We used 2 caddisflies, Drusus discolor and Drusus romanicus, to test explicitly whether closely related species that occupy similar niches and occur in partial sympatry maintain comparable population structure and share a similar population history. We used mitochondrial sequence data to analyze and compare the population structure and the phylogeography of 105 specimens of D. discolor and 74 individuals of D. romanicus collected in southeastern Europe. We examined the relationship between both species with phylogenetic inference and coalescent modeling and used the results to assign larvae to species. We were able to unambiguously assign larvae to species level based on clearly defined association criteria within a phylogenetic analysis of all specimens. The species were closely related and not reciprocally monophyletic in our haplotype phylogeny. One D. romanicus haplotype from the Bucegi Mountains was nested within D. discolor, a result that suggests isolation with migration, introgression, or incomplete lineage sorting between the 2 species. For each species, we examined population genetic structure with median joining networks, analysis of molecular variance (AMOVA), exact tests of population differentiation, and Mantel tests of isolation by distance. We used tests for selective neutrality (Tajima's D, Fu's F) to infer potential population growth and expansion. Species differed in their genetic population structure. Drusus discolor had haplotype overlap among several mountain ranges in the study region. No D. romanicus haplotypes were shared among any regions examined, and levels of divergence between haplotype clades exceeded those of D. discolor by a factor of up to 2.1. The different degree of population differentiation and divergence of both species probably reflects different Pliocene/Pleistocene population histories and might be related to differences in dispersal capabilities or competitive exclusion of D. romanicus by D. discolor in the mountain ranges north and west of the Western Carpathians. Based on our results, we discuss the importance of the Carpathian Mountains and Bulgarian highlands as Pliocene/Pleistocene refugia and centers of diversification.

Despite the global occurrence of braided rivers and the frequency with which they are anthropogenically modified, the benthic diversity of their floodplains and, in particular, lateral and longitudinal patterns in their communities have been neglected. A spatially nested, hierarchical survey was conducted of 11 braided rivers in the South and North Islands of New Zealand to investigate benthic invertebrate diversity at multiple spatial scales. In each river, 6 reaches and up to 5 floodplain habitat types within each reach were sampled. From a total of 203 sites, 145 taxa were identified. Benthic invertebrate diversity was highly variable at the island, river, reach, and habitat scales. Diversity differences between islands were driven by high regional endemism. Diversity at the whole-river scale ranged from 99 taxa in the Wairau River to only 56 in the Waiapu River. At the reach scale, no significant differences in diversity or abundance were found longitudinally down the rivers. At the habitat scale, lateral floodplain habitats, particularly spring creeks, consistently had greater richness and abundance than did adjacent main channel habitats. Comparisons of Whittaker's multiplicative β at different spatial scales revealed that the greatest range and mean value of taxon turnover was among habitats within reaches. Additive partitioning of β diversity indicated that diversity values at the broad spatial scales of reach and river contributed disproportionately to overall diversity. Our results indicate that river managers concerned with maintaining diversity should focus their efforts at the whole-river scale rather than attempting management of smaller units representing local scales. However, at finer scales, lateral floodplain habitats (particularly springs and floodplain ponds) are hot spots of diversity and protection and remediation of these habitats are particularly important.

Detritus processing, the breaking down of organic matter into smaller particles, is an essential operation in aquatic systems because it provides resources to filter feeders and accelerates nutrient release by microorganisms. Detrital foodweb dynamics are influenced by both consumption (top-down) and production (bottom-up) effects. We tested the effects of predators and detritivores on the abundance of microorganisms in an inquiline community in pitcher plants. We manipulated densities of mosquitoes (top predator) and midges (processing detritivore) in a factorial press experiment and measured the response (density) of bacteria, protozoa, and rotifer populations over several generations. We hypothesized that: 1) midges would have a positive effect on microorganisms by increasing nutrient availability (bottom-up effects), 2) mosquitoes would depress microorganism populations through consumption (top-down effects), 3) top-down and bottom-up effects would operate independently, and 4) would attenuate with trophic position. Mosquitoes (predators) had a negative effect on all measured populations. Midges (processing detritivores) had a positive effect on bacteria, but a negative effect on rotifers and some protozoan taxa. The increase in bacterial density probably was the result of nutrient enrichment from detritus processing, whereas the decrease in rotifers seems to have been the result of consumption by midges. Our study shows that the role of processing detritivores is complex and can enhance both bottom-up and top-down effects. Specifically, omnivory can complicate simple top-down and bottom-up predictions. Although they accelerate decomposition by microorganisms and, thereby, can increase resource availability, processing detritivores can also be important consumers in detrital food webs.

