Benthic Communities In Streams Research Articles

Exploring the Influence of Industrial and Climatic Variables on Communities of Benthic Macroinvertebrates Collected in Streams and Lakes in Canada’s Oil Sands Region

Identifying and tracking the influence of industrial activities on streams and lakes is a priority for monitoring in Canada’s oil sands region (OSR). While differences in indicators are often found in waterbodies adjacent to mining facilities, the confounding influence of natural exposures to bitumen and other stressors can affect the identification of industrial effects. However, recent work suggests metrics of industrial activity at individual facilities, including production and fuel consumption, may be used in site-specific analyses to identify influence of the industry as a whole as well as individual operations. This study further examined the potential relationships between industrial and climatic variables on benthic communities from 13 streams and 4 lakes using publicly available data from the minable region and the Elastic Net (EN) variable selection technique. From the full set of possible industrial and climate variables, the EN commonly identified the negative influence of plant and fuel use of petroleum coke at the Suncor Basemine on benthic communities in streams and lakes. The fuel/plant use of petroleum coke at Suncor likely reflects the emission and regional deposition of delayed coke fly ash. Among the other industrial variables, crude bitumen production at Syncrude Mildred Lake and other facilities, steam injection rates, and petroleum coke stockpiling were also selected for some benthic invertebrate indices at some sites. Land disturbance metrics were also occasionally selected, but the analyses largely support the predominant influence of industrial facilities via (inferred) atmospheric pathways. While climate variables were also commonly selected by EN and follow-up work is needed, this study suggests that integrating industrial performance data into analyses of biota using a site-specific approach may have broad applicability in environmental monitoring in the OSR. More specifically, the approach used here may both resolve the long-standing challenge of natural confounding influences on monitoring the status of streams in the OSR and track the influence of industrial activities in biota below critical effect sizes.

Impacts of industrial atmospheric emissions on watershed export of dissolved ions in coastal streams: a Bayesian modeling approach.

Anthropogenic atmospheric emission and subsequent deposition of sulfur (S) has been linked to disrupted watershed biogeochemical processes through soil and surface water acidification. We investigated watershed-scale impacts of acidic deposition on tributary concentrations and watershed exports of major nutrients and ions for the Kitimat River Watershed, British Columbia. Since the 1950s,the Kitimat watershed hadan aluminum smelting facility with substantial emissions at the river estuary. Emissions loadthe airshed overlying the watershed and potentially impact western tributaries leaving eastern tributaries available as reference. We assessed concentrations and export of key compounds in three reference and six potentially impacted tributaries and watersheds in 2015 and 2016. Sulfate (SO4), fluoride (F), nitrate (NO3), and chloride (Cl) were significantly higher in impacted tributaries. F concentrations exceeded the Canadian Council of Ministers of the Environment guideline for aquatic life in 83% of samples collected from impacted streams. Watershed export and associated uncertainty were determined by bootstrapped flow-stratified Beale's unbiased estimator. Impact of emissions on watershed export was modeled in a Bayesian approach to include variance in the export estimate to inform the uncertainty of model parameters. Export of SO4 and Ca increased significantly within 16 km and 8 km, respectively, toward the smelter emissions. The corresponding impacted area for SO4 and Ca was approximately 100 km2 and 45 km2, respectively. SO4 export is likely due to direct impacts of S deposition, with excess S being flushed from the watersheds. Ca export patterns likely result from indirect impacts of S deposition on soil chemistry and flushing of Ca. These impacts may contribute to effects within tributaries on benthic stream communities and regionally important juvenile Pacific salmon.

