Spatial and temporal trends in stream macroinvertebrate communities: the influence of catchment disturbance

Macroinvertebrate communities of five headwater streams in catchments disturbed by wildfire were compared with five similar streams with no catchment disturbance. Over the five years of observation, communities in disturbed streams were more similar to one another than they were to reference streams. Communities in disturbed streams exhibited more year-to-year variation than reference streams, although some indication of decreasing variation was evident through time, and species richness was greater in reference streams than disturbed streams. No increasing trend in richness over time was observed in disturbed streams. Stability of the relative abundance structure and persistence of dominant taxa through time may be characteristic of temperate streams over moderate time intervals. Local effects of catchment-wide disturbance have persistent effects that alter these trends.     

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).     

Short-term recovery of benthos following disturbance from stream habitat rehabilitation

The recovery of benthic macroinvertebrates after disturbance from stream rehabilitation was studied in the River Livojoki, northern Finland. The stream that had been channelized for log transport was rehabilitated on 1 July 1992 by digging holes and inserting boulders. We measured habitat characteristics and sampled benthic animals before and after rehabilitation, including an unrehabilitated control site. The immediate effect of rehabilitation was a slight decrease in the abundances of benthic insects. Recolonization occurred rapidly, within 10 days. Disturbance of the rehabilitation did not have a detectable effect on the macroinvertebrate community. Most species-level changes and community patterns reflected seasonal life history events. Timing of such rehabilitation work can be critical for the recovery rate, which depends on the colonization abilities of the species present after disturbance. We suggest that many disturbances (including minor floods and moderate rehabilitation procedures) may have only small, short-term effects on benthic communities. We emphasize the importance of considering seasonality in studies of disturbance in streams.     

Local disturbance history affects patchiness of benthic river algae

1. Recent research has shown that high‐flow events in streams leave a small‐scale mosaic of bed patches that have experienced scouring, sediment deposition (fill), or remained stable. Few studies have investigated if this ‘local disturbance history’ contributes to the patchy distribution of benthic organisms in streams and rivers. 2. In the present research, we demonstrate that local disturbance history in a mid‐sized river can have both short‐ and long‐term effects on epilithic algae. Chains buried vertically in the substratum of the river bed (236 in a 800‐m reach) indicated that two floods (return periods ≤1 year) caused a mosaic of bed patches with different disturbance histories. Once after the first and twice after the second flood, we sampled epilithic algae (mainly diatoms) in replicate patches that had been scoured, filled, or remained stable during the respective event. Algal biomass and cell density per substratum area were determined. 3. Three months after the first flood, algal biomass, total diatom density, diatom taxon richness, and densities of six of nine most common taxa were highest in fill patches. Six days after the second flood, biomass was highest in stable patches, indicating a refugium function of these patches. The refugium patches consisted of average‐sized stones, in contrast to previous studies of flood refugia for benthic algae in which these refugia were always large and/or immobile substrata. Four weeks after the second flood, diatoms tended to be most abundant in scour patches. With one exception, these differences between patch types could not be attributed to differences in local near‐bed current velocity or water depth. 4. The effects of disturbance history were more complex than a simple refugium function of stable patches because algal patterns changed with time since the last disturbance, possibly depending on the successional state of the algal mats.     

