Nutrient enrichment experiments were conducted in 1.2-m deep enclosures in 2 shallow, oligotrophic, mountain lakes. ¹⁵N-NO₃ isotope tracer was used to compare the importance of phytoplankton and benthic compartments (epilithon, surface sediment [epipelon], and subsurface sediment) for NO₃ uptake under high and low NO₃ conditions. NO₃ uptake approached saturation in the high-N lake, but not in the low-N lake. The capacity of phytoplankton and benthic compartments to take up NO₃ differed among treatments and between lakes, and depended on water-column nutrient conditions and the history of NO₃ availability. Phytoplankton productivity responded strongly to addition of limiting nutrients, and NO₃ uptake was related to phytoplankton biomass and photosynthesis. However, more NO₃ usually was taken up by benthic compartments (57–92% combined) than by phytoplankton, even though the response of benthic algal biomass to nutrient additions was less pronounced than that of phytoplankton and benthic NO₃ uptake was unrelated to benthic algal biomass. In the low-N lake where NO₃ uptake was unsaturated, C content or % was related to NO₃ uptake in benthic substrates, suggesting that heterotrophic bacterial processes could be important in benthic NO₃ uptake. These results suggest that phytoplankton are most sensitive to nutrient additions, but benthic processes are important for NO₃ uptake in shallow, oligotrophic lakes.

The region of surface water–groundwater interaction in streams, the hyporheic zone, is important for biogeochemical processes and provides habitat for specialized microbial and invertebrate assemblages. Although hyporheic invertebrates contribute little biomass and respiration relative to microbes in stream sediments, invertebrate effects on biogeochemical processes may be disproportionately large. We tested how various interstitial invertebrate assemblages affected N cycling and respiration in flow-through microcosms filled with alluvial sediment in the laboratory. Average invertebrate biomasses in low and high invertebrate treatments were 0.20 and 19 mg dry mass/L sediment, respectively. Average net NO₃⁻ regeneration/uptake rate increased with increasing invertebrate biomass, showing invertebrates suppressed NO₃⁻ uptake or stimulated in situ NO₃⁻ production. Average respiration (normalized for sediment organic matter) and particulate organic matter (POM) increased 51% and 33%, respectively, with increasing invertebrate biomass, suggesting direct contribution to hyporheic metabolism and/or stimulation of microbial activity and an accumulation of POM driven by invertebrates. We suggest that interstitial invertebrates can substantially alter biogeochemical processes in hyporheic zones.

Stable isotope enrichment experiments offer a potentially powerful way to examine the base of aquatic food webs, but interpretation of the data from these experiments may be confounded by problems such as selective ingestion/assimilation of bulk food sources by consumers, variable tracer enrichment over time, and the failure of consumers to approach isotopic equilibrium with the tracer in their diets over the course of the experiment. Our study examined data from a stable isotope addition experiment in which ¹⁵N-labeled NH₄ was added to a midwestern US stream. A compartment model was used to provide insights into the kinetics of ¹⁵N uptake and release from algae, heterotrophic microbes colonizing detritus, and invertebrate consumers. The model accounted for temporal variation in the degree of isotopic enrichment and did not require the assumption of isotopic equilibrium between consumers and their diets. The importance of instream production (i.e., growth of algae and microbes within the study reach during the experiment) relative to allochthonous and upstream inputs was ∼38 to 50% for heptageniids and Psephenus, 10 to 20% for Orconectes propinquus, Gammarus, hydropsychids, and larval Stenelmis, and <10% for the filter-feeding Similium and the unionid mussel Pleurobema sintoxia. The alternative choices of algae or heterotrophic microbes as the basis of consumer diets made little difference in these estimates, even though the microbes became more ¹⁵N-enriched than the algae because microbes had higher turnover rates. These results were subject to a number of caveats, and guidelines for experimental design are suggested for future studies to help address some of these problems.