The combination of varying aquatic habitats and flow regimes creates a complex stimulus environment from which sensory information can be extracted. Previous studies with crayfish in artificial stream settings have shown that altering the temporal and spatial structure of an odor plume modifies orientation behavior. Exposure to more temporally complex odor signals enables crayfish to locate food more efficiently. To link these studies to a more natural setting, we examined how odor signals are dispersed in 3 physically different habitats within the Maple River in Pellston, Michigan, USA, by simultaneously measuring flow patterns and odor plume characteristics. These microhabitats consisted of sections of the stream with gravel, transition (gravel and sand), or sand substrate. Flow measurements were taken using an Acoustic Doppler Velocimeter while simultaneous in situ odor plume measurements were made with an Epsilon electrochemical system. Flow had more turbulent energy in gravel and transition habitats than in sand habitat. Mean velocity was significantly higher in gravel (67.8 ± 2.00 cm/s) than in transition (30.5 ± 0.46 cm/s) and sand (30.6 ± 0.27 cm/s) habitats. In addition, eddy diffusivity rates were higher in gravel (173.3 cm²/s³) than in sand (9.74 cm²/s³) and transition (129.7 cm²/s³) habitats. Differences in turbulent flow profoundly affected the fine-scale distribution of the chemical signal. The concentration of dopamine tracer in odor pulses was significantly lower in sand (range: 8.1–6.9 µmol/L) and transition (range: 26.2–8.9 µmol/L) habitats than in gravel (range: 46.5–5.39 µmol/L) habitat. Background (natural) concentrations of dopamine are undetectable with our current technology. Furthermore, spectral density of odor pulses showed larger fluctuations in chemical concentration at all frequencies (0.1–10 Hz) in gravel habitat than in sand or transition habitats. Our results illustrate the dynamic nature of turbulence structure within natural stream systems and its influence on the distribution of chemical odor signals. Our results provide evidence that natural stream settings influence both the distribution and concentration of chemical odors available for ecological and behavioral interactions. The combination of turbulence and odor plume variation creates habitat-specific information that influences orientation behavior of organisms within a river.

We compared spatial patterns of N (NO₃⁻-N and NH₄ -N) and Cl⁻ variability in 2 reaches with different hydrological regimes (permanent vs intermittent) of a typical Mediterranean semiarid stream under different hydrological conditions (baseflow, high flow, and drought). We hypothesized that stream-flow intermittency would affect N spatial variability. We expected higher spatial variability of solute concentrations in intermittent reaches than in permanent reaches because local environmental conditions (advection, water residence time, temperature, biotic properties, sediment–water interactions, or environment redox condition) change through space as flow discharge does. We expected spatial heterogeneity of solute concentrations to change more through seasons, as do stream hydrological conditions (high flood periods and drought), in the intermittent than in the permanent reach. We also expected seasonal variability to be higher in the intermittent than in the permanent reach. We used the coefficient of variation (CV) to measure the spatial and seasonal variability of solute concentration. We used a null model to test whether the differences in CVs among reaches were statistically significant. NH₄ -N was the most spatially variable form of N in the intermittent reach. NO₃⁻-N and Cl⁻ were seasonally more variable in the intermittent reach than in the permanent one. We propose that NO₃⁻-N and Cl⁻ temporal variations are generated mainly by seasonal changes in advection, as shown by the synchrony between solute concentrations and rainfall. However, we propose that NH₄ -N seasonal variation is generated mainly by the variation of biotic (uptake and dissimilatory reduction) and abiotic drivers (geochemical processes, such as chemical sorption–desorption) over time as hydrological conditions change. We also analyzed the effect of hydrological conditions on N spatial variability. We predicted that N spatial variability would decrease after rainfall and would increase during drought periods, especially in the intermittent reach. The results obtained in the intermittent reach fulfilled this prediction. However, the increase in runoff after the spring and autumn rainfall in the permanent reach increased the spatial variability in both N forms and Cl⁻. For our scale of comparison (5–8 km, 13 mo), NO₃⁻-N and Cl⁻ were more variable spatially than seasonally in the permanent reach, whereas they were more variable seasonally than spatially in the intermittent reach. NH₄ -N concentrations in both reaches were more variable seasonally than spatially. Our study emphasizes the potential effects of changes in climate regimes on the spatial and seasonal variability of N and, ultimately, on stream ecosystem functioning.

