Invertebrate responses to short-term water abstraction in small New Zealand streams

1. Small permanent streams are coming under increasing pressure for water abstraction. Although these abstractions might only be required on a short‐term basis (e.g. summer time irrigation), the highest demand for water often coincides with seasonal low flows. 2. We constructed weirs and diversions that reduced discharge in three small streams (<4 m width) to test the hypotheses that short‐term water abstractions would decrease habitat availability and suitability for invertebrates, resulting in increased invertebrate drift, reduced taxonomic richness and decreased benthic invertebrate densities. 3. We sampled benthic invertebrates, invertebrate drift and periphyton at control (upstream) and impact (downstream) sites on each stream before and during 1 month of discharge reduction. 4. Discharge decreased by an average of 89–98% at impact sites and wetted width decreased by 24–30%. Water depth decreased by 28–64% while velocity decreased by 50–62%. Water conductivity, temperature and dissolved oxygen showed varying responses to flow reduction among the three streams, whereas algal biomass and pH were unaffected in all streams. 5. The densities of invertebrate taxa tended to increase in the impact reaches of these streams, even though invertebrate drift increased at impact sites in the first few days following discharge reduction. There were a higher proportion of mayflies, stoneflies and caddisflies at the impact site on one stream after flow reduction. There were no changes to the number of taxa or species evenness at impact sites. 6. Our results suggest that for these small streams, the response of invertebrates to short‐term discharge reduction was to accumulate in the decreased available area, increasing local invertebrate density.     

Active entry of stream benthic macroinvertebrates into the water column

Field experiments investigated the possible active entry of stream benthos into the water column. Over a 1-year period, sediment baskets were suspended for 24 hours in a stream pool so that only swimming or floating organisms, essentially unaided by current, could colonize them. A variety of benthos, including taxa characteristic of riffles, colonized the baskets, with colonization highest in late summer and negligible in winter.A modified drift net towed through the pool was used to quantitatively sample benthic animals actually in the water column. Nighttime tows captured a diverse, abundant fauna and indicated densities substantially higher than invertebrate drift densities reported in the literature. Daytime tows yielded little. Estimated percentages of the benthos in the water column at a given time were generally < 1.0%.These findings suggest that not all invertebrate drift is the result of passive mechanical removal from the substrate by current.     

Invertebrate drift lost to the sea during low flow conditions in a small coastal stream in Western Canada

The invertebrate drift leaving the lower end of a small coastal stream on the east coast of Vancouver Island was recorded during low flow conditions. 20,156 animals, weighing 0.96 g (dry weight), were collected in drift nets over a 5 day period in early summer. High and low drifting taxa are listed. Considerable daily variation in total captures occurred and was attributed to a few dominant taxa (primarily harpacticoid copepods, mites and chironomid larvae) exhibiting atypical drift patterns. 0.004% of the stream's invertebrate standing crop was estimated to be in the water column at any instant in time. The possible use of the outgoing animals as food for juvenile salmon in the estuary is discussed.     

Selective predation by reintroduced juvenile Lake sturgeon (<Emphasis Type="Italic">Acipenser fulvescens</Emphasis>) in Ft. Loudoun reservoir,Tennessee (USA)

Following water quality and minimum flow improvements to the impounded Tennessee and Cumberland Rivers, juvenile lake sturgeon (Acipenser fulvescens) have been restocked annually since 2000. Our goal was to seasonally assess foraging mode of this recovering population in Ft. Loudoun Reservoir in the Upper Tennessee River. During 2014–15, individuals were captured using trot-lines in a 13-km reach that supports the greatest density of lake sturgeon. We used colonic flushing and gastric lavage techniques to obtain diet. We took systematic benthic sediment grabs along multiple transects throughout the reach and opportunistically deployed rock cages filled with hard substrates to assess potential prey that colonize hard surfaces. Foraging modes of lake sturgeon were determined by comparing relative abundances of invertebrate taxa in the gut contents (6581 invertebrates) of 28 fish to the relative abundances of the same invertebrate taxa collected from the resource base (1667 invertebrates). Proportional similarity, Levin’s niche breadth, and Manly’s index were used to assess the degree of prey selectivity. Lake sturgeon fed selectively on a narrow range of available prey consisting mostly of larval chironomids (93% composition by number during warm season, 96% during cool season), some genera of which they prey upon selectively, primarily Chironomus sp., but to a lesser extent Procladius, Ablabesmyia, Coelotanypus, and Cryptochironomus spp. Meanwhile, other abundant taxa in the resource base were avoided, such as Oligochaetes, Hexagenia mayflies, and the chironomid Glyptotendipes. Our results illustrate that assessing seasonally available prey from habitat utilized by lake sturgeon is important when investigating diet preference.     

