Floristic studies indicate the abundance of microalgae in northern boreal peatlands, but we know relatively little about their ecology or how they will respond to changes in environmental conditions expected in this region as climate changes. We examined changes in algal community structure at sites exposed to a long-term water-table manipulation, including drought (lowered water-table treatment), flooding (raised water-table treatment), and control (no manipulation) treatments in an Alaskan fen. In previous years, continuous algal colonization typically would have occurred only in the raised water-table treatment, but a spring flood inundated experimental plots and provided a unique opportunity to examine algal community response to rewetting after long-term drought. This event allowed us to investigate how much ecosystem memory of the antecedent water-table manipulations regulated the ability of taxa to recolonize sites after prolonged drought compared to sites that had been continuously flooded. Despite no differences in water-table position among treatments at the time of sampling, surface-water nutrient concentrations were higher in the lowered water-table treatment relative to the other treatments after the spring thaw. This difference corresponded with greater algal abundance and biovolume in the lowered water-table treatment relative to the control and raised water-table treatments. Higher abundance and biovolume was driven mainly by filamentous green algae (Chlorophyta), especially Oedogonium, Spirogyra, and Microspora. Diatoms were most abundant in the raised water-table treatment, whereas chrysophytes were most abundant in the control treatment. Across all water-table treatments, N-fixing cyanobacteria increased as nutrients and water-table position declined over time. The differences in algal community structure among water-table treatments suggest that alternating drought and flooding events expected with climate change may significantly alter algal-driven functions in boreal wetlands.

We examined the effect of vertical hydrological exchange on physicochemical variables, periphyton biomass (ecosystem properties), and sediment respiration (ecosystem process) at 2 spatial scales along a large gravel-bed river (Tagliamento River, northeastern Italy). At the regional scale, we investigated a 29-km-long, expanding and contracting, losing (average flow decrease 2.5 m³ s⁻¹ km⁻¹) and a 12.5-km-long, stable, gaining (average increase 0.3 m³ s⁻¹ km⁻¹) reach. At the local scale, we sampled riffle head and riffle tail units nested within the losing and gaining reaches. At the regional scale, we characterized vertical exchange by strong downwelling (negative vertical hydraulic gradient; VHG) in the losing and moderate upwelling (positive VHG) in the gaining reach. Nutrients, such as NO₃-N and dissolved N (DN), and periphyton biomass (AFDM) showed a significantly (p < 0.05) lower concentration in the losing than in the gaining reach. A contrasting pattern emerged for sediment respiration. Expansion and contraction dynamics (i.e., drying and rewetting) in the losing reach negatively affected chlorophyll a and AFDM. At the local scale, VHG was negative at riffle heads and positive at riffle tails nested in the gaining reach, but variables did not respond to vertical exchange at riffle heads and riffle tails in the losing or gaining reach. However, averaged data from riffle heads and riffle tails in the losing reach compared to averaged data for riffle heads and riffle tails in the gaining reach showed the same pattern as found in the regional investigation. Vertical exchange was apparently a major factor influencing ecosystem properties and ecosystem processes hierarchically across different spatial scales.

Little research has been conducted on the influence of microbial communities, sediment disturbances, and their interaction on the Vertical Water Flux (VWF) across a hydraulically complex streambed. Our study was aimed at the effects of microbial activity and shallow-sediment disturbance on VWF in sandbed flumes. We assessed the dynamics of VWF and the development of a microbial community during 30 d (June–July 2010) in 16 outdoor flumes with 2 types of bedform shape (level and ripple). We operated 8 flumes in constant darkness (no-light), and 8 in daylight to gain information on the relative significance of algae and heterotrophic microorganisms. We simulated a shallow-sediment disturbance after 21 d and compared microbial activity and VWF before and after the disturbance. We measured organic matter content, abundance of organisms, microbial activity, precipitation of CaCO₃, and O₂ bubbles resulting from primary production before and after the disturbance. VWF differed among treatments after 13 d. Algal and bacterial cells embedded in an extracellular polymer network, algal and microbial precipitation of CaCO₃, and production of O₂ bubbles in the uppermost sediment blocked the sediment pore space and disrupted VWF under daylight conditions, whereas bacterial cells and microbial precipitation of CaCO₃ reduced VWF in no-light flumes. Microbial activity and organic matter were not affected by the shallow-sediment disturbance, but VWF was restored. VWF can be controlled by microbial activity and shallow-sediment disturbances and should be seen as spatially heterogeneous and fluctuating in time on small scales.

