The role of predators (particularly top predators such as fish) in structuring stream communities has been debated for 2 decades. Much of the debate may have been caused by the lack of a conceptual framework for evaluating predator effects in stream communities. First, I propose a general conceptual model of the factors (abiotic, such as stream permanence and disturbance regime; biotic, such as predation) that can influence community structure, and the conditions in which these various factors would be expected to be important. Hydrologic permanence and disturbance transitions separate streams where abiotic factors are most important in determining community structure from streams with relatively benign disturbance regimes where predation may be more important. Second, I focus on the potential effect of predators in perennial streams with relatively benign disturbance regimes. Such streams are divided longitudinally into sections where different types of predators might be important in determining community structure. Large invertebrates (stoneflies, dragonflies, shrimp, and crayfish) and salamanders may be the dominant benthic predators affecting species composition in small perennial fishless streams. A transition from invertebrate- and amphibian-dominated to fish-dominated systems may occur in larger, downstream sections (predator transition 1). In addition, longitudinal transitions in fish-assemblage structure from upstream tributaries to downstream main-channel fish assemblages (predator transition 2) may affect community structure. I present evidence supporting the above model and suggest experimental approaches to test the model. This conceptual framework may help in understanding the role of specific predators in determining prey distributions in many stream communities.

The influence of the net-spinning caddisfly larva Stenopsyche marmorata on streambed substratum stability was examined in 14 riffles of 5 rivers in northern Hokkaido, Japan. In each riffle, 6 representative stones were measured and 7 variables were quantified for each stone: 1) ambient flow velocity, 2) ambient depth, 3) extent of embeddedness, 4) force required to initiate shifting of the stone parallel to the flow direction, 5) biomass of S. marmorata living on and under the stone, 6) wet mass of the stone, and 7) size of the stone. Regression analyses were used to examine which of the variables best predicted the coherent strength of a particle with the stream bed (F_C). Riffle-scale patterns derived from the means of 6 stones showed significant influences of biomass of S. marmorata/stone and embeddedness on F_C; however, the collinearity of these 2 variables hindered evaluation of their relative importance. At the stone scale, biomass of S. marmorata/stone was positively related to F_C when standardized for the degree of stone embeddedness. F_C of stones inhabited by S. marmorata at the maximum range of their biomass is predicted to be 50 to 260% greater than F_C of stones without S. marmorata. The effects of biological consolidation of substratum particles by S. marmorata probably are substantial, even in inherently heterogeneous natural environments.

Dam removal is becoming an increasingly common management solution for aging dams, and evaluation of the impact of dam removal on basic attributes of streams such as nutrient uptake and transport is essential. The removal of 2 small dams from a forested, 2^nd-order stream in Wisconsin allowed us to study how nutrient dynamics were influenced by changes in channel geometry and bed material. We calculated P uptake and transient storage metrics from a series of 19 short-term injections, measured cross-sectional profiles, and determined benthic sediment size at regular intervals for 2 mo before and after the removals. We also repeated these measurements over the same time period the following year. Nutrient uptake was highly variable, and the stream changed from being a soluble reactive P (SRP) sink to being a source several times over the study period. Uptake lengths increased immediately after dam removal but differences between measurements made before and after removal were not significant. We found no significant relationship between uptake length and benthic sediment composition, channel geometry, or water residence time over the course of the dam removals. Our results indicate that changes in physical channel attributes did not play an important role in controlling SRP retention, probably because the impact of dam removal was small compared with the natural variability of this system.

We examined changes in stream chemistry following the removal of a 2-m-high dam on Manatawny Creek in southeastern Pennsylvania. Our primary objective was to determine the effect of small dam removal on the concentrations and forms of C, N, and P. Dissolved and particulate constituent concentrations were monitored at sites upstream and downstream of the dam and impoundment. Seasonal changes in alkalinity and N and P concentrations were observed before and after dam removal. However, the proportions of NO₃⁻ NO₂⁻, NH₄ , and dissolved organic N (DON), and soluble reactive P (SRP) and dissolved organic P (DOP) in the total dissolved pools of N and P did not change seasonally. The dam and dam removal did not influence C, N, or P concentrations and forms (except for NH₄ ) in this stream. The lack of significant changes probably was a result of the short hydraulic residence time (<1.5 h at base flow), infrequent temperature stratification, and potential C limitation of bacterial activity in the small impoundment. Our results suggest that alterations in N and P concentrations and forms following dam removal may be site-specific. The characteristics of the dam, watershed, and impoundment (e.g., the amount of wetted streambed, hydraulic residence time, and organic enrichment of sediment and water) probably determine the influence of dam removal on stream water chemistry.

