Spatial responses of hyporheic invertebrates to seasonal changes in environmental parameters

1. Spatial relationships between hyporheic invertebrates and subsurface water flow patterns, sediment characteristics, water physicochemical parameters and several possible food sources were compared over three seasons at one site beneath a riffle. Measures of food sources included particulate organic matter (POM), bacterial activity (aerobic respiration, nitrate respiration and mineralisation of organic nitrogen) and microbial abundance. 2. Patterns of water flow changed significantly over the 9‐month study period, from predominantly upwelling beneath the entire riffle in spring, to distinct differentiation between downwelling and upwelling zones in summer and autumn. Water physicochemical parameters changed accordingly, showing gradually weaker correlations with depth and stronger correlations with zone between spring and autumn. 3. Despite these changes, depth remained the strongest predictor of invertebrate richness, density and taxon composition throughout the study period. However, invertebrate distributions were secondarily correlated with water physicochemical parameters, and a minor gradient in invertebrate distributions between downwelling and upwelling zones became stronger from spring to summer. 4. The correlations between invertebrates and physicochemical parameters changed in both magnitude and direction with season. In spring, invertebrates showed a negative correlation with surface water infiltration, whereas in summer and autumn, the correlation was positive. Correlations were strongest in summer, when interstitial dissolved oxygen concentrations were lowest. 5. No relationships were found between hyporheic invertebrates and POM, microbial abundance or activity. This suggests that at this site, proximity to the streambed surface and physicochemical variables are more important than the abundance of food in controlling invertebrate distributions.     

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.     

Hydraulic parameters and benthic invertebrate distributions in two gravel-bed New Zealand rivers

1. Benthic invertebrates were sampled over a matrix of about eighty combinations of mean velocity (10–150cms^?1) and depth (10–150cm) in two rivers that differed in substrate size variability. Vertical velocity profiles were measured at each sample site and substratum roughness was measured and estimated from percentage cover by stone size classes. The influence of depth on periphyton biomass was also measured. 2. The hydraulic and substrate data were used to investigate the correlations between conventional (mean velocity, depth, substrate size) and complex hydraulic variables (Froude number, shear velocity, and water column and boundary Reynolds number) that were either calculated from direct measurements or inferred from mean velocity, depth, kinematic viscosity and substrate roughness. The ecological relevance of these hydraulic variables was investigated by comparing their degree of correlation with invertebrate densities and community metrics. 3. The invertebrate variables had similar correlations with mean velocity and the complex near-bed hydraulic variables in the river with uniform cobble substrates. In the river with diverse substrates, however, average correlations with Froude number, and inferred shear velocity and boundary Reynolds number were 25–45% higher than with velocity. Of all the individual hydraulic parameters, the boundary Reynolds number, calculated from simple measures, was most strongly correlated with benthic invertebrate distributions and taxa richness. However, invertebrate distributions were more strongly correlated with predictions of multiple regression models, incorporating substrate size, depth and mean velocity, than with any single hydraulic variable. 4. Hydraulic influences on food availability and oxygen concentration in the benthos are likely mechanisms affecting the hydraulic preferences of several taxa. Lower periphyton biomass with depth, partly attributable to light attenuation, appeared to have a non-hydraulic influence on a collector-browser species.     

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.     

Macroinvertebrate community structure along gradients of hydraulic and sedimentary conditions in a large gravel-bed river

1. The spatial distribution of macroinvertebrate species was examined in relation to hydraulic and sedimentary conditions in a large gravel‐bed river, the Fraser River, Canada. Mean annual discharge in the Fraser River is 2900 m3 s?1 and annual flood discharge, due to snowmelt in May and June, averages 8760 m3 s?1.
2. Invertebrates were sampled from four water depths (0.2, 0.5, 1.5, 3.0 m) at various levels of discharge that together captured the spatial and temporal variability of the physical habitat. Several hydraulic (near‐bed shear velocity, Boundary Reynolds number, turbulence intensity, depth‐averaged velocity, Froude number, Reynolds number) and substratum variables (mean grain size, Trask's sorting coefficient, Nikuradse's roughness, percentage of fine sediment, and Shields entrainment function) were measured for each sample of macroinvertebrates. Concentrations of fine and coarse particulate organic matter were also assessed.
3. The physical habitat was characterized by a major gradient of hydraulic conditions that corresponded positively with increasing water depth and accounted for 52% of the total variation in the habitat data. Substratum conditions and the concentration of organic matter explained 24% of the total variation in the habitat data.
4. The distribution of invertebrates was correlated significantly with hydraulic variables and suggests that hydraulic conditions represent a major physical gradient along which the benthic community is organized. The distribution of organic matter and substratum texture were also important for some species. The spatial distribution of most species reflected morphological and trophic suitability to particular habitat conditions.
5. Hydraulic stress associated with foraging and maintaining position, as well as organic matter retention in coarse substrata, are probable mechanisms affecting the spatial distribution of macroinvertebrates.     