Temperature is a primary driver of the structure and function of stream ecosystems. However, the lack of stream temperature (ST) data for the vast majority of streams and rivers severely compromises our ability to describe patterns of thermal variation among streams, test hypotheses regarding the effects of temperature on macroecological patterns, and assess the effects of altered STs on ecological resources. Our goal was to develop empirical models that could: 1) quantify the effects of stream and watershed alteration (SWA) on STs, and 2) accurately and precisely predict natural (i.e., reference condition) STs in conterminous USA streams and rivers. We modeled 3 ecologically important elements of the thermal regime: mean summer, mean winter, and mean annual ST. To build reference-condition models (RCMs), we used daily mean ST data obtained from several thousand US Geological Survey temperature sites distributed across the conterminous USA and iteratively modeled ST with Random Forests to identify sites in reference condition. We first created a set of dirty models (DMs) that related STs to both natural factors (e.g., climate, watershed area, topography) and measures of SWA, i.e., reservoirs, urbanization, and agriculture. The 3 models performed well (r²  =  0.84–0.94, residual mean square error [RMSE]  =  1.2–2.0°C). For each DM, we used partial dependence plots to identify SWA thresholds below which response in ST was minimal. We then used data from only the sites with upstream SWA below these thresholds to build RCMs with only natural factors as predictors (r²  =  0.87–0.95, RMSE  =  1.1–1.9°C). Use of only reference-quality sites caused RCMs to suffer modest loss of predictor space and spatial coverage, but this loss was associated with parts of ST response curves that were flat and, therefore, not responsive to further variation in predictor space. We then compared predictions made with the RCMs to predictions made with the DMs with SWA set to 0. For most DMs, setting SWAs to 0 resulted in biased estimates of thermal reference condition.

Stream metabolism is used to characterize the allochthonous and autochthonous basis of stream foodweb production. The metabolic rates of respiration and gross primary production often are estimated from changes in dissolved O₂ concentration in the stream over time. An upstream–downstream O₂ accounting method (2-station) is used commonly to estimate metabolic rates in a defined length of stream channel. Various approaches to measuring and analyzing diel O₂ trends have been used, but a detailed comparison of different approaches (e.g., required reach length, method of measuring aeration rate [k], and use of temperature-corrected metabolic rates) is needed. We measured O₂ upstream and downstream of various reaches in Kings Creek, Kansas. We found that 20 m was the approximate minimum reach length required to detect a significant change in O₂, a result that matched the prediction of a calculation method to determine minimum reach length. We assessed the ability of models based on 2-station diel O₂ data and k measurements in various streams around Manhattan, Kansas, to predict k accurately, and we tested the importance of accounting for temperature effects on metabolic rates. We measured gas exchange directly with an inert gas and used a tracer dye to account for dilution and to measure velocity and discharge. Modeled k was significantly correlated with measured k (Kendall's τ, p < 0.001; regression adjusted R²  =  0.70), but 19 published equations for estimating k generally provided poor estimates of measured k (only 6 of 19 equations were significantly correlated). Temperature correction of metabolic rates allowed us to account for increases in nighttime O₂, and temperature-corrected metabolic rates fit the data somewhat better than uncorrected estimates. Use of temperature-correction estimates could facilitate cross-site comparisons of metabolism.

Elevated suspended solids are a widespread stressor of aquatic ecosystems, but their effects on growth and reproduction in freshwater mussels are largely unknown. We fertilized experimental ponds to create a gradient in total suspended solids (TSS) and examined the effects of TSS on growth, nutritional status, reproduction, and clearance rate in Ligumia subrostrata. The number of females that became gravid declined sharply with increasing TSS, and no gravid females were found in the highest TSS treatments. The proportion of gravid females was not related to the TSS organic∶inorganic ratio. Fertilization was an all-or-nothing phenomenon. In all females that did become gravid, 98 to 99% of eggs were fertilized regardless of TSS, and total fecundity was unrelated to TSS. Clearance rates declined sharply as TSS increased but showed a threshold relationship in which clearance was uniformly low at TSS > ∼8 mg/L. Reproductive failure probably was not caused by poor body condition or nutritional status because growth (length and mass) and energetic status (measured as caloric density) were not related to TSS. We propose 2 mechanisms that implicate interference of TSS with fertilization as the cause of reproductive failure. Reduced clearance rate could decrease the chance of females encountering suspended sperm during filter feeding, or an increase in pseudofeces production could bind sperm in mucus and lead to its egestion before fertilization. Interruption of fertilization coincident with high TSS is a potential mechanism to explain the lack of mussel recruitment in many locations. Monitoring and reduction of TSS, especially during the spawning season, may help create conditions necessary for maintenance and recovery of mussel populations. More research is needed to explore the generality of this pattern across a broad range of mussel species including those adapted to lotic environments or that use different brooding strategies.