We investigated the effects of autotrophy on short-term variations in nutrient dynamics by measuring diurnal and day-to-day variations in light level, primary productivity, and NO₃⁻ uptake during early and late spring in 2 forested streams, the East and West Forks of Walker Branch in eastern Tennessee, USA. We predicted that diurnal and day-to-day variations in NO₃⁻ uptake rate would be larger in the West Fork than in the East Fork in early spring because of higher rates of primary productivity resulting from a more stable substratum in the West Fork. We also predicted minimal diurnal variations in both streams in late spring after forest leaf emergence when light levels and primary productivity are uniformly low. Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O₂ change technique, and reach-scale rates of NO₃⁻ uptake were determined by tracer ¹⁵N-NO₃⁻ additions. In the West Fork, significant diurnal and day-to-day variations in NO₃⁻ uptake were related to variations in light level and primary productivity in early spring but not in late spring, consistent with our predictions. In early spring, West Fork NO₃⁻ uptake rates were 2 to 3× higher at midday than during predawn hours and 50% higher on 2 clear days than on an overcast day several days earlier. In the East Fork, early spring rates of GPP were 4 to 5× lower than in the West Fork and diurnal and day-to-day variations in NO₃⁻ uptake rates were <30%, considerably lower than in the West Fork. However, diurnal variations in NO₃⁻ uptake rates were greater in late spring in the East Fork, possibly because of diurnal variation in water temperature. Our results indicate the important role of autotrophs in nutrient uptake in some forested streams, particularly during seasons when forest vegetation is dormant and light levels are relatively high. Our results also have important implications for longer-term assessments of N cycling in streams that rely on daytime measurements or measurements only under limited weather conditions (i.e., clear days).

Sediments in the backwaters of the Upper Mississippi River (UMR) are highly organic and provide an optimal environment for N removal. We monitored an 8.6-ha UMR backwater site near La Crosse, Wisconsin, for nearly 3 y to assess temporal variability, seasonal trends, and the factors regulating denitrification. We measured rates of unamended denitrification (DEN) and denitrification enzyme activity (DEA) rates at ambient temperature and DEA at 30°C (DEA30). Seasonal mean (±1 SE) DEN rates ranged from 0.041 ± 0.015 to 0.47 ± 0.23 μg N cm⁻² h⁻¹ and were highest in winter and lowest in autumn. Seasonal rates of DEA exhibited a different pattern with the highest rates in summer (25.6 ± 3.4 μg N cm⁻² h⁻¹) and the lowest rates in winter (10.6 ± 2.1 μg N cm⁻² h⁻¹). The overall mean DEA30 rate was 31.0 ± 1.9 μg N cm⁻² h⁻¹ but showed no significant seasonal pattern. Short-term (weekly) and seasonal variability exhibited by rates of DEN and DEA were best explained by water-column NO₃⁻ concentration and temperature, respectively. No environmental variables explained a significant amount of variability in DEA30. Our results suggest that nutrient (i.e., NO₃⁻) availability and temperature are both regulators of denitrification, with NO₃⁻ concentration being the most important limiting factor in this system. The high DEN rates during winter were in response to elevated NO₃⁻ concentrations resulting from a chain reaction beginning with algal blooms creating oxic conditions that stimulated nitrification. Increasing hydrological connectivity in large rivers as a river management tool to reduce N flux to downstream areas may be beneficial.

Aquatic shredders depend mostly on terrestrial leaf litter as a food resource, and differences between the C:N ratio of their food resource and their bodies may affect nutrient excretion and the composition of their feces. Laboratory experiments were used to test how the stonefly Klapopteryx kuscheli rebalances the high C:N ratio of its food. An experiment was designed in which K. kuscheli was fed 5 different leaf litters across a gradient of C:N ratios and the elemental ratios of the food, insect bodies, and feces were analyzed. Klapopteryx kuscheli regulated its internal elemental composition by changing its excretion of N depending on the N content of its food. An inverse relationship was found between N content of most foods and excreted NH₄ . However, K. kuscheli feces were rich in N and the insect excreted very small amounts of NH₄ when fed Nothofagus pumilio, probably because of refractory N-based compounds in the leaves of this species. Homeostatic regulation of N content by invertebrate shredders may influence N dynamics in small nutrient-poor streams.

