Hydraulic microhabitats and the distribution of macroinvertebrate assemblages in riffles

1. The influence of hydraulic conditions on the spatial distribution of macroinvertebrate assemblages was investigated in three riffles in a perennial Australian river. 2. Velocity, depth and variability of substrate roughness were measured at each of 56 macroinvertebrate sampling locations. Complex hydraulic variables (roughness Reynolds number, shear velocity, Froude number) were calculated from combinations of two or three of the directly measured variables. The biological significance of directly measured and complex hydraulic variables was determined by a combination of univariate and multivariate statistical procedures. 3. Macroinvertebrate abundance, number of taxa and community composition were significantly different between the identified roughness Reynolds number, Froude number, velocity and shear velocity microhabitats throughout the studied riffles. 4. Regression analysis showed macroinvertebrate abundance and number of taxa were negatively related to roughness Reynolds number, shear velocity, velocity and Froude number. Depth was negatively related to abundance. In general, the majority of the macroinvertebrate community preferred the areas of riffles with the lowest near‐bed turbulence. 5. Roughness Reynolds number explained more of the spatial variation in invertebrate abundance, number of taxa and community composition than the other hydraulic variables, either directly measured or calculated. Of the directly measured variables, velocity had the greatest explanatory power, which was marginally less than roughness Reynolds number and shear velocity. 6. This study demonstrated that small‐scale differences in hydraulic conditions created by combinations of velocity, depth and substrate roughness have an important role in the spatial distribution of macroinvertebrate assemblages in riffle habitats.     

Association of macroinvertebrate assemblages with dissolved oxygen concentration and wood surface area in selected subtropical streams of the southeastern USA

Woody debris (CWD) is an important habitat component in northern Gulf of Mexico coastal plain streams, where low gradients and low flows allow accumulation of CWD and promote low dissolved oxygen (DO) concentrations. We tested the influences of CWD and DO on stream macroinvertebrates experimentally by placing two surface area CWD treatments each in three concentrations of ambient DO in two streams in Louisiana, USA, with macroinvertebrates collected from ambient woody debris used as a control. We also sampled macroinvertebrates in benthic and woody debris habitats in three streams twice yearly over 2 years to examine the applicability of the experimental results. Total abundance, richness (generic), and Shannon–Wiener diversity were all higher in lower DO conditions during the experiment, and total abundance was higher in the larger CWD treatment. Stream sampling corroborated the relationship between higher diversity and low DO in both benthic and woody debris habitats, but the relationship between richness and low DO only was supported in benthic habitats. Few taxa correlated with DO or CWD in the experiment (5 of 21 taxa) or stream survey (2 of 54 taxa). Whereas most taxa were uncorrelated with experimentally manipulated and in-stream measured variables, we suggest these taxa respond as generalists to stream habitat and physicochemistry. Based on this experiment and stream sampling, we believe the majority of macroinvertebrates in these streams are tolerant of seasonally low DO conditions.     

Effects of channelisation on stream habitat in relation to a spate and flow refugia for macroinvertebrates in northern Japan

1. The effects of channelisation on macroinvertebrates were examined in relation to a spate and flow refugia. Habitat components that can function as flow refugia were identified in a small, low‐gradient stream in northern Hokkaido, Japan. 2. Macroinvertebrates and their habitat characteristics (depth, current velocity and substratum) were sampled and measured in natural and channelised sections on three occasions: before, during and immediately after a spate. For macroinvertebrate sampling and habitat measurements, five (riffle, glide, pool, backwater and inundated habitats) and three (channelised‐mid, channelised‐edge and inundated habitats) habitat types were classified in the natural and channelised section, respectively. 3. The rate of velocity increase with discharge was compared among habitat types to determine which habitat types were less affected by increased discharge. The rate was the highest in riffles followed by glides and channelised‐mids. Backwaters maintained low current velocity even at high flow. In addition, current velocity in both natural and channelised inundated habitats was low relative to other habitat types during the spate. 4. Through the spate, total density of macroinvertebrates in channelised‐mids and taxon richness in both channelised‐mids and edges decreased. In the natural section, however, such a significant decrease was not found except for taxon richness in pools. This indicated that the spate had a greater impact on assemblages in the channelised section. Riffle assemblages exhibited a rapid recovery immediately after the spate, suggesting the existence of flow refugia in the natural section. Among the habitat types we examined, backwaters and inundated habitats appeared to have acted as flow refugia, because these habitats accumulated macroinvertebrates during the spate. 5. The lower persistence of the macroinvertebrate assemblage in the channelised section was attributable to the lower availability of flow refugia such as backwaters and inundated habitats. Our results emphasised the importance of considering flow fluctuations and refugia in assessing the effects of channelisation. In addition, the lateral heterogeneity of stream channels should be considered in stream restoration and management.     