Self-purification is a key ecosystem service provided by riverine biota that is particularly important in polluted water bodies serving multiple societal uses, but the extent to which self-purification may be influenced by human uses is unknown. We studied a eutrophic lowland river used for drinking water and recreation to identify the maximal sustainable extent of human use. We recorded filtration by mussels and modeled the disturbance to mussels caused by wave action induced from recreational boating. Filtration was significantly affected by shear stress produced from boats down to a depth of 2.7 m. Threshold values for the intensity of wave disturbance ranged from 0.21 N/m² (Unio tumidus) to 0.43 N/m² (Anodonta anatina) for moderate effects and from 0.02 N/m² (U. tumidus) to 0.13 N/m² (Dreissena polymorpha) for no effects on filtration. Anodonta anatina and D. polymorpha showed a significantly lower degree of shell closing and a higher predicted medium-effect shear stress (the shear stress associated with 50% of the maximum shell closing duration) than U. tumidus and Unio pictorum, which probably results from differences in the species position in and above the sediment. Coupled hydraulic–ecological modeling showed that typical boating activity may reduce self-purification activity by mussels, with the extent of disturbance depending on mussel species, river depth, boating frequency, and cruising speed. Single passages of boats reduced daily mussel filtration rates by 0.02% for muscle-driven boats, 0.45% for yachts, 0.68% for motor boats, and 0.69% for motorized rafting and rowing boats. Depending on total daily boat traffic and hydrological conditions, a reduction in the daily filtration rate by mussel populations within the studied river section was estimated at 6.9%. We conclude that self-purification activity of this lowland river section is not significantly affected by recreational boating, but might be affected by more intense recreational boating under altered river flow conditions.

Crayfish are ecologically important in freshwater ecosystems, but few investigators have evaluated landscape patterns of their prevalence and abundance in temperate lakes. We report results of a crayfish survey for 100 lakes in the Puget Sound lowlands of Washington State (USA) serving as a census of the distributions of the signal crayfish Pacifastacus leniusculus and previously unknown populations of nonnative crayfish, including the red swamp crawfish Procambarus clarkii. We modeled crayfish presence and abundance with classification and regression trees based on multivariate ordinations of lake-, riparian-zone-, and watershed-scale predictors. We found 4 nonnative crayfish in 15% and P. leniusculus in 53% of surveyed lakes. The presence of P. leniusculus was best predicted by a lake-scale gradient of greater depth (present) to greater productivity (absent), but forested riparian zones with firm or rocky substrates also supported crayfish presence. No models adequately characterized patterns of P. leniusculus abundance measured as catch-per-unit effort from traps. Exclusion of data on predatory fish communities and lake pH and Ca may have impeded our ability to characterize patterns of crayfish abundance in the study lakes. Prevalence and abundance of P. leniusculus were low relative to what has been reported for native crayfish in other lake districts or for P. leniusculus in its Asian and European invasive ranges. We hypothesize that the erratic distribution of P. leniusculus in Puget Sound lakes may be attributable to challenges of postglacial colonization in this region and potentially to human introductions of this crayfish. Our study expands the geographic and taxonomic scope of inquiry into relationships between crayfish populations and lakes at landscape scales and provides a baseline for monitoring and understanding crayfish in an increasingly urban and invaded region of North America.

Benthic algal nutrient bioassays traditionally have been done by measuring periphytic algal biomass that has grown on fertilized or unfertilized patches of habitat produced by nutrient-diffusing substrata (NDS). This method requires destruction of the accumulated periphyton communities and, thus, does not allow for convenient monitoring through time. Variable fluorescence methods of estimating algal biomass and photosynthetic activity have been used in aquatic environments, but generally not over different nutrient treatments and not for a substantial duration. We evaluated the use of a pulse amplitude modulated (PAM) fluorometer for measuring algal biomass and photosynthetic activity in conjunction with NDS over several weeks in a wetland under several nutrient-addition treatments. We were able to detect a significant fluorometric response as early as 1 wk into the study with addition of both N and P. Wetland periphyton was co-limited by N and P. Dark-acclimated minimal fluorescence was highly correlated with chlorophyll a in different nutrient treatments. Our results suggest that active fluorometry is a useful method for measuring periphytic responses to nutrients and for evaluating the effect of nutrient additions on overall photosynthetic efficiency.