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     

Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages

• 1 It is axiomatic that unusually long dry periods (droughts) adversely affect aquatic biota. Recovery after drought is rapid by macroinvertebrates that possess strategies to survive drying or are highly mobile but other taxa take longer to recolonise depending on the timing, intensity, and duration of the dry phase.
• 2 Although drought acts as a sustained ‘ramp’ disturbance, impacts may be disproportionately severe when certain critical thresholds are exceeded. For example, ecological changes may be gradual while a riffle dries but cessation of flow causes abrupt loss of a specific habitat, alteration of physicochemical conditions in pools downstream, and fragmentation of the river ecosystem. Many ecological responses to drought within these habitats apparently depend on the timing and rapidity of hydrological transitions across these thresholds, exhibiting a ‘stepped’ response alternating between gradual change while a threshold is approached followed by a swift transition when a habitat disappears or is fragmented.
• 3 In two Australian intermittent streams, drought conditions eliminated or decimated several groups of macroinvertebrates, including atyid shrimps, stoneflies and free‐living caddisflies. These taxa persisted during the early stages of the drought but did not recruit successfully the following year, despite a return to higher‐than‐baseflow conditions. This ‘lag effect’ in response to drought emphasises the value of long‐term survey data. Although changes in faunal composition were inconsistent among sites, marked shifts in taxa richness, abundance and trophic organisation after the riffle habitat dried provide evidence for a stepped response.
• 4 Responses by macroinvertebrate assemblages to droughts of differing severity in English chalk streams were variable. The prolonged 1988–92 drought had a greater impact than shorter droughts in the early 1970s but recovery over the next 3 years was swift. Effects of the 1995 summer drought were buffered by sustained groundwater discharge from the previous winter. These droughts tended to reduce available riverine habitats, especially via siltation, but few taxa were eliminated because they could recolonise from perennial sections of the chalk streams.
• 5 In the contrasting environments of the intermittent streams studied in England and Australia, there are parallels in the rapid rates of recolonisation. However, recruitment by taxa that lack desiccation‐resistant stages or have limited mobility is delayed. Currently, long‐term data on these systems may be insufficient to indicate persistent effects of droughts or predict the impacts of excessive surface or groundwater abstraction or the increased frequency and duration of droughts expected with global climate change.

     

Effects of natural disturbance on stream communities: a habitat template analysis of arctic headwater streams

1. We used the habitat template approach to test the hypothesis that substratum stability, freezing and nutrient supply were determinants of community structure in 19 headwater streams of arctic Alaska. Streams were selected from five categories: glacier (n = 3), mountain (4), mountain spring (4), tundra spring (4) and tundra (4). 2. Bed movement among streams ranged from 0 to 97% during a ～2‐month summer season. Glacier and mountain streams had significantly higher bed movement than tundra and spring streams (P < 0.001). 3. All glacier and tundra streams froze solid during winter; all mountain spring streams remained unfrozen. Freezing among mountain and tundra spring streams was variable, with a subset of streams flowing throughout winter. 4. With the exception of glacier streams, which showed high concentrations of NH₄⁺ and NO₃^? (P < 0.001), differences in nutrient concentrations among stream types were not significant. 5. Algal taxon richness was greatest in tundra springs (13 taxa) and lowest in glacier streams (five taxa, P < 0.001), as was algal biovolume (7350 versus 687 mm³ m^?2, P < 0.001). Macroinvertebrate taxon richness was lower in glacier streams (4.7 ± 1.7, P < 0.005) than the other stream types (20.5–25.0 taxa), and biomass was greater in mountain springs (4837 mg m^?2) and tundra springs (3367 mg m^?2, P < 0.001). 6. Multidimensional scaling and multiple regression analyses of macroinvertebrate (biomass) and periphyton (biovolume) indicated that a 2‐dimensional habitat template with bed movement and freezing as axes provides an accurate model of major factors controlling the community structure of headwater streams in arctic Alaska.     

Habitat heterogeneity and disturbance influence patterns of community temporal variability in a small temperate stream

