We analyzed nutrient data from the National Lakes Assessment (NLA), a probability survey of 1028 lakes >4 ha in lake area across the conterminous USA to quantify and contrast different methods of setting nutrient criteria. We calculated potential nutrient criteria for total P (TP), total N (TN), and chlorophyll a (Chl a) by 4 methods (25^th percentile of population, 75^th percentile of least-disturbed reference sites, diatom-based paleolimnological reconstruction, and stressor modeling) and compared them to existing draft US Environmental Protection Agency (EPA) criteria within national nutrient ecoregions. At the national scale, the ecoregional criteria derived from the different approaches were highly correlated. However, absolute values differed widely among approaches within ecoregions. Population 25^th percentiles were lower (often by a factor of 2–6) than values obtained with other approaches in almost all ecoregions, results indicating that population 25^th percentiles cannot be used as a surrogate for reference-site or paleolimnological approaches. Stressor regression models often did not explain much of the variance in nutrient concentration, especially in the less-disturbed ecoregions. For TP, diatom-inferred paleolimnological criteria were higher than reference-site-based criteria, which were higher than stressor-model criteria. For TN, these 3 approaches were very comparable. For Chl a, the reference-site and stressor-model approaches gave similar criterion values in low-nutrient ecoregions, but the reference-site 75^th-percentile approach had much higher criterion values than the stressor-modeling approach in the high-nutrient ecoregions. Use of NLA reference-site 75^th percentiles as nutrient criteria showed that 42% of the assessed lakes exceeded TP criteria, 47% exceeded TN criteria, and 32% exceeded Chl a criteria. Survey results also suggest that most lakes are P limited. Ninety-three percent of the lakes in the population had molar TN∶TP ratios > 16, and 52% had TN∶TP > 50.

One of the biggest challenges when conducting a national-scale assessment of lakes, such as the 2007 US National Lake Assessment (NLA), is finding enough reference lakes to set appropriate expectations for the assessed sites. In the NLA, a random design was used to select lakes for sampling to make unbiased estimates of regional condition. However, such an approach was unlikely to yield enough minimally impacted lakes to use as reference sites, especially in disturbed regions. We developed a 3-stage process to select candidate reference lakes to augment the NLA probability sample in the northeastern USA (Northeast). Screening included a water-chemistry database filter, landuse evaluation, and analysis of aerial photographs. In the Northeast, we assembled a database of 2109 lakes >4 ha in surface area, of which 369 passed the water-chemistry screen. Of these, 220 failed the watershed landuse screen and 60 failed the aerial photograph screen, leaving a set of 89 optimal candidate reference lakes. Twenty of these lakes were sampled as potential reference lakes in the NLA. Based on a wide variety of indicators, NLA field measurements indicated that almost all (85–100%) of the chosen candidate reference lakes had least-disturbed water chemistry, although somewhat fewer had least disturbed physical habitat (74–79%) and biology (68–78%). Nevertheless, our 3-stage screening process was an efficient method for identification of good candidates for reference-lake sampling. The reference-lake selection process used in our study can be done in the office and relatively inexpensively. As such, it is very useful for large-scale regional or national studies encompassing areas too large to census. It also has the advantage of adding a level of consistency and quantification to the reference-site selection process.

Physical changes to rivers associated with large dams (e.g., water temperature) directly alter macroinvertebrate assemblages. Large dams also may indirectly alter these assemblages by changing the food resources available to support macroinvertebrate production. We examined the diets of the 4 most common macroinvertebrate taxa in the Colorado River through Glen and Grand Canyons, seasonally, at 6 sites for 2.5 y. We compared macroinvertebrate diet composition to the composition of epilithon (rock and cliff faces) communities and suspended organic seston to evaluate the degree to which macroinvertebrate diets tracked downstream changes in resource availability. Diets contained greater proportions of algal resources in the tailwater of Glen Canyon Dam and more terrestrial-based resources at sites downstream of the 1^st major tributary. As predicted, macroinvertebrate diets tracked turbidity-driven changes in resource availability, and river turbidity partially explained variability in macroinvertebrate diets. The relative proportions of resources assimilated by macroinvertebrates ranged from dominance by algae to terrestrial-based resources, despite greater assimilation efficiencies for algal than terrestrial C. Terrestrial resources were most important during high turbidity conditions, which occurred during the late-summer monsoon season (July–October) when tributaries contributed large amounts of organic matter to the mainstem and suspended sediments reduced algal production. Macroinvertebrate diets were influenced by seasonal changes in tributary inputs and turbidity, a result suggesting macroinvertebrate diets in regulated rivers may be temporally dynamic and driven by tributary inputs.

