size classes of IG predators (small or large Anax junius) in a 2 × 2 × 2 factorial design. Mortality rates of the shared resource in all single-predator treatments were significantly greater than in nonpredator controls, and risk reduction was observed when the shared resource was exposed to combinations of predators. The significant negative effect of large A. junius on P. longipennis survival and activity level was greater than that of small A. junius. Cannibalism occurred between large and small A. junius in size-structured IG predator treatments, and the effects of the size classes were not additive for the survival of IG prey. Cannibalism was not solely responsible for risk reduction in IG prey, and reduced activity level of small A. junius in the presence of larger conspecifics probably had a positive influence on P. longipennis survival. My results demonstrate that cannibalism among IG predators can influence the survival of IG prey and might contribute to coexistence among predators in systems with strong IGP.

Increased need to quantify adult insects emerging from streams as a part of foodweb and ecosystem studies has placed new demands on techniques used to sample adults. The population sampled must be better understood to establish the scope of inferences that may be drawn from emergence data. We used data from 2 different studies to compare the structure of insect assemblages represented by benthic samples and emergence-trap samples and to compare adult insect assemblages collected in emergence traps placed at mid-channel vs streambank locations. Nonmetric multidimensional scaling (NMDS) ordination showed that some components of the benthic assemblage were underrepresented in the emergence-trap samples and others were underrepresented in benthic samples. These differences were mainly caused by taxa that emerged by crawling out on the stream bank (e.g., Plecoptera) or whose larvae reside in habitats, such as stream margins, that are underrepresented with traditional benthic sampling (e.g., Dixidae). The flux of insects into traps placed mid-channel did not differ significantly from the flux into traps placed along the stream bank. Taxa collected by mid-channel and streambank traps overlapped considerably, but midchannel traps tended to collect proportionally more Trichoptera, Ephemeroptera, and Diptera, whereas streambank traps collected proportionally more Plecoptera. Our results can be used to improve trapping designs for future assessments of aquatic insect emergence in studies of insect behavior and life histories and as part of foodweb and ecosystem research.

We evaluated several lines of evidence to identify bedded fine sediment levels that should protect and maintain self-sustaining populations of native, sediment-sensitive aquatic species in the western US. To identify these potential criterion values for streambed sediments ≤0.06 mm (fines) and ≤2 mm (sand and fines) diameter, we examined: 1) the range of areal % fines and areal % sand and fines values at 169 least-disturbed reference sites in our sample, 2) sediment tolerance values calculated for a selection of sediment-sensitive aquatic vertebrate and macroinvertebrate taxa for both particle size ranges, 3) quantile regression predictions of the declines in vertebrate and macroinvertebrate Indices of Biotic Integrity (IBIs) at progressively higher ambient levels of streambed sediment from synoptic survey data acquired in 557 mountain stream sampling sites in 12 western states, 4) a literature review of the effects of sand and fines on the survival of salmonid eggs to hatching, and 5) a literature review of studies that quantitatively linked macroinvertebrate response to the pertinent size ranges of streambed sediment in mountain streams. Predicted maximum vertebrate Index of Biotic Integrity (IBI) declined 4.4 points (SE  =  1.0) and macroinvertebrate IBI declined 4.0 points (SE  =  0.60) for each 10% increase in % fines. Similarly, for each 10% increase in % sand and fines, the predicted maximum vertebrate IBI decreased 3.7 points (SE  =  0.50) and macroinvertebrate IBI decreased 3.0 points (SE  =  0.50). Combining all lines of evidence, we concluded that for sediment-sensitive aquatic vertebrates, minimum-effect sediment levels were 5% and 13% for % fines and % sand and fines, respectively, both expressed as areal percentages of the wetted streambed surface. For aquatic macroinvertebrates, minimum-effect levels for the 2 sediment size classes were 3% and 10%, respectively. We encourage managers to consider these biologically based minimum-effect values when developing sediment criteria for mountain streams. Quantifying and comparing both vertebrate and macroinvertebrate assemblage responses to streambed sedimentation informs the criteria-setting process and allows managers to set stream restoration priorities.