Both habitat heterogeneity and disturbance can profoundly influence ecological systems at many levels of biological and ecological organization. However, the joint influences of heterogeneity and disturbance on temporal variability in communities have received little attention despite the intense homogenizing influence of human activity. I performed a field manipulation of substrate heterogeneity in a small New England stream, and measured changes in benthic macroinvertebrate communities for 100 days—a period that included both a severe drought and a flood. Generally, community variability decreased with increasing substrate heterogeneity. However, within sampling intervals, this relationship tended to fluctuate through time, apparently tracking changes in hydrology. At the beginning of the experiment, community temporal variability clearly decreased along a gradient of increasing substrate heterogeneity—a result consistent with an observational study performed the previous year. During the subsequent weeks, droughts and flooding created exceptionally high variability in both hydrology and benthic macroinvertebrate community structure resulting in the disappearance of this relationship. However, during the last weeks of the experiment when hydrologic conditions were relatively more stable, the negatively sloped relationship between community temporal variability and habitat heterogeneity reemerged and mimicked relationships observed both early in the experiment and in the previous year’s study. High habitat heterogeneity may promote temporal stability through several mechanisms including stabilization of resources and increased refugia from minor disturbances or predation. However, the results of this experiment suggest that severe disturbance events can create large-scale environmental variability that effectively swamps the influence of habitat heterogeneity, illustrating that a thorough understanding of community temporal variability in natural systems will necessarily consider sources of environmental variability at multiple spatial and temporal scales. Handling editor: L. M. Bini     

Spatial and temporal impacts of a diesel fuel spill on stream invertebrates

1. We assessed the effects of a 26 500 L diesel fuel spill on the macroinvertebrate fauna of a small trout stream in central New York, U.S.A. To determine the spatial extent of the spill we sampled three locations (0.7, 5.0 and 11.8 km downstream of the spill), each containing a reference site (unaffected tributary) and an impact site (downstream of spill). Sampling was repeated four times over a 15‐month period to assess temporal recovery. 2. Immediately after the spill, invertebrate density at all three locations below the spill was significantly lower than reference density. Three months after the spill, density up to 5 km below the spill was still far lower (<100 individuals per sample) than reference density (800–1200 individuals per sample). A year after the spill, density was similar between reference and impact sites, suggesting that invertebrates had recovered numerically. 3. Taxonomic richness up to 5.0 km below the spill was less than half the reference taxonomic richness and this difference persisted for at least 3 months. Some significant differences between reference and impact sites were observed after 15 months, but these differences could not be attributed to the oil spill. 4. For at least 3 months following the spill, the site immediately downstream of the spill was dominated by Optioservus, a petrochemical‐tolerant riffle beetle. Twelve to 15 months after the spill, both the reference and impact sites near the spill were dominated numerically by the mayfly Ephemerella, but the degree of dominance was twice as large at the impact site. 5. We concluded that the diesel fuel spill significantly reduced the density of invertebrates (by 90%) and taxonomic richness (by 50%) at least 5.0 km downstream, but density recovered within a year. Throughout the study, however, the fauna immediately below the spill was species poor and significantly over‐represented by a single dominant taxon, suggesting that 15 months was not sufficient for full community recovery from the oil spill.     

Habitat heterogeneity drives the geographical distribution of beta diversity: the case of New Zealand stream invertebrates

To define whether the beta diversity of stream invertebrate communities in New Zealand exhibits geographical variation unexplained by variation in gamma diversity and, if so, what mechanisms (productivity, habitat heterogeneity, dispersal limitation, disturbance) best explain the observed broad‐scale beta diversity patterns. We sampled 120 streams across eight regions (stream catchments), spanning a north–south gradient of 12° of latitude, and calculated beta diversity (with both species richness and abundance data) for each region. We explored through a null model if beta diversity deviates from the expectation of stochastic assembly processes and whether the magnitude of the deviation varies geographically. We then performed multimodel inference analysis on the key environmental drivers of beta diversity, using Akaike's information criterion and model and predictor weights to select the best model(s) explaining beta diversity. Beta diversity was, unexpectedly, highest in the South Island. The null model analysis revealed that beta diversity was greater than expected by chance in all eight regions, but the magnitude of beta deviation was higher in the South Island, suggesting differences in environmental filtering and/or dispersal limitation between North and South Island. Habitat heterogeneity was the predominant driver of beta diversity of stream macroinvertebrates, with productivity having a secondary, and negative, contribution. This is one of the first studies accounting for stochastic effects while examining the ecological drivers of beta diversity. Our results suggest that local environmental heterogeneity may be the strongest determinant of beta diversity of stream invertebrates, more so than regional‐ or landscape‐scale variables.     