Temporal shifts in physical habitat of the crayfish,Orconectes neglectus (Faxon)

A total of 300 samples was collected from February 1985 to August 1986 in a medium order Ozark Mountain stream. Physical habitat measurements of temperature, mean water column velocity, depth, and substrate character were recorded for each of the 25 monthly samples along with length and sex of all individuals of Orconectes neglectus (Faxon). Analysis of habitat utilization and suitability (or preference) was conducted using exponential polynomial models of hydraulic stress models. There appeared to be equal preference for depth over the range measured. Both substrate and velocity preference curves were bimodal with each mode designating certain crayfish size classes. Young-of-the-year were found primarily in cobbled, high velocity areas while adults were found in low velocity, macrophyte beds. Utilization curves for laminar sublayer thichness also reflected size-dependent phenomena where young-of-the-year were found in thin sublayer areas and adults were found primarily in thick sublayers. When separated by time and size, adults were found to occupy higher velocity, cobbled habitats during at least two months. This time period corresponded with the time of egg-bearing and further analysis yielded a time-dependent habitat suitability surface which accounted for this movement pattern. We suggest that the application of these suitability surfaces, which reflect habitat changes during the annual life cycle, will produce more accurate predictions of density and will allow better habitat management decisions under various regulated flow scenarios.     

Spatial and temporal variability of macroinvertebrates in spawning and non-spawning habitats during a salmon run in Southeast Alaska

Spawning salmon create patches of disturbance through redd digging which can reduce macroinvertebrate abundance and biomass in spawning habitat. We asked whether displaced invertebrates use non-spawning habitats as refugia in streams. Our study explored how the spatial and temporal distribution of macroinvertebrates changed during a pink salmon (Oncorhynchus gorbuscha) spawning run and compared macroinvertebrates in spawning (riffle) and non-spawning (refugia) habitats in an Alaskan stream. Potential refugia included: pools, stream margins and the hyporheic zone, and we also sampled invertebrate drift. We predicted that macroinvertebrates would decline in riffles and increase in drift and refugia habitats during salmon spawning. We observed a reduction in the density, biomass and taxonomic richness of macroinvertebrates in riffles during spawning. There was no change in pool and margin invertebrate communities, except insect biomass declined in pools during the spawning period. Macroinvertebrate density was greater in the hyporheic zone and macroinvertebrate density and richness increased in the drift during spawning. We observed significant invertebrate declines within spawning habitat; however in non-spawning habitat, there were less pronounced changes in invertebrate density and richness. The results observed may be due to spawning-related disturbances, insect phenology, or other variables. We propose that certain in-stream habitats could be important for the persistence of macroinvertebrates during salmon spawning in a Southeast Alaskan stream.     

You don’t belong here: explaining the excess of rare species in terms of habitat,space and time

Ecological communities are composed of a few common and several rare species. Many studies have evaluated the shape of abundance distribution curves, but few studies have assessed the causes of rarity. Using a dataset of stream macroinvertebrates, we investigated whether the excess of rare species in three focal communities of stones in riffles were common 1) in other habitats at the same stream site and period of sampling (environment), 2) in other stream sites in the same habitat and period of sampling (space), and 3) in other years in the same stream site and habitat (time). We observed that around 28% of the rare species were common in other habitats (environment), stream sites (space) or years (time). Among the three factors, rarity was mostly explained by habitat type, whereas a significant portion of the rare species in riffles were common in pools, submerged roots of terrestrial plants or in partially submerged moss patches. This result suggests that the presence in non‐optimum habitat is a strong determinant of the rarity observed in natural communities and most rare species are due to sampling artifacts or accidentally sampled transient species.     