Benthic macroinvertebrate communities in sites with native forest presence and absence in north Patagonia

ABSTRACT The benthic fauna in northern Patagonian streams is characterized by presence of water quality indicator species. The aim of the present study was characterized the benthic communities in streams associated to sites with presence of Nothofagus forests and with human intervention without Nothofagus forests using null models in ecology, unified neutral theory of biodiversity (UNTB) and unweighted pair group method with arithmetic mean (UPGMA). The results of species co-occurrence null model revealed that species associations are random, whereas the results of niche overlap null models revealed the presence of niche overlap. The UNTB results revealed low species number. The results of UPGMA revealed that species composition is specific for each site existing marked differences. The exposed results agree with results observed for central and northern Patagonian Chilean rivers, where the species composition is variable in according to surrounding basin properties and seasonal conditions.

Effects of pulse and press drying disturbance on benthic stream communities

Natural disturbance is an integral component of most ecosystems and occurs in 3 different forms: pulse, press, and ramp. In lotic ecosystems, seasonal drought is a major form of disturbance, particularly in intermittent headwater streams, which often are reduced to pools that serve as refuges for biota. We used simulated intermittent stream pools to compare the effects of control, pulse, and press drying on growth and survival in 3 fish species (Lepomis megalotis, Campostoma anomalum, and Etheostoma spectabile) commonly found together in drought-prone streams in the Ozark Highlands, USA. We also compared effects on benthic community structure, including periphyton and chironomid density and sediment in deep (permanently watered) and shallow (intermittently dewatered) habitat. Only one species, L. megalotis, showed a significant reduction in length and mass growth in press drying compared with control treatments. Drying and type of drying had no effect on survival of any fish species. Drying and type of drying had strong overall effects on periphyton growth in shallow habitats, where ash-free dry mass decreased and the autotrophic index (the ratio of chlorophyll a to total biomass) increased significantly in drying relative to control and in press relative to pulse treatments. Drying negatively affected sediment accumulation in shallow habitat and chironomid density in deep habitat. Drying in intermittent streams has species-dependent effects on fish growth and benthic structure, and pulse and press drying differ in their effects on periphyton in these systems. These effects may have important consequences in seasonally drying streams as anthropogenic influence on stream drying increases.

Perspectives on the context‐dependency of stream community responses to contaminants

Summary Natural variation in the structure and function of aquatic communities along environmental or spatiotemporal gradients can influence how systems respond to biotic and abiotic disturbances. Although these context‐dependent responses are a well‐established element of basic ecology, they have received little attention from an ecotoxicological perspective. As the scope of freshwater bioassessment expands from single catchments to broad‐scale, regional surveys, it has become apparent that community responses to contaminants can vary significantly among locations. This variability is generally considered a nuisance and an impediment to recognising ecological patterns; however, natural spatiotemporal variation in communities also provides opportunities to test hypotheses about context‐dependent responses to contaminants in aquatic ecosystems. This study presents three examples of context‐dependent responses of benthic stream communities to contaminants. We show how longitudinal variation in community composition, the development of tolerance to contaminants and exposure to multiple stressors influence the historical context of a community and hence may determine community responses to contaminants. Aquatic ecotoxicologists have made significant progress identifying abiotic factors that influence contaminant bioavailability and toxicity. A similar effort could be undertaken to identify specific ecological factors (e.g. community composition, natural disturbance regimes and life‐history characteristics) that contribute to variation in susceptibility of communities to contaminants. Understanding the nature of context‐dependent responses and the mechanisms responsible for variation among communities should be a key element of ecological risk assessment and a research priority in ecotoxicology.

Non-native rainbow trout change the structure of benthic communities in headwater streams of the Cape Floristic Region, South Africa

Introduced rainbow trout Oncorhynchus mykiss have invaded many headwater streams in the Cape Floristic Region (CFR) and depleted, or elimi- nated, native fish populations. However, the question of whether trout invasions also have consequences for lower trophic levels in these systems has not been addressed. We used a broad-scale comparative study in the upper Breede River catchment (CFR) to evaluate differences in benthic community structure between sites on headwater streams with and without trout, and thereby infer community-level impacts of trout. There were differences in invertebrate abundance and assem- blage composition, and algal biomass, between sites with and without trout. Specifically, the abundance of certain herbivorous invertebrate taxa was higher, and the biomass of benthic algae lower, at sites invaded by trout. This pattern implies that trout have induced a trophic cascade by releasing herbivorous invertebrates from predation, leading to an increase in grazing pressure and a consequent indirect decrease in the biomass of benthic algae; a pattern that contrasts with the majority of studies investigating community-level impacts of introduced trout elsewhere. These findings, together with comparisons of environmental condi- tions between invaded and uninvaded sites, indicate that trout invasions have changed the structure and function of benthic communities in these streams.