Effects of habitat complexity on benthic assemblages in a variable environment

1. Habitat complexity is thought to exert a significant influence on ecological communities, but its operation under variable natural conditions is not well understood, particularly in freshwater. To elucidate the role of habitat complexity, in particular the fractal structure of surface irregularity, in a stream system, field colonisation experiments were conducted at three times of year (summer, winter and spring) using natural substrates with different levels of fractal dimension in a small coastal mountain stream of southern Japan. 2. In the winter experiment, comparison was also made between the standard (control) treatment and the resource‐preconditioning treatment whereby experimental plates were conditioned in the natural stream environment to allow the accumulation of potential food resources (algae and detritus) for 1 month prior to the experiment. 3. Species abundance patterns observed at different times of year showed little systematic variation with levels of habitat complexity but largely followed the patterns expected from, or lying in between, the Random Assortment model and the random fraction model. 4. Taxon richness and density increased with habitat complexity in all seasons except for density in spring. Different taxa showed different patterns of change with habitat complexity, which also varied with seasons. Biomass of invertebrates showed no systematic trend with an increase in habitat complexity. 5. Chlorophyll‐a concentrations tended to be lower in more complex habitats, particularly in summer. In contrast, fine particulate organic matter (FPOM) tended to increase with habitat complexity. However, the relationship between these potential food resources and invertebrate assemblages remain unclear. 6. While there were no significant differences in taxon richness and biomass of invertebrates between the resource‐preconditioning and the control treatment, density was higher in the former than in the latter. The abundance of relatively large, surface‐dwelling animals showed more marked temporal variation over the entire period of colonisation in the resource‐preconditioning treatment than in the control treatment. 7. Body size of invertebrates tended to decline with fractal complexity, indicating that crevice sizes could affect habitat use by benthic animals of different sizes. In addition, body size was larger in the resource‐preconditioning treatment than in the control treatment, suggesting that body size in invertebrate assemblages was controlled by a mixture of factors. Thus, the present study demonstrates that habitat structure affects benthic invertebrate assemblages in a complex manner.     

Effects of floods versus low flows on invertebrates in a New Zealand gravel-bed river

1. Floods and low flows are hydrological events that influence river ecosystems, but few studies have compared their relative importance in structuring invertebrate communities. Invertebrates were sampled in riffles and runs at eight sites along 40 km of a New Zealand gravel‐bed river every 1–3 months over 2.5 years, during which time a number of large flood and low flow events occurred. Flows were high in winter and spring, and low in summer and autumn. Four flow‐related variables were calculated from hydrological data: flow on the day of sampling (Q_sample), maximum and minimum flow between successive samples (Q_max and Q_min, respectively), and the number of days since the last bed‐moving flood (N_days). 2. The invertebrate community was summarised by relative densities of the 19 most abundant taxa and four biotic metrics [total abundance, taxon richness, the number of Ephemeroptera, Plecoptera and Trichoptera taxa (i.e. EPT richness), and per cent EPT]. Invertebrate density fluctuated greatly, and was high in summer and autumn, and low during winter and spring. Stepwise multiple regression (SMR) analysis was used to investigate relationships between the invertebrate community and season, flow, habitat and water temperature. 3. Seasonal variables were included in almost 50% of the SMR models, while flow‐related variables were included in >75% of models. Densities of many taxa were negatively correlated to Q_min and Q_max, and positively correlated to N_days, suggesting that while high flows reduced invertebrate densities, densities recovered with increasing time following a flood. Although season and flow were confounded in this study, many of the taxa analysed display little seasonal variation in abundance, suggesting that flow‐related variables were more important in structuring communities than seasonal changes in density associated with life‐cycles. 4. Five discrete flood and low flow events were identified and changes to invertebrate communities before and after these events examined. Invertebrate densities decreased more commonly after floods than after low flows, and there was a significant positive relationship between the number of taxa showing reductions in density and flood magnitude. Densities of most invertebrates either remained unchanged, or increased after low flow events, except for four taxa whose densities declined after a very long period (up to 9 months) of low flow. This decline was attributed to autogenic sloughing of thick periphyton communities and subsequent loss of habitat for these taxa. 5. Invertebrate communities changed more after floods and the degree of change was proportional to flood magnitude. Community similarity increased with increasing time since the last disturbance, suggesting that the longer stable flows lasted, the less the community changed. These results suggest that invertebrate communities in the Waipara River were controlled by both floods and low flows, but that the relative effects of floods were greater than even extended periods of extreme low flow. 6. Hydraulic conditions in riffles and runs were measured throughout the study. Riffles had consistently faster velocities, but were shallower and narrower than runs at all measured flows. Invertebrate density in riffles was expressed as a percentage of total density and regressed against the flow‐related variables to see whether invertebrate locations changed according to flow. Significant negative relationships were observed between the per cent density of common taxa in riffles and Q_sample, Q_max and Q_min. This result suggests either that these animals actively drifted into areas of faster velocity during low flows, or that their densities within riffles increased as the width of these habitats declined.     