In summer, streams in Portuguese eucalyptus forests frequently experience drought resulting in isolated pools, frequently saturated with leaf litter, in which the leaf leachates may generate toxic and hypoxic conditions. We assessed the ecological effects of Eucalyptus globulus leachate with and without aeration (i.e., low flow vs pool scenarios) on microbial decomposition of eucalyptus leaves and on toxicity to and survival, avoidance, and feeding behavior of Chironomus riparius Meigen (Diptera:Chironomidae), Echinogammarus meridionalis Pinkster (Amphipoda:Gammaridae), and Sericostoma vittatum Rambur (Trichoptera:Sericostomatidae). Eucalyptus globulus leaves immersed in a gradient of aerated or nonaerated eucalyptus leachate (100, 40.0, 16.0, 6.40, 2.50% volume/volume) were colonized by species-poor fungal assemblages. Leaf mass loss did not differ among treatments, but hypoxia suppressed conidia production and negatively affected fungal biomass. A concentration-dependent effect on fungal biomass was observed in aerated leaf extracts. A trade-off was found between the stimulatory effect of leachate nutrients and inhibitory effects of secondary compounds at leachate concentrations of 16 to 40%, and microbial respiration was depressed at concentrations >16% in nonaerated conditions. Only S. vittatum discriminated leaves conditioned in aerated water from leaves conditioned in nonaerated leachates. Leachates negatively affected all species, mostly in nonaerated conditions. Maximum leachate concentration caused mortality of 100% of E. meridionalis and 26 to 40% of C. riparius, regardless of aeration, and 70% of S. vittatum in nonaerated conditions. Sericostoma vittatum avoided the highest leachate concentrations, but C. riparius and E. meridionalis did not. Impoverished microbial communities and invertebrate assemblages with dissimilar tolerance to leachate may maintain functional properties and processes during drought disturbances in eucalyptus streams.

Species traits have been identified as a component of biodiversity that is worthy of investigation as a tool for exploring ecological effects of anthropogenic disturbances. We used meta-analysis to show how selected species traits of Ephemeroptera and Plecoptera can be used to examine the ecological effects of flow and water-level regulation in freshwater ecosystems used for hydropower generation. We hypothesized that hydrological regulation would favor an opportunistic ecological strategy and bring about increased similarity in species composition across samples. We found higher reproductive capacity, faster life cycles, and a tendency toward a changed composition of functional feeding groups in lotic stonefly assemblages in response to hydrological regulation. However, the effects of hydrological regulation on mayfly species traits and lentic stonefly species traits were limited. We found lower species turnover in mayflies and stoneflies from regulated than from unregulated lotic and lentic sites, a result suggesting selection of a subset of mayflies and stoneflies from the regional species pool by flow and water-level regulation. The limited support for our predictions probably reflects the comparatively low diversity of aquatic insect species in Norwegian freshwater ecosystems for biogeographical and historical reasons, phenotypic plasticity of the insects' life histories and feeding habits, low trait diversity, trade-offs among species traits in a marginal region, restrictions to trait combinations and, perhaps, random extinction of mayflies and stoneflies caused by hydrological regulation. We advocate inclusion of species-trait variables that may affect ecosystem-level ecological processes in environmental assessment studies because a better understanding of trait-mediated ecological functioning should facilitate assessment of the ecological consequences of anthropogenic perturbations of freshwater ecosystems.