An experimental test of the effects of inorganic sediment addition on benthic macroinvertebrates of a subtropical stream

Inorganic sediments of terrestrial origin may impact stream macroinvertebrate communities. Although input of terrestrial sediments to streams may occur naturally, human-induced activities in the catchment amplify this input greatly. We used an in-stream experiment to investigate whether short-term additions of terrestrial sediments of two size classes affected stream macroinvertebrates. The experiment was designed in blocks to minimize the influence of flow velocity and other environmental variables. Four treatments were employed: (i) addition of fine sand (0–0.24 mm), (ii) coarse sand (0.25–0.8 mm), (iii) fine+coarse sand, and (iv) control (water only). Macroinvertebrates were sampled immediately after the addition of sediments (or water). The experiment consisted of 20 blocks. We analyzed the response of the macroinvertebrate fauna in terms of abundance and species richness. Since species richness is strongly dependent on number of individuals sampled, we also analyzed rarefied species richness. Community structure was evaluated using a distance-based Manova on presence/absence and abundance data. The addition of coarse and fine+coarse sand reduced the abundance and species richness of macroinvertebrates in relation to the control. The response in terms of rarefied species richness in the treatments did not differ from the control, indicating that reduction in species richness was a sampling artifact resulting from decreased sample abundance. The Manova analyses indicated that coarse-sand addition caused changes in both species composition and community structure. Addition of fine and fine+coarse sand affected only slightly species composition and community structure. We concluded that even short-term input of terrestrial sediments causes impacts on benthic macroinvertebrates, and recommend that land-use management of tropical catchments should employ practices that reduce input of terrestrial sediments to streams. Handling editor: K. Martens     

Microhabitat preferences of benthic fauna in a woodland stream

Estimates of numbers, biomass, and diversity of benthic macroinvertebrates were made quarterly over a two-year period to investigate microhabitat preferences. Although biomass of most taxa was significantly different among sampling times, physical factors also appeared to be important in determining abundance of many taxa. Optimum depth, velocity, substrate type, and turbulence were determined for major taxa. Optimum conditions for diversity appeared to be 34 cm depth, 60 cm s^?1 velocity, and rubble and boulder substrate type. Habitat preference functions were derived for several taxa based on significant polynomial regressions of biomass on depth, velocity, substrate, and Froude number (turbulence). The relationship between abundance and physical habitat conditions was tested by using the product of the preference factors (range: 0–1) for depth, velocity and substrate type as a measure of habitat suitability (joint preference factor). There were significant correlations between biomass [transformed by log_e (x + 1)] of 10 benthic species and the joint preference factor. The joint preference factors accounted for from 11 to 61% of the variation of biomass of the 10 benthic species. The intercepts of the relationships between biomass of individual species and the joint preference factor were not significantly different from zero for any species. Therefore, the joint preference factors appear to be valid indicators of biomass. The preference functions have utility in habitat assessment studies, specifically with regard to minimum instream flow determinations.     

The influences of habitat and seasonal sampling regimes on the ordination and classification of macroinvertebrate assemblages in the catchment of the River Wye,Wales

Assemblages of benthic macroinvertebrates were assessed in relation to environmental variables at 45 sites in the catchment of the Welsh River Wye in 1982.Samples were collected from three habitats (margins, riffles, flats) and in two seasons (Spring and Autumn), separated and combined data sets being ordinated by DECORANA and classified by TWINSPAN. Correlations between primary ordinations were high (p < 0.001) and always strongly related to total hardness or pH; secondary ordinations were also intercorrelated and were related to stream slope.In each instance, total hardness and slope were the strongest discriminants between site groupings produced by TWINSPAN. It is concluded that relationships between faunal assemblages, stream chemistry and stream physiography would be effectively detected in the Wye catchment in either Spring or Autumn and in each of the habitats sampled. However, the most precise categorization of assemblage type required a sampling strategy which combined habitat and seasonal data.     

An ecologically useful classification of mean and near-bed flows in streams and rivers

