Selection of appropriate scales of measurement is fundamental to the interpretation and understanding of river systems. Principles of hierarchy theory can be used to guide the selection of scales of measurement in stream ecological studies because hierarchy incorporates relational links between nested levels. However, the scales of measurement used in macroinvertebrate distribution studies are not cast strongly into a hierarchical context. Scales are usually nested spatially, in a logical manner, but are not accompanied by an explanation of underlying relational links among scales. We describe 2 approaches to the selection of scales of measurement that incorporate hierarchical principles. The parallel hierarchy approach uses a hierarchy of river system organization, derived from the parallel discipline of fluvial geomorphology, to identify scales of measurement that may correspond with the distribution of macroinvertebrates. The self-emergence approach circumvents any a priori imposition of scales of measurement and allows scales to self-emerge from a data set. The use of these approaches shifts focus onto the detection of levels of hierarchical organization, or holons, and may advance our understanding of river ecosystems by elucidating the way in which levels of ecological and physical organization are juxtaposed within the river system.

The McMurdo Dry Valleys of South Victoria Land, Antarctica, contain numerous glacial meltwater streams that drain into lakes on the valley floors. Many of the streams have abundant perennial mats of filamentous cyanobacteria. The algal mats grow during streamflow in the austral summer and are in a dormant freeze-dried state during the rest of the year. NO₃ and soluble reactive P (SRP) concentrations were lower in streams with abundant algal mats than in streams with sparse algal mats. NO₃ and SRP concentrations were higher in the hyporheic zone of a stream with abundant algal mats than in the stream itself. An experimental injection of LiCl, NaNO₃, and K₃PO₄ was conducted in Green Creek, which has abundant algal mats. Substantial hyporheic exchange occurred. The NO₃ and PO₄ concentrations at 50 m below the injection were 55 μM and 18 μM, respectively, during the experiment. NO₃ and PO₄ concentrations were below the detection limit of 1 to 2 μM at a site 497 m below the injection during the Cl tracer arrival, indicating a high capacity for nutrient uptake by algal communities. NO₂ and NH₄ were present at sites 226 and 327 m below the injection, indicating that, in addition to denitrification and algal uptake, dissimilatory NO₃ reduction to NO₂ and NH₄ may be a NO₃ sink during transport. Transport modelling with nutrient uptake represented as a 1^st-order process yielded reach-scale parameters of 4.3 × 10⁻⁵ to 3.9 × 10⁻⁴/s and 1.4 × 10⁻⁴ to 3.8 × 10⁻⁴/s for uptake of NO₃ and PO₄, respectively. The best match with the observed data was a model in which PO₄ uptake occurred only in the main channel and NO₃ uptake occurred in the main channel and in the hyporheic zone. Hyporheic NO₃ uptake was 7 to 16% of the total uptake for the different stream reaches. These results demonstrate that nutrient flux to the lakes is controlled by hyporheic exchange and nutrient uptake by algal mats in dry valley streams. Streams without algal mats contribute more nutrients to the lakes than streams with algal mats.

The breakdown rates of natural branch wood were studied in 2 streams (lowland and upland), and the current knowledge on breakdown rates of wood in streams was reviewed. The breakdown coefficient of natural alder branches, k = 0.0650/y, in an unpolluted upland stream corresponded well to the few published data on breakdown rates of natural wood substrates in streams. Breakdown rates of Scots pine (Pinus sylvestris, k = 0.0698/y), bark-covered black alder (Alnus glutinosa, k = 0.0859/y), and debarked black alder branches (k = 0.0717/y) in a nutrient-rich sandy lowland stream indicated that breakdown rates differ among tree species. Data from the present and previous studies on the breakdown of natural wood and commercial wood products (wood veneer, tongue depressors, ice-cream sticks) showed that breakdown is slower for natural wood than for commercial wood products, in part because the commercial wood products used in the studies had high surface to volume ratios. A strong relationship exists between surface to volume ratio of the substrate and breakdown rates. The breakdown rates of wood in streams were associated with several abiotic factors, including the physical and chemical properties of the wood substrates used, dissolved nutrients, and water temperature. Overall, the review of published breakdown rates emphasized that the current knowledge on the breakdown rates of wood in streams, especially the temporal patterns of microbial activities, is very limited, and heterogeneous experimental designs in previous studies prevent general conclusions. Nevertheless, measuring wood breakdown rates could be a promising tool for practical application in assessment of the ecological integrity of streams.

