The influence of introduced trout on the benthic communities of paired headwater streams in the Sierra Nevada of California

1. Non‐native trout have been stocked in streams and lakes worldwide largely without knowledge of the consequences for native ecosystems. Although trout have been introduced widely throughout the Sierra Nevada of California, U.S.A., fishless streams and their communities of native invertebrates persist in some high elevation areas, providing an opportunity to study the effects of trout introductions on natural fishless stream communities. 2. We compared algal biomass and cover, organic matter levels and invertebrate assemblages in 21 natural fishless headwater streams with 21 paired nearby streams containing stocked trout in Yosemite National Park. 3. Although environmental conditions and particulate organic matter levels did not differ between the fishless and trout streams, algal biomass (as chlorophyll a concentration) and macroalgal cover were, on average, approximately two times and five times higher, respectively, in streams containing trout. 4. There were no differences in the overall densities of invertebrates in fishless versus paired trout streams; however, invertebrate richness (after rarefaction), evenness, and Simpson and Shannon diversities were 10–20% higher in fishless than in trout streams. 5. The densities of invertebrates belonging to the scraper‐algivore and predator functional feeding guilds were higher, and those for the collector‐gatherer guild lower, in fishless than trout streams, but there was considerable variation in the effects of trout on specific taxa within functional feeding groups. 6. We found that the densities of 10 of 50 common native invertebrate taxa (found in more than half of the stream pairs) were reduced in trout compared to fishless streams. A similar number of rarer taxa also were absent or less abundant in the presence of trout. Many of the taxa that declined with trout were conspicuous forms (by size and behaviour) whose native habitats are primarily high elevation montane streams above the original range of trout. 7. Only a few taxa increased in the presence of trout, possibly benefiting from reductions in their competitors and predators by trout predation. 8. These field studies provide catchment‐scale evidence showing the selective influence of introduced trout on stream invertebrate and algal communities. Removal of trout from targeted headwater streams may promote the recovery of native taxa, community structure and trophic organisation.     

Recolonization by benthic invertebrates after experimental disturbance in a Swiss prealpine river

1. Although the crucial point of disturbance experiments in streams is the extent to which they can simulate an actual spate, this aspect has been widely neglected in the design of such studies. Similarly, the influence of the specific hydrological disturbance regime of a stream on its benthic community has received much theoretical attention in recent years, but hypotheses have rarely been tested in the field. 2. Our field experiment compared the structure of the benthic invertebrate community in the prealpine River Necker in north-eastern Switzerland with predictions of the patch dynamics concept about the faunal composition of frequently disturbed streams. We also compared the resistance and resilience of the invertebrates between two sites in the River Necker. A similar substratum composition at both sites, but higher shear stress values both at baseflow and bankfull discharge at site 2, implied a higher disturbance frequency at the latter site. Five patches of stream bed of ≈ 9 m² were disturbed by kicking and raking at each site, while five similar areas served as controls. From each plot, six Surber samples were taken: the first immediately after the disturbance, and the following five 1, 3, 6, 10 and 30 days later. 3. Resilience of the total benthic invertebrate fauna was high. The total number of individuals recovered to undisturbed densities within 30 days at site 1 and 6 days at site 2. Taxon richness recovered within 3 days. In accordance with theory, taxa with high recolonization rates made up a major percentage of the total number of individuals, especially in disturbed plots. However, this percentage was lower at site 2 in spite of the higher disturbance frequency at this site. Rhithrogena spp., Leuctra spp. and the Simuliidae recovered faster to undisturbed densities at site 2. In contrast, absolute recolonization rates of these taxa were higher at site 1, where total invertebrate densities were more than twice as high as at site 2. 4. Our results suggest that the time since the last disturbance should be considered as an important factor in studies of benthic invertebrate communities in prealpine rivers, because disturbances can alter the community structure. In frequently disturbed streams, very short sampling intervals may be needed to detect differences in taxon-specific colonization rates. The specific hydrological disturbance regime of such streams is also important, because even within-stream differences in the resilience of the benthic invertebrate community are possible.     