Stream invertebrate communities are the product of multiple-scale habitat filters, and a major goal in community ecology is to determine which scales most influence the relationships between habitat and community structure. We analyzed the relationships between habitat and the functional structure of invertebrate communities at 3 spatial scales (microhabitat, stream reach, basin), using 264 samples collected from 38 stream reaches in 2 basins in France. The habitat was described in terms of hydraulic conditions (Froude number), substrate size, and benthic particulate organic matter. The functional structure of the invertebrate communities was described using 60 categories of 12 biological traits. Relationships between habitat and traits were significant for 96 of 180 tests (r ² ≤ 26%) at the microhabitat scale (within reaches). Relationships between habitat and traits were significant for 46 of 180 tests (r² ≤ 34%) at the reach scale (within basins), and the effects of habitat variables on traits did not differ between the 2 basins. Invertebrate traits differed between basins in 34 of 60 trait categories (r² ≤ 62%). Relationships between habitat and traits were consistent between the microhabitat and reach scales for all traits except body flexibility, number of reproductive cycles/y, and respiration patterns. At both scales, maximum size, body form, mode of attachment to the substrate, feeding habits, reproduction, lifespan, and strategies of dissemination were significantly correlated with habitat variables, especially hydraulics. These results illustrate adaptation to habitat characteristics in terms of resistance to drag and foraging strategies. However, overall differences between basins were inconsistent with patterns observed at smaller scales. In summary, the functional variability of invertebrate communities in stream reaches depended largely on microhabitat filters but also on other filters prevailing at the reach or larger scales.

We used 2 approaches to investigate possible mechanisms underlying positive responses of chironomids to fragmentation of resources in streambeds. We manipulated arrays of leaf patches on a sandy streambed and asked whether responses of chironomids to patch arrangement and leaf species in patches were genus or instar specific. We manipulated densities of Chironomus riparius instars I and III in laboratory microcosms and assessed density dependent dispersal and settlement behaviors. In streambed arrays, chironomid densities in leaf packs were ∼2× higher in fragmented than in aggregated arrays, but density was not affected by leaf species. Density of larvae in sand cores did not differ with respect to patch arrangement or leaf species. Genus and instar composition differed between sand cores and leaf packs, but did not differ with respect to leaf patch arrangement or leaf species in arrays. Resource partitioning in streambed arrays appeared to occur among congeneric instars rather than among genera. In laboratory microcosms, instar III larvae dispersed from high-quality patches into poor-quality patches when larval density was high, and the presence of instar III larvae strongly inhibited settlement of instar I larvae into high-quality patches. We concluded that recruitment of larvae to leaf patches from the regional species pool (drift) probably was random, but intraspecific competitive interactions and density dependent dispersal may be important mechanisms structuring chironomid assemblages in leaf patches. Therefore, larval abundances should be high in streambeds where resources such as leaf patches have spatial arrangements that reduce density dependent dispersal and recruitment inhibition, mitigate the effects of dispersal, and enable congeneric instars to partition the streambed habitat.

We compared densities of the caddisflies Ironoquia punctatissima (Walker) and Neophylax mitchelli (Carpenter) in microhabitats (cobbles, leaf litter, bedrock) nested within pool and run macrohabitats with different bedforms (stony [cobble-dominated] or flat bedrock) before and immediately after 4 floods in a north-central Ohio intermittent stream. Our goal was to identify which microhabitat and macrohabitat types acted as flow refugia. Ironoquia densities were highest on leaf litter in stony pools prior to floods. Flooding significantly lowered larval densities in stony and bedrock pools, where most leaf litter occurred, but not in runs. The most marked changes in pre- and postflood Ironoquia densities occurred in leaf litter, where densities decreased by ∼65 to 97% in 3 of 4 floods. In contrast, preflood densities of Neophylax were highest in bedrock runs and pools. However, floods significantly decreased Neophylax densities in these macrohabitats, suggesting bedrock macrohabitats provided larvae little shelter from high-flow forces. Measurement of cobble and leafpack tracer particles confirmed that hydraulic forces were highest in bedrock runs and that cobbles were far more stable than leaf packs in all macrohabitat types. Both species apparently face conflicting demands between residing in habitat patches that are food-rich but have poor flow refugia. Behavioral observations in a laboratory flume, where larvae were exposed to incrementally increased flows, revealed that Neophylax was more resistant than Ironoquia to dislodgment from cobbles and simulated bedrock, largely because of differences in body morphology and mass. These differences also partially explain why stony macrohabitats in the stream provided better flow refugia for Neophylax than for Ironoquia. Ironoquia, but not Neophylax, laid silk draglines, which helped prevent flow-induced dislodgment in low to moderate current speeds (≤0.4 m/s). Our study demonstrates how spatial attributes, scales, and species traits interact to determine or mediate flood effects on these caddisflies.