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream. 2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m²) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM). 3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study. 4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters. 5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).     

Dynamics of Invertebrate Benthic Communities and Drift in a Regulated River of Central Spain

We document invertebrate benthic and drift dynamics in a regulated river in central Spain at two temporal scales: seasonal (for both benthos and drift) and daily (for drift). The benthic abundance of individuals and taxon richness generally increased in the summer. Drift abundance showed no seasonal or daily variation, but taxon richness of drifting individuals was higher in the spring. Both ben‐thos and drift showed clear seasonal changes in taxonomic composition. Interestingly, some benthic taxa showed their highest abundances in the spring, while others were more abundant in the summer. In contrast, most drifting taxa were more abundant in the spring. Different functional feeding groups showed different patterns of variation throughout the year, both in the benthos and the drift. Daily variations in drift were present in very few taxa and functional feeding groups, and only in some seasons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long‐term patterns of invertebrate abundance and relationships to environmental factors in arid Australia

Resource pulses are a key feature of semi‐arid and arid ecosystems and are generally triggered by rainfall. While rainfall is an acknowledged driver of the abundance and distribution of larger animals, little is known about how invertebrate communities respond to rain events or to vegetative productivity. Here we investigate Ordinal‐level patterns and drivers of ground‐dwelling invertebrate abundance across 6 years of sampling in the Simpson Desert, central Australia. Between February 1999 and February 2005, a total of 174 381 invertebrates were sampled from 32 Orders. Ants were the most abundant taxon, comprising 83% of all invertebrates captured, while Collembola at 10.3% of total captures were a distant second over this period. Temporal patterns of the six invertebrate taxa specifically analysed (Acarina, ants, Araneae, Coleoptera, Collembola and Thysanura) were dynamic over the sampling period, and patterns of abundance were taxon‐specific. Analyses indicate that all six taxa showed a positive relationship with the cover of non‐Triodia vegetation. Other indicators of vegetative productivity (seeding and flowering) also showed positive relationships with certain taxa. Although the influence of rainfall was taxon‐dependent, no taxon was affected by short‐term rainfall (up to 18 days prior to survey). The abundance of Acarina, ants, and Coleoptera increased with greater long‐term rainfall (up to 18 months prior to survey), whilst Araneae showed the opposite effect. Temperature and dune zone (dune crest vs. swale) also had taxon‐specific effects. These results show that invertebrates in arid ecosystems are influenced by a variety of abiotic factors, at multiple scales, and that responses to rainfall are not as strong or as predictable as those seen for other taxa. Our results highlight the diversity of invertebrates in our study region and emphasize the need for targeted long‐term sampling to enhance our understanding of the ecology of these taxa and the role they play in arid ecosystems.     

Invertebrate hydraulic preferences and predicted impacts of changes in discharge in a large river

• 1 Instream habitat models are attractive tools for assessing the impacts of flow restoration on aquatic organisms, but have been rarely used for invertebrates due to the lack of knowledge of their hydraulic preferences.
• 2 In this study, we provide quantitative models of the hydraulic preferences for 66 invertebrate taxa found in the Upper Rhône River (France), and combine these preferences with a statistical habitat model (FSTress) to predict the effects of minimum flow restoration on invertebrate abundances in two bypassed sections of the river.
• 3 Many taxa show marked preferences for particular hydraulic conditions. Average preferred hydraulic conditions were significantly related to those observed in smaller streams (R² = 0.37).
• 4 Our model predicted that habitat values of 27 taxa should be enhanced by restoration operations, but also predicted a decrease of habitat values in the main channel for 12 limnophilic taxa. However, these taxa are expected to maintain populations by finding refuges in restored cut‐off channels and overall invertebrate diversity is predicted to increase in the two bypassed sections studied here.