Land-use influences macroinvertebrate community response following a pulse disturbance

1. We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press‐pulse disturbance) and native forest (pulse disturbance) catchments before and after a one‐in‐28‐year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways. 2. Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance. 3. Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2‐year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies. 4. Taxa numbers and densities recovered to preflood levels within 5–7 months at both sites, but a range of taxa‐specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood. 5. Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site. 6. Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non‐Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera. 7. This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land‐use impacts.     

Use of both benthic and drift sampling techniques to assess tropical stream invertebrate communities along an altitudinal gradient, Costa Rica

1. Two sampling techniques were used to characterize invertebrate communities in eight, low-order streams along an altitudinal gradient in Costa Rica that represents the last continuous tract of primary forest spanning such extremes in elevation (i.e. near sea level to 2900 m a.s.l.) along the Caribbean Slope of Central America. A standard Surber sampler was used to sample invertebrates on the stream bottom, and drift sampling nets were used to sample invertebrates drifting in the stream flow. 2. Sites were established at 30, 50, 700 1800 and 2700 m a.s.l. In one to two streams per site, six Surber samples were collected, and drift was sampled every 3 h over one 24-h period between April and August 1994. All sites were in primary forest, with the exception of the lowest elevation site (30 m) which was located in banana plantations. 3. Both sampling techniques indicated that Diptera (Chironomidae) and Ephemeroptera were the dominant insect groups at all sites. Disturbed streams draining banana plantations were dominated by Chironomidae and had lower taxon richness and diversity than other sites. 4. While data from benthic samples indicated that insects were the major faunal component (> 90%) at all sites, drift samples were dominated by larval shrimps (> 50%) at the 30 m and 50 m sites. 5. Drift periodicity of invertebrates was observed at those sites characterized by predaceous fishes: nocturnal drift densities were higher than diurnal densities at 30, 50 and 700 m a.s.l., however, no periodicity was observed at 1800 and 2700 m a.s.l. where fish were absent. 6. This study shows the importance of measuring invertebrate drift, in addition to directly sampling the benthos. Drift sampling provided data on a major community component (shrimps) of lowland tropical streams, that would have been overlooked using traditional benthic sampling techniques, and in some cases provided additional information on taxon richness. 7. Based on results of the present study, it is recommended that drift sampling be included as a standard complementary tool to benthic sampling in biological assessments (e.g. bioassessment protocols) of tropical streams, which are often characterized by migratory invertebrate species such as shrimps. Drift samples provide critical information on the presence or absence of shrimps and also on the timing and magnitude of their migration which is an important link between many tropical rivers and their estuaries.     

Permafrost thaw and intense thermokarst activity decreases abundance of stream benthic macroinvertebrates

Intensification of permafrost thaw has increased the frequency and magnitude of large permafrost slope disturbances (mega slumps) in glaciated terrain of northwestern Canada. Individual thermokarst disturbances up to 40 ha in area have made large volumes of previously frozen sediments available for leaching and transport to adjacent streams, significantly increasing sediment and solute loads in these systems. To test the effects of this climate‐sensitive disturbance regime on the ecology of Arctic streams, we explored the relationship between physical and chemical variables and benthic macroinvertebrate communities in disturbed and undisturbed stream reaches in the Peel Plateau, Northwest Territories, Canada. Highly disturbed and undisturbed stream reaches differed with respect to taxonomic composition and invertebrate abundance. Minimally disturbed reaches were not differentiated by these variables but rather were distributed along a disturbance gradient between highly disturbed and undisturbed sites. In particular, there was evidence of a strong negative relationship between macroinvertebrate abundance and total suspended solids, and a positive relationship between abundance and the distance from the disturbance. Increases in both sediments and nutrients appear to be the proximate cause of community differences in highly disturbed streams. Declines in macroinvertebrate abundance in response to slump activity have implications for the food webs of these systems, potentially leading to negative impacts on higher trophic levels, such as fish. Furthermore, the disturbance impacts on stream health can be expected to intensify as climate change increases the frequency and magnitude of thermokarst.     

Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

1. Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.
2. Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in-stream predators such as fishes.
3. We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.
4. Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.
5. There was an increase over time in algal biomass (chlorophyll-a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.
6. Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

     

Disturbance frequency and functional identity mediate ecosystem processes in prairie streams

A major consequence of climate change will be the alteration of precipitation patterns and concomitant changes in the flood frequencies in streams. Species losses or introductions will accompany these changes, which necessitates understanding the interactions between altered disturbance regimes and consumer functional identity to predict dynamics of streams. We used experimental mesocosms and field enclosures to test the interactive effects of flood frequency and two fishes from distinct consumer groups (benthic grazers and water-column minnows) on recovery of stream ecosystem properties (algal form and biomass, invertebrate densities, metabolism and nutrient uptake rates). Our results generally suggest that periphyton communities under nutrient limitation are likely to recover more quickly when grazing and water-column minnows are present and these effects can diminish or reverse with time since the disturbance. We hypothesized that increased periphyton production and biomass was the result of increased nutrient turnover, but decreased light limitation and indirect effects on other trophic levels are alternative explanations. Recovery of stream ecosystem properties after a natural flood differed from mesocosms (e.g. lower algal biomass and no long algal filaments present) and species manipulations did not explain recovery of ecosystem properties; rather, ecosystem processes varied along a downstream gradient of increasing temperature and nutrient concentrations. Different results between field enclosures and experimental mesocosms are attributable to a number of factors including differences in algal and invertebrate communities in the natural stream and relatively short enclosure lengths (mean area=35.8 m²) compared with recirculating water in the experimental mesocosms. These differences may provide insight into conditions necessary to elicit a strong interaction between consumers and ecosystem properties.     

Long-term effects of local disturbance history on mobile stream invertebrates

The patchy distribution of benthic invertebrates in streams and rivers is an important and widely researched phenomenon. Previous studies on reasons for this patchiness have neglected the potential role of local disturbance history, probably because most lotic invertebrates are mobile and any effect of disturbance history was thought to be short-lived. Here we demonstrate for a New Zealand gravel-bed stream that local disturbance history can have long-term effects on the distribution of highly mobile stream invertebrates. Buried scour chains (100 at each of three 20-m sites within a 350-m reach) indicated that a spate with a return period of 5 months caused a mosaic of bed patches with different stabilities. More than 2 months after the spate, we took random, quantitative samples at each site from five patches that had experienced 4 cm or more of scour during the spate, from five patches with 4 cm or more of fill, and from five stable patches. Density of the dominant invertebrate taxon, the highly mobile mayfly Deleatidium spp., and densities of another three of the seven most common taxa differed significantly between patch stability categories. Larvae of Deleatidium, the black fly Austrosimulium spp. and the dipteran Eriopterini were most abundant in fill patches, whereas Isopoda were most abundant in scour patches. Total invertebrate densities and densities of six common taxa also differed between sites, although these were only 95–120 m apart. These results show that local disturbance history can have long-term effects on lotic invertebrates and be an important cause of invertebrate patchiness. The observed effects might have been even stronger had we sampled sooner after the spate or after a large flood. Disturbance history may influence invertebrates both directly (through dislodgement or mortality) and indirectly, through effects on the spatial distribution of their resources. Our results suggest that the role of disturbance in structuring animal communities dominated by mobile species may be more important than previously thought. Received: 25 January 2000 / Accepted: 14 April 2000     

Effects of Tea Plantations on Stream Invertebrates in a Global Biodiversity Hotspot in Africa