Benthivorous fish reduce stream invertebrate drift in a large-scale field experiment

Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or to escape after encountering a predator. While the control of the diurnal patterns of invertebrate drift activity by fish predators has received considerable interest, it remains unclear whether benthivorous fish reduce or increase drift activity. We performed a large-scale field experiment in a second-order stream to test if invertebrate drift was controlled by two benthivorous fish species (gudgeon Gobio gobio and stone loach Barbatula barbatula). An almost fishless reference reach was compared with a reach stocked with gudgeon and loach, and density and structure of the invertebrate communities in the benthos and in the drift were quantified in both reaches. The presence of gudgeon and stone loach reduced the nocturnal drift of larvae of the mayfly Baetis rhodani significantly, in contrast to the findings of most previous studies that fish predators induced higher night-time drift. Both drift density and relative drift activity of B. rhodani were lower at the fish reach during the study period that spanned 3 years. Total invertebrate drift was not reduced, by contrast, possibly due to differences in vulnerability to predation or mobility between the common invertebrate taxa. For instance, Chironomidae only showed a slight reduction in drift activity at the fish reach, and Oligochaeta showed no reduction at all. Although benthic community composition was similar at both reaches, drift composition differed significantly between reaches, implying that these differences were caused by behavioural changes of the invertebrates rather than by preferential fish consumption. The direction and intensity of changes in the drift activity of stream invertebrates in response to the presence of benthivorous fish may depend on the extent to which invertebrate taxa can control their drifting behaviour (i.e. active versus passive drift). We conclude that invertebrate drift is not always a mechanism of active escape from fish predators in natural streams, especially when benthos-feeding fish are present.     

Meiofauna constitute a considerable portion of invertebrate drift among moss-rich patches within a karst hydrosystem

Aiming to establish the most frequent invertebrate taxa in drift at the small spatial scale within a moss-rich karst tufa-precipitating hydrosystem, we sampled drift among microhabitats differing in substratum type and flow conditions along a tufa barrier-cascading lotic reach. Additionally, we addressed the question of the contribution and the potential significance of meiofauna within the overall invertebrate drift at the small spatial scale. During the study period, a total of 60 invertebrate taxa were recorded in the drift. Six of these taxa belonged to the annelid/arthropod meiofauna and they represented 35% of total drift density. Macroinvertebrates found in drift were represented mainly by larval insects. The composition of the most abundant taxa in total drift was as follows: Alona spp. (Cladocera 26.7%), Riolus spp. (Coleoptera: Elmidae 13.2%), Simulium spp. (Diptera: Simuliidae 12.2%), Enchytraeidae (Oligochaeta 10.4%), Hydrachnidia (6.3%), Orthocladinae (Diptera: Chironomidae 3.9%) and Naididae (Oligochaeta 3.6%). Faunal drift densities and amounts of transported particulate matter (PM) were highest at the fast-flowing sites located at the barriers and lowest at the slow-flowing sites within pools. Similarly to the seasonal amounts of transported PM, faunal drift was lowest in winter, and peaked in autumn and in late spring/early summer. Correlation between flow velocity and PM-faunal drift densities suggested a significant effect of the dislodged PM, though a minor influence of discharge and flow velocity on faunal drift. We suggest that the small-scale habitat heterogeneity and the respective feeding and refugial strategies of the fauna, as well as faunal passive dislodgement initiated by the shear forces of the flow were the most important drivers of observed drift patterns.     