Estimating demographic values and rates for populations of cryptic stream species frequently is difficult because of prohibitively low capture probabilities. When assessing cryptic populations, researchers often are forced to make simplifying assumptions that could alter their conclusions about a population. We constructed models based on different assumptions about cohort structure, behavioral responses to capture, temporary emigration, and survival in a cryptic, larval population of red salamanders (Pseudotriton ruber) and fitted models to capture-mark-recapture data. Overall, models based on 2 cohorts were favored over models based on 1 cohort. Models based on assumptions of constant survival, behavioral responses to capture, and random temporary emigration were ranked higher than models lacking these assumptions. Consistent behavioral responses to capture demonstrated that using uncorrected counts to assess trends for this, and perhaps other larval amphibian populations, yields misleading results. Counts that are not corrected for trap-shy behavior may inherently show negative temporal trends. Temporary emigration was a critical assumption when describing larval salamander demography because only 27% of the larvae were active on the surface (the rest were in substratum habitats). Our study demonstrates the importance of making appropriate assumptions about demographic parameters and shows how population models can quantify aspects of the natural history of cryptic species.

Downed wood pieces are key links between terrestrial and aquatic ecosystems. They promote organic matter retention, create habitat, and potentially increase stream productivity. The stock of downed wood in a river system is a product of the interaction between wood supply, transport, in situ losses, and retention characteristics of the system. Fire and forest management are important disturbances that influence the amount and organization of stream wood with boom-and-bust periods of recruitment and fluvial transport processes. We examined 1^st- through 3^rd-order Portuguese streams flowing through 3 common silvicultural systems in southern Europe: forests of cork oak, eucalyptus, and maritime pine. Our data set included 1483 pieces of wood in 27 streams, all of which had experienced extensive wildfires within the previous 6 y. We used binned neighbor-k analysis to assess wood organization (segregated, random, or aggregated). We then used linear mixed-effects modeling to evaluate the effects of stream order, forest type, and their interaction on wood volume and organization. The best predictor of wood volume and organization was the interaction between forest type and stream order. Most wood pieces were burned and organization was low, suggesting that arrangement of wood was largely a product of input dynamics rather than transport processes at this time. Potential drivers of across-system variability included vegetation obstructions, wood length∶channel width ratios, management actions, and effects of fire. Climate models predict more droughts in the Euro-Mediterranean region in the future, with implications for wood volume, transport, and function as terrestrial vegetation invades intermittent stream channels and plant communities shift from managed forests to shrublands with few trees.

Theory predicts that different components of species diversity should increase with environmental heterogeneity. Our main aim was to examine the relationship between β-diversity and environmental heterogeneity in a system with high habitat heterogeneity and very small spatial distances between sites. This system allowed us to examine the effect of habitat heterogeneity on β-diversity in the absence of dispersal limitation. We surveyed 100 riffle sites (10 riffles in each of 10 streams) for benthic macroinvertebrates in a boreal drainage basin. Streams differed in average community composition (based on canonical analysis of principal coordinates) and heterogeneity in community composition (based on test of homogeneity of dispersion). These results were robust regardless of the distance measures used in distance-based multivariate analyses. β-diversity was not significantly correlated with stream habitat heterogeneity, despite the fact that the latter was quantified by a large set of environmental variables deemed important for species occurrence in our study streams. Thus, we suggest that the relationship between β-diversity and habitat heterogeneity was masked by individual species–environment responses and mass effects. Thus, the β-diversity–habitat heterogeneity relationship may not always be significant, a result that may have important consequences for understanding the structure of community patterns. Despite the absence of a significant β-diversity–habitat heterogeneity relationship, community structure was significantly associated with environmental factors (e.g., moss cover, stream width, velocity) across the streams in distance-based redundancy analysis. This finding suggests that different ways to associate β-diversity, community structure, and environmental conditions may yield different insights into the structure of biotic communities.

We explored changes in trophic structure and species composition of freshwater nematodes in response to nutrient enrichment in microcosms for 16 mo. Succession of functional groups differed significantly among the 5 nutrient treatments, which ranged from oligotrophic to polytrophic conditions, in the 2^nd half of the experiment. Differences among nutrient levels were most important for representatives of lower trophic levels, such as bacterial and algal feeders, whereas suction feeders and predators remained relatively unaffected. In contrast to our expectations, density and biomass of bacterial and algal feeders increased in the nutrient-poor treatments because of the presence of a high fraction of edible unicellular green algae and diatoms, whereas large standing stocks of inedible macrophytes developed in the highly nutrient-enriched treatments. Nematode assemblages in microcosms with intermediate or high nutrient levels were characterized by a large proportion of omnivores and predators, respectively, because of relatively low total densities. Our results also suggest a gradual change in species composition along the nutrient gradient because the degree of pairwise assemblage dissimilarity increased with corresponding differences in nutrient enrichment. Our study highlights the potential of bottom-up forces in shaping freshwater nematode assemblages and provides further evidence of pelagic–meiobenthic coupling.