Tropical stream ecosystems in montane forest watersheds are important centers of endemism and diversity and provide essential ecosystem services. These habitats are subject to a variety of stressors, including the conversion of adjacent terrestrial habitats from forest to agriculture, but the impacts of these anthropogenic effects are largely unknown because of the paucity of studies in these systems. In montane habitats in the wet tropics, large-scale cultivation of tea is common and can represent an important source of income at local and national scales. However, little is known about how tea cultivation impacts adjacent stream ecosystems. In this study, we examine stream macroinvertebrate assemblages in a biodiversity hotspot the East Usambara Mountains, Tanzania. Specifically, we compare diversity of macroinvertebrate assemblages found on cobbles in stream riffles in watersheds dominated by forest with those surrounded by tea cultivation. We found that streams surrounded by tea were characterized by significantly lower dissolved oxygen and had lower total estimated species richness and number of families. Furthermore, the richness of invertebrate taxa known to be sensitive to anthropogenic disturbance were substantially reduced in tea streams and general assemblage-level analysis shows significant differences in the composition of macroinvertebrate assemblages between tea and forested streams. Our results suggest that tea cultivation may reduce stream habitat quality and biodiversity in the East Usambaras. Further research is needed to evaluate the effects of tea cultivation on streams over longer times scales and to address methods for minimizing negative effects of agriculture on montane stream communities.     

Temporal and altitudinal variations in the attached algae of mountain streams in Colorado

Attached algae were examined from eight sites in Central Colorado streams (five sites on St. Vrain Creek and three on the Snake River) between June and November, 1995. The sites ranged in elevation from 1600 m to 3500 m a.s.l.(plains zone to alpine zone). Seasonal variation in algal communities were qualified in forms of species composition and abundance (cell counts and biovolumes). The Ochiai coefficient of similarity was used to estimate degree of similarity between communities at different times, on different streams, and at different locations on the same stream. The composition of communities across streams for alpine and foothills zones was very close, but montane communities differed greatly across streams (similarity < 0.1). Temporal variability of attached algae was almost absent in the alpine zone, but was high in other zones. The composition of the alpine community in August developed downstream with a temporal shift: October in the montane and foothills zones, November in the plains zone. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling the hydraulic preferences of benthic macroinvertebrates in small European streams

1. Relating processes occurring at a local scale to the natural variability of ecosystems at a larger scale requires the design of predictive models both to orientate stream management and to predict the effects of larger scale disturbances such as climate changes. Our study contributes to this effort by providing detailed models of the hydraulic preferences of 151 invertebrate taxa, mostly identified at the species level. We used an extensive data set comprising 580 invertebrate samples collected using a Surber net from nine sites of second and third order streams during one, two or three surveys at each site. We used nested non‐linear mixed models to relate taxon local densities to bed shear stresses estimated from FliesswasserStammTisch hemisphere numbers. 2. An average model by taxon, i.e. independent from surveys, globally explained 25% of the density variations of taxa within surveys. A quadratic relationship existed between the average preferences and the niche breadth of taxa, indicating that taxa preferring extreme hemisphere numbers had a reduced hydraulic niche breadth. A more complete model, where taxa preferences vary across surveys, globally explained 38% of the variation of taxa densities within surveys. Variations in preferences across surveys were weak for taxa preferring extreme hemisphere numbers. 3. There was a significant taxonomic effect on preferences computed from the complete model. By contrast, season, site, average hemisphere number within a survey and average density of taxa within a survey used as covariates did not consistently explain shifts in taxon hydraulic preferences across surveys. 4. The average hydraulic preferences of taxa obtained from the extensive data set were well correlated to those obtained from two additional independent data sets collected in other regions. The consistency of taxon preferences across regions supports the use of regional preference curves for estimating the impact of river management on invertebrate communities. By contrast, the hydraulic niche breadths of taxa computed from the different data sets were not related.     