We compared genetic lineages in the mayfly genus Ephemerella (Ephemeroptera:Ephemerellidae) identified from mitochondrial DNA (mtDNA) to current taxonomy in 9 morphological taxa, including 2 geographically widespread species, Ephemerella invaria ( = E. inconstans, E. rotunda, E. floripara) and Ephemerella dorothea ( = E. infrequens). Maximum likelihood and parsimony analyses of the mtDNA sequences placed E. inconstans and E. invaria in a well-supported clade; however, mean Kimura 2-parameter genetic distance between the lineages was high (5.2%) relative to distance within lineages (1.3%). The phylogenetic relationships of synonyms E. rotunda and E. floripara were not resolved, but estimates of mean genetic distance to E. invaria were high for both (8.5% and 11.6%, respectively). Populations of E. dorothea were grouped in 2 well-supported clades (12.9% mean divergence) that did not include the synonym E. infrequens (20.9% mean divergence, based on a single sample). A large genetic distance (18.6%) also was found between eastern and western populations of Ephemerella excrucians. Western samples of Ephemerella aurivillii were so genetically distant from all other lineages (32.2%) that doubt about its congeneric status is raised. mtDNA data have been useful for identifying genetic lineages in Ephemerella, but our results do not support use of cytochrome oxidase I (COI) as a DNA barcode to identify species in this genus because we also found evidence of incomplete mtDNA lineage sorting in this gene. Use of the barcoding gene rediscovered some old taxonomic problems in Ephemerella, a result that emphasizes the importance of completing empirical systematic description of species before using single-character systems for identification.

We examined the relationships between channelization, environmental characteristics, and plant communities in 4 streams (Betz, Cléry, Lunain, and Ecole Rivers) southeast of Paris, France, with the goal of assessing stream health. Seventeen 100-m-long reaches, each divided into a pool/riffle sequence, were monitored in 2006. Each reach had 0 to 3 channelization structures (i.e., embanking, resectioning, and other man-made structures). Redundancy analysis and mean-comparison tests done at 2 spatial scales (reach scale and pool/riffle sequence scale) indicated that channelization significantly affected flow velocity, depth, substrate type, and number of pools/riffles (4 of 24 variables considered). Physical characteristics of the streams were significantly more affected by channelization than were chemical characteristics. Moreover, different channelization structures affected the streams differently. Resectioning and embanking reduced flow velocity and the number of riffles and increased the number of pools. However, man-made structures either accelerated (downstream) or reduced flow velocity (upstream). Channelization strongly affected floristic richness on the basis of biological type (vascular plants, bryophytes, macroalgae) and ecomorphological type (hydrophytes, helophytes) at the pool/riffle sequence scale. Channelization led to taxonomic shifts and loss of biodiversity. Vascular plant taxa such as Helosciadium nodiflorum and Berula erecta were replaced by opportunist taxa such as Potamogeton crispus in channelized reaches. Combinations of channelization structures affected the stream plant assemblages less than did individual structures, a result that suggested compensatory effects. Our study supports the idea that channelization must be accompanied by measures that preserve the initial physical conditions of streams and the natural plant community composition.