Invertebrate drift along a longitudinal gradient in a Neotropical stream in Serra do Cipó National Park,Brazil

The diversity and composition of drift invertebrate assemblages were evaluated along a longitudinal gradient of an altitudinal stream in southeastern Brazil. The main goal of this study was to evaluate the influence of seasonality, stream order, and some abiotic factors on invertebrate drift and the use of drifting invertebrate assemblages to assess aquatic invertebrate diversity. Drift samples were collected over a 24 h period using nets (open area of 0.08 m²; mesh 0.250 mm), partially submerged (60%) in the water column. Taxonomic richness, Pielou evenness (J), Shannon–Wiener diversity (H), and total density of drift invertebrate assemblages were used in unpaired t-tests, Kruskal–Wallis and stepwise multiple regression analysis. The results showed a high taxonomic richness of aquatic invertebrates, with 91 taxa found. Chironomidae and Ephemeroptera represented together c. 80% of the total density of drift organisms. The drift approach allowed the collection of new and rare taxa, besides the knowledge of pupae stage of several chironomid genera. Significant differences in the taxonomic richness and diversity of drift invertebrate assemblages were found between the rainy and dry periods, indicating a significant influence of seasonality. An increase in water flow and electrical conductivity were associated with the increase in the taxonomic richness and diversity in the rainy period. No significant differences were found among the other abiotic variables among the stream orders.     

Effects of an experimental acid pulse on invertebrates in a high altitude Sierra Nevada stream

The effects of pulsed acidification on invertebrate densities and drift, and water chemistry, in a high altitude Sierra Nevada stream were measured using artificial stream channels. Water was diverted from the Marble Fork of the Kaweah River, California, U.S.A., through twelve replicate channels; however, low flow in the summer of 1985 eliminated all but four of these channels. Channels were stocked with natural substrates and organisms from the Marble Fork of the Kaweah River. After a three week acclimation period, we simulated a low pH rain event by adding acid (H₂SO₄ and HNO₃) to two of the channels, reducing pH to 5.0 for 6 hours. The other two channels acted as controls (pH 6.4). During acid additions, Baetis spp. drift in acidified channels was ca. 7 times higher than in control channels (F = 39.02, p < 0.025; data fourth root transformed, ANOVA), and the percentage of drifting baetids that was dead was significantly higher in acidified than control channels (46% vs. 0%, F = 29.86, p < 0.05; arcsine square root transformed data, ANOVA). Other taxa showed no significant drift responses, and benthic densities of all taxa showed no effects two days after acidification, probably owing to rapid recolonization by invertebrate drift in influent waters. Stream chemistry data are presented; heavy metal concentrations did not significantly increase in the 2 m stream channels.     

Variability in the response of amphipods and macroinvertebrate assemblage structure to prolonged drought in forested upland streams

With climate change set to increase the frequency and severity of drought in many parts of the world, there is a need to better understand the effects of drying on stream ecosystems. We investigated the long-term effects of drought on two amphipod taxa Paramoera fontana (Pontogeneiidae) and Austrogammarus australis (Paramelitidae) and macroinvertebrate assemblage structure through an analysis of 13 years of data collected from four forested stream reaches (Victoria, Australia). Abundances of A. australis and P. fontana in the lower reach of Lyrebird Creek declined to zero following surficial streambed drying. Similar declines in abundances were not observed in Sassafras Creek or the two headwater springs, which continued to flow throughout the drought. P. fontana was detected again in the lower reach of Lyrebird Creek 12 months after the final cease-to-flow, however A. australis remained undetected 5 years later, despite an upstream source population within 2 km. Both the entire and shredder macroinvertebrate assemblage structure in Sassafras Creek and the lower reach of Lyrebird Creek shifted significantly pre- and post-surficial streambed drying in the lower reach of Lyrebird Creek. Despite signs of recovery following a return to more average flows, assemblage composition remained considerably different. The substantial differences in the recovery of the two species indicates varying resistance and resilience traits. The failure of A. australis to recolonize after 5 years indicates an absence of any significant resistance or resilience traits. In contrast, the rapid re-colonization of P. fontana may indicate poor resistance traits, but strong resilience traits. The sensitivity of A. australis to cease-to-flow events points to the need to carefully manage water extraction to protect this threatened species. The effective management of macroinvertebrate assemblages in the face of drought requires a clear understanding of their response to drying, the conservation of refugia and the minimization of additional stressors which reduce ecosystem resilience.     