Beta diversity of aquatic macroinvertebrate assemblages associated with leaf patches in neotropical montane streams

Over 70% of the total channel length in all river basins is formed by low order streams, many of which originate on mountaintops. Headwater streams play fundamental roles in processing and transporting terrestrial and aquatic organic matter, often harboring high biodiversity in bottom leaf patches deposited from riparian vegetation. The objective of this study was to assess the variation in taxonomic composition (measured by beta diversity of aquatic macroinvertebrates) among stream sites located in the Espinhaço Meridional Mountain Range, part of a UNESCO Biosphere Reserve in eastern Brazil. We tested two hypotheses. (a) Taxa turnover is the main reason for differences in aquatic insect assemblages within stream sites; we predicted that turnover would be higher than nestedness in all stream sites. (b) Stream site altitude and catchment elevation range are the main explanatory variables for the differences in beta diversity; we predicted that local stream site variables would account for only minor amounts of variation. In both dry and wet seasons, we sampled twice in two habitat types (five leaf patches in pools and five in riffles) in each of nine stream sites distributed in three different river basins. We computed average pairwise beta diversity among sampling stations and seasons in each stream site by using Jaccard and Bray–Curtis indices, and calculated the percentages of diversity resulting from turnover and nestedness. Finally, we tested the degree that local‐ or catchment‐level predictor variables explained beta diversity. We found that turnover was the main component of beta diversity and that both dissolved oxygen and elevation range best explained Bray–Curtis beta diversity. These results reinforce the importance of leaf patches in montane (sky islands) Neotropical savanna streams as biodiversity hotbeds for macroinvertebrates, and that both local and landscape variables explained beta diversity.     

The influence of microhabitat on availability of drifting invertebrate prey to a net-spinning caddisfly

SUMMARY. 1. Invertebrate drift was sampled at both a rockface and a deep pebble riffle site in streams draining both a clearcut and a forested catchment.
2. A sampler was designed to separate the bottom 2 cm of flow, encompassing the effective range of caddisfly (Hydropsychidae: Trichoptera) catchnets. from upper flow.
3. No significant difference in drift density (numbers per cubie metre) was seen between sites within each stream. However, numbers per square centimetre intake area per day at the rockface sites were 4 times higher in the clearcut and 10 times higher in the forested stream than at the pebble-riffle site.
4. Rockface habitat which had highest drift availability was also the site of maximum secondary production of the predaceous collector- filterer Parapsyche cardis in both streams studied.
5. Increased sediment load in the clearcut stream may influence the efficiency of utilization of invertebrate drift by collector-filterers.     

Biotic disturbance and benthic community dynamics in salmon-bearing streams

1. Organisms can impact ecosystems via multiple pathways, often with positive and negative impacts on inhabitants. Understanding the context dependency of these types of impacts remains challenging. For example, organisms may perform different functions at different densities. 2. Anadromous salmon accumulate > 99% of their lifetime growth in marine ecosystems, and then return to spawn, often at high densities, in relatively confined freshwaters. While previous research has focused on how salmon nutrients can fertilize benthic communities, we examined how an ecosystem engineer, sockeye salmon Oncorhynchus nerka, influences seasonal dynamics of stream benthic communities through their nest-digging activities in south-western Alaska, USA. Benthic invertebrate and algal abundance were quantified every 7-14 days during the open water seasons of 10 streams in riffle and run habitats across multiple years, leading to 25 different stream-year combinations that spanned a large gradient of salmon density. 3. In streams with few or no salmon, benthic algal and insect biomass were fairly constant throughout the season. However, in streams with more than 0.1 salmon m(-2), algal and insect biomass decreased by an average of 75-85% during salmon spawning. Algal biomass recovered quickly following salmon disturbance, occasionally reaching pre-salmon biomass. In contrast, in streams with more than 0.1 salmon m(-2), aquatic insect populations did not recover to pre-salmon levels within the same season. We observed no positive impacts of salmon on algae or insects via fertilization from carcass nutrients. 4. Salmon, when their populations exceed thresholds in spawning density, are an important component of stream disturbance regimes and influence seasonal dynamics of benthic communities. Human activities that drive salmon densities below threshold densities, as has likely happened in many streams, will lead to altered seasonal dynamics of stream communities. Human activities that alter animal populations that are sources of biogenic disturbance can result in shifts in community dynamics.     