Predation effects in streams can cascade to terrestrial food webs through the flux of organisms that develop in the stream and emerge as adults to the terrestrial system. This emergence subsidizes some terrestrial predators, an effect that generally varies based on the magnitude of the subsidy. Factors regulating this magnitude are relatively well known, but factors regulating the trophic structure of the subsidy are not. I tested the hypothesis that predatory fish in natural stream pools alter the biomass and trophic structure (proportion of predatory adults) of emerging aquatic insects. I created a 13× gradient of predatory fish biomass (4 species of Lepomis sunfish and the minnow Notropis boops; within the range of natural variation) across 10 pools in Brier Creek, Oklahoma (USA). Pool area and substrate composition varied naturally, so I also measured their effect on insects. At the end of the experiment after the stream became intermittent, fish reduced benthic insect biomass but not emergence to the terrestrial habitat. The proportion of predatory insects emerging from pools was positively associated with pool area, but was unaffected by fish density. The best predictor of emergence biomass among pools on any date was the standing crop of benthic insects before fish manipulation, a result suggesting a time lag between measured benthic standing crop in the stream and subsequent emergence. Fish manipulations occurred during the end of peak summer insect emergence, which may have limited my ability to detect fish effects on emergence. My study demonstrates that variation in the timing of predation may constrain the spatial scale of fish effects in aquatic and terrestrial food webs and suggests that pool size can influence the trophic structure of emerging aquatic insects.

The highly endemic diversity of the Yun-Gui plateau lakes has decreased severely in recent decades. Protecting these native species is a challenge, in part because their biology and population ecology remain unexplored. We investigated 1 focal endangered gastropod species (Margarya melanioides) in 1 plateau lake: 1) to quantify spatiotemporal variation in population abundance and size structure, and 2) to identify the key variables influencing these variations. Density was 0.068 individuals/m² with an estimated total population size of 2.1 × 10⁷ individuals. The size-frequency distribution was skewed toward larger, mature individuals (40.0–60.0 mm), a result indicating that the population is aging. Population size structure showed high spatial heterogeneity, and density was highest in the north-central region of the lake. Redundancy analysis indicated that human-induced environmental degradation strongly controlled the spatial patterns in size-structure. This species has experienced a remarkable decrease in range and population size over the past 50 y, and the population probably will collapse by 2015. The decline could be attributed to demographic drift, environmental filtering, human harvesting, and its limited dispersal ability. Our studies should give useful information for building efficient conservation strategies for M. melanioides and other threatened plateau gastropods. Further studies are needed on the population ecology and reproductive biology of M. melanioides, particularly on its reproductive behavior and size-specific survivorship.

The effects of nutrient enrichment on aquatic ecosystems have been well documented, but less is known about how these effects vary in response to additional stressors. I used experimental mesocosms to assess how nutrient enrichment affected zooplankton density and community structure in the presence and absence of invasive zebra mussels (Dreissena polymorpha). The density of several pelagic zooplankton taxa increased in response to nutrient enrichment, but these increases were offset or reduced in mesocosms that contained zebra mussels. At least at the beginning of the experiment, zebra mussels reduced the amount of algae that developed in response to nutrient enrichment and potentially were in competition with pelagic zooplankton. In contrast, zebra mussels did not reduce the total density of zooplankton in the absence of nutrient enrichment. Instead, the density of several littoral taxa increased in mesocosms with zebra mussels, presumably because they were able to use attached algae that also developed in mesocosms with zebra mussels. Combined, these results suggest that the overall effects of nutrient enrichment on plankton communities should be considered in the context of multiple stressors including the presence or absence of invasive species.

Identification of ecological thresholds is important both for theoretical and applied ecology. Recently, Baker and King (2010, King and Baker 2010) proposed a method, threshold indicator analysis (TITAN), to calculate species and community thresholds based on indicator species scores adapted from Dufrêne and Legendre (1997). We tested the ability of TITAN to detect thresholds using models with (broken-stick, disjointed broken-stick, dose-response, step-function, Gaussian) and without (linear) definitive thresholds. TITAN accurately and consistently detected thresholds in step-function models, but not in models characterized by abrupt changes in response slopes or response direction. Threshold detection in TITAN was very sensitive to the distribution of 0 values, which caused TITAN to identify thresholds associated with relatively small differences in the distribution of 0 values while ignoring thresholds associated with large changes in abundance. Threshold identification and tests of statistical significance were based on the same data permutations resulting in inflated estimates of statistical significance. Application of bootstrapping to the split-point problem that underlies TITAN led to underestimates of the confidence intervals of thresholds. Bias in the derivation of the z-scores used to identify TITAN thresholds and skewedness in the distribution of data along the gradient produced TITAN thresholds that were much more similar than the actual thresholds. This tendency may account for the synchronicity of thresholds reported in TITAN analyses. The thresholds identified by TITAN represented disparate characteristics of species responses that, when coupled with the inability of TITAN to identify thresholds accurately and consistently, does not support the aggregation of individual species thresholds into a community threshold.