Nutrient-diffusing substrata were used to determine if periphyton communities in lakes of different benthic productivity respond similarly to nutrient inputs, and if their responses corresponded with inference models developed from a calibration set of 32 Irish lakes. In addition, the attributes of the periphyton community most effective for detecting changes in periphyton associated with nutrient addition were examined. P-addition treatments had significantly higher algal biovolume than N-addition or control treatments. Canonical correspondence analysis using nominal variables to define treatments indicated that both nutrient addition and lake (= benthic productivity) significantly affected taxonomic composition and the proportion of algal growth forms. The more productive lakes had a greater relative abundance of filamentous chlorophytes, cyanobacteria, and mobile and stalked diatoms. Within lakes, nutrient addition was associated with an increase in filamentous chlorophytes and decreases in nonmobile prostrate growth forms and Achnanthes minutissima. Transfer functions from previously developed inference models were capable of inferring relative differences in total P concentrations among the lakes but lacked precision when predicting responses to nutrient addition. The relationship between normalized size spectra of algae and nutrient status was contradictory. Algae were significantly larger in nutrient-addition treatments within one lake. However, among lakes, larger size classes were most abundant in the least productive lake. Overall, the results indicate that increased nutrient loading should increase periphyton area-specific biovolume, increase filamentous chlorophytes, and potentially shift size spectra to larger classes over a broad range of benthic productivities in hardwater lakes. Changes in algal growth form appeared to be the most expedient attribute to measure when using periphyton communities to assess nutrient loading. Examining the response of benthic freshwater communities to experimental addition of nutrients in situ may help to better refine calibration sets and improve inference models, especially if the data set is restricted to lakes with similar watershed-scale characteristics.

Grazer–periphyton interactions are shaped, in part, by indirect effects of nutrient regeneration. They are an important model system with which to test predictions of ecological stoichiometry and the Growth Rate Hypothesis. We conducted a laboratory experiment to test how nutrient enrichment and grazer identity interact to regulate the nutrient content and stoichiometry of both periphyton and consumers. We considered a situation in which P concentration in the water column was high, in contrast to previous experiments in which P was the limiting nutrient. We added N and P, alone and in combination, to the water in experimental aquaria that contained periphyton communities on clay tiles and grazers (1 of 3 snail species) or no grazers (ungrazed control). Benthic algae incorporated nutrients in close proportion to their availability in each nutrient treatment. Algal biomass increased significantly with N P enrichment, but not with N or P enrichment alone. Grazers had no effect on periphyton C:N ratios and positive effects on periphyton C:P and N:P ratios. P content of grazers (% dry mass) increased and C:P and N:P molar ratios of grazers decreased in response to N enrichment of the water. Grazer P content increased in response to N enrichment, probably because of increased grazer growth rates. We hypothesize that the addition of N under N-limiting conditions led to increased P uptake or retention by grazers because of high growth rates and RNA production, consistent with the Growth Rate Hypothesis.

The salt marshes of the Empordà wetlands are subjected to both physical variability, caused by hydrological fluctuations, and nutrient variability. We assessed which of these sources of variability most influenced nematode assemblages in 5 basins in the wetlands. One of these basins was permanently inundated, but the hydroperiods of the others were variable. Taxonomic composition and the Maturity Index, which is based on the ecological characteristics of nematodes (r- or K-strategists), were used to describe the assemblages. Variation decomposition analysis showed that nematode assemblages of the Empordà wetlands were more influenced by physical variability (18.2% of explained variation), caused by changes in variables such as conductivity and granulometry, than by nutrient variability (3.4% of explained variation). The Maturity Index of the assemblages was related to hydrological disturbance, but not to eutrophy. Cluster analysis based on taxonomic composition separated the permanent basin from the temporary basins. Nematode diversity and genus richness were significantly higher in the permanent basin than in the temporary basins. We conclude that the Maturity Index is valid for assessing hydrological disturbances in fluctuating Mediterranean environments, whereas assemblage parameters such as diversity and richness may not be because they responded to water permanence but not to other environmental factors associated with hydrologic variability.

Substrate interstices influence the microdistribution and survival of benthic invertebrates. The benefits of interstitial refugium availability were quantified over 4 wk under baseflow conditions that are common to lowland streams in northwestern Europe. Effects on growth, feeding, and behavior of the amphipod Gammarus pulex L. were studied in 8 indoor artificial stream channels. Combinations of 2 near-bed flow velocities (3 and 9 cm/s) and 2 substrate types (medium sand and coarse gravel) were assigned to 32 experimental compartments. Growth of G. pulex was greater at 9 cm/s than at 3 cm/s, and G. pulex avoided bare sand and used gravel patches at both flow velocities. Food consumption was lower and individual growth was greater in gravel than in sand, a result that suggests that energetic costs were lower in gravel than in sand. We suggest that the interstitial refugia in gravel may have benefits beyond providing protection from flow. The presence of interstitial refugia may be of key importance to G. pulex, and the underlying mechanism of interstitial refugium use might be associated with the adaptation of organisms to seek refuge from predators.

A numerical model for simulation of freshwater mussel dynamics was used to investigate the effects of substrate and hydrodynamic conditions on the formation of mussel beds in a 10-km reach of the Upper Mississippi River (UMR). Suitable habitats for mussel survival were identified by creating a dimensionless parameter (shear stress ratio) combining shear force and substrate type. This parameter is a measure of substrate stability that could be used in many different applications. Dispersal of juvenile mussels with flow as they detach from their fish hosts was simulated by a particle-tracking mechanism that identified suitable areas for colonization with the potential to evolve into mussel beds. Simulated areas of mussel accumulation coincided with reported locations of mussel beds, and simulated densities were in the range of abundant mussel beds in other reaches of the UMR. These results, although more qualitative than quantitative, provide insight into factors influencing the formation of mussel beds in a large river.