Species traits and channel architecture mediate flow disturbance impacts on invertebrate drift

相似文献   

Natural flow regime,temperature and the composition and richness of invertebrate assemblages in streams of the western United States

1. We tested how strongly aquatic macroinvertebrate taxa richness and composition were associated with natural variation in both flow regime and stream temperatures across streams of the western United States. 2. We used long‐term flow records from 543 minimally impacted gauged streams to quantify 12 streamflow variables thought to be ecologically important. A principal component analysis reduced the dimensionality of the data from 12 variables to seven principal component (PC) factors that characterised statistically independent aspects of streamflow: (1) zero flow days, (2) flow magnitude, (3) predictability, (4) flood duration, (5) seasonality, (6) flashiness and (7) base flow. K‐means clustering was used to group streams into 4–8 hydrologically different classes based on these seven factors. 3. We also used empirical models to estimate mean annual, mean summer and mean winter stream temperatures at each stream site. We then used invertebrate data from 63 sites to develop Random Forest models to predict taxa richness and taxon‐specific probabilities of capture at a site from flow and temperature. We used the predicted taxon‐specific probabilities of capture to estimate how well predicted assemblages matched observed assemblages as measured by RIVPACS‐type observed/expected (O/E) indices and Bray–Curtis dissimilarities. 4. Macroinvertebrate taxon richness was only weakly associated with streamflow and temperature variables, implying that other factors more strongly influenced taxa richness. 5. In contrast to taxa richness, taxa composition was strongly associated with streamflow and temperature. Predictions of taxa composition (O/E and Bray–Curtis) were most precise when both temperature and streamflow PC factors were used, although predictions based on either streamflow PC factors or temperature alone were also better than null model predictions. Of the seven aspects of the streamflow regime we examined, variation in baseflow conditions appeared to be most directly associated with invertebrate biotic composition. We were also able to predict assemblage composition from the conditional probabilities of hydrological class membership nearly as well as Random Forests models that were based directly on continuous PC factors. 6. Our results have direct implication for understanding the relative importance of streamflow and temperature in regulating the structure and composition of stream assemblages and for improving the accuracy and precision of biological assessments.     

The relation between invertebrate drift and two primary controls,discharge and benthic densities,in a large regulated river

相似文献   

Can biological traits of stream invertebrates help disentangle the effects of multiple stressors in an agricultural catchment?

相似文献   

The invasive mud crab enforces a major shift in a rocky littoral invertebrate community of the Baltic Sea

In rocky littoral communities, intense herbivory allows for the occurrence of trophic cascades where higher trophic levels influence producer communities. Invasive predators can be especially effective in imposing trophic cascades. The North American mud crab Rhithropanopeus harrisii is a recent invader in the Baltic Sea, with an expanding distribution range. Here, we document the effects of mud crab on the native invertebrate community associated with the key foundation species Fucus vesiculosus. During the initial 3 years of invasion, mud crab abundance in F. vesiculosus increased from 2 % to about 25 % of the algae being inhabited by crabs. Simultaneously, the invertebrate community underwent a major transition: Species richness and diversity dropped as a consequence of decreasing abundance and the loss of certain taxa. The abundance of gastropods decreased by 99 % and that of crustaceans by 75 %, while chironomids completely disappeared. Consequently, the community dominated earlier by herbivorous and periphyton-grazing gastropods and crustaceans shifted to a mussel dominated community with overall low abundances of herbivores. At the same time filamentous epiphytic algae prospered and the growth rate of F. vesiculosus decreased. We suggest that this shift in the invertebrate community may have far reaching consequences on ecosystem functioning. These arise through changes in the strength of producer–herbivore interaction, caused by mud crab predation on the dominating grazer taxa. This interaction is a major determinant of ecological function of ecosystems, i.e. productivity and energy flow to higher trophic levels. Therefore, the decrease in herbivory can be expected to have a major structuring role in producer communities of the rocky littoral macroalgal assemblages.     

Responses of hyporheic invertebrate assemblages to large-scale variation in flow permanence and surface–subsurface exchange