We investigated the effect of a flood on particulate organic matter (POM) dynamics in the floodplain and active channel of the Little Tennessee River in western North Carolina. We measured litterfall, leaf breakdown, and floodplain litter (before and after the flood) at 12 sites. Annual litterfall (256–562 g m⁻² y⁻¹) was typical of a temperate deciduous forest but lower than lowland floodplain forests in the eastern US. Leaf breakdown rates of 4 tree species (Acer rubrum, Carpinus caroliniana, Juglans nigra, and Platanus occidentalis) ranged from 0.001 to 0.010/d. We separated the 12 sites into 2 groups (inundated and non-inundated) based on the degree of flooding after a flood on 8 January 1998 and determined POM exchange between the active channel and floodplain. Significant transport of leaves from the floodplain to the river occurred at inundated sites, but transport of herbaceous or woody material did not occur. The flood increased leaf breakdown rates of all 4 tree species. Our measurements of litterfall, leaf breakdown, and floodplain litter standing crop suggest that leaves entrained from the floodplain of Little Tennessee River during floods are a source of POM to the active channel. However, flood input of leaves to the river were a small source of POM compared to direct leaffall.

Managed flow releases can influence biomass and productivity of littoral biofilms in regulated lowland rivers by varying the frequency and duration of inundation and desiccation. I examined the effects of inchannel water level variability on the productivity of littoral biofilms growing on woody substrata in the highly regulated Murrumbidgee River, southeastern Australia. Wooden blocks of river red gum (Eucalyptus camaldulensis) were placed at a subsurface depth and used to experimentally simulate 3 regimes of water level variability common to the Murrumbidgee River: 1) low inundation/emersion frequency (permanent inundation), 2) high inundation/emersion frequency (5 d inundated, 9 d emersed), or 3) intermediate inundation/emersion frequency (11 d inundated, 21 d emersed). Maximum net primary productivity of biofilm was achieved after 29 d of permanent inundation. After 75 d, a heterotrophic biofilm developed on blocks exposed to the high frequency of inundation and emersion, whereas an autotrophic biofilm developed on blocks exposed to the intermediate frequency of inundation and emersion. In regulated lowland rivers, water level variations from managed-flow releases of intermediate frequency and duration have the potential to provide maximum rates of biofilm productivity on woody substrata.

We conducted an enclosure/exclosure experiment in a forested, headwater stream to determine if crayfish acted as ecosystem engineers in this community. Three sets of channels (4 channels per set) were placed in separate pools. All channels contained cobble-filled baskets. Large crayfish (Cambarus bartonii) or a 5 g chestnut oak (Quercus prinus) leaf pack were added to some channels. The 4 treatments in the experiment were: 1) no crayfish/no leaf pack, 2) no crayfish/leaf pack, 3) crayfish/no leaf pack, and 4) crayfish/leaf pack. This design allowed us to assess the effects on the benthic community of adding crayfish and a leaf pack. It also allowed us to assess the effect of large crayfish on leaf pack breakdown. Leaf pack breakdown rate was significantly faster in the presence of crayfish. Crayfish did not have a significant effect on the abundance of any invertebrate taxon associated with leaf packs. However, few large (≥3.0 mm long) chironomids were found in leaf packs exposed to crayfish after 8 wk. Crayfish significantly reduced the abundance of fine particulate matter (FPM, a mix of fine inorganic sediment and fine particulate organic matter), chironomids, and harpacticoid copepods in the baskets. Heptageniid mayfly larvae, however, were found only in channels in which crayfish had reduced the abundance of FPM. Few large chironomids were found in baskets exposed to crayfish. Only harpacticoid copepods displayed a significant response to the addition of a leaf pack; this effect was observed only after 4 wk in channels containing crayfish. Our results demonstrate that crayfish can function as ecosystem engineers in forested, headwater streams because they can influence detrital processing rates and the distribution of FPM. In addition to acting as ecosystem engineers, crayfish may affect some invertebrate taxa directly through size-selective predation.