Cuffney and Qian (2013) performed numerous simulations to demonstrate potential flaws in Threshold Indicator Taxa Analysis (TITAN), a method for interpreting taxon contributions to community change along novel environmental gradients. Based on their simulations, they concluded that: 1) TITAN is not an effective method for detecting different types of statistical thresholds in trend lines, 2) permutation results in highly significant p-values even for splits that are not thresholds, and 3) coincident change points may arise as an artifact of inaccuracies, imprecision, and systematic bias in both change-point estimation and TITAN's bootstrap. The critique raises some important concerns, but because of significant misunderstanding, it is based on analyses that violate basic assumptions of both TITAN and indicator species analysis (IndVal), and thus, constitutes a straw man that cannot be used to evaluate their performance. We demonstrate that the critique: 1) fundamentally misrepresents TITAN's primary goals; 2) simulates taxon abundances based on unrealistic statistical models that fail to represent important empirical patterns present in Cuffney and Qian's own published data sets (i.e., negative binomial distributions, frequent absences a function of the predictor); 3) tests TITAN's ability to identify breaks in trend lines distorted by log-transformation that do not match the greatest change in the simulated response, leading to misinterpretation of expected and previously documented behavior by TITAN as errors; 4) misinterprets TITAN's use of p-values while ignoring diagnostic indices of purity and reliability for identifying robust indicator taxa; and 5) asserts that bootstrapped change-point quantiles in TITAN are too narrow despite published results to the contrary. Last, in contrast to the claim that change-point synchrony may be an artifact of the technique, we show that: 6) analysis of published data using completely independent methods (i.e., scatterplots of abundance data or generalized additive models) also reveals synchrony in the nonlinear decline of numerous taxa in corroboration of TITAN and its underlying conceptual model. Thus, Cuffney and Qian have not identified any serious limitations of TITAN because their critique is based on misinterpretation of TITAN's assumptions and primary objectives. However, their critique does highlight the need for clarification of the appropriate uses, potential misuses, and limitations of TITAN and other methods for ecological analysis.

The fraction of gross primary production (GPP) that is immediately respired by autotrophs and their closely associated heterotrophs (AR_f) is unknown. This value is necessary to calculate the autotrophic base of food webs, which requires knowing production available for grazers. AR_f is also necessary for estimating heterotrophic respiration (HR) which is needed to calculate C spiraling in streams and rivers. We suggest a way to estimate AR_f from daily metabolism data using quantile regression between GPP and 90% quantile of ecosystem respiration (ER). We reasoned that autotrophic respiration represents the lower limit for ER on any one day and used quantile regression to estimate the relationship of the lower quantile of ER with respect to GPP. We examined this approach with simulation modeling and application of quantile regression to estimates of continuous GPP and ER from >20 streams. Simulation modeling showed that low-uncertainty estimates of AR_f required large variation in daily GPP. Covariance between HR and GPP, which might be observed if the processes were temperature controlled, biased estimates of AR_f. Seasonal estimates of AR_f were robust to daily variation in AR_f as a function of GPP. AR_f calculated from previously published estimates of daily metabolism from streams averaged 0.44 (SD  =  0.19) with high variation among streams. This value is higher than most physiological measurements, probably because of light limitation of algae and from HR closely associated with daily GPP. How much of AR_f was from algal respiration vs closely associated heterotrophic respiration is not known, but we suggest that the value (1 – AR_f)GPP represents the amount of C available to animals.