     

Seasonal and diel patterns of invertebrate drift in different alpine stream types

1. We examined the seasonal and diel patterns of invertebrate drift in relation to seston and various habitat characteristics in two each of four different kinds of alpine streams [rhithral (snow‐fed) lake outlets, rhithral streams, kryal (glacial‐fed) lake outlets and kryal streams]. Samples were collected at four times of the day (dawn, midday, dusk and midnight) during three seasons (spring, summer and autumn). 2. Habitat characteristics differed mainly between rhithral and kryal sites, with the latter having higher discharge and turbidity, lower water temperature, and higher concentrations of ammonium, and particulate and soluble reactive phosphorus. Seasonality in habitat characteristics was most pronounced for kryal streams with autumn samples being more similar to rhithral sites. 3. The concentration of seston was lowest in the glacial‐influenced lake outlets and slightly higher in the stream sites; no seasonal or diel patterns were evident. 4. The density of drifting invertebrates averaged less than 100 m^?3 and was lowest (<10 m^?3) at three of the four kryal sites. Taxon richness and diversity were lowest at rhithral lake outlets. Chironomidae dominated the drift as well as benthic communities and <30% of benthic taxa identified were found in the drift. 5. Drifting invertebrates showed no consistent seasonal pattern. However, density tended to be highest in spring at rhithral sites and in autumn at kryal sites. No diel periodicity in drift density was found at any site and the lack of diel pattern may be a general feature of high altitude streams. 6. Glacially influenced habitat parameters were a major factor affecting drift in these alpine streams, whereas no clear differences were observed between streams and lake outlets. Our findings indicate that invertebrate drift in alpine streams is primarily influenced by abiotic factors, and therefore, substantially differs from patterns observed at lower altitude.     

Drought ecohydrology alters the structure and function of benthic invertebrate communities in mountain streams

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Resistance to ship-induced waves of benthic invertebrates in various littoral habitats

1. Ship‐induced waves disturb benthic invertebrate assemblages colonizing littoral zones of lakes and rivers. However, the impact of ship‐induced waves on invertebrates has rarely been quantified, and the influencing factors have not been addressed. 2. In an experimental wave tank, five benthic invertebrate species, Bithynia tentaculata, Calopteryx splendens, Dikerogammarus villosus, Gammarus roeseli and Laccophilus hyalinus, were exposed to waves of increasing shear stress (0.43–2.19 N m^?2). Mean number of detached individuals was recorded for five littoral habitats [coarse woody debris (CWD), reeds, sand, stones and tree roots], representing different levels of structural complexity as quantified by their fractal dimensions (FD). 3. Results showed that detachment of invertebrates was significantly related to shear stress in all habitats except tree roots. Detachments averaged for the five species were significantly lower in habitats with a high degree of structural complexity, decreasing in the habitat sequence: sand, CWD, stones, reeds and tree roots. 4. Consistent with their different morphologies and methods of attachment to substrates, the five species displayed differences in their response to hydraulic stress that were dependent on habitat. 5. The increasing sheltering effect of structural habitat complexity was mirrored by increasing dissipation of the kinetic energy of waves; i.e. the FD of the habitat was positively correlated with shear stress reduction due to the flow resistance of the habitat. 6. Network habitats such as tree roots provided the best sheltering conditions against hydraulic disturbance, because they combined good refuge availability for all studied invertebrate species and maximal dissipation of kinetic wave energy. Consequently, persistent anthropogenic impacts, such as lakeshore modification or long‐term exposure to ship‐induced waves, which cause disappearance of complex littoral habitats such as tree roots or dense reed belts, will drastically increase the adverse effects of boating and ship traffic on littoral invertebrate assemblages.     