SUMMARY. 1. Mean motion and near-bed flows in streams and rivers can be described using a classification derived from fairly simple field measurements. Our proposed classification is ecologically useful because it incorporates the combined effects of velocity, depth and substrate roughness to provide a means of quantifying the flow regimes occurring within the microhabitats of stream benthos. 2. Mean motion is characterized by the Reynolds number and the Froude number. Both are easily calculated, and because they are dimensionless they provide a means of comparing flows at different sites. 3. Five categories of near-bed flows (i.e. the flow microenvironments of stream benthos) are recognized. Flow may be hydraulically smooth or hydraulically rough and the latter category is subdivided further into: chaotic flow, wake interference flow, isolated roughness flow and skimming flow. Hydraulically smooth flows occur in sections of a river bed with fine sediments (e.g. sands, muds and clays). over flat sheets of bedrock, or in association with the flat blades of submerged macrophytes. Hydraulically rough flows occur where the substrate elements are larger (e. g. pebbles, cobbles and boulders) and are a function of substrate roughness and the depth of flow relative to the height of the roughness elements. Chaotic flows and wake interference flows predominate in riffles whilst isolated roughness flows and skimming flows are more likely to be a feature of runs. 4. Conventional stream sampling methods (e.g. the Surber and box or cylinder samplers) may collect across several different flow microhabitats. Our classification should enable different flow microenvironments to be recognized and so sampled more appropriately which, in turn, may reduce apparent clumping and the wide confidence intervals of benthic population estimates. Because our classification identifies ‘patches’ within the flow regime associated with the stream bed it enables stream ecologists to generate testable hypotheses regarding the distribution and abundance of benthic species in response to flow. 5. Our classification identifies spatial patterns in the flow regimes associated with the stream bed. Temporal patterns have not been identified: however, predictable changes in spatial patterns will resuh from temporal changes in stream discharge.     

Presence or absence of fish as a cue to macroinvertebrate abundance in boreal wetlands

Waterfowl that eat macroinvertebrates must select among potential nesting or brood-rearing habitats that may vary in food abundance over the season. We compared the reliability of predicting the relative abundance of macroinvertebrates in boreal wetlands using either the number of macroinvertebrates collected at one sampling period, or presence or absence of fish. Wetlands with fish had fewer macroinvertebrates than fishless wetlands in all five sampling periods. Predictions of the relative abundance of invertebrates in a wetland at other sampling periods based on the presence or absence of fish, were equal to or better than predictions based on the actual number of macroinvertebrates collected during one sampling period. These results suggest that fish status of a wetland is a reliable cue to invertebrate abundance in boreal wetlands.     

Implications of a Hierarchical Relationship Among Channel Form,Instream Habitat,and Stream Communities for Restoration of Channelized Streams

Hierarchy theory provides a conceptual framework for understanding the influence of differently scaled processes on the structure of stream communities. Channel form, instream habitat, and stream communities appear to be hierarchically related, but the strength of the relationships among all components of this hypothesized hierarchy have not been examined. We sampled channel form, instream habitat, fishes, and macroinvertebrates in a channelized stream in Mississippi and Alabama to examine the hypothesis that a hierarchical relationship exists among channel form, instream habitat, and stream communities. Instream habitat, fishes, and macroinvertebrates were sampled in May, July, and September 2000. Measurements of channel form were obtained in July 2000. Mantel tests, multiple regressions, and correlation analyses were used to assess strength of the relationships among channel form, instream habitat, and stream communities. Positive correlations were observed between channel form and instream habitat, and correlations observed between these factors were the greatest observed in our study. Overall, fish and macroinvertebrate communities exhibited stronger relationships with instream habitat than with channel form. Species richness, evenness, and abundance tended to exhibit greater correlations with instream habitat, while species composition had greater correlations with channel form. We concluded that channel form, instream habitat, and stream communities were hierarchically related.     

The fish assemblage on a coralligenous shallow shelf off the Mediterranean coast of northern Israel

The fish assemblage on the shallow coralligenous shelf (16-30m depth) off Haifa, Israel was sampled using trammel nets throughout a period of 1 year. Complementary data were obtained via underwater censuses of fish on an artificial reef established later in the sampling area. Fortythree species of fish were sampled by trammel nets, 79% of which were observed also during the underwater censuses. Although fish of Red Sea origin constituted only 11.6% of the species composition in the net samples, they contributed 46.2% of the fish abundance and 40.6% of the biomass in these samples. This was supported by the finding that species of Red Sea origin contributed 64% of the abundance of large fish counted on the artificial reef. Siganus luridas, S. rivulatus and Sargocentron rubrum are the main contributors in number and biomass among fish of Red Sea origin. It is suggested that the biogenic rocky bottom of this area contains several components which are similar to biogenic habitats favoured by these benthic species in the Red Sea; this may explain the high abundance of these species in the studied area.     