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.

Influence of a tributary stream on benthic communities in a Didymosphenia geminata impacted stream in the Sierra Nevada, USA

The freshwater stalked diatom Didymosphenia geminata can alter benthic community structure. However, the numbers of D. geminata that must be present to do this is not clear. Is it possible that benthic communities in streams with D. geminata are not altered? A comparison of benthic communities in two similar, adjacent streams (one with and one without D. geminata) was conducted to identify the effects of D. geminata presence on benthic community structure. An additional comparison was made below the confluence of the two streams to determine the influence of an unregulated tributary stream on the temporal and spatial patterns of potential impacts of D. geminata on benthic communities. Benthic algal biomass (chlorophyll a and ash-free dry weight) in the D. geminata stream (dominated by D. geminata and Achnanthidium spp. throughout study period) was an order of magnitude higher. High D. geminata growth altered the physical environment of the stream benthos resulting in fewer grazing invertebrates and greater densities of burrowing invertebrates such as Chironomidae and Oligochaeta. Below the confluence, benthic community structure became more similar to the non-D. geminata stream. Results show that extensive growth of D. geminata can alter benthic communities, however, an unregulated non-D. geminata tributary stream can assuage impacts and establish benthic community structure resembling the non-D. geminata tributary stream over a relatively short distance. This study provides insight into the extent and mechanisms by which growth of D. geminata affects benthic communities and helps determine the influence of tributary stream on D. geminata growth and patterns in benthic communities.

Application of the Maximum Convex Sum Algorithm in Determining Environmental Variables that Affect Nigerian Highland Stream Benthic Communities

In this paper, we utilise the newly developed method K-Disjoint Maximum Convex Sum Problem (K-DMCSP) in a real-life application, which investigates the effects of land use changes on benthic stream communities in highland tropical streams of Nigeria. A collaboration between computer scientists and freshwater biologists was established to implement and examine the robustness of this approach compared to a traditional method that used rectangular shape in the K-Disjoint Maximum Sub-Array algorithm (K-DMSA). This novel approach uses the K- DMCSP, which utilises a convex shape as compared to a rectangular shape. In the above comparison the new approach using the convex shape optimises the sum and yields more accurate and precise results. This is because using the rectangular shape is limiting and is therefore not flexible enough to cover various sets of data distributions. The new approach was applied to data that were collated from 55 tropical highland streams on the Mambilla Plateau, Nigeria to investigate the interactions between substrate index or dissolved oxygen and temperature with number of macroinvertebrates (taxa). The K-DMCSP method located the K-maximum threshold values. This was achieved by defining the substrate index or dissolved oxygen associated most with the temperature by maximising the sum of elements of a selected portion of a two dimensional array. The K-DMCSP successfully detected the various temperatures between the different categories of the substrate index or dissolved oxygen, and how those variables affect numbers of macroinvertebrates. Furthermore, applying the algorithm revealed that the number of macroinvertebrates differs according to land use (e.g. forestry and agriculture). The new method used in this paper is encouraging in its capability to find the relationship between various environmental parameters and macroinvertebrate distribution and diversity. This method can potentially be applied to other real-life applications requiring finding associations between different parameters.