1. Flow permanence (the proportion of time that flowing water is present) strongly influences benthic invertebrate assemblages in ephemeral and intermittent river reaches. Effects of varying flow permanence on hyporheic invertebrate assemblages are not well understood, and have not previously been studied at large spatial scales. 2. We used a 52‐km long flow‐permanence gradient in the alluvial Selwyn River, New Zealand to assess hyporheic assemblage responses to variation in flow permanence and surface–subsurface exchange. The Selwyn mainstem consists of perennial and temporary reaches embedded in longer downwelling (losing) and upwelling (gaining) sections. 3. We predicted that hyporheic invertebrate diversity, density and assemblage stability would increase with increasing flow permanence. We further predicted that assemblage structure would be influenced by the relative contribution of downwelling and upwelling water at the reach‐scale. 4. Hyporheic invertebrates were collected at 15 river cross‐sections over a 13‐month period. As predicted, hyporheic taxon richness, density and assemblage stability varied directly with flow permanence. The distribution of taxa along the flow permanence gradient appeared to be related to desiccation resistance. However, it is possible that proximity to colonist sources also contributed to distribution patterns. 5. Taxon richness was significantly higher at sites in the gaining section compared with the losing section. Sites with high flow permanence in the gaining and losing sections supported distinct hyporheic assemblages, characterised by amphipods and isopods in the gaining section, and ostracods, Hydra sp. and the mayfly Deleatidium spp. in the losing section. 6. Results of the study suggest an expansion of the scope of the Hyporheic Corridor Concept to include large hyporheic flowpaths associated with unbounded alluvial plains rivers. Hyporheic assemblages in alluvial rivers are strongly influenced by large‐scale flow permanence gradients, large‐scale surface water–groundwater exchange, and their interactions.     

The implications of succession after fire for the conservation management of moorland invertebrate assemblages

Fire management in protected areas requires an understanding of the consequences of fire regimes. Invertebrates are a key component of biological communities, but studies of fire impacts on diverse invertebrate assemblages over long timeframes are rare. The responses of ground- and foliage-active invertebrate assemblages to fire in buttongrass moorlands were investigated using a space-for-time design. Assemblages in recently burnt moorlands were distinct from those in older moorlands. Contrary to expectations, ground-active invertebrate abundance, but not taxon richness, was greatest in young regrowth (2–3 years since last fire), owing to large populations of Formicidae, Orthoptera, Collembola and Diptera. Foliage-active invertebrate assemblages followed the expected trend with least numbers of invertebrates and taxa in young regrowth. Very few taxa (n?=?9) were absent from young successional stages and none were absent from later successional stages. Invertebrate assemblages in moorlands on low productivity soils took approximately twice as long to return to their pre-fire state than assemblages on moderate productivity soils. The shifts in invertebrate composition were associated with shifts in vegetation composition. Vegetation density was found to be a potentially important predictor of invertebrate compositional variation. Fire in buttongrass moorland appears to have a limited impact on ground-active and foliage-active invertebrate assemblages, suggesting that these components of the invertebrate fauna are resilient to fire (i.e. able to return to the pre-fire state). Given that fire impedes successional processes that convert moorlands into rainforest, and eliminate many of the moorland invertebrate species, conservation management of moorlands should involve the acceptance or imposition of fire.     

Spatial and temporal patterns of invertebrate drift in streams draining a Neotropical landscape

1. Invertebrate drift in streams draining a tropical landscape in Costa Rica was studied to assess differences in assemblage composition above and below a major gradient break in geomorphic landform and to assess temporal patterns of drift in lowland reaches below the gradient break. The gradient break (～50 m a.s.l.) is the point at which the foothills of the Costa Rican Cordillera Central (piedmont) merge with the Caribbean coastal plain (lowlands).
2. Spatial patterns were assessed along two streams by sampling drift over 24 h once a month for 3 months in both the piedmont (90 m a.s.l.) and lowlands (30 m a.s.l.). Temporal patterns of drift were assessed through monthly diel sampling of three lowland sites over 8–10 months, encompassing both ‘dry’ (<400 mm precipitation per month, November to May) and wet (July to October) seasons.
3. Drift composition was insect dominated in piedmont sites and larval shrimp dominated in the lowlands. Percent similarity of assemblages between piedmont and lowland sites was low (range 26–43%) because of high larval shrimp densities in lowland versus piedmont sites.
4. Drift densities were higher during night than day, with peaks at sunset on all dates and at all sites. Diel patterns in drift agree with previous observations for the study area and support the ‘risk of predation’ hypothesis.
5. Analysis of monthly patterns in lowland sites showed high variability in drift densities; however, all major taxa were found every month. Overall, there was a trend for high invertebrate densities during the ‘dry’ season, but these trends were not significant.
6. Observed changes in drift composition support the concept of river zonation, which predicts a change in community composition along the stream continuum due to geomorphic features. Drift at lowland sites below the gradient break was dominated by shrimps, which are linked to marine environments via their migratory behaviour.     