Plant—herbivore interactions in streams near Mount St Helens

1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during 1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frog Ascaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated using streamside channels and in-stream manipulations. In the experimental channels, abundances of periphyton and small benthic invertebrates declined significantly with increasing density of these larger herbivores. 2. In eleven small, high-gradient streams affected to varying degrees by the May 1980 eruption, in-stream platforms were used to reduce grazing by A, truei tadpoles on tile substrates. Single platforms erected in each tributary and compared to grazed controls revealed only minor grazing effects, and no significant differences among streams varying in disturbance intensity (and, consequently, tadpole density). However, results probably were confounded by high variability among streams in factors other than tadpole abundance. 3. Grazing effects were further examined in two unshaded streams with different tadpole densities, using five platforms per stream. In the stream with five tadpoles m^?2, grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the stream lacking tadpoles, no significant grazing effects were revealed. Low algal abundance on both platforms and controls, and high invertebrate density in that stream (c. 30000m^?2) suggests that grazing by small, vagile invertebrates was approximately equivalent to that of tadpoles. 4. The influence of large benthic herbivores on algal and invertebrate communities in streams of Mount St Helens can be important, but reponses vary spatially in relation to stream disturbance history, local environmental factors, and herbivore distributional patterns and abundance.     

Seasonal flow variation allows 'time-sharing' by disparate aquatic insect communities in montane desert streams

1. Flow variation can drive major abiotic changes in stream environments between seasons. Theoretically, disparate biotic communities could be maintained during different seasons at a single site if suitable refuges and colonist sources were available. Using isolated montane desert streams in south‐east Arizona as a model system, we hypothesised that two disparate aquatic insect faunas (montane temperate and neotropical) could be maintained at the same sites through strong seasonal variation in abiotic conditions. 2. We collected aquatic insects representing 59 families from seven streams during high‐flow (March–April) and low‐flow (June) sampling periods across two years. We assessed changes in aquatic insect community and functional feeding group composition by habitat (riffle, pool) and season (high flow, low flow). 3. Within sites, wetted stream area decreased by an average of 97% between high‐flow (predominately riffles) and low‐flow (predominately pools) seasons. Community composition likewise showed strong seasonal patterns; the montane temperate fauna was strongly associated with the high‐flow season while neotropical hemipterans and coleopterans were associated with the low‐flow season. Increased water temperature was significantly associated with this shift from temperate to neotropical assemblages. 4. Functional feeding group composition shifted dramatically by season. The proportion of predators increased from 24.5% (high flow) to 75.2% (low flow) while collector–filterers and shredders declined from 38.4% (high flow) to 1.7% (low flow). 5. We suggest that habitat ‘time‐sharing’ by disparate communities is facilitated via strong seasonal variation in temperature and flow and the presence of high elevation refuges or diapause stages for temperate montane taxa to survive the dry season.     

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.     

The Value of Long-Term Stream Invertebrate Data Collected by Citizen Scientists

The purpose of this investigation was to systematically examine the variability associated with temporally-oriented invertebrate data collected by citizen scientists and consider the value of such data for use in stream management. Variability in invertebrate data was estimated for three sources of variation: sampling, within-reach spatial and long-term temporal. Long-term temporal data were also evaluated using ordinations and an Index of Biotic Integrity (IBI). Through two separate investigations over an 11-year study period, participants collected more than 400 within-reach samples during 44 sampling events at three streams in the western United States. Within-reach invertebrate abundance coefficient of variation (CV) ranged from 0.44–0.50 with approximately 62% of the observed variation strictly due to sampling. Long-term temporal CV ranged from 0.31–0.36 with 27–30% of the observed variation in invertebrate abundance related to climate conditions (El Niño strength) and sampling year. Ordinations showed that citizen-generated assemblage data could reliably detect differences between study streams and seasons. IBI scores were significantly different between streams but not seasons. The findings of this study suggest that citizen data would likely detect a change in mean invertebrate density greater than 50% and would also be useful for monitoring changes in assemblage. The information presented here will help stream managers interpret and evaluate changes to the stream invertebrate community detected by citizen-based programs.     