A new species of the genus Nyctiophylax Brauer 1865, Nyctiophylax (Paranyctiophylax) gaditana sp. n., endemic to the southern Iberian Peninsula, is described and illustrated. To date, Nyctiophylax is known in the Western Palearctic region only through fossil representatives, some of which are preserved in Baltic amber. This finding is the first record of a living species of the genus Nyctiophylax for the European fauna. A key is provided to distinguish adult Nyctiophylax from other European genera of Polycentropodidae based on wing venation characters. The new species appears to be related to 2 Eastern Palaearctic species: Nyctiophylax (Paranyctiophylax) hjangsanchonus Botosaneanu 1970 from Korea and Nyctiophylax (Paranyctiophylax) adaequatus Wang and Yang 1997 from China, but it also could be related to the African species. Last, the ecology of the new species is described.

We carried out a set of experiments in a small clear-water lake in northern Sweden during summer 2010 to assess the effect of organic C (OC) released from epipelic algae on pelagic bacterial production (BP). The release rate of OC (dissolved and particulate) from epipelic algae was ∼45.4 ng C m⁻² h⁻¹. Bacterioplankton uptake of dissolved OC was P-limited, and pelagic primary production (PP) was colimited by N and P. Pelagic BP (3.2 ± 6 µg C L⁻¹ h⁻¹) exceeded pelagic PP (0.012 ± 0.008 µg C L⁻¹ h⁻¹). Pelagic BP was higher in lake water in contact with sediments and the epipelic algae growing on their surface than in water separated from the sediments. Epipelic algae release OC to lake water and potentially stimulate pelagic BP. However, exploitation of benthic OC probably is suboptimal because of nutrient limitation (primarily by inorganic P) of BP.

Stage-specific interactions can have large effects on food webs and ecosystem processes. We investigated stage-specific interactions between signal crayfish (Pacifastacus leniusculus) and cutthroat trout (Oncorhynchus clarkii), the 2 dominant consumers coexisting in a small stream ecosystem. We used enclosure experiments to assess effects of young-of-the-year (YOY) or adult crayfish on individual growth of YOY or adult fish and vice versa. Adult crayfish had adverse effects on the individual growth of YOY fish. Adult fish had no significant effect on YOY crayfish. There were reciprocal negative effects of YOY crayfish and YOY trout on each other's growth rates. Adult crayfish had the largest effect on leaf-litter loss rate, whereas fish and YOY crayfish had no significant effect. However, both adult and YOY crayfish reduced benthic invertebrate abundances in leaf packs to <⅓ those of the controls, whereas fish had no significant effects. Neither consumer species had significant effects on the invertebrates in gravel baskets in the central portion of the enclosures. These results show that ontogenetic-stage-specific effects complicate the characterization of foodweb interactions between species. Ontogenetic stage is important in determining magnitude of the interactions between these 2 consumers.

Unionoids are important in aquatic ecosystems, but despite their continued decline in diversity, few multifactorial studies have been done to identify determinants of their distribution and diversity. We studied the effects of multiple environmental factors on species richness and abundance at 65 sites in 6 major watersheds in the Pine Hills region, Louisiana. We surveyed in-stream habitat variables, land use/cover, and the co-occurring fish assemblages in 2^nd- through 6^th-order streams. A structural equation model suggested that 2 major latent variables were important: 1) reduced habitat disturbance, influenced by lower current velocity, substratum composed mostly of fine sediments, and increasing stream order, and 2) agricultural land use in riparian corridors and associated reduced water quality. These 2 latent variables explained 84% of mussel species richness and 48% of total mussel abundance. Mussel, but not host-fish species richness and abundance increased with stream order. Sites in the lower river basins had increased amounts of fine sediments and lower current velocities. We suggest that the lower river basins have extensive riparian wetlands that ameliorate the effects of frequent floods, thus mediating hydrologic disturbance and increasing mussel diversity.

The National Lakes Assessment (NLA) used relative and attributable risks to measure the apparent nationwide effects of excess N, reduced lakeshore habitat, and other stressors on planktonic assemblages in lakes. The risk measures, borrowed from human health research, use nontechnical language to compare the apparent effects of disparate lake stressors, thereby helping the public and policymakers identify stressors of greatest national concern. However, the investigators for the NLA and similar prior surveys of streams did not adjust their risk estimates of each stressor for possible confounding by other covarying stressors. As a result, the NLA point estimates overstate the risks of individual stressors and, thus, are unreliable for assessing their importance relative to that of closely related stressors. I used NLA data from 966 lakes to illustrate existing statistical methods of risk adjustment. Point estimates of adjusted relative risk for 7 stressor variables were 15 to 64% lower than unadjusted estimates. Adjusted attributable risks also were lower than unadjusted values, but for some stressors, they were very inconsistent across 3 adjustment methods. In addition, adjusted risk estimates used only part of the available data because each estimate was adjusted for many (6) covarying stressors. Closely related stressor variables (for example, N, P, and turbidity) can be combined into a bundle representing a broader type of stress (reduced water quality). For the NLA data, adjusted risk estimates for the stressor bundles (water quality and habitat) were more consistent across estimation methods and had lower relative uncertainty than estimates for their component stressors. In addition, closely related stressors with similar sources and modes of impact are more likely to be managed together rather than individually. For these reasons, I suggest evaluating stressor bundles in future aquatic surveys.