Most freshwater zooplankton species produce dormant eggs or other long-lived resistant life stages to bridge periods that are unsuitable for growth and reproduction. One of the principal cues that determine activation of these dormant stages is light exposure. Contrary to the analogous process of seed germination in plants, relatively little is known about the mechanism of light activation and its potential adaptive value in freshwater zooplankton. We made use of a temporary pool model to investigate the hatching response of resting eggs of the fairy shrimp Branchipodopsis wolfi to a specific set of selected light regimes and to evaluate the importance of egg pigmentation for mediating susceptibility to light cues. Our results suggest the presence of a critical light threshold for activation leading to maximal hatching. Moreover, the process of light activation does not appear to be purely cumulative because light administered in a single continuous pulse resulted in much higher hatching than the same amount of light administered in several 2-h pulses. Last, darkly pigmented eggs were less sensitive than lightly pigmented eggs to light exposure, resulting in delayed hatching of darker eggs during an experimental inundation. This result indicates that natural variation in egg pigmentation could be a risk-spreading mechanism that trades off the risks of mortality caused by kin competition during the nutrient-limited conditions that typically prevail early during inundations on one hand and the risk of mortality from premature pond drying on the other hand. Overall, our results indicate that light-energy activation thresholds and variation in pigmentation of the eggshell can be important traits that may contribute to variation in hatching phenology among zooplankton lineages.

Nondestructive tissue sampling is desirable for genetic or physiological studies of endangered freshwater mussels. We used the freshwater pearl mussel (Margaritifera margaritifera) to evaluate 4 sampling methods (haemolymph extraction, foot scraping, mantle biopsy, and viscera swabbing) with regard to their effectiveness for deoxyribonucleic acid (DNA) analyses and their effects on sampled mussels. One hundred twenty-eight days subsequent to tissue sampling (1 June–7 October), all sampled individuals were alive, and average growth of sampled individuals was not significantly different from unsampled, control individuals, except that the viscera-swabbing group had lower growth. The magnitude of decreased growth in viscera-swabbed individuals was small (∼0.5 mm less than controls), and the biological significance of this result is unclear. DNA yields from haemolymph extraction and foot scraping were significantly lower and more variable than yields from the other methods. Genotyping success was lowest for haemolymph extraction and mantle biopsy, but was high for the other methods. Viscera-swab samples stored in lysis buffer at room temperature prior to DNA extraction had higher DNA yield than samples stored in buffer at 4°C or samples stored dry, but genotyping success was equivalent among storage methods. On the basis of these results, we recommend use of the noninvasive viscera-swabbing method.

The use of species traits in basic and applied ecology is expanding rapidly because trait-based approaches hold the promise to increase our mechanistic understanding of biological responses. Such understanding could transform descriptive field studies in community ecology into predictive studies. Currently, however, trait-based approaches often fail to reflect species–environment relationships adequately. The difficulties have been perceived mainly as methodological, but we suggest that the problem is more profound and touches on the fundamentals of ecology and evolution. Selection pressures do not act independently on single traits, but rather, on species whose success in a particular environment is controlled by many interacting traits. Therefore, the adaptive value of a particular trait may differ across species, depending on the other traits possessed by the species and the constraints of its body plan. Because of this context-dependence, trait-based approaches should take into account the way combinations of traits interact and are constrained within a species. We present a new framework in which trade-offs and other interactions between biological traits are taken as a starting point from which to develop a better mechanistic understanding of species occurrences. The framework consists of 4 levels: traits, trait interactions, trait combinations, and life-history strategies, in a hierarchy in which each level provides the building blocks for the next. Researchers can contribute knowledge and insights at each level, and their contributions can be verified or falsified using logic, theory, and empirical data. Such an integrated and transparent framework can help fulfill the promise of traits to transform community ecology into a predictive science.

Omnivory is prevalent in terrestrial and aquatic food webs. However, the extent and seasonality of predatory feeding by omnivores in stream food webs is largely unknown. To understand better these aspects of omnivory in stream food webs, we investigated seasonal changes in the trophic positions of 2 omnivores in a small forested stream. We selected the amphipod, Gammarus pulex, and the caddisfly larvae, Hydropsyche spp., as key organisms because both taxa are common, reach high biomasses in many stream ecosystems, and have broad food spectra. We used stable-isotope analysis of the most prevalent taxa in the benthic macroinvertebrate community to assess the trophic positions of the 2 omnivorous taxa in different seasons. We estimated the degree of predatory feeding by quantifying the importance of different food resources with a stable-isotope mixing model (IsoSource). The predation capacity of omnivores, defined as the fraction of omnivore biomass associated with predation, was compared to the biomass of strictly predatory invertebrates. Our analysis indicated a predation capacity of Hydropsyche spp. similar to that of strict invertebrate predators, whereas the predatory biomass of G. pulex was even higher than those of the other invertebrate predators because of a combination of high biomass and large proportions of animal prey (50–90%) in the diets of both omnivores. The trophic position of omnivores in winter was comparable to that of invertebrate predators (trophic level 3), whereas in summer, their trophic position was comparable to primary consumers (level 2). These shifts might be caused by seasonally varying prey availability for Hydropsyche spp. and life-cycle patterns for G. pulex. Predation by omnivores was generally high in our study stream and probably has been underestimated in other stream ecosystems where omnivores, such as Hydropsyche spp. and G. pulex, are common.