We studied aspects of the nymphal biology and ecology of 3 Perlodidae species (Guadalgenus franzi, Hemimelaena flaviventris, and Isoperla curtata) in a Mediterranean seasonal stream in the southern Iberian Peninsula. Their life-history strategies were greatly influenced by the characteristics of their environment, i.e., a summer dry period with relatively warm temperatures, but strategies differed among species. Guadalgenus franzi was semivoltine and probably underwent nymphal quiescence when the stream was dry. Hemimelaena flaviventris and I. curtata had relatively short univoltine life cycles that overlapped, but I. curtata was slightly ahead of H. flaviventris. Both species passed the dry period in the egg stage, probably with a diapause phase. Growth rates of H. flaviventris and I. curtata peaked before emergence, whereas growth rate of G. franzi peaked immediately before and after the dry period. The 3 species had relatively short flight periods compared with other species from seasonal streams. Guadalgenus franzi and I. curtata were mainly scrapers that fed on diatoms, whereas H. flaviventris was mainly predatory. Diet changed somewhat in relation to size, and prey electivity patterns differed among species. All 3 species preferred Chironomidae, but G. franzi preferred Chironomidae principally when smaller. All 3 species had high secondary production relative to other stonefly species from both temporary and permanent waters.

Loss of riparian vegetation surrounding streams can affect instream biota by altering stream characteristics, such as terrestrially derived detrital inputs and instream productivity. Omnivorous crayfish can be a dominant component of stream biota and are considered a keystone species because of their ability to forage at multiple trophic levels. Resource shifts caused by changes in riparian canopy have the potential to influence crayfish diet and growth. We investigated the effects of changes in canopy cover on the crayfish Cherax destructor in a southeastern Australian lowland stream. We compared the diet of C. destructor between sites with and without riparian cover and determined how differences in the quantity of food resources between sites affected crayfish growth. The availability of basal (plant, algae, and detrital) resources was related to the presence of a riparian canopy. Aquatic macrophytes were more common at sites with no canopy cover and terrestrially derived leaf litter was more abundant at sites with an intact canopy. Stable isotope and gut content analyses of crayfish diet indicated a shift toward autochthonous food sources in individuals from sites with no canopy cover. In laboratory feeding trials, crayfish had higher growth rates when fed macrophyte material than when fed terrestrially derived leaf litter. Insights gained into resource use by crayfish, particularly the importance of aquatic invertebrates in crayfish diet, emphasize the merits of conducting both gut content and stable isotope analyses to assess short- and longer-term aspects of diet. Further structural and functional impacts of changes to riparian condition should be investigated, but the trophic role of C. destructor in stream food webs appears to be sensitive to alterations in the dominant basal resources associated with changes in riparian canopy.

Crayfish must locate food resources in a chemically complex environment in which chemicals from various sources interact and mix. We tested the hypothesis that an additional chemical food source of leachate from detritus (dissolved organic matter [DOM]) would affect crayfish orientation behavior toward a fish stimulus. In addition, we predicted that the effect of DOM would differ depending on whether the detritus was produced under ambient (AMB) or elevated (ELEV) CO₂ because of changes in foliar chemistry that accompany elevated CO₂ levels. DOM was collected from leaf litter derived from quaking aspen (Populus tremuloides) foliage that was produced at either AMB or ELEV CO₂ and was presented at 2 concentrations (3 mg/L, 6 mg/L). Stimulus treatments were: 1) CON (control; fish odor only), 2) AMB-low (fish AMB DOM at 3 mg/L), 3) AMB-high (fish AMB DOM at 6 mg/L), 4) ELEV-low (fish ELEV DOM at 3 mg/L), and 5) ELEV-high (fish ELEV DOM at 6 mg/L). Crayfish (Orconectes virilis) were observed in a recirculating flume in which the fish odor source was placed downstream of the DOM odor source. Behavioral responses measured were % success in locating the fish odor source, time to find source, walking speed, walking speed toward the source, turning angle, heading angle relative to the source, heading angle relative to upstream, and net-to-gross ratio (NGR). Crayfish in the AMB-high treatment were most successful in locating the source. Animals had higher turning angles in the ELEV-high treatment than in all treatments except the control and had increased heading angles relative to the source and decreased heading angles relative to upstream. No differences were found for walking speed or NGR. These results indicate that crayfish orientation to a fish odor source is affected by the presence of DOM from detritus, but only when it is presented at the high end of a natural range of DOM concentration. It also appears that the presence of ambient CO₂ detritus (with a lower concentration of secondary defensive compounds) increases success in finding the source, whereas the presence of elevated CO₂ detritus does not. Possibly, in an elevated-CO₂ future, behaviors associated with foraging and selection of food types by crayfish might be altered, and therefore affect the aquatic community. These effects might be seasonal and synchronized with the natural fluctuations of DOM found in lotic systems.