A comparison of the relative contributions of temporal and spatial variation in the density of drifting invertebrates in a Dorset (U.K.) chalk stream

1. Invertebrate drift is commonly investigated in streams, with the majority of studies focussed on temporal (typically diel) variation. In comparison, few studies have investigated spatial variation in drift and there is little consensus among them. We tested the hypothesis that spatial variation in invertebrate drift is as important as temporal variation. 2. The density of drifting invertebrates in a chalk stream was sampled using an array of nets arranged to determine vertical, lateral and longitudinal variation. Samples were collected at dawn, during the day, at dusk and by night, on four separate monthly occasions. Insecta and Crustacea were analysed separately to identify the effect of differing life history strategies. The density of drifting debris was also recorded, to act as a null model. 3. Time of day and vertical position together explained the majority of the variance in invertebrate drift (79% for Insecta and 97% for Crustacea), with drift densities higher at dusk and night, and nearer the stream bed. Independently, time of day (38%, Insecta; 52%, Crustacea) and vertical position (41%, Insecta; 45%, Crustacea) explained a similar amount of the observed variance. Month explained some of the variance in insect drift (9%) but none for Crustacea. 4. Variation in the density of drifting debris showed little in common with invertebrate drift. There was little variation associated with time of day and only 27% of the observed variation in debris could be explained by the factors investigated here, with month explaining the largest proportion (20%). We suggest the difference in drifting debris and invertebrates provides further evidence for a strong behavioural component in invertebrate drift. 5. Spatial variation in invertebrate drift can be of the same order of magnitude as the much‐described diel temporal variation. The extent of this spatial variation poses problems when attempting to quantify invertebrate drift and we recommend that spatial replication should be incorporated into drift studies.     

Impacts of fine sediment addition to tussock, pasture, dairy and deer farming streams in New Zealand

1. Increased fine sediment input caused by agricultural development is expected to act as a stressor for stream ecosystems. In a large‐scale field experiment, we added fine river sand to 50‐m reaches of three second‐order streams in each of four categories of catchment development (ungrazed tussock grasslands, grazed pasture, dairying and deer farming) and measured the responses of macroinvertebrates and aquatic moss. 2. Before addition, fine sediment cover differed between land uses, being lowest in tussock (7%), intermediate in pasture (30%) and dairy (47%) and highest in deer streams (88%). Sediment addition increased cover by one land‐use category (e.g. augmented sediment cover in tussock streams was similar to pre‐existing cover in pasture streams), and cover remained high in impact reaches (compared with controls) throughout the 5‐week experiment. Sediment addition did not change concentrations of phosphate, nitrate and ammonium, which were generally highest in dairy streams and lowest in tussock streams. 3. Aquatic mosses (most common in tussock, absent in dairy and deer), invertebrate density (highest in deer, lowest in tussock), taxon richness (highest in pasture, lowest in deer) and diversity (highest in pasture and tussock, lowest in dairy and deer) all differed between land uses. Sediment addition resulted in reductions of moss cover, invertebrate taxon richness and richness of Ephemeroptera, Plecoptera and Trichoptera in impact relative to control reaches. 4. The impact of sediment addition was strongest in pasture streams where pre‐existing sediment cover was moderate and richness and diversity of the invertebrate community highest. However, even in the already sediment‐rich and species‐poor deer streams, density of one common taxon was reduced significantly by sediment addition, and another two were affected in the same way in dairy streams, the second‐most intense land use. 5. Our experiment has disentangled the impact of sediment addition from other concomitant land‐use effects that could not be reliably distinguished in previous research, which has mainly consisted of correlative studies or unrealistically small‐scale experiments.