We compared hydrological environments and macroinvertebrate communities in 1^st- and 2^nd-order streams between a deer-excluded catchment (EC) and a control catchment (CC) to test effects of deer-induced hillslope soil erosion and sedimentation on macroinvertebrates. Overland flow contribution to the streams was greater in CC than in EC, and substrate in 1^st-order streams contained more fine sediment in CC than in EC, whereas fine sediment in substrate in 2^nd-order streams was similar between catchments. Macroinvertebrate community structure in 2^nd-order streams was similar between catchments, but community structure in 1^st-order streams differed between catchments. In 2^nd-order streams, grazer and predator taxa predominated in both catchments, whereas in 1^st-order streams, a clinger taxon predominated in EC and a burrower taxon predominated in CC. Diversity of macroinvertebrates in 1^st-order streams was 1.14× higher in EC than in CC. We suggest that effects of deer on macroinvertebrates were less apparent in 2^nd- than in 1^st-order streams because fine sediments did not accumulate in 2^nd-order streams exposed to deer browsing. Our results suggest that effects of sediment addition caused by deer browsing depends on the hydrogeomorphic properties of headwater streams.

Coarse particulate organic matter (CPOM) transport and retention were evaluated in 14 small streams in central Massachusetts. Seven streams were within the greater Worcester metropolitan area, and 7 were within protected areas north of the city. Sites drained areas with a range of watershed and riparian disturbance levels. Surrogate materials (wooden dowels that simulated small branches and diamond-shaped acetate strips that simulated leaves) of similar size and density to natural CPOM particles were released into 50-m reaches during low-flow conditions to measure retention rates. Cobble and boulders were the most important retention structures at all sites, and debris dams were important at some sites. Acetate diamonds had significantly shorter travel distances than dowels because particle flexibility and surface area reduced particle travel distance. Fewer acetate diamonds were retained in urban than in forested streams, and debris-dam density was lower in urban than in forested streams. The amount of developed area in the watershed and vegetation characteristics of the riparian zone contributed to the difference in retention. My findings indicate that restoration and management efforts for improving particle retention in urban streams should include reach- (e.g., riparian tree density) and watershed- (e.g., % urban or impervious surface) scale features during mitigation and when evaluating the success of the mitigation effort.

Acidification of surface water is a persistent threat to aquatic ecosystems throughout the world. Several international policies have led to successful reductions of emissions of acidifying gases, but recovery of surface-water quality has been modest. Associated biological communities continue to suffer, and research on ecosystem functional response is sparse. We investigated shifts in macroinvertebrate communities, shifts in whole-stream metabolism, whether whole-stream metabolism is as sensitive an indicator of acidification as is benthic community structure, and shifts in stream food webs in 9 small forested streams exhibiting a pH gradient from 4.0 to 7.7 and belonging to the GEOchemical MONitoring (GEOMON) Network in the Czech Republic. Acidification was related to changes in macroinvertebrate communities including reduced taxon richness and reduced Ephemeroptera family richness and abundance. Whole-stream metabolism was not significantly related to stream pH and may be less sensitive to acidification than macroinvertebrate community metrics. ¹³C and ¹⁵N isotopic signatures of the Chironomidae and Leuctridae shifted away from periphyton isotopic signatures and toward isotopic signatures of allochthonous food sources at acidified sites, a result suggesting either species-level dietary changes or replacement of species by those that rely more heavily on detritus. Our results document long-lasting changes in the ecosystem structure of these small streams in response to acidification. Differences in the responses of structural and functional metrics to acidification have implications for stream-monitoring programs.