Mountaintop removal and valley fill (MTR/VF) coal mining has altered the landscape of the Central Appalachian region in the USA. Among the changes are large-scale topographic recontouring, burial of headwater streams, and degradation of downstream water quality. The goals of our study were to: 1) compare the structure and function of natural and constructed stream channels in forested and MTR/VF catchments across ephemeral, intermittent, and perennial flow regimes and 2) assess the relationship between leaf litter breakdown and structural measures, such as the habitat assessments currently used by regulatory agencies. Specific conductance of stream water was, on average, 36 to 57× higher at perennial reaches below valley fills than at perennial reaches in forested catchments, whereas pH was circumneutral in both catchment types. Channel habitat and invertebrate assemblages in litter bags differed between forested streams and constructed channels in VF catchments. Invertebrate density, diversity, and biomass were typically higher in litter bags from forested catchments than from VF catchments. No differences in fungal biomass, estimated as ergosterol concentration, were detected between litter bags from forested and VF catchments. Breakdown of oak (Quercus alba) leaves was slower at perennial and intermittent reaches in VF catchments than at perennial and intermittent reaches in forested catchments. However, breakdown rates did not differ between ephemeral reaches on VFs and in forested catchments. Breakdown rates of oak leaves were significantly correlated to conductivity at perennial and intermittent reaches and to shredder diversity across all reaches, but were not correlated with habitat assessment scores currently being used to determine compensatory mitigation. Landuse changes associated with MTR/VF have detrimental consequences to headwater stream function that are not adequately evaluated using the prevalent habitat assessment.

In Pacific Northwest (USA) streams, historical levels of marine-derived subsidies of C, N, and P from spawning salmon and steelhead are either greatly diminished or no longer present. A novel nutrient enrichment form, pasteurized salmon carcass analogue (SCA), has been developed as a management tool to increase freshwater productivity. We analyzed macroinvertebrate assemblages in 4 central Idaho (USA) streams to explore the relative influence of reach-scale SCA treatments on macroinvertebrate assemblages. We stratified study streams into upstream (control) and downstream (treatment) reaches and applied a single SCA treatment in mid-September to the downstream reaches of 2 treatment streams. We measured chemical, physical, and biological variables in all study streams and used nonmetric multidimensional scaling (NMDS) ordination of macroinvertebrate communities to illustrate relationships to foodweb and environmental variables. Macroinvertebrate assemblages differed significantly among streams (p < 0.001) and between treated and control reaches in treatment streams (p < 0.030). No reach-level differences were found in control streams (p > 0.458) or in treatment streams before SCA additions (p > 0.130). Variables that were significantly and positively correlated with NMDS ordination scores and suggested an SCA response included presence of SCA, elevated periphyton and macroinvertebrate δ¹⁵N, increased periphyton ash-free dry mass, and increased relative abundances of dipterans, collectors, and Chironomidae. A weaker autotrophic response in 1 treatment stream relative to the other appeared to be explained partially by differences in canopy shading, as indicated by periphyton autotrophic index values. Increased autotrophic productivity and the absence of major shifts in treatment stream macroinvertebrate community composition and structure suggest that SCA is a viable nutrient-enrichment strategy.