Competitive interactions, including interference and intraguild predation, occur in many aquatic food webs and influence large-scale patterns of abundance, especially in lentic habitats. In a study of small coastal streams in eastern Canada, we found patterns suggesting that abundances of a predatory invertebrate, Sweltsa onkos (Plecoptera:Chloroperlidae), are influenced by another predator, Rhyacophila vibox (Trichoptera:Rhyacophilidae). Larval densities of the 2 species were inversely correlated, and mortality of larval S. onkos tended to be higher in streams with many R. vibox. Abundance and mortality of a 3^rd predatory species, Isoperla montana (Plecoptera:Perlodidae), were unrelated to abundances of the other species. Laboratory behavioral observations confirmed that interactions between R. vibox and S. onkos were asymmetrical; high densities of R. vibox led to higher injury and mortality rates and lower growth rates of S. onkos, whereas S. onkos had no effect on R. vibox growth or mortality. Previous work has shown that R. vibox reaches high densities only in streams with few or no fish, and we conclude that the relative abundances of S. onkos and R. vibox in the study streams might be influenced by differences in vulnerability to intraguild and fish predation.

We developed and tested a species-level Bray–Curtis (BC) similarity to reference index and a genus-level diatom model affinity (DMA) index to quantify agricultural and acid mine drainage (AMD) impacts on streams in the Western Allegheny Plateau of southeastern Ohio. Decreased similarity to references sites was found in impaired streams, and diatom metrics further indicated how assemblages were impacted. Sites identified by index scores as impaired had significantly greater conductivity, PO₄-P, Ca² , Mg² , Na , Cl⁻, % pasture in the upstream watershed, and forest fragmentation than minimally impaired sites (p < 0.05). Percent acidophilic diatoms significantly increased with reduced alkalinity caused by mining impacts (p < 0.01). Relative abundances of species indicating low P and N status decreased with increased pasture and row crops in upstream watersheds (p < 0.01), whereas abundance of motile diatoms increased (p < 0.01). In a case study, diatom, macroinvertebrate, and fish assemblages were compared among 18 sites in a single watershed (6 sites along an AMD impact gradient, 6 NaOH-treated AMD sites, and 6 sites with no mining impacts). All assemblages indicated severe impairment by AMD, but fish were least useful because they were absent from 5 of the 6 sites. Macroinvertebrates indicated unimpaired conditions at treated sites, but fish signaled potential problems (lower than expected biomass and presence of deformities) despite high species richness and index of biotic integrity scores. Diatom indices indicated significant impairment at AMD and treated sites (p < 0.01). Diatom metrics and indices very effectively signaled agricultural and mining impacts at the regional scale and were especially useful in the case study because they provided finer resolution of AMD effects and additional information of ecological importance missed by or in conflict between macroinvertebrate and fish assemblages. The DMA and BC indices responded similarly to stressors at the regional scale, but BC more effectively signaled impacts at the watershed scale. However, DMA was an effective assessment tool that might be used more easily than BC by novice phycologists and watershed groups because it requires less taxonomic expertise. If implemented in future watershed or regional studies, diatom indices could greatly benefit policy, management plans, and current monitoring efforts.