High spatial variability biases the space-for-time approach in environmental monitoring

The space-for-time approach is widely used in fundamental and applied ecology but assemblages from some habitats are highly variable. For example, streams may show marked spatio-temporal changes in the taxonomic composition of the macroinvertebrate assemblages. We exemplify the effect of the temporal component ‘season’ on some assemblage-derived stream quality assessment metrics under the assumptions of the space-for-time and the replicated samples approaches. Benthic macroinvertebrates were sampled in spring, summer, and fall from two stream types, namely streams in the Pleistocene sediments of the alpine foothills and small fine substrate dominated siliceous highland rivers in southern and central Germany. As exemplified for ASPT and the German multimetric index (MMI), the data showed no effect of season when samples were regarded as independent, whereas stream quality decreased between spring and fall in the replicated samples approach. The transformation of MMI to rank-ordered stream quality classes depicted a decrease in perceived stream quality in 29% and 54% of the sites by summer and early fall, respectively, when compared to spring samples. We thus suggest (1) to test seemingly robust metrics in a repetitive measures approach for other stream types and regions, and (2) to standardize the sampling season for ecological quality assessment. Based on this example, we assume that many subtle, but significant, environmental trends are still to be detected in highly heterogeneous habitats from various ecosystems.     

Effects of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM) on the macroinvertebrates within the hyporheic region of a small stream

The effect of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM), on the benthic macroinvertebrates within the hyporheic region of Dam Creek, Ontario was investigated. Organisms were regularly found to a depth of 70 cm in the substrate throughout the study period. Of the eight major taxa found at both the treated and untreated sites only Tubificoidea exhibited a decrease in abundance attributable to TFM.One day after treatment, TFM concentrations were greatest at a depth of 55 cm, the greatest depth to which water samples were taken. The movement of TFM into the hyporheic region during the present study may be due to the large convective forces created by the rapid decrease in surface water temperature. These convective forces are usually greatest in the late fall and winter when TFM is not applied. The greater part of TFM stream treatments are carried out during the summer when surface water temperatures remain fairly constant. It is suggested that the hyporheic region may act as a refuge zone for benthic macroinvertebrates against the lampricide during most of the TFM application season.     

Hydrology and substrates: determinants of oligochaete distribution in lowland streams (The Netherlands)

In most soft-bottomed, lowland streams in the Netherlands discharge regimes largely follow the precipitation pattern. Winter discharges are higher and much more dynamic then summer discharges, although rain storms throughout the year cause unexpected peak flows. Minimal precipitation, reduced stream flow and droughts can occur during the summer months. Lowland stream habitat, particularly in The Netherlands, is hydrologically dynamic, with substrates frequently moved or disturbed. Differences in discharge patterns in Dutch soft-bottomed lowland streams are expected to affect distribution patterns of macroinvertebrates and thus oligochaetes. Ten small to medium sized lowland streams, differing from one another in hydrological regime, were studied. Five major habitats in each stream were assessed on three occasions over a 15-month period. Each habitat sampled with a micro-macrofauna shovel; during each sampling period, several environmental parameters, especially hydrological and substrate parameters, were measured. Ordination (CANOCO) (Ter Braak, 1989) and statistical tests (chi-squared test) (Lindgren & McElrath, 1970) were used to determine the major oligochaete distribution patterns between streams as well as between habitats within each stream. Each stream was characterized by its oligochaete assemblage; general distribution patterns and individual oligochaete–substratum relationships were documented. Hydro-morphological parameters together explained the differences in major distribution patterns. Preferences of oligochaetes for specific structural habitats are discussed.     

Do stream macroinvertebrates use instream refugia in response to severe short-term flow reduction in New Zealand streams?

1. Demand for water is increasing and water managers need to know how much they can remove from a stream before there are significant detrimental effects on its biological integrity. Flow reduction alters a number of habitat variables known to be important to aquatic invertebrates such as depth, velocity, temperature and fine sediment accumulation. Some taxa may attempt to use instream refugia to mitigate the effects of flow reduction.
2. We experimentally manipulated flows by constructing weirs and diversions in three small New Zealand streams. Discharge was reduced by 88–96%. We tested the hypothesis that macroinvertebrates would use pools and the hyporheic zone as refugia during short-term (1-month) periods of reduced flow.
3. We sampled hyporheic invertebrates with colonization chambers and pool invertebrates with kick nets within a before-after, control-impact (BACI) experimental design. A suite of physicochemical parameters was measured concurrently including surface and hyporheic temperatures.
4. Flow reduction significantly decreased velocity (60–69%) in all streams. Depth (18–61%) and wetted width (24–31%) tended to decrease but these changes were not always significant. Sediment cover increased the most in farmland streams (10–80%). Apart from decreasing temperature range (18–26%), flow reduction had little impact on the surface water temperatures.
5. Flow reduction had no impact on the abundance of common pool macroinvertebrates or on the abundance, vertical distribution or community composition of hyporheic macroinvertebrates.
6. Our results suggest that aquatic macroinvertebrates are resistant to short-term, severe flow reduction as long as some water remains.     