We measured nutrients and dissolved organic matter (DOM) in surface water and floodplain sediments to evaluate biogeochemical linkages between streams and floodplains in 3 streams of increasing size in a tropical catchment in northwestern Australia. We hypothesized that stream–floodplain biogeochemical connectivity, measured as similar DOM concentrations and spectroscopic properties of streamwater and floodplain sediment leachate, would decrease with increasing stream size. We expected decreasing connectivity to shift support of aquatic foodweb structure from mainly allochthonous C in small streams to algal C in larger streams. Streamwater and sediment leachate concentrations of NH₄-N, NO₃-N, soluble reactive P, and DOM varied several-fold across the 3 sites but showed no strong pattern in stream–floodplain connectivity with increasing stream size. Fluorescence index, specific ultraviolet absorbance, and δ¹³C-dissolved organic C (DOC) for stream water and sediment leachate were most similar to each other at the mid-size Adcock River, indicating tight river–floodplain biogeochemical connectivity via input of allochthonous DOM and NH₄-N to stream water. δ¹³C and δ¹⁵N signatures for key producers and consumers for all 3 streams showed that invertebrate consumers probably were supported by a range of in-stream (e.g., benthic and filamentous algae) and floodplain (allochthonous leaf litter) sources, whereas fish strongly reflected δ¹³C-enriched sources (e.g., benthic algae). Allochthonous C was probably only a minor energy source for metazoa, but we propose that river–floodplain biogeochemical linkages may be important for other aspects of ecosystem productivity, such as input of inorganic nutrients to support in-stream primary production. Terrestrial–aquatic biogeochemical linkages may be critical but poorly quantified components of river–floodplain ecosystems.

Watershed development alters hydrology and delivers anthropogenic stressors to streams via pathways affected by impervious cover. We characterized relationships of diatom communities and metrics with upstream watershed % impervious cover (IC) and with riparian % forest and wetland cover in 120-m buffers along each side of upstream networks. Threshold Indicator Taxa ANalysis (TITAN) identified potential threshold responses of diatom communities at 0.6 and 2.9% IC. Boosted regression trees (BRTs) indicated potential thresholds between 0.7 and 4.5% IC at which relative abundances of low-nutrient diatoms decreased and those of high-nutrient, prostrate, and motile diatoms increased. These individual thresholds indicated that multiple stressors or magnitudes of stressors related to increasing watershed % IC differentially affected relative abundances of taxa, and these differential effects probably contributed to a more gradual, but still substantial, change in overall community structure. BRTs showed that near-stream buffers with >65% and ideally >80% forest and wetland cover were associated with a 13 to 34% reduction in the effects of watershed % IC on diatom metrics and community structure and with a 61 to 68% reduction in the effects of watershed % pasture on motile and high-P diatom relative abundances. Watershed % IC and riparian % forest and wetland cover probably affect hydrologic, nutrient, and sediment regimes, which then affect diatom community physiognomy and taxa sensitive to nutrients and conductivity. Our results emphasize the importance of implementing mindful development and protective measures, especially in watersheds near watershed % IC thresholds. Effects of development potentially could be reduced by restoring and conserving near-stream forests and wetlands, but management and restoration strategies that extend beyond near-stream buffers are needed.

Physicochemical conditions in refugia must remain within the tolerance limits of aquatic invertebrates if they are to persist in intermittent streams when surface water is absent. We investigated shallow streambed sediments as a refuge for aquatic invertebrates and survival of invertebrates at high temperatures during the dry period in 3 headwater intermittent streams in New Zealand. Sediments from dry stream beds inundated in the laboratory contained 37 of the 53 taxa found in nearby flowing streams, a result suggesting that many taxa attempt to use the dry stream bed as a refuge during dry periods. Relative abundances of microcrustacea (copepods and ostracods), Gastropoda and Coleoptera (mostly Scirtidae) were greater in rewetted sediments than in sediments from nearby flowing streams, and several Ephemeroptera, Plecoptera, and Trichoptera taxa were present in low densities in rewetted sediments. All taxa that became active in rewetted sediments, except Hydrobiosidae, Tarapsyche olis (Trichoptera), Lymnaea (Gastropoda), Chironomini, and Cladocera, were active within a few hours of rewetting. We tested invertebrate survival in rewetted sediments at high temperatures (comparable to near-surface temperatures of dry stream beds in the field) in 2 experiments. In experiment 1, abundances of all major taxa were 11 to 94% lower in warm (air temperature  =  30°C, sediment temperatures  =  22–25°C for 1 h/d) than in cooler treatments. In experiment 2, abundances of most major taxa except Coleoptera and Acari were >75% lower in warm (air temperature  =  37°C, sediment temperature  =  32°C for 1 h/d) than in cooler treatments. Mid-summer daily maximum temperatures in the top 5 cm of the stream bed were 35 to 42°C in open pasture, 17 to 19°C under dense forest canopy, and 22 to 29°C under sparse riparian trees. Results suggest that maintaining the microclimate provided by riparian forest is important for protecting the aquatic invertebrate community in intermittent streams.