The percentage of emergent mosquitoes (Culex spp.) collected, temporal dynamics of collection by pyramidal emergence traps, and the effect of a chemical deterrent (pest control strips) for terrestrial scavengers, which remove emergent mosquitoes from collection jars, on the number of emergent mosquitoes collected were studied in ponds in southern California. On average, 75.5% of emergent mosquitoes were collected in jars at the apex of 0.25-m² emergence traps placed on the water surface. Recently emerged mosquitoes spent ∼1 d in emergence traps before moving into the collection jars. Pest control strips (2.54 cm² of Vaportape II: 10% 2,2-dichlorovinyl dimethyl phosphate), which have been used in collection jars during previous studies to deter foraging on newly emerged mosquito adults by Argentine ants (Linepithema humile Mayr), did not significantly reduce the number of adult mosquitoes in collection jars and did not delay the movement of adult mosquitoes into collection jars. The cumulative proportion of adult mosquito production sampled by the pyramidal emergence traps increased at a decreasing rate across time (proportion of total production = −0.314 1.021(1 − e^−0.370t) where t is days of trapping). The proportion of adult mosquito production collected during 3- to 5-d collection periods ranged between 0.37 and 0.55 during periods that Culex emergence was continuous from aquatic habitats.

It is generally accepted that juvenile salmonid production is linked, via bottom-up pathways, to marine-derived nutrient (MDN) inputs from spawning salmon. Many studies have used standing stock biomass estimates of aquatic macroinvertebrates to infer relationships between MDN and secondary production in streams that receive spawners. However, no study has measured aquatic macroinvertebrate secondary production in relation to MDN. To assess the relationship between MDN and aquatic insect production, we measured secondary production of the 5 dominant mayfly genera (Baetis spp., Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) and chironomids throughout the primary growing season in 2 streams in southeastern Alaska. Both streams had upstream reaches blocked from spawning salmonids by a waterfall barrier and downstream reaches that received large spawning runs of pink and chum salmon. Four of the mayfly genera studied (Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) had significantly greater production in upstream than in spawning reaches. Secondary production of Baetis spp. was similar between upstream and spawning reaches. Chironomid production was significantly greater in spawning than in upstream reaches. However, biomass of each taxon was maximized in the spring and summer before the primary period of MDN input. These patterns indicate that another factor, primarily spawning disturbance, is an important driver of benthic insect secondary production in these streams and might provide the community structure within which MDN subsidies occur. If these patterns are common in streams that receive salmon runs, then secondary production-mediated links between MDN and juvenile salmonid production might be mostly the result of responses of chironomids and other benthic organisms with similar life histories.

Daily fluctuations of primary production can result in diurnal variations in labile dissolved organic C (DOC) and N (DON) concentrations, but we know little about how biotic uptake of labile DOC and DON changes from day to night. We compared daytime vs nighttime uptake of ammonium (NH₄ ), acetate (labile DOC), and glycine (labile DON) using short-term nutrient releases conducted in 6 open-canopy streams in northwestern Wyoming, USA. NH₄ uptake was primarily a function of autotrophic demand because it was often higher during the day, and daytime uptake was driven by gross primary production when placed in context of prior data from the region (r² = 0.60, p < 0.001). However, acetate uptake showed no significant diurnal change and the quality of fine benthic organic matter explained diurnal variation in uptake across streams, indicating control via heterotrophic demand (r² = 0.94, p = 0.006). Patterns in glycine uptake were distinct from both NH₄ and acetate uptake. Glycine uptake decreased as the ratio of total dissolved N to soluble reactive P increased (r² = 0.72, p = 0.002), indicating that DON uptake might be P-limited. Uptake did not differ significantly by solute type, suggesting labile forms of DOC and DON can be cycled as rapidly as NH₄ , a solute traditionally considered to be extremely bioreactive in headwater streams. Thus, demand for labile DOC and DON was high despite the fact that bulk DOC and DON concentrations were high relative to inorganic N. This result suggests that ambient availability of labile organic nutrients might not satisfy demand. Last, diurnal variation in NH₄ , DOC, and DON demand was as high as annual variation in some forested streams; therefore, future stream nutrient uptake studies should be consider how and when uptake is measured, particularly in streams with open canopies.