Global climate change threatens to affect negatively the structure, function, and diversity of aquatic ecosystems worldwide. In alpine systems, the thermal tolerances of stream invertebrates can be assessed to understand better the potential effects of rising ambient temperatures and continued loss of glaciers and snowpack on alpine stream ecosystems. We measured the critical thermal maximum (CTM) and lethal temperature maximum (LTM) of the meltwater stonefly (Lednia tumana), a species limited to glacial and snowmelt-driven alpine streams in the Waterton–Glacier International Peace Park area and a candidate for listing under the US Endangered Species Act. We collected L. tumana nymphs from Lunch Creek in Glacier National Park, Montana (USA) and transported them to a laboratory at the University of Montana Flathead Lake Biological Station, Polson, Montana. We placed nymphs in a controlled water bath at 1 of 2 acclimation temperatures, 8.5 and 15°C. We increased water temperature at a constant rate of 0.3°C/min. We calculated the average CTM and LTM (± SD) for each acclimation temperature and compared them with Student's t-tests. Predicted chronic temperature maxima were determined using the ⅓ rule. Mean LTMs were 32.3 ± 0.28°C and 31.05 ± 0.78°C in the 8.5 and 15°C acclimation treatments, respectively. CTM and LTM metrics were lower in the 15 than in the 8.5°C acclimation treatment, but these differences were not statistically significant (p > 0.05). The predicted chronic temperature maxima were 20.6 and 20.2°C for the 8.5 and 15°C acclimation treatments, respectively. More research is needed on the effects of chronic exposures to rising stream temperatures, but our results can be used to assess the potential effects of warming water temperatures on L. tumana and other aquatic macroinvertebrates in alpine ecosystems.

The relative influence of landscape vs local processes on variation in stream C and energy sources to lotic consumers remains unresolved, but is a key component in understanding pathways through stream food webs. I investigated potential linkages between the distribution of δ¹³C signatures of primary producers (periphyton), terrestrial organic matter (detritus), and secondary consumers (aquatic invertebrates and fish) and: 1) drainage area and 2) stream reach geomorphology in 3 mountain watersheds of northern Idaho, USA. When considered alone, drainage area explained variation in the δ¹³C signatures of periphyton (R²  =  0.30) and sculpin (R²  =  0.15) only. Subsequent General Linear Models (GLMs) showed that stream morphology explained significant variation in δ¹³C of periphyton (79%), Diptera (93%), sculpin (51%), and trout (63%). Drainage area was not significant in these models, a result indicating that once δ¹³C signatures are explained as a function of geomorphic type, the contribution of drainage area becomes less important. On the whole, consumer δ¹³C signatures related more strongly to periphyton δ¹³C than detrital δ¹³C, though this relationship varied among geomorphic types. Collectively, results indicate that the morphology of a stream reach (including factors such as substrate size and storage elements, floodplain development, etc.) may be an important factor in the assimilation and distribution of C in food webs in forested mountain streams.

Small dams enhance the development of patchy microenvironments along stream corridors by trapping sediment and creating complex streambed morphologies. This patchiness drives intricate hyporheic flux patterns that govern the exchange of O₂ and redox-sensitive solutes between the water column and the stream bed. We used multiple tracer techniques, naturally occurring and injected, to evaluate hyporheic flow dynamics and associated biogeochemical cycling and microbial reactivity around 2 beaver dams in Wyoming (USA). High-resolution fiber-optic distributed temperature sensing was used to collect temperature data over 9 vertical streambed profiles and to generate comprehensive vertical flux maps using 1-dimensional (1-D) heat-transport modeling. Coincident with these locations, vertical profiles of hyporheic water were collected every week and analyzed for dissolved O₂, pH, dissolved organic C, and several conservative and redox-sensitive solutes. In addition, hyporheic and net stream aerobic microbial reactivity were analyzed with a constant-rate injection of the biologically sensitive resazurin (Raz) smart tracer. The combined results revealed a heterogeneous system with rates of downwelling hyporheic flow organized by morphologic unit and tightly coupled to the redox conditions of the subsurface. Principal component analysis was used to summarize the variability of all redox-sensitive species, and results indicated that hyporheic water varied from oxic-stream-like to anoxic-reduced in direct response to the hydrodynamic conditions and associated residence times. The anaerobic transition threshold predicted by the mean O₂ Damköhler number seemed to overestimate the actual transition as indicated by multiple secondary electron acceptors, illustrating the gradient nature of anaerobic transition. Temporal flux variability in low-flux morphologies generated a much greater range in hyporheic redox conditions compared to high-flux zones, and chemical responses to changing flux rates were consistent with those predicted from the empirical relationship between redox condition and residence time. The Raz tracer revealed that hyporheic flow paths have strong net aerobic respiration, particularly at higher residence time, but this reactive exchange did not affect the net stream signal at the reach scale.