Exploring stream communities in a tropical biodiversity hotspot: biodiversity,regional occupancy,niche characteristics and environmental correlates

Exploring and describing biodiversity and the mechanisms structuring it is fundamental to advancing ecology. This is particularly pertinent in understudied biogeographical regions, such as the Afrotropics, that are characterised by strong seasonal climatic shifts. We investigated the characteristics of stream biodiversity in the Niger Delta region of Nigeria, a tropical biodiversity hotspot, by examining patterns in 20 stream invertebrate communities across both the wet and dry seasons. For this, we took a multi-faceted approach accounting for the three levels of biodiversity (α, β and γ), including partitioning the nestedness and turnover components of β diversity, regional occupancy-abundance patterns, niche characteristics, and the environmental drivers of community structure. α diversity was low in these streams, with strong turnover between sites leading to high β diversity contributing to regional biodiversity, but there was little variation in communities between seasons. The proportion of sites occupied by taxa declined with increasing niche position, and decreasing niche breadth. Occupancy was predicted well by a combination of these two factors (niche position and breadth), but not mean local abundance, as the abundance-occupancy link was an upper-limit unimodal relationship. On average, community structure was linked more strongly to environmental variables in the wet season. Our findings demonstrate the clear role of spatial, but not temporal, turnover in assemblages, which likely reflects the environmental heterogeneity of this region. This is further supported by the fact that regional occupancy was mostly related to niche characteristics, particularly niche position. We emphasise the importance of continued basic and applied ecological work in this important biogeographic region to enable better protection of its biodiversity.     

Trends in water quality and discharge confound long-term warming effects on river macroinvertebrates

1. Climate‐change effects on rivers and streams might interact with other pressures, such as pollution, but long‐term investigations are scarce. We assessed trends among macroinvertebrates in 50 southern English streams in relation to temperature, discharge and water quality over 18 years (1989–2007). 2. Long‐term records, coupled with estimates from inter‐site calibrations of 3–4 years, showed that mean stream temperatures in the study area had increased by 2.1–2.9 °C in winter and 1.1–1.5 °C in summer over the 26 year period from 1980 to 2006, with trends in winter strongest. 3. While invertebrate assemblages in surface‐fed streams were constant, those in chalk‐streams changed significantly during 1989–2007. Invertebrate trends correlated significantly with temperature, but effects were spurious because (i) assemblages gained taxa typical of faster flow or well‐oxygenated conditions, contrary to expectations from warming; (ii) more invertebrate families increased in abundance than declined and (iii) concomitant changes in water quality (e.g. declining orthophosphate, ammonia and biochemical oxygen demand), or at some sites changes in discharge, explained more variation in invertebrate abundance and composition than did temperature. 4. These patterns were reconfirmed in both group‐ and site‐specific analyses. 5. We conclude that recent winter‐biased warming in southern English chalk‐streams has been insufficient to affect invertebrates negatively over a period of improving water quality. This implies that positive management can minimize some climate‐change impacts on stream ecosystems. Chalk‐stream invertebrates are sensitive, nevertheless, to variations in discharge, and detectable changes could occur if climate change alters flow pattern. 6. Because climatic trends now characterize many inter‐annual time‐series, we caution other investigators to examine whether putative effects on ecological systems are real or linked spuriously to other causes of change.     

Temporal consistency in the long-term spatial distribution of macroinvertebrate drift along a stream reach

Previous studies of the spatial pattern of stream invertebrate drift have focused on spatial variation at microhabitat scales or landscape scales, or temporal variation over diel or seasonal scales. None have examined consistency in spatial variation over longer time scales (>1 year). This study examined invertebrate drift density and composition at fixed locations (terminal ends of 10 riffles) each month at day and night along a 1 km reach of a 2nd order stream over a period of nearly 2 years. Consistent differences in the density of macroinvertebrate drift between riffles over 2 years were observed. The only habitat characteristic observed to be related to invertebrate drift density was the length and size of riffles above sampling sites, with larger and longer riffles producing the highest drift densities. Consistent differences in the supply of drifting macroinvertebrates along a stream reach may have implications for the supply of colonists to substrate patches and the profitability of feeding positions for drift-feeding fish and other predators. Handling editor: D. Dudgeon