Temporally variable macroinvertebrate–stone relationships in streams

Stones were used to sample macroinvertebrates and characterise microhabitats at monthly or bimonthly intervals in six Ecuadorian streams covering a gradient in four different stability measures and other stream characteristics. The physical variables current velocity, water depth, horizontal position, embeddedness and size were measured to characterise stone microhabitats and presumed to be affected by or related to physical impact during hydrological disturbances. My first objective was to analyse how density, the number of families and a richness measure (residuals from a power regression of families vs. individuals) were related to the physical characteristics of individual stone habitats. My second objective was to quantify temporal variability in fauna–stone relationships and to analyse if such variability was related to overall stability of stream reaches. Partial Least Squares (PLS) multiple regression analyses showed high temporal variability between sampling dates in factor loadings of specific stone micro habitat variables. In spite of this, there was a clear negative effect of depth and a positive effect of current on density and number of families. Stone size was consistently negatively related to density and positively related to number of families. Patterns were less clear for richness residuals. Simple linear regressions of fauna vs. stone parameters generally confirmed the results reached by the PLS analysis, although few of the regressions were significant. For all fauna–stone regressions the variability in slopes was much higher among sampling dates within streams (temporal variability) than among streams (spatial variability), and significant slopes were even inverted on different sampling dates. Although the coefficients of variation (CV) of slopes of a given combination of fauna parameter and stone variable from different sampling dates (n=9–11) were rarely correlated to any of the measures of stream stability, this study has demonstrated high temporal variability in fauna–stone relationships (CV’s of regression slopes). Consequently, temporally un-replicated studies of such relationships do not necessarily reveal general patterns.     

Distribution and abundance of macroinvertebrates within two temporary ponds

We investigated the distributions of macroinvertebrates within two temporary ponds (Spring Peeper Pond and Taylor-Ochs Pond) in central Ohio and examined what environmental factors may be driving those distributions. We sampled macroinvertebrates in Spring Peeper Pond three times from May to July 2001, and Taylor-Ochs Pond two times from May to June 2001. Macroinvertebrate distributions were significantly aggregated on all sampling dates in both ponds. Bivalve abundance in Spring Peeper Pond was higher in shallower water. The distribution of bivalves in Taylor-Ochs Pond was not correlated with any variable we measured. Dragonfly nymph abundance in Taylor-Ochs Pond decreased between the first and second sampling dates, whereas in Spring Peeper Pond no factor examined was correlated with dragonfly nymph density. Snail densities in Spring Peeper Pond were negatively related to dissolved oxygen and depth. In Taylor-Ochs Pond, snail abundance was positively related to temperature. The densities of damselfly nymphs in Spring Peeper Pond were positively related to dissolved oxygen and depth and declined across the study. In Spring Peeper Pond, hemipteran densities were negatively related to depth and increased across the study. Damselfly nymphs and hemipterans were not common enough in Taylor-Ochs to analyze. In general, the abiotic and biotic factors we examined explained relatively little (<37% in all cases) of the within pond distribution of the macroinvertebrates in our two study ponds.     

A technique for predicting in-stream flow requirements of benthic macroinvertebrates

SUMMARY. A total of 225 benthic samples was taken at various riffles along the Tongue River in Montana during three separate weekly intervals in 1975. For each sample, depth, current velocity, substrate microprofile, turbulence (Froude number), and the number and kind of species present were considered.
The conditions of highest faunal diversity were 75–125 cm/s current velocity at 20–40cm depth. The optimum condition appeared to be 76 cm/s at a depth of 28 cm over medium cobble substrates.
The centroids for optimum conditions of depth and current velocity for thirty-six common macroinvertebrates are also presented.
Representative diagrams of the tolerance ranges of some common invertebrate species are presented. Rhithrogena hageni Eaton was chosen as the indicator species since its tolerances closely matched those of the highest community diversity. The presence of Rhithrogena in future samples would indicate maintenance of adequate flow conditions for the present benthic community.
A method for applying these data to produce a minimum flow recommendation is discussed. If the optimum conditions can be plotted on composite hydrologic maps of depth and velocity in riffle areas at various discharges, one can predict the discharge at which the optimum conditions are eliminated and, thus, the recommended minimum discharge.