Various components of island stream faunas, including caridean shrimps, fish, and gastropods, undertake obligate amphidromous migration, whereby larvae are released in upstream freshwater reaches, drift downstream to estuaries or marine waters, then migrate upstream as postlarvae to freshwater adult habitats. Longitudinal migration from estuaries to headwaters is well documented for many amphidromous species, but the degree of among-river marine dispersal is poorly known for most species. We need better understanding of the potential for marine dispersal in population processes of amphidromous species, particularly recolonization and population recovery in impacted lotic systems, such as those on Puerto Rico, because some theories of dispersal for species with marine larvae predict high rates of self-recruitment. We tested population genetic predictions for widespread marine larval dispersal and self-recruitment to the natal river for 11 amphidromous species, including shrimps, fish, and a gastropod, in Puerto Rico. Population genetic analysis of mitochondrial DNA data showed high rates of gene flow among rivers and indicated that marine dispersal determines the population genetic structure of all 11 species. Difficulty in recruiting to oceanic currents promotes closed population structures in some marine species, but larvae of amphidromous species entrained in downstream river flow might be delivered more readily to ocean currents. Population recovery processes occurred at the island scale rather than at the river scale, but further studies are needed to identify whether population recovery processes are likely at larger spatial scales (e.g., among islands). River management strategies should maintain environmental flows that allow larval export, maintain longitudinal dispersal pathways over dam spillways and via subterranean passages, and maintain open and healthy estuaries.

Aquatic communities are highly threatened by anthropogenic and climate change. However, despite their importance in these communities, information regarding temporal changes in populations and assemblages of North American crayfish is scarce. Long-term monitoring of crayfish populations in south-central Ontario, Canada, indicates that the populations are in a significant state of decline. We sought to determine whether these population declines are spatially and taxonomically broad, and if so, what factors might be associated with the declines. We sampled crayfish abundance (catch per unit effort) in 100 lakes, and compared current abundances to survey results from the early 1990s. Abundances of all species (natives and nonnatives) declined significantly during this interval. Declines were both severe (63–96% loss of abundance) and geographically widespread for all species. Previous studies have documented native species declines caused by the invasive crayfish Orconectes rusticus, but this species was absent from almost all lakes and was not a factor in the declines. We hypothesize that the introduction of predatory smallmouth bass (Micropterus dolomieui), increases in Al concentrations, and reduced Ca concentrations in these lakes are negatively affecting crayfish populations.

Aquatic food webs use C derived from 2 sources: autochthonous (derived from inside the water body) and allochthonous (derived from outside the water body). Various autochthonous sources are available to consumers (e.g., algae, mosses, macrophytes), and stable isotope analysis can be used to differentiate among C sources and to determine which sources sustain aquatic communities. The goal of our study was to determine which C sources contribute to the autochthonous pathways in 2 oligotrophic rivers, the Upper Salmon and Big Salmon, in New Brunswick (Canada). Samples were taken from headwaters to river mouth (above head of tide) at 17 study sites distributed across subbasins and stream orders. Vegetation samples included macrophytes, filamentous algae, biofilm, and aquatic bryophytes. Macroinvertebrate taxa in the scraper (grazer) functional feeding group were collected to represent primary consumers that feed primarily on autochthonous food sources. Macrophyte and cyanobacteria δ¹³C values were correlated with scraper δ¹³C values (r  =  0.55 and 0.47, respectively), but δ¹³C of most scrapers was more depleted than δ¹³C of these food sources (>40% and >90%, respectively). Trophic fractionation is primarily an enrichment process, so these results indicated that macrophytes and cyanobacteria were not important food sources for primary consumers. Filamentous algae and biofilm δ¹³C values were poorly correlated with scraper δ¹³C values (r  =  0.36 and 0.40, respectively), and >60% of the values were too depleted for these sources to be important food sources for scrapers. Ninety-eight percent of scraper δ¹³C values were enriched relative to bryophyte δ¹³C values, and bryophyte δ¹³C values were correlated with scraper δ¹³C values (r  =  0.53), particularly when samples from slow-flowing habitats were removed from the analysis (r  =  0.76). Spatial distributions of 2 important bryophyte taxa differed. Fontinalis sp. was abundant in headwater streams, and Drepanocladus sp. was abundant in low-order streams. Scrapers in low-order streams seemed to depend more on Fontinalis sp. than scrapers in high-order streams depended on Drepanocladus sp. Our results suggest that in low-productivity, nutrient-limited rivers, reduced availability of more preferred food sources (e.g., epilithic algae) might cause primary consumers to switch to marginal food sources, such as bryophytes. Further research is required to confirm this hypothesis and to eliminate confounding factors.