The relationship between the widespread and common oligochaete Chaetogaster limnaei limnaei (CLL) and its snail hosts is usually considered commensal or mutualistic. However, its effects on snails have rarely been measured. We used laboratory experiments to examine the fitness-relevant behavior, growth, and reproductive output parameters of host snails relative to different degrees of CLL infestation. The pulmonate freshwater snail Physa acuta was used as the host species. At high infestation rates, snails used a smaller proportion of their time for foraging and a larger proportion for resting. Consequently, the infested snails had lower growth rates. Moreover, their overall reproductive output was reduced and their reproductive strategy was shifted toward producing a larger number of egg masses that contained fewer and smaller eggs. Furthermore, fewer of these eggs survived. Based on our results, the relationship between CLL and P. acuta can be described as epizoic antibiosis at high infestation rates. Our study demonstrates the context dependency of the relationship between CLL and their hosts because other investigators found that snails can profit from CLL infestation in the presence of parasitic trematodes or remain unaffected by CLL. CLL could be a much more important factor influencing snail communities than previously assumed given this context-specific potential to influence the fitness of its hosts strongly and its host-species selectivity.

Seasonal variation in macroinvertebrate assemblages usually is attributed to environmental variability, but this relationship is unclear in temporary ponds because they are highly dynamic across time. We assessed the influence of environmental variables and biotic interactions on macroinvertebrate community structure in 22 temporary ponds sampled monthly over 2 successive years that differed in rainfall. We hypothesized that abiotic and biotic variables would have different influences on macroinvertebrates having different dispersal abilities (capable of dispersal or not) and dietary modes (predaceous or not). Constrained Analysis of Principal Coordinates showed mainly seasonal influences on community assembly. During the filling phase, water-column total P (TP) and pH were important, whereas maximum depth, pH, electrical conductivity, and dissolved O₂ were important during the aquatic phase. When ponds were close to desiccation, water-column electrical conductivity and TP and sediment organic matter were most influential. Predation by urodele adults occurred early in the inundation cycle and by urodele larvae when the ponds were close to desiccation. Negative species checkerboards (patterns of species noncoexistence) revealed that primarily competitive exclusion was in operation immediately after pond inundation, whereas competitive exclusion and habitat segregation were operating in tandem during the subsequent months. During the drying phase, general deterioration of environmental conditions and high pressure exerted by biotic interactions may trigger macroinvertebrate strategies to survive desiccation. Variability in pond characteristics allows macroinvertebrate species to assemble and disassemble in response to pond inundation–desiccation cycles and, thus, supports high biodiversity in the area.

Few investigators have examined simultaneous bacterial and fungal responses to leaf-litter chemistry in fresh water. We tested the hypothesis that bacteria would be more abundant on labile litter with lower concentrations of defensive compounds, whereas fungi would be more abundant on recalcitrant litter. We used quantitative-polymerase chain reaction (qPCR) to measure the abundance of bacterial 16S and fungal 18S ribosomal ribonucleic acid (rRNA) genes and found that these groups responded differently to leaf chemistry. Bacterial 16S rRNA genes were 4× more abundant on labile than on recalcitrant litter. In contrast, fungal 18S rRNA genes were 8× more abundant on recalcitrant than on labile litter. Peak bacterial gene abundances on day 6 were related to leaf-litter % bound condensed tannin (r²  =  0.38), and peak fungal gene abundances on day 14 were related to % soluble condensed tannin (r²  =  0.49), % bound condensed tannin (r²  =  0.34), and % lignin (r²  =  0.33). Leaf-litter C∶N ratios were not associated with microbial gene abundance. The ratio of fungal 18S:bacterial 16S genes also increased along the 1^st axis in a principal components analysis of phytochemical variables. The early peak in bacterial rRNA gene abundance may indicate the role of bacteria in the early decomposition of leaf litter. rRNA gene abundance patterns demonstrate that bacteria and fungi have different patterns of growth and productivity in response to leaf-litter chemistry.