Few studies have examined the effects of low flows on stream biota—a paradoxical situation considering the mounting pressure on streams as a result of increased human abstraction. We present a conceptual model describing processes operating in rivers during low flows and argue that such processes reflect both direct effects of reduced flow on benthic invertebrates and indirect effects of enhanced plant growth occurring during such periods. Our model suggests that the longer the duration of low flow, the more the plant community will change and, in turn, the more habitat quality will change, with consequences to benthic invertebrate communities and higher trophic levels. A fundamental part of the model is the recognition of different stream types, based on a habitat-template matrix of resource supply, interflood velocity, and substrate stability during flood events. This habitat template results in 12 stream types, each of which supports specific plant communities that set the antecedent plant conditions prior to a low-flow period. During low flows, hydraulic factors interact with the antecedent plant communities, which undergo specific responses. For example, in low-nutrient streams with gravel substrates and dominated by diatoms, low-flow conditions will cause little or no change to the antecedent plant community because this plant community is structured by top-down grazing pressure. Consequently, benthic invertebrate composition will not change. In contrast, cover and biomass of filamentous green algae in higher-nutrient gravel-bed streams will increase during low-flow events, and this increase will affect habitat suitability for invertebrates over and above those caused solely by hydraulic changes. The greatest changes to invertebrate communities are expected in macrophyte-dominated streams because these plants change instream habitat conditions the most during extended periods of low flow. Therefore, current flow-management techniques must consider the type of plant communities that exist prior to low flows and be cognizant of that fact that these plant communities can have dramatic influences on benthic communities during low-flow periods.

Flow variation associated with hydropower production causes periodic exposure of zones along the banks of regulated rivers. These zones have reduced algal biomass and lower productivity. We investigated whether algal assemblages in regulated rivers differed in tolerance to aerial exposure because understanding such tolerance could be used to alter water releases to improve downstream productivity. In field experiments, we tested algal tolerance to aerial exposure in 3 assemblages: green filamentous and cyanobacterial assemblages in Thailand and a mixed (filamentous green cyanobacteria) assemblage in New Zealand. Algae-bearing stones were exposed at night, during the day (in sun or shade, with or without simulated rain), or continuously, or were continuously submerged (with a handling control). Exposure reduced cyanobacterial chlorophyll a, and reduction was greater with day than with night exposure. Mixed algal assemblages showed a similar pattern in chlorophyll a, but ash-free dry mass (AFDM) did not change (bleached algae remained). In contrast, green filamentous algae survived better when exposed than when submerged, and when exposed during the day than at night. Snails consumed green filamentous algae, and submersion resulted in high grazing loss of filamentous algae but not cyanobacteria. Grazing pressure was presumably lower at night during water release. For cyanobacteria, shade and rain slightly increased chlorophyll a retention. For filamentous green algae, shade had no effect, and rain was beneficial in the shade but not the sun. Our experiments demonstrated an interaction among variable hydraulic conditions, activity of grazing snails, and availability of grazing-susceptible algae. A longer wet–dry cycle, daytime water release, and a shaded riparian zone reduced aerial exposure effects on algal assemblages, indicating that water releases can be regulated to improve productivity in the downstream exposed zone.

Proliferations of filamentous chlorophytes and mats of cyanobacteria (hereafter termed metaphyton) are increasingly observed in rivers, lakes, wetlands, and estuaries undergoing eutrophication, but their contribution to invertebrate production and overall ecological significance remains poorly understood. In Lake Saint-Pierre, a shallow widening of the St. Lawrence River (Québec), vascular macrophytes (mainly Vallisneria americana) grow in combination with filamentous chlorophytes (Hydrodictyon, Oedogonium) in the upstream reach, which is fed by nutrient-rich waters from the tributaries, and in association with filamentous cyanobacteria (Lyngbya wollei) in the chronically NO₃⁻-depleted downstream reach. We hypothesized that different vegetation types (macrophytes, filamentous chlorophytes, and cyanobacteria) would support macroinvertebrate communities with different biomasses and taxonomic compositions. We expected a higher invertebrate biomass in the upstream reach and, within the reach, a higher biomass on metaphyton than on macrophytes. Total macroinvertebrate biomass was significantly higher at the enriched stations in the upstream reach (75–100 mg/g vegetation, dry mass) than farther downstream (8–38 mg/g). In addition, macrophytes and metaphyton in the upstream reach sustained taxonomically different invertebrate assemblages. Gastropods dominated the fauna associated with macrophytes throughout the lake (43–73%) and probably benefitted from a structurally simple and solid substratum on which to crawl and feed. Small mobile taxa, such as cladocerans, copepods, chironomids, and ostracods, were more abundant on filamentous metaphyton, both up- and downstream. Amphipods were dominant (59%) in metaphytic mats of L. wollei. At the scale of the river reach, macrophytes supported most of the invertebrate biomass. Chlorophytes in the upstream reach contributed <5% of the total biomass, representing an alternative, albeit temporary, habitat. In contrast, in the downstream reach where macrophytes were scarce, cyanobacterial mats hosted a significant fraction of macroinvertebrates (36%). Shifts in vegetation between the 2 reaches affected the quantity and availability of prey items for fish predators.