Complex hydromorphology of meanders can support benthic invertebrate diversity in rivers

In freshwater environments, high biodiversity is commonly associated with habitat heterogeneity. River bends and meanders are particularly complex morphodynamic elements of watercourses. However, the specific spatio-temporal interactions between hydromorphology and the resident biota have scarcely been studied. This article reviews the relationships between hydraulic processes, and morphological units that are typical for meanders, and analyzes the concomitant spatial and temporal dynamics of habitats suitable for aquatic invertebrates. Flow in river bends is characterized by significant cross-stream velocities, which modify primary flow patterns, and create helical flow trajectories. Consequently, boundary shear stresses at the river-bed are altered, so that complex erosion, transport, and accumulation processes characteristically shape bed and bank morphology. The diversity of substrate types and complex bathymetry in meanders provide a large variety of habitat conditions for benthic invertebrates within a relatively small spatial domain, which are connected via hydraulic pathways. Periodic reversal of hydro-morphological processes between low and high flow, and seasonal growth of aquatic macrophytes creates spatio-temporal dynamics at the meso- and microhabitat scales. Such habitat dynamics increases benthic invertebrate diversity to the extent it is consistent with spatio-temporal scales of invertebrate mobility and life cycle. Furthermore, the presence of flow refugia, and hydraulic dead zones in meanders is essential to sustain species richness. This study concludes that meanders are highly complex morphodynamic elements that exhibit several self-regulating principles supporting invertebrate diversity and resilience in fluvial ecosystems.     

Effects of fuel reduction burning on epigeal arthropods and earthworms in dry sclerophyll eucalypt forest of west-central Victoria

The effects of prescribed low-intensity burning during spring and autumn on invertebrates in litter/upper soil were assessed in dry sclerophyll mixed eucalypt forest near Daylesford, west–central Victoria. The 4-year study was based on 68 848 arthropod specimens representing 29 ordinal or lower level taxa contained in 1980 pitfall trap samples, and on in situ counts of earthworms (Annelida) in 2220 litter/upper soils samples. The spring bum caused short-term reductions in activity among the common ‘major’ taxa Collembola (springtails) and Diptera (flies), and among the rarely trapped ‘minor’ taxa Opilionida (harvestmen), Lepidoptera (moths) and Apocrita (parasitic wasps) for up to one year. These reductions were associated with low fine fuel loads in the first year after the fire. Populations of earthworms also declined substantially, but recovered within 3 years of the burn. The autumn burn suppressed the Collembola and the ‘minor’ taxa Blattodea, Polydesmida, Thysanura and Tettigoniidae for up to 10 months. Earthworms were not affected. Very dry soil conditions were associated with depressed collembolan activity at study sites irrespective of burning. Given the importance of Collembola, larval Diptera and earthworms among decomposers in forest litter, it appears that the spring burn, and to a lesser extent the autumn burn, may have temporarily reduced the decomposer cycle. Further research on individual species is required to substantiate this conclusion, and also on the effects of high frequency burning. In the interim, any broadscale fuel reduction burning in forest ecosystems similar to that studied here should be scheduled for autumn rather than spring to protect earthworms and no burning should be permitted during drought periods, to minimize adverse impacts on the overall invertebrate fauna inhabiting litter/upper soil.     

Habitat‐specific invertebrate responses to hydrological variability,anthropogenic flow alterations,and hydraulic conditions

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Spatial and temporal occurrence of invertebrates in a chalk stream,Berkshire, England

The invertebrate assemblages of the River Lambourn and its tributary, the Winterbourne stream, were investigated as part of a broad ecological study prior to water resource development in the catchment. The longitudinal distribution of the invertebrate fauna within the Winterbourne stream was examined in detail and supporting data for the R. Lambourn revealed that the upstream intermittent section of each water course had an invertebrate assemblage which was distinct from the fauna downstream where flow was permanent. Within the perennial zone taxon richness tended to increase downstream.A one-year programme of monthly quantitative sampling on five distinct biotopes (Ranunculus, Berula, Callitriche, gravel and silt) at two contrasted sites (shaded and unshaded) was undertaken in the lower perennial of the R. Lambourn below the confluence with the Winterbourne. On each study site, the taxon richness, numerical abundance and biomass of invertebrates were significantly higher on the three macrophytes than on gravel and silt. There were also statistically significant seasonal changes in the biomass of invertebrates on each site. Estimates of total biomass of invertebrates per site revealed a late spring peak (May/June) and a late autumn (November/December) peak, neither of which coincided with the summer peak biomass of macrophytes. This suggested that, in the lower perennial, the level of food resources (epiphytic algae in late spring and fallen tree leaves plus decaying macrophytes in autumn) rather than available habitat, were a major influence on population levels for most of the year.Quantitative sampling of each biotope on each site continued in June and December only for a period of 8 years (1971–78), during which time the river experienced a minor drought (1973) and a severe drought (1976). The Chironomidae showed a strong positive response to conditions of low flow in June of 1973 and 1976, when exceptionally high densities of larvae were recorded. Although further between-year differences were recorded in several other major components of the invertebrate fauna, which may have been related to conditions of low discharge, they did not have long-term consequences for the fauna in the lower perennial of the R. Lambourn.In contrast, in the upper perennial, the prolonged drought of 1976 resulted in the loss of wetted perimeter and macrophytes, coupled with deposition of silt, all of which had deleterious effects on the invertebrate assemblages. The following year, despite a normal discharge regime and the removal of silt, some submerged macrophytes were slow to return to their pre-drought condition. In the intermittent section, the drought of 1976 resulted in the total loss of flow for a 16 month period instead of a more typical four month period of water loss.Thus, chalk streams encompass within a single system both stable and extreme hydrological regimes which offer unique opportunities for investigating the processes of community formation and maintenance.     