Freshwater mussel assemblages in the Flint River Basin (FRB) of southwestern Georgia are among the most diverse in the southeastern Coastal Plain of North America. Historically, 29 species, including 7 endemics, occurred in the FRB. A drought during the summer of 2000 caused record low flows and many perennial streams dried or became intermittent. Predrought surveys conducted in 1999 allowed an assessment of the impact of the drought on mussel assemblages. During 2001, 21 stream reaches that had abundant or diverse mussel assemblages in 1999 were resurveyed. Study sites were classified as flowing or non-flowing during the drought based on data from stream gauging stations or visual observation of study reaches. Mussels were classified by conservation status, either stable, special concern, or federally endangered. Greater than 90% of the mussels observed in the lower FRB were species with stable conservation status. Special-concern species represented 5 to 6% and endangered species represented 1% of mussel abundance. Sites that ceased flowing during the drought had significant declines in the abundance of stable species and in taxa richness. Endangered species also showed evidence of a decline in non-flowing sites. Sites that maintained flow had increases in stable species and no change in special concern, endangered species, or species richness through the drought. Sites that showed declines in mussel abundance had a significantly lower frequency of wood debris than other sites. Field observations suggested that shallow depressions beneath wood debris may act as refuges for freshwater mussels during stream drying. Greatest declines in mussel abundance usually occurred in the mid-reaches of the major tributaries of the lower Flint River. These reaches depend on the Upper Floridan aquifer, which is heavily used for irrigation, to maintain base flows. Declines in mussel populations appear to be associated with unusual climatic conditions and increasing demand on the area streams and the regional aquifer system for irrigation water supply.

During the early 1990s, 2 Eurasian macrofouling mollusks, the zebra mussel Dreissena polymorpha and the quagga mussel D. bugensis, colonized the freshwater section of the St. Lawrence River and decimated native mussel populations through competitive interference. For several years, zebra mussels dominated molluscan biomass in the river; however, quagga mussels have increased in abundance and are apparently displacing zebra mussels from the Soulanges Canal, west of the Island of Montreal. The ratio of quagga mussel biomass to zebra mussel biomass on the canal wall is correlated with depth, and quagga mussels constitute >99% of dreissenid biomass on bottom sediments. This dominance shift did not substantially affect the total dreissenid biomass, which has remained at 3 to 5 kg fresh mass /m² on the canal walls for nearly a decade. The mechanism for this shift is unknown, but may be related to a greater bioenergetic efficiency for quaggas, which attained larger shell sizes than zebra mussels at all depths. Similar events have occurred in the lower Great Lakes where zebra mussels once dominated littoral macroinvertebrate biomass, demonstrating that a well-established and prolific invader can be replaced by another introduced species without prior extinction.

The reproduction, demography, and propagation of the endangered dromedary pearlymussel (Dromus dromas) (Lea, 1834) were studied in the Clinch and Powell rivers, Tennessee. Viable populations of the dromedary pearlymussel now occur only in the Clinch and Powell rivers; the species has been extirpated from the remaining portions of its range in the Cumberland and Tennessee river drainages. Females are long-term winter brooders, and they are gravid from October to June. Glochidia are contained in conglutinates that are red to white and resemble freshwater leeches or flatworms. Conglutinates are 20 to 40 mm long and are released through the excurrent aperture. Estimates of fecundity based on 7 gravid females collected from the Clinch River were 55,110 to 253,050 glochidia/mussel. The ages of 66 valves of D. dromas were determined by thin-sectioning and ranged from 3 to 25 y. Annual growth averaged 5 mm/y until age 10 and decreased to ∼1.2 mm/ y thereafter. Nineteen fish species were tested for suitability as hosts for glochidia. Ten were confirmed as hosts through induced infestations of glochidia: black sculpin (Cottus baileyi), greenside darter (Etheostoma blennioides), fantail darter (Etheostoma flabellare), snubnose darter (Etheostoma simoterum), tangerine darter (Percina aurantiaca), blotchside logperch (Percina burtoni), logperch (Percina caprodes), channel darter (Percina copelandi), gilt darter (Percina evides), and Roanoke darter (Percina roanoka). Juveniles produced from these hosts were cultured in dishes held in nonrecirculating aquaculture systems containing fine sediment (<105 µm) and were fed the green alga Nannochloropsis oculata every 2 d. Survival of 2810 newly metamorphosed juveniles was 836 (29.7%) after 1 to 2 wk.

The larva, adult male, and adult female of Nectopsyche waccamawensis n. sp. are described. This new species appears to be endemic to Lake Waccamaw in Columbus County, North Carolina, and brings to 9 the number of Nectopsyche species now known east of the Mississippi River. The larva of Nectopsyche sp. A, a probable new species, is also described. Descriptions, illustrations, and a key are provided for the larvae of all eastern North American species with the exception of N. spiloma (Ross), but including the previously undescribed larva of N. paludicola Harris.