Disturbances from flooding are a dominant feature of the habitat template in streams. Frequent floods created by recreational releases (1–2 d between releases/floods) from Abanakee Dam in the Adirondack Mountains, USA, result in a static mosaic of scoured patches in areas of high shear stress (low diatom densities) and mats of algae in areas of low shear stress (refugia); this pattern is different from the shifting mosaic of patches more typical of streams recovering from floods. Predictable, recreational release floods and similar bed size particle distributions in this stream enabled us to use a reciprocal replacement experiment to examine macroinvertebrate community responses in high scour and flow refugia areas without the confounding variable of different substrata. Trays of substrata were embedded in both high shear stress (HSS) and low shear stress (LSS) areas. Half of the trays from each area were then reciprocally exchanged while resident trays remained in their original positions. After 2, 6, and 12 releases, 20 trays for each of 4 treatments were sampled. Lower macroinvertebrate densities in the LSS area reflected an absence or loss of filter-feeders that require higher current velocities than were present between releases, and this absence probably was responsible for the lower species diversity in the LSS area. The net-spinning caddisfly, Macrostemum, dominated the HSS area because its retreat design protects the fine-meshed net from high velocities. The filter-feeding mayfly, Isonychia, thrived in the HSS area because it could seek refuge during releases and could benefit from the potentially higher particle delivery in the higher flow velocities between releases. Ordinations showed that community composition shifted quickly (2–12 d) in trays moved from the LSS area to the HSS area because of the rapid colonization by filter-feeders in the HSS area. However, trays from the HSS area moved to the LSS area were slower (>21 d) to change to the LSS community composition. We suggest that hydrologic conditions strongly influenced macroinvertebrate composition with filter-feeders dominating where shear stresses scoured periphyton. However, in LSS areas where periphyton was abundant, the community was dominated by collector-gatherers and scrapers. Also, HSS substrata transferred to the LSS areas were slow to be colonized by gatherers and scrapers probably because periphyton had not yet developed on the HSS rocks.

Agricultural drainage can contribute excess N to aquatic ecosystems. The objective of our study was to investigate the capacity of agricultural ditches to remove NO₃⁻ via denitrification in maintained 1-stage and naturalized 2-stage agricultural ditches. We hypothesized that maintenance of ditches limits the potential for denitrification by removing in-stream and riparian vegetation and excavating fine sediments. We quantified denitrification rates in the sediments collected from ten 1-stage and ten 2-stage headwater ditches with 2 methods (sediment static core and denitrification enzyme activity) and over 4 sampling periods. We also measured water, plant, and sediment characteristics. With both methods, denitrification rates from sediments collected inside the channel (i.e., not along the slope and the bench) did not differ between the 2 types of ditch. We used a series of enrichment treatments of sediment slurries with C and NO₃⁻ to determine that denitrification was limited by NO₃⁻ on the bench of the 2-stage ditches and by both NO₃⁻ and C on the slope of the 1-stage ditches. We hypothesize that greater denitrification rates measured with the static core method in the slope might have been linked to greater % fine sediments in the slope of the 1-stage ditches and, thus, more developed anaerobic conditions in the sediment cores. Accumulation of organic matter in the benches that form in unmaintained ditches is favorable to denitrification, as shown by greater denitrification measured in the sediment slurries unamended with C.