Microform bed clusters: are they preferred habitats for invertebrates in a flood‐prone stream?

SUMMARY 1. Microform bed clusters are bedform microunits in streams. They consist of an obstacle clast (a large surface stone) against which other stones are stacked in a specific manner. Stream ecologists have suggested that these clusters are better flood refugia and more valuable habitats for invertebrates than single surface stones because of their higher stability during high‐flow events and their greater diversity of microhabitats available for colonisation. 2. To test these predictions in the Schmiedlaine, a flood‐prone prealpine stream, we sampled invertebrates on clustered and single stones on a total of seven occasions including before and after a moderate spate, two major floods and a minor spate. We also monitored surface particle stability during the second major flood and the minor spate. 3. Before the second flood (return period 1–2 years), we determined the exact positions of 60 clusters (60 obstacle clasts with 136 associated smaller particles) and 50 single stones in a 250‐m reach. We sampled the fauna on stones in the clusters, on well‐embedded single stones, and on loose single stones. Only six obstacle clasts and one single stone remained after the flood, implying movement of almost the entire surface layer. Therefore, we sampled the invertebrates on stones in newly formed clusters, on well‐embedded and single loose stones 3 and 11 days after the flood. The minor spate, in contrast to the second flood, moved only one of 495 monitored surface particles. 4. Three days after the flood, invertebrate density on clustered stones and both single stone categories were similar and equalled 34% of the mean pre‐flood density. Eight days later, density had almost doubled. The relatively high survival and rapid recovery suggest that invertebrates found refugia during the flood. However, the very low stability of clusters and single stones implied that surface particles were unimportant as refugia. 5. Total invertebrate density and taxon richness were never higher on clustered than on single stones (regardless of the timing of sampling relative to the last previous high‐flow event). Densities of the seven most common taxa and invertebrate community structure were also generally similar between particle types. We conclude that microform bed clusters cannot be regarded as more valuable invertebrate habitats than single surface stones in the Schmiedlaine.     

Drought refuges,spatial scale and recolonisation by invertebrates in non‐perennial streams

1. If resistance traits drive recolonisation after drought, then drought refuges should contribute strongly to assemblage composition within streams. If resilience traits drive recolonisation, macroinvertebrates emerging from refuges may disperse widely, colonising many streams. To determine whether the contribution of drought refuges to macroinvertebrate recolonisation in non‐perennial streams was mostly local (within stream) or broader scale (across streams), we measured the association between the composition of invertebrate assemblages in different types of in‐stream drought refuge and the assemblage composition of streams when flow resumed. 2. We sampled 16 streams of varying hydrological regime on the western side of the Victoria Range in the Grampians National Park, Victoria, Australia. Drought refuges (perennial pools, dry sediment, damp sediment, seeps, patches of leaf litter, beneath stones) were identified and sampled during autumn. Most taxa were found in perennial pools; few taxa were found aestivating beneath stones or having desiccation‐resistant stages in dry sediment. Perennial pools and perennially flowing reaches were the refuges that harboured the greatest diversity of macroinvertebrate taxa. 3. Streams were sampled again during spring. Assemblage composition of non‐perennial reaches in spring was unrelated to composition in nearby refuges in the previous autumn. In contrast, assemblage composition in perennial reaches during spring was strongly correlated with composition during autumn. Therefore, drought refuges did not directly influence assemblage composition locally within non‐perennial streams. Rather, both perennially flowing reaches and perennial pools acted as drought refuges across the broader landscape. Resilience traits are likely to drive recolonisation in these streams. 4. Monitoring of drought refuges in a particular stream will therefore not predict species composition when flow resumes. Drought refuges are likely to sustain biodiversity over larger spatial scales such as groups of streams or whole drainage networks. Consequently, stream networks will need to be managed as entities rather than as single waterways and the focus of drought refuge protection should be on perennial pools and reaches.