Natural disturbances can provide insights into mechanisms organizing communities by perturbing systems at larger scales and more realistic intensities than can often be achieved otherwise. We took advantage of 2 winterkills of fish, a common disturbance of small lakes on the Boreal Plains of northern Alberta (Canada), to assess the effects of sudden, large reductions in fish densities on littoral macroinvertebrate assemblages. Winterkill nearly eliminated the native fish assemblages (dominated by northern pike, Esox lucius, and yellow perch, Perca flavescens) in the 2 lakes, whereas 2 nearby lakes with similar fish assemblages were unaffected and served as references. Environmental characteristics of both winterkill and reference lakes changed little from year to year. Uni- and multivariate analyses of macroinvertebrates revealed some inherent among-lake differences; however, strong and parallel changes in the invertebrate assemblages occurred only in the 2 winterkill lakes, congruent with the winterkills of fish. Decreases in fish biomass were generally accompanied by increases in macroinvertebrate density, particularly among taxa (e.g., amphipods, leeches, chironomids) commonly eaten by the native fish. As a result, analyses of matrix concordance and variance partitioning showed variation in macroinvertebrate assemblages was related to both temporal changes in fish density and environmental differences among lakes but that the fish and environmental matrices were not strongly concordant. Our serendipitous study of community-level disturbance revealed that winterkill-induced reductions of fish densities can have strong, cascading effects on littoral macroinvertebrates in these boreal lakes.

The effect of different dissolved oxygen (DO) concentrations on the macroinvertebrate assemblages from 2 Australian tropical streams (1 upland, 1 lowland) was measured using artificial stream mesocosms. Responses to 5-d exposures were tested. Both the upland and lowland assemblages showed a similar response. Most taxa tolerated all but very low DO levels (<10% saturation), although a reduction in emergence of insect taxa at intermediate levels (25–35% and 10–20% saturation) was observed. Mayflies showed the highest sensitivity to low oxygen conditions, and lethal effects were observed at DO levels <20% saturation for several upland and lowland species. For other taxa, including several Chironomidae, mortality was observed when oxygen concentrations were below 8% saturation. A drift response was observed only when oxygen concentrations reached near lethal levels (≤10% saturation). The lack of a drift response at DO concentrations of 25 to 35% and 10 to 20% saturation indicates that, in moderately poor oxygen conditions, macroinvertebrates will remain at a location and, hence, experience sublethal effects such as suppressed emergence. It is clear that these animals can persist in hypoxic conditions in the short term. However, because of sublethal effects, understanding how low DO concentrations affect natural assemblages of aquatic macroinvertebrates may require studies of populations over several generations.

We conducted an enclosure/exclosure experiment during summer 1998 and 1999 to test the strength of top-down control by slimy sculpins (Cottus cognatus) on benthic macroinvertebrates and periphyton. We used 1-m² cages to manipulate slimy sculpin density within 6 riffles in Valley Creek, Minnesota. Each riffle contained an enclosure stocked with 3 slimy sculpins, a fishless exclosure, and an open control cage. In 1998, densities of total invertebrates and grazers on clay tiles were significantly higher in exclosures than enclosures and control cages; however, densities of individual grazer taxa (Glossosoma and Baetis) on tiles did not differ significantly among treatments. Fish had no significant effect on invertebrates inhabiting gravel substrate. In 1999, densities of total invertebrates and grazers on tiles and inhabiting gravel substrate did not differ significantly among treatments. Slimy sculpins did not have a detectable effect on the drift propensity of invertebrates during either year. Regardless of the effect of fish on grazers in either year, periphyton biomass did not differ significantly among the fish treatments even though there was a strong, negative correlation between Glossosoma density and periphyton biomass. Periphyton biomass was significantly higher on tiles with a barrier that excluded Glossosoma larvae from the surface, indicating that Glossosoma limited periphyton biomass. We conclude that the strength of top-down control by slimy sculpins on invertebrates was weak. We did not detect an algae-based trophic cascade during either year because Glossosoma larvae were relatively invulnerable to fish predation and did not alter their behavior in the presence of fish.