Measuring standardized effect size improves interpretation of biomonitoring studies and facilitates meta-analysis

Intersite differences in benthic communities detected by tests of null hypotheses are used routinely to infer effects of habitat perturbation. The statistical outputs from these tests are often treated as binary results (presence or absence of detectable effects), and the sizes and potential biological importance of detected differences or effects are frequently ignored. This situation can be remedied by measuring standardized effect sizes of detected differences. To demonstrate the benefits of standardized effect sizes, we compared benthic communities in streams draining forested and perturbed catchments based on kick-net samples, and samples from bricks and Hester–Dendy multiplate samplers. We complemented null hypothesis testing by calculating standardized effect sizes (Cohen’s d) and their confidence intervals (CIs) to rank 14 benthic metrics and the 3 sampling techniques. Despite having higher variance than metrics from brick or Hester–Dendy samplers, metrics from kick-net samples better separated sites than did metrics from bricks or Hester–Dendy samples. Metrics from brick samples separated sites more often and by a larger number of standard deviations than did metrics from Hester–Dendy samplers. Metrics that included mayfly abundance or richness produced the largest d-values, particularly when calculated from kick-net samples. Metric rankings were inconsistent among techniques. Successional changes over the 30-d study were subtle or absent, but generally consistent among sampling techniques. Differences detected with few replicate kick-net samples were consistent in direction but smaller than differences detected with more replicates. In some cases, differences with large d-values were not detectable with small sample sizes and standard null-hypothesis-testing approaches. These differences were consistently confirmed by addition of replicates. d-values and their CIs add value to data sets, particularly given the small number of replicates common in labor-intensive ecological studies. This approach expresses biological differences in a common currency that can be compared across studies regardless of units of measurement, scale, or technique.

Consequences of acid mine drainage for the structure and function of benthic stream communities: a review

Streams affected by acid mine drainage (AMD) are highly stressed ecosystems and occur worldwide. These streams typically have low pH, high concentrations of dissolved metals, and substrata coated with metal hydroxide precipitates. This combination of chemical and physical stressors creates a challenging environment for aquatic biota. We provide a synthesis of the effects of AMD on stream food webs to provide a holistic perspective of these highly stressed ecosystems. First, we reviewed the effects of AMD on the structure and function of algal, microbial, invertebrate, and fish communities. Then, we used this published information to propose generalized food webs and identify areas for future research. In general, AMD-affected streams have depauperate communities that are dominated by a few tolerant species, and ecosystem processes (e.g., decomposition) are often impaired. Biota respond differently to the individual stressors (e.g., pH compared to precipitates), which may complicate remediation efforts that focus primarily on neutralizing acidity and removing metals from mine discharges. Food webs in these streams are substantially altered because basal resources are less productive or inaccessible, microbial processing of organic matter is slow, many grazers and shredders are absent, and fish are replaced by invertebrates as top predators. Structurally, declines in species diversity and the loss of fish shorten and simplify food webs by decreasing the number of interactions among species. Functionally, most energy pathways are weakened by disrupted trophic links, and this problem should to be a key target of restoration efforts. We think research that focuses on species interactions in a foodweb context is needed to provide a better understanding of community organization and functioning in these highly stressed ecosystems.

Learning to Avoid Dangerous Habitat Types by Aquatic Salamanders,Eurycea tynerensis

AbstractThere should be intense selection for predation avoidance mechanisms when prey live in close proximity to their predators. Prey individuals that can learn to associate habitat features with high levels of predation risk should experience increased survival if they subsequently avoid those habitats. We tested whether or not habitat learning occurred in a benthic stream community consisting of adult Oklahoma salamander (Eurycea tynerensis) prey and a syntopic predatory fish, the banded sculpin (Cottus carolinae). We exposed individual salamanders to chemical stimuli from sculpin, non‐predatory tadpoles, or a blank control in training tanks containing either rocks or grass. Two days later, the salamanders were tested in tanks that offered a choice of rocks or grass. Salamanders showed significant avoidance of the habitat where they had previously encountered chemical cues from sculpin in comparison to the non‐predatory controls. Learning to avoid dangerous habitats may be particularly important for prey whose predators are visually cryptic ambush foragers, such as sculpin.