Freshwater mussels are long-lived and relatively stationary organisms that form annual rings, so they provide excellent opportunities for studying how population properties such as age structure relate to environmental influences on recruitment and mortality. We investigated population dynamics of the freshwater pearl mussel, Margaritifera falcata, in 2 northern California Coast Range rivers, the South Fork Eel and the Navarro. We used field observations and inverse theoretical modeling to assess age structures and identify factors that control population demographics. Distinctly different mortality rates for young (<20 y) and old (≥20 y) mussels were observed in the South Fork Eel, and recruitment and mortality both were related to river discharge regimes. Margaritifera falcata recruited more successfully during low-discharge than during high-discharge years, and a 2-fold increase in discharge caused a 60% decline in recruitment success. Active annual recruitment (˜1100–1800 ind./y) was observed in the South Fork Eel, whereas recruitment has been low or nonexistent during the past 30 y in the Navarro. This difference probably is a consequence of the influence of 2 non-mutually exclusive factors: landuse history and host-fish abundance. The Navarro watershed has extensive timber harvesting and orchard/vineyard agriculture and has had severe declines in fish populations, whereas large sections of the South Fork Eel pass through protected land and the river has retained its historical fish populations. Investigation of the factors that shape local M. falcata population age structures yielded insights into the influence of environmental variables and history on mortality and recruitment that will aid conservation of this endangered family.

Unionid mussels often occur as multispecies aggregates called mussel beds and in dense patches within the mussel beds themselves. Thus, their distributions are patchy at 2 spatial scales. We examined the association between mussel assemblage structure and macroinvertebrate assemblage structure at these 2 spatial scales in rivers of the Ouachita Highlands, Arkansas and Oklahoma, USA. We used multivariate variation partitioning techniques to relate variation in benthic macroinvertebrate distribution and abundance to variation in mussel assemblages, environmental variables, spatial variables, and overlapping or shared variation between these components. At the patch scale, total densities of macroinvertebrates and dominant groups (Oligochaeta, Chironomidae, Ephemeroptera, and Trichoptera) were significantly higher in patches containing mussels than where mussels were absent, and densities of macroinvertebrates were positively correlated with unionid density. In variation partitioning analyses, mussel assemblages explained almost ½ of the variation in macroinvertebrate assemblages at both spatial scales, even after removing effects of similar habitat (environmental variables) and biogeographic history (spatial variables).

A common objective when surveying freshwater mussels is to detect the presence of rare populations. In certain situations, such as when endangered or threatened species are potentially in the area of a proposed impact, the survey should be designed to ensure a high probability of detecting species presence. Linking survey design to probability of detecting species presence has been done for quantitative surveys, but commonly applied designs that are based on timed searches have not made that connection. I propose a semiquantitative survey design that links search area and search efficiency to probability of detecting species presence. The survey can be designed to protect against failing to detect populations above a threshold abundance (or density). I illustrate the design for surveys to detect clubshell (Pluerobema clava) and northern riffleshell (Epioblasma torulosa rangiana) in the Allegheny River. Monte Carlo simulation indicated that the proposed survey design performs well under a range of spatial distributions and low densities (<0.05 m²) where search area is sufficient to ensure that the probability of detecting species presence is predicted to be ≥0.85.

We compared the performance of Test Site Analysis (TSA) to 2 existing bioassessment methods: the BEnthic Assessment of SedimenT (BEAST), and the multimetric approach. In TSA, simple spreadsheet calculations are used to: 1) derive an overall multivariate measure of dissimilarity between a potentially impaired test site and a reference benchmark using a number of biological summary metrics, 2) classify each site as impaired, potentially impaired, or in reference condition with formal hypothesis tests, and 3) identify the metrics important in distinguishing a significantly impaired test site from the reference benchmark. We used data collected as part of the Fraser River Action Plan (British Columbia, Canada) to compare the biological condition of 15 potentially impaired test sites known to be exposed to agriculture, logging, or mining to the condition of 61 minimally disturbed reference sites with comparable habitat. When TSA was used, the false-positive (i.e., Type-1 error) and false-negative (i.e., Type-2 error) rates were as low as or lower than when the other methods were used. Using TSA enhances our ability to identify impairments because it accounts for correlations among metrics and uses a statistically defined potentially impaired category. TSA integrates formal hypothesis testing with the strengths of many existing approaches, does not require specialized software, and enhances our ability to detect and diagnose biological impairments.