We used stable isotope analysis to investigate dietary differences between 2 sympatric species of tubicolous chironomid larvae, Chironomus plumosus and C. anthracinus, from the profundal sediments of 6 eutrophic lakes in the UK and Germany. We found striking variation between lakes in both δ¹³C (−29.8 to −56.3‰) and δ¹⁵N (−7.8 to 14.7‰) in the 2 chironomid species. Moreover, C. plumosus was consistently depleted in both ¹³C and ¹⁵N relative to C. anthracinus. Our data support previous reports of interspecific dietary variations between the 2 species, which suggest niche separation partly on the basis of diet. However, reported differences in the feeding modes of the 2 species could not explain the extreme ¹³C- and ¹⁵N-depletion observed in chironomid larvae from several of the lakes. We suggest that the low δ¹³C signatures result from the ingestion of methanotrophic bacteria and subsequent incorporation of biogenic methane-derived C. Further, the chironomid larvae enhance methanotrophic activity via bioturbation of the surrounding sediment. Significant isotope differences between the 2 species may result from their variable tube morphologies or physiology.

Larval Glossosoma intermedium are dominant benthic grazers that often deplete their food resources (periphyton). We observed G. intermedium grazing periphyton from conspecific cases, a possible result of limited resources on stream cobbles, and we hypothesized that case grazing increases when periphyton resources become scarce. This hypothesis was tested by monitoring frequency of case grazing among G. intermedium in 3 streams in southwest Wisconsin for 1 y, and also in the laboratory using time-lapse video of larvae on tiles with and without periphyton. In situ periphyton biomass was higher on larval cases than on the stream cobbles to which larvae had access, irrespective of density, age, or size structure of G. intermedium. Case grazing was positively related to larval densities at 2 of the 3 streams. In laboratory experiments, larval encounter rates were similar in both tile treatments, although frequency and duration of case grazing increased on tiles lacking periphyton. These results suggest that periphyton on G. intermedium cases can provide an important resource patch for this species, especially when periphyton biomass is low in the ambient environment.

Understanding how lotic communities respond to combinations and gradients of physical disturbance and nutrient inputs is important for the practical management of stream ecosystems, and to add to our knowledge of stream ecology. We used covariance structure analysis to examine the direct and indirect effects of a gradient in nutrient concentration and hydrologic disturbance on benthic community structure in 133 riffles in 97 midwestern US streams. Model results indicated that the relationship between nutrient concentration, algal biomass, and primary consumer biomass was mediated by hydrologic regime. Algal biomass was strongly dependent on nutrient supply in streams where scouring floods reduced primary consumer biomass. In more stable streams, primary consumers depressed algal standing stocks regardless of nutrient concentration. Our results imply that the response of stream benthic communities to nutrient stimulation is highly variable and dependent on physiography and climate because they strongly influence hydrologic disturbance regimes.

Relationships among landscape attributes, instream dissolved organic C (DOC) concentrations, and metal bioaccumulation in Arctopsyche grandis and Drunella doddsi were investigated in the Upper Arkansas River Basin, Colorado. A Geographic Information System (GIS) was used to calculate landscape attributes in 16 watersheds where DOC concentrations were measured from May to August 2001. Metal concentrations in Arctopsyche and Drunella were related to physicochemical characteristics measured in these streams and in the Arkansas River. Results of multiple linear regression showed that % forested area explained 47% and 59% of the variation in maximum and mean DOC concentrations, respectively. Maximum DOC was negatively associated with the concentration of Zn (R² = 0.25) and Cd (R² = 0.39) in Arctopsyche. DOC concentration did not describe metal concentrations in Drunella, which accumulated significantly more Zn, Cd, and Cu than Arctopsyche. The higher metal concentrations and the absence of a DOC effect on metal uptake in Drunella most likely resulted from dietary exposure to metal-enriched detritus and periphyton. Our results indicate that % forested area within a watershed can be used to describe DOC concentrations, which in turn influence metal bioaccumulation in Arctopsyche. To our knowledge, this study is the first to quantify the relationship between landscape attributes and DOC, and to demonstrate the influence of DOC on metal bioaccumulation in benthic macroinvertebrates in the field.