Effects of clogging on stream macroinvertebrates: An experimental approach

The influence of streambed sediment clogging on macroinvertebrate communities was investigated in the Lemme creek (NW Italy). To assess how fine sediment accumulation can influence the colonisation process and community composition of macroinvertebrates, we placed 48 traps in the riverbed. The traps consisted of boxes built with metal net (mesh 1 cm, height 15 cm, sides 5 cm) covered with nylon net except for the apex, allowing access exclusively from the top. We created four trap types filled with 100% gravel, 30% sand and 70% gravel, 70% sand and 30% gravel and 100% sand. After 20 and 40 days, we removed 6 traps/type. Macroinvertebrates rapidly colonised the traps, as we found no significant community differences between the two removal dates. Among the four trap types, we found significant differences in taxa number and abundance, which both decreased with increasing clogging. Thus, our study supports the hypothesis that clogging and the accumulation of fine substratum elements strongly affects benthic stream communities.

Impact of acid mine drainage on benthic communities in streams: the relative roles of substratum vs. aqueous effects

Restoration of streams impacted by acid mine drainage (AMD) focuses on improving water quality, however precipitates of metals on the substrata can remain and adversely affect the benthos. To examine the effects of AMD precipitates independently of aqueous effects, four substrata treatments, clean sandstone, clean limestone, AMD precipitate-coated sandstone and coated limestone, were placed in a circumneutral stream of high water quality for 4 weeks. Iron and Al were the most abundant metals on rocks with AMD precipitate, and significantly decreased after the exposure. Precipitate on the substrata did not significantly affect macroinvertebrate or periphyton density and species composition. In an additional experiment, percent survival of caged live caddisflies was significantly lower when exposed in situ for 5 days in an AMD affected stream than in a reference stream. Caddisfly whole-body concentrations of all combined metals and Fe alone were significantly higher in the AMD stream. Whole-body metal concentrations were higher in killed caddisflies than in live, indicating the importance of passive uptake. The results suggest the aqueous chemical environment of AMD had a greater affect on organisms than a coating of recent AMD precipitate on the substrata (ca. 0.5 mm thick), and treatment that improves water quality in AMD impacted streams has the potential to aid in recovery of the abiotic and biotic benthic environment.

Predator-prey interactions in a benthic stream community: a field test of flow-mediated refuges.

Ecological theory suggests that the impact of predation can be strongly modified by the existence of regions of the environment in which prey are less accessible to predators, which underscores the need for empirical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic microhabitats with high flows provide suspension-feeding larval black flies (Simulium␣vittatum) with a spatial refuge in which the negative impact of predatory flatworms (Dugesia dorotocephala) is reduced. We conducted a short-term field experiment in Chester Creek (southeastern Pennsylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in flatworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sites with high flows to provide larvae with a refuge from these predators. Multiple regression analysis revealed that the final abundance of larvae exhibited a significant negative relationship to flatworm abundance and a significant positive relationship to current speed. After adjusting for variations in elapsed time and initial larval abundance, flow and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundance and flow had two components: a positive, direct effect of flow on larvae, which arises because these food-limited consumers prefer to reside within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larvae, which combine to yield a positive indirect effect of flow on larvae. This indirect effect demonstrates the existence of flow-mediated refuges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportion of total variation in larval abundance. A consideration of biomechanical relationships suggests that microhabitats with high flows are likely to create prey refuges in a wide range of freshwater and marine benthic environments. In particular, predators will often experience greater dislodgement forces than prey because of their larger size and because they project farther above the bed where current speeds are faster. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi- sessile.