Previous studies on the effects of logging on streams have suggested that light and water temperature were important variables structuring stream communities but, in many cases, these effects were confounded. We observed pronounced gradients in the flux of solar energy and water temperature in an earlier large-scale experiment in which we manipulated the width of riparian buffers along headwater streams. Associated with these abiotic changes were increases in periphyton biomass and primary consumer abundance. We present results from a study in streamside channels that was designed to isolate the effects of light on stream communities, while holding water temperature constant. Light treatments in the channel experiment simulated inputs of solar radiation created during the prior watershed-scale experiment. Results from the present study suggested that consumers limited periphyton biomass early in the study; however, a rainstorm midway through the experiment reduced periphyton biomass and insect consumer abundance. Following this disturbance, chlorophyll a biomass was 2 to 4 times higher in the full sunlight treatment compared to the 2 lowest light treatments. At the end of the study, primary consumer abundance, biomass, survival, and growth rate were positively related to light and periphyton resources. Therefore, we inferred biotic control of periphyton during the early part of the channel study, whereas light appeared to control periphyton at the end of the study. Results from the large-scale and channel experiments suggested that light was the primary constraint on periphyton biomass accrual. Moreover, both experiments, especially the channel study, showed that light indirectly influenced consumer performance as mediated by increased primary production.

Most floodplain sediments of the rivers Rhine and Meuse in The Netherlands are moderately polluted with trace metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other chemicals. The effects of these sediment-bound contaminants on the productivity of benthic macroinvertebrates are unclear. Sixteen locations along a pollution gradient were investigated in creeks in the Biesbosch floodplain area. Sediment samples were analyzed for bulk sediment characteristics and contaminants (total and bioavailable concentrations of trace metals, PAHs, and PCBs). Exclosures were used to study the effect of predation by fish and birds on macroinvertebrates. Macroinvertebrates were sampled and identified to species level, and production was estimated from biomass increases inside the exclosures during a 1-mo interval in spring. Benthic macroinvertebrate species richness was negatively affected by sediment contamination. Production of oligochaetes and chironomids was not correlated with levels of contamination, but production of gastropods was negatively correlated with contamination. Environmental variables that reflected food availability (seston and sediment organic C) were positively correlated with contamination. Predation significantly reduced invertebrate biomass, but the effects of predation and sediment contamination were not correlated with each other. Our study suggested that the moderate levels of contamination affected the structure but not the productivity of the benthic macroinvertebrate community, probably because of the counteracting effects of contamination and associated surplus of food.

Landscape classifications, such as ecoregions, are widely promoted as spatial frameworks for aquatic conservation and environmental management. The River Environment Classification (REC) is an alternative approach to landscape-scale river classification. The REC classifies individual sections of river networks according to the climate, topography, geology, and land cover of the unique upstream watershed of each section. The classification is hierarchical and is based on the assumption that climate, topography, geology, and land cover are dominant causes of spatial variation in physical and biological characteristics of rivers at 4 spatial scales. The ability of the REC to explain variation in invertebrate samples collected from 237 sites in New Zealand was compared to that of region-based classifications using classification strength tests. Classifications with differing numbers of classes were compared using relative classification strengths calculated by expressing absolute classification strength as a proportion of the classification strength reached by an a posteriori classification with the same number of classes. The a posteriori classifications were derived by clustering sites by their invertebrate assemblage, so the classifications were close to the maximum possible strength attainable for a set number of classes. The relative classification strength of the REC was compared with an existing ecoregion classification and with classes that were defined by clustering sites based on intersite distances. The relative classification strength of the REC was higher (>55% of maximum attainable) than the strengths of classifications based on ecoregions (45%) or distance-based classes (<47%) when presence/absence data were used. The relative classification strength of the REC also was higher (>28% of maximum atainable) than the strengths of classifications based on ecoregions (22%) or distance-basedt classes (<24%) when relative abundance data were used. The higher classification strength of the REC was attributed to its explicit consideration of the causes of spatial variation in ecological characteristics of rivers. In addition, the REC was more likely than ecoregion classification to differentiate among geographically close sites that had contrasting watershed conditions because the REC represented the network spatial structure of rivers. However, the classification strength of the REC was low, indicating that it provided a general representation of ecological patterns, but could not be used to predict assemblage characteristics reliably for a specific site.