Freshwater mussels (Bivalvia:Unionidae) are among the most imperiled organisms in North America. Information on the spatial scale of reproduction and population connectivity will better enable mussel conservation programs to sustain long-term population viability, particularly restocking and recovery programs. Here we used genetic methods to characterize population structure, dispersal potential, and reproductive strategies in the freshwater mussel Lampsilis cardium from Twin Creek and Big Darby Creek (Ohio, USA). We genotyped adults and individual glochidia at 12 microsatellite loci to assess local population structure relative to within-population patterns of relatedness and parentage. Local populations within watersheds were weakly structured, and within-population estimates of relatedness identified probable full- and half-siblings several kilometers apart. Parent–offspring comparisons provided evidence of multiple paternity in single broods and identified the likely father of 3 glochidia from 1 female's brood 16.2 km upstream of the mother, indicating that long-distance transport of spermatozoa can promote population connectivity within watersheds. Given that lampsilines and other unionoids exhibit similar reproductive strategies, it is possible that other species are capable of long-distance fertilization. If so, fertilization in populations of many freshwater mussels might not be limited by local density of breeding adults. Therefore, the prospects for recovery of imperiled freshwater mussels might be better than what is now expected.

Three enduring conceptual models make predictions regarding the basal production sources supporting the upper food web of rivers: the River Continuum Concept, the Flood Pulse Concept, and the Riverine Productivity Model. Sources of primary production supporting metazoan biomass might best be understood by using a pluralistic approach that views basal sources as a dependent variable and key physicochemical and hydrological factors as independent variables. Here, I review studies in which chemical markers (stable-isotope and fatty-acid analyses) were used to estimate dominant primary producers contributing to consumer biomass in large rivers and evaluate associated independent variables (e.g., hydrologic regime, turbidity, concentration of dissolved organic matter, floodplain vegetation, lateral connectivity, and upstream impoundment) that have been hypothesized to contribute to variation in basal production sources. My review shows that C₄ grasses rarely support riverine metazoans and that algae are the predominant source of C supporting upper trophic levels of large rivers worldwide. However, I also found that many consumers assimilate material from C₃ plants in rivers with high sediment loads and low transparency during high-flow pulses. Exceptions to this pattern occur when river reaches are downstream from an impoundment, in which case, algae assume greater importance. Terrestrial C₃ plants also subsidize consumers in rivers with high dissolved organic matter concentrations via the microbial loop and, in other rivers, after periods of high discharge or leaf fall that increase the amount of terrestrial material in the particulate organic matter pool. I highlight the natural causes of differences in turbidity and dissolved organic matter among large rivers and the importance of transported materials as a source of nutrients for ecologically and economically important fish species.

Parasites are ubiquitous members of ecological communities but have only recently been recognized as key players in broader interactions and ecosystem dynamics, such as foodweb structure and energy flow. Ecological stoichiometry provides a framework for placing parasites in an ecosystem perspective by considering elemental imbalances and their consequences. I measured the elemental content of trematodes and their gastropod hosts and then estimated the nutrient-recycling effects of parasitism. N∶P of all tissue types in trematodes of the freshwater pulmonate snail, Physa acuta, was similar to N∶P of gastropod gonadal tissue. However, N∶P of trematodes and gastropod gonads was lower than N∶P of other gastropod tissues, a result suggesting an elemental imbalance between parasite and host. N∶P of excreta of P. acuta increased with the N∶P content of their algal food, consistent with consumer-driven nutrient recycling theory. However, gastropods with patent infections of Trichobilharzia physellae excreted greater N∶P than did uninfected snails, a result indicating that infected and uninfected gastropods were functionally different. Overall, these results suggest a significant role for parasites in nutrient recycling.