Competition and the Structure of a Benthic Stream Community

The mechanisms structuring benthic stream communities composed largely of highly mobile taxa are poorly known. I studied the role of competition in structuring the benthic community of hard substrates in a coldwater stream dominated by mobile insects that grazed upon periphyton and by relatively sessile filter—feeding insects. I expected competitive interactions among periphyton grazers to be significant and chronic because the community was not subject to frequent flow—related disturbances and grazer densities were high despite consistently low periphyton availability. Laboratory and field experiments were performed to determine: (1) the potential for, and relative strengths of, competition within and between two common, widely distributed grazers, and caddisfly Glossosoma nigroir and the mayfly Baetis tricaudatus, (2) the role of Glossosoma in affecting the organization of the community, and (3) the mechanisms of competition. Strong intraspecific effects on Baetis survivorship were observed in laboratory enclosures within the range of natural densities, but interspecific effects were not detected. Glossosoma survivorship was not affected by competition with itself or with Baetis. Individuals of both species experienced significant intra— and interspecific effects on growth, but the relative strengths of these effects differed between species. The effects of Baetis on Glossosoma growth were stronger than the effects of Glossosoma on itself. In contrast, Baetis growth was much less affected by the presence of Glossosoma than by conspecifics. Both species strongly depressed periphyton abundance, indicating that observed competitive effects resulted, in part, from exploitation. Exclusion of Glossosma from habitats in the field resulted in a marked increase in periphyton biomass throughout a 10—mo study in which the movements of other taxa were not restricted. Most periphyton grazers responded to this increase in resource availability with increased densities in the Glossosoma exclusion treatment relative to the control. The size distributions of several chironomid grazers suggested that larval growth rates were greater in the absence of Glossosoma than with Glossosoma present. The larvae of two relatively small—bodied filter—feeders (Simulium, Rheotanytarsus) responded positively to the removal of Glossosoma, but larger bodied species (Hydropsyche, Brachycentrus) were not affected. There was not strong evidence for seasonal variation in the intensity of competition. Effects of Glossosoma on filter—feeders appeared to result from a form of interference competition, but there was no evidence for the operation of interference among grazers. Glossosoma had no effect on the colonization of habitats by grazers, and the feeding rates of Baetis and Glossosoma were not affected by the presence of conspecifics or heterospecifics. Contrasting patterns in periphyton abundance between the laboratory and field experiments suggest that, when their movements are unrestricted, Baetis departs periphyton patches at higher food densities than does Glossosoma to search for food elsewhere. Thus, strong competitive effects of Baetis on Glossosoma in the laboratory experiments resulted, in part, from limited Baetis dispersal in the enclosures. Baetis and Glossosoma appear to exhibit trade—offs in feeding efficiency and the costs of exploiting food patches that should help to ensure their coexistence on a spatially variable food resource. Results of this study suggest that the structure of benthic communities in coldwater streams composed largely of mobile taxa may often be strongly influenced by Glossosoma. A similar role has been observed for sessile, territorial grazers in other systems. Glossosoma's dominance does not result from aggressive behavior, but from physiological, behavioral, and life cycle attributes that allow it to regulate periphyton biomass throughout the year.

Flow Mediates Predator-Prey Interactions between Triclad Flatworms and Larval Black Flies

The velocity preferences of suspension-feeding larval black flies (Simulium vittatum) and predatory triclads (Dugesia dorotocephala), both alone and together, were determined in a laboratory flow chamber in which a gradient of current velocities was maintained and manipulated (...) These results clearly demonstrate the potential importance of flow in mediating predator-prey interactions in benthic stream communities

Effect of hydropower peaking flow fluctuations on community structure and feeding guilds of invertebrates colonizing artificial substrates in a large impounded river