Sets of reference sites often are used to create a benchmark for biological assessment of river condition. Unfortunately, human modification of river systems is now so widespread and complex that this practice can result in comparisons against a reference condition that embodies an appreciable and poorly defined degree of human impact. Our paper describes an alternative means of deriving reference conditions that does not require the use of reference sites. Instead, the tolerances and preferences of each taxon in the regional taxon pool are established for various environmental factors. These factors can be regarded as filters that exclude members of the regional pool from sites where environmental conditions are incompatible with their tolerances and preferences. The suite of taxa that can occupy an assessment site under natural conditions can be estimated if the natural values of the environmental filters at that site can be determined. A comparison between this suite of potential taxa and the taxa actually collected at an assessment site provides an indication of human impact on taxonomic richness. This approach is illustrated for aquatic macroinvertebrate families in rivers in northeastern New South Wales, Australia, and the environmental filters of annual water temperature range, flow regime, and river bed type. An assessment based on this approach showed substantial correlation with several independent measures of human influence, in contrast to very little correlation for a previous macroinvertebrate bioassessment based on reference sites. The performance of the filters approach probably could be improved with further development of the underlying techniques and data.

The related topics of spatial variability of aquatic invertebrate community metrics, implications of spatial patterns of metric values to distributions of aquatic invertebrate communities, and ramifications of natural variability to the detection of human perturbations were investigated. Four metrics commonly used for stream assessment were computed for 9 stream reaches within a fairly homogeneous, minimally impaired stream segment of the San Pedro River, Arizona. Metric variability was assessed for differing sampling scenarios using simple permutation procedures. Spatial patterns of metric values suggest that aquatic invertebrate communities are patchily distributed on subsegment and segment scales, which causes metric variability. Wide ranges of metric values resulted in wide ranges of metric coefficients of variation (CVs) and minimum detectable differences (MDDs), and both CVs and MDDs often increased as sample size (number of reaches) increased, suggesting that any particular set of sampling reaches could yield misleading estimates of population parameters and effects that can be detected. Mean metric variabilities were substantial, with the result that only fairly large differences in metrics would be declared significant at α = 0.05 and β = 0.20. The number of reaches required to obtain MDDs of 10% and 20% varied with significance level and power, and differed for different metrics, but were generally large, ranging into tens and hundreds of reaches. Study results suggest that metric values from one or a small number of stream reach(es) may not be adequate to represent a stream segment, depending on effect sizes of interest, and that larger sample sizes are necessary to obtain reasonable estimates of metrics and sample statistics. For bioassessment to progress, spatial variability may need to be investigated in many systems and should be considered when designing studies and interpreting data.

Best Management Practice (BMP) guidelines have been developed to reduce the negative impacts of timber harvest on streams. BMPs are widely implemented, but the effectiveness of timber harvest BMPs has not been evaluated using modern biological monitoring techniques. Most current biological monitoring is based on 1 of 2 main approaches: multimetric monitoring or predictive modeling. These approaches differ considerably, and their respective merits and failings have been debated extensively in the literature. Our review evaluated the ability of these biological monitoring approaches to detect timber harvest effects and to assess the effectiveness of BMPs. Both techniques detect impairment via changes in macroinvertebrate community structure, despite their differences in approach. Most of the negative effects of timber harvest result in changes in the macroinvertebrate community, so we have concluded that both techniques should be effective for the evaluation of timber harvest and BMPs.

Stream restoration attempts to reverse the global degradation of rivers and streams, but rigorous evaluations are needed to advance the science. We evaluated a 3^rd-order channelized Indiana (USA) stream that was restored in 1997 by constructing two meanders, each ∼400 m long. Pool and riffle sequences were constructed, coarse substrate and wood were added to the channel, banks were stabilized and revegetated, and sedimentation was reduced by creating a sediment retention basin upstream. Habitat, periphyton, macroinvertebrates, and fishes were measured before restoration and for 5 y after restoration in the restored reaches and in an upstream, unrestored reach. Restoration improved habitat conditions (e.g., more pools, fewer fine sediments) in both restored reaches compared to the unrestored reach. Within 1 y after restoration, major trophic groups (i.e., periphyton, macroinvertebrates, and fishes) recovered to or exceeded levels in the degraded, unrestored reach. However, biotic responses varied with time, trophic level, and community parameter measured. Five years after the restoration, habitat quality, algal abundance, and macroinvertebrate density remained higher in the restored reaches, whereas macroinvertebrate diversity and fish abundance in the restored reaches were similar to or below levels in the unrestored, channelized reach. Although biotic recovery was relatively rapid, long-term persistence is uncertain because of continued sedimentation at a watershed scale. In many instances, reach-scale restorations may be ineffective in the face of basin-wide degradation. This study illustrates the importance of conducting long-term assessments of stream restorations, which can improve both knowledge and management of stream ecosystems.