Macroinvertebrate assemblages and environmental variables were evaluated at 45 stream sites throughout the Puget Sound Basin, Washington, USA. Environmental variables were measured at 3 spatial scales: reach, local, and whole watershed. Macroinvertebrate distributions were related to environmental variables using canonical correspondence analysis to determine which variables and spatial scales best explained the observed community composition and to identify biota–land cover optima. The calculation of a biota–land cover optimum was a 2-step process. First, an individual taxon's optimum was estimated for a particular land cover by weighting the mean value for that land cover by the abundance of that taxon at all sites. Second, the biota–land cover optimum was determined as the point at which the greatest numbers of taxa, at their calculated optima, appeared for a particular land cover. Sampling reaches were located on streams in watersheds with varying levels of forest, agriculture, and urban/suburban land cover that represented the full range of physical conditions typically found in Puget Sound streams. At the reach scale, taxa composition was correlated with conductivity and mean velocity. At the local and whole-watershed scales, taxa composition was correlated with % forest and agricultural land cover and % forest and bedrock land cover, respectively. For all of the scales, the dominant environmental variables represented an anthropogenic gradient. There was little difference in the amount of variability explained by each spatial scale. At the local-watershed scale, a biota–land cover optimum of ∼80 to 90% forest land cover was identified. The total number of taxa at their optima declined rapidly as forest land cover within the local scale declined below 80 to 90%. At the whole-watershed scale, a biota–land cover optimum of 70 to 80% forest land cover was identified. The total number of taxa at their optima declined rapidly as forest land cover within the whole watershed declined below 70 to 80%. Our results suggest that macroinvertebrates can be used both as quantitative indicators of environmental conditions at multiple scales and indicators of land cover optima. Further examination of these optima could be used to establish priorities for conservation and restoration efforts.

We evaluated the influence of various sources of sampling error on the precision, accuracy, and sensitivity of bioassessments based on River Invertebrate Prediction and Classification System (RIVPACS)-type models. We used data from 98 minimally altered streams in western Oregon and Washington to generate 18 models representing 2 field collection techniques (fixed-area riffle and multiple-habitat collections) and 9 levels of subsampling (50, 100, …450 fixed counts). For each model, we generated 2 observed-to-expected taxa ratios (O/E). The 1^st O/E was based on all predicted taxa and the 2^nd excluded rare taxa (i.e., those taxa with predicted probabilities of occurrence <0.5). We then compared O/E values to determine the extent to which subsampling effort, field collection method, different field personnel, and individual site characteristics altered model performance. We also generated O/E values for 63 streams with varying amounts of watershed and channel alteration (test sites) to quantify the extent to which sampling error influenced the sensitivity of these models in detecting biological impairment. Model precision improved with increased sampling effort. However, neither collection method consistently led to more precise models. Model accuracy generally was not affected by subsampling effort, sample collection techniques, or different sample collectors. However, ∼50% of the error in predictions was associated with unexplained characteristics of individual sites. Average assessment values across all test sites were robust to both collection method and subsampling effort. However, inferences about the biological condition of some test sites varied among models. Precision, accuracy, and sensitivity all improved with the exclusion of rare taxa, although O/E values for individual test sites were more variable among models based on different subsample counts when rare taxa were excluded from model calculations. Overall, we found that the effects of sampling error on RIVPACS model performance can be minimized by constructing models from subsamples of ≥350 individuals and by excluding rare taxa from calculations of O/E.

Replicate samples of cobbles and loose inorganic and organic matter collected from 3 stream-riffle sites with different periphyton communities were passed through a geometric series of 9 sieves (0.063–16 mm mesh) to quantify sieve retention probabilities of benthic invertebrates. Sieves retained all organisms with a body length >10× the mesh size. Logistic regression models were estimated to describe retention probabilities as functions of body length and mesh size. Retention probability functions differed slightly but significantly among sites, operators, and taxa. Retention probabilities were higher for samples containing filamentous algae, which entangled invertebrates. Size distributions of straight, elongate invertebrates (e.g., oligochaetes, midge larvae) retained by sieves were more variable than distributions of more spherical organisms (e.g., gastropods). On average, the 1-mm sieve retained >90% of invertebrate biomass but <33% of individuals retained by a 63-μm sieve. Size distribution of organisms retained by coarse sieves (≥1 mm), combined with logistic functions predicting retention probability, can be used to describe abundance, biomass, and size distribution of organisms retained by fine sieves. These results suggest that unbiased descriptions of benthic communities can be obtained with relatively little effort by using a minimum sieve mesh size of 1 mm.