Artificial substrates were used to monitor the invertebrate communities below a power peaking impoundment and a flow re-regulating impoundment on the Missouri River on northeastern Nebraska, USA. Invertebrate communities on shallow samplers subjected to exposure from diel fluctuations in flow averaged 3 taxa per sampler and 91 organisms per square meter. In the absence of diel fluctuations, number of taxa per sampler increased to 12 and mean densities increased to 743 per square meter. Fluctuating discharges had no significant effect on numbers of taxa or densities on continually submerged artificial substrates. However, greater numbers of taxa and total densities were observed on deep (107 cm) versus shallow (30 cm) samplers below both impoundments. Polycentropodidae (Trichoptera), Chironomidae (Diptera) and Oligochaeta were observed to tolerate diel fluctuations and exposure below Fort Randall Dam while Hydropsychidae (Trichoptera) and Heptageniidae (Ephemeroptera) were extremely intolerant. Collector-gatherers and predator-engulfers were the predominant functional groups colonizing samplers below both impoundments. Higher numbers of collector-gatherers, collector-filterers and scrapers were observed from samplers in the absence of diel flow fluctuations. This study corroborates results from a number of other investigations, separated in space and time. Examined collectively, these studies provide the framework for understanding power peaking as a stress to benthic stream communities.

Competition for Space, Disturbance, and the Structure of a Benthic Stream Community

Interspecific competition for space limits the distribution and abundance of many benthic insects on stones in a western Montana stream. The sessile caddisfly larva Leucotrichia pictipes is territorial and aggressively eliminates conspecifics and other species from its foraging territories. Territoriality produces an intraspecific pattern of regular spacing and negative spatial associations between Leucotrichia and several other sessile insects: Parargyractis confusalis, Rhetanytarsus sp. and Eukiefferiella sp. Experimental removals of Leucotrichia resulted in higher densities of other sessile species and several mobile insects, including Baetis, Glossosoma, and Simulium. These insects show broad microhabitat overlap with Leucotrichia; competition with Leucotrichia limits their distributions and abundances within otherwise suitable microhabitats. Leucotrichia is the only species that monopolizes large areas of space. Physical disturbances interrupt the formation of competitive monopolies by Leucotrichia. During seasonal reduced flows, Leucotrichia is eliminated from stones and boulders in shallow water. Greater densities of a short—lived sessile species with multiple annual generations occupy this ephemeral spatial resource. On permanently submerged stones, Leucotrichia competitively affects a large part of the benthic fauna. On small stones that overturn with higher frequencies during flooding, Leucotrichia densities are reduced and species' abundance are highly equitable. Larger, more stable substrates with greater densities of Leucotrichia are characterized by lower species evenness.

Secondary Production of Epilithic Fauna in a Woodland Stream

Epilithic fauna were sampled by placing three sets of 40 cobbles in transects across a riffle in Factory Brook, Massachusetts. One set was sampled monthly from September 1973 to August 1975, allowing 3 months for recolonization. Three Surber samples were collected concurrently during the last 12 months to provide biomass estimates of infauna. Length-wet weight regression equations and dry weight conversion factors were computed for genera; caloric equivalents were obtained from the literature. Surface area of cobbles was determined by a foil-areagraph technique. Secondary production on cobbles was calculated mainly by genus with the modified Hynes method. Mean standing stock in dry weight (DW) was 0.65 gDW/m2 of cobble surface in 1974 and 0.61 gDW/m2 in 1975. Insect production on cobbles in 1974 was 4.80 gDW/m2-year and 4.32 gDW/m2-year in 1975, or between 22.3-24.7 kcal/m2-year. Approximately half of the mean biomass and production on cobbles was attributed to four genera, Epeorus, Ephemerella, Hydropsyche and Acroneuria. Dipterans constituted 56% of the organisms collected but less than 20% of production; mayflies represented 53% of epilithic production. Annual turnover ratios were 7.4 and 7.1 in 1974 and 1975, respectively, and are a first estimate of community-based P/B for stream benthos in New England. Trophic level production efficiency between primary and secondary consumer groups agreed closely with previous studies on benthic communities in streams and may provide a simplified method for estimating secondary production. Total benthic production on the riffle, based on Surber and cobble sampling, was approximately 12.2 gDW/m2-year.