Streams draining urban heat islands tend to be hotter than rural and forested streams at baseflow because of warmer urban air and ground temperatures, paved surfaces, and decreased riparian canopy. Urban infrastructure efficiently routes runoff over hot impervious surfaces and through storm drains directly into streams and can lead to rapid, dramatic increases in temperature. Thermal regimes affect habitat quality and biogeochemical processes, and changes can be lethal if temperatures exceed upper tolerance limits of aquatic fauna. In summer 2009, we collected continuous (10-min interval) temperature data in 60 streams spanning a range of development intensity in the Piedmont of North Carolina, USA. The 5 most urbanized streams averaged 21.1°C at baseflow, compared to 19.5°C in the 5 most forested streams. Temperatures in urban streams rose as much as 4°C during a small regional storm, whereas the same storm led to extremely small to no changes in temperature in forested streams. Over a kilometer of stream length, baseflow temperature varied by as much as 10°C in an urban stream and as little as 2°C in a forested stream. We used structural equation modeling to explore how reach- and catchment-scale attributes interact to explain maximum temperatures and magnitudes of storm-flow temperature surges. The best predictive model of baseflow temperatures (R²  =  0.461) included moderately strong pathways directly (extent of development and road density) and indirectly, as mediated by reach-scale factors (canopy closure and stream width), from catchment-scale factors. The strongest influence on storm-flow temperature surges appeared to be % development in the catchment. Reach-scale factors, such as the extent of riparian forest and stream width, had little mitigating influence (R²  =  0.448). Stream temperature is an essential, but overlooked, aspect of the urban stream syndrome and is affected by reach-scale habitat variables, catchment-scale urbanization, and stream thermal regimes.

Community dynamics in temporary waters are constrained by the hydroregime (depth, timing, duration, frequency, and predictability of water in an aquatic habitat), which in turn is influenced by climatic patterns and anthropogenic use of water in the landscape. Declining rainfall in regions with a Mediterranean climate, such as southwestern Australia, has decreased the depth and duration of water in temporary wetlands, potentially altering the composition of invertebrate communities. We used a long-term data set (6–25 y) to examine temporal changes in hydroregimes and aquatic invertebrate diversity (based on species presence/absence) at 9 seasonal wetlands. The study wetlands maintained distinctly seasonal hydroregimes, despite declining rainfall and the contraction of wetland hydroperiods. Distance-based redundancy analysis (dbRDA) indicated that conductivity, NO₃⁻ NO₂⁻, and turbidity were the most important factors explaining the changes in invertebrate community composition over time. Allocation of species into 4 trait-based groups based on their resilience to or resistance of drought and their mode of recolonization of a water body upon rewetting revealed that the fauna is dominated by active dispersers. This result suggests that the proximity of source wetlands from which mobile invertebrate species and vertebrate vectors, such as waterbirds, can recolonize seasonal wetlands is an important factor influencing the invertebrate community response to rewetting. Despite the decline in water availability, we found little evidence of a shift to a more arid-adapted fauna. We suggest that the maintenance of a mosaic of wetlands of varying hydroregimes at the whole-landscape scale will be critical to the future persistence of aquatic invertebrate communities in Mediterranean regions where the frequency and intensity of droughts is predicted to increase.

Riparian-zone vegetation can influence terrestrial and aquatic food webs through variation in the amount, timing, and nutritional content of leaf and other litter inputs. We investigated how riparian-forest community composition, understory density, and lateral slope shaped vertical and lateral litter inputs to 16 streams in the Oregon Coast Range. Riparian forests dominated by deciduous red alder delivered greater annual vertical litter inputs to streams (504 g m⁻² y⁻¹) than did riparian forests dominated by coniferous Douglas-fir (394 g m⁻² y⁻¹). Deciduous forests also contributed greater lateral litter inputs per meter of stream bank on one side (109 g m⁻¹ y⁻¹) than did coniferous forests (63 g m⁻¹ y⁻¹). Total litter inputs from deciduous forests exceeded those from coniferous forests most strongly in November, coincident with an autumn peak in litter inputs. Lateral litter inputs contributed most to total inputs during winter in both forest types. Annual lateral litter movement increased with slope at deciduous sites, but only in spring/summer months at coniferous sites. Neither experimental removal of understory vegetation nor installation of mesh fences to block downslope litter movement affected lateral litter inputs to streams, suggesting that ground litter moves <5 m downslope annually. N concentrations of several litter fractions were higher at deciduous sites and, when combined with greater litter amounts, yielded twice as much total litter N flux to streams in deciduous than coniferous sites. The presence of red alder in riparian forests along many small streams of the deeply incised and highly dendritic basins of the Oregon Coast Range enhances total fluxes and seasonality of litter delivery to both terrestrial and aquatic food webs in this region and complements the shade and large woody debris provided by large coniferous trees.