Flood effects on invertebrates, sediments and particulate organic matter in the hyporheic zone of a gravel-bed stream

1. We investigated the effects of a flood on the fauna and physical habitat of the hyporheic zone of the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. Freeze core hyporheic samples (to 50 cm depth) and benthic samples (to 10 cm) were taken, along with measurements of vertical hydrological gradient, before, 2 days after and 1 month after the flood (estimated return period: 1.5 years, estimated Q_max = 10.4 m³ s^?1). 3. The composition of the hyporheos differed over the three sampling occasions with fewer taxa collected immediately postflood than preflood. The equitability of the community was higher on both postflood occasions, consistent with the reduced densities of two abundant taxa (Leptophlebiidae and Copepoda). 4. Total invertebrate abundance was lower on the postflood occasions than preflood in both benthic (0–10 cm) and hyporheic (10–50 cm) sediments. Several taxa, including asellotan isopods and amphipods, recovered within 1 month of the event. Hyporheic densities of larval Hydora and nematodes did not differ among the three sampling occasions, but the water mite Pseudotryssaturus was more abundant 1 month after the flood than preflood. There was no evidence of vertical movements (to 50 cm) by any taxa in response to the flood. 5. The proportion of fine sediments (<1 mm) in the subsurface sediments (10–50 cm) increased over the three sampling occasions and median particle size declined, but sediment porosity did not change. More particulate organic matter was found in the sediments after the flood. 6. Our study provides little evidence that the hyporheic zone (to 50 cm) acted as a significant refuge during the flood event, although movements to or recolonisation from sediments deeper than 50 cm could explain the recovery of many crustacean and mite taxa within 1 month.     

The effects of disturbance on a herbivore-epilithon interaction in an upland stream

Two experiments were performed in an upland stream to determine the effects of the frequency of physical disturbance on the relationship between an abundant glossosomatid trichopteran (Agapetus monticolus; Banks) and the epilithon upon which it feeds. Artificial cobbles with an established epilithic community were tumbled either every 1, 2 or 4 weeks. The first experiment failed to detect any significant effects of rock tumbling on the abundance of A. monticolus or the epilithon: a result due to several spates. The first experiment did reveal that disturbances may disrupt the ability of A. monticolus to locate patches of abundant food. The second experiment found that although the abundance of A. monticolus was not affected by the disturbances, periphyton abundance was significantly reduced. Increasing the frequency of disturbance did not magnify this effect. Comparisons of these results with other studies of disturbance in streams indicate that the effects of disturbance on herbivory may be highly variable. A variety of factors, such as the relative resistances of the herbivores and the epilithon, need to be examined before the effects of disturbances on lotic herbivorous interactions can be completely understood.     

Impact of simulated drought on ecosystem biomass production: an experimental test in stream mesocosms

Climate models predict widespread shifts in precipitation patterns and increases in the frequency of extreme events such as droughts, but consequences for key processes in affected ecosystems remains poorly understood. A 2‐year manipulative experiment used a series of stream mesocosms to test the effect of recurrent drought disturbance on the composition and secondary production of macroinvertebrate consumer assemblages and functional groups. On average, secondary production in drought‐disturbed communities (mean 4.5 g m^?2 yr^?1) was less than half of that that in controls (mean 10.4 g m^?2 yr^?1). The effects of the drought differed among functional feeding groups, with substantial declines for detritivore shredders (by 69%) and engulfing predators (by 94%). Contrasting responses were evident among taxa within most functional feeding groups, ranging from extirpation to irruptions in the case of several small midge larvae, but production of most species was suppressed. Taxon‐specific responses were related to body mass and voltinism. The ratio of production to biomass (community P/B) increased under drought, reflecting a shift in production from large long‐lived taxa to smaller taxa with faster life cycles. This research provides some of the first experimental evidence of the profound effects that droughts can have on both the structure and functioning of aquatic ecosystems.     

Urbanisation and earthquake disturbance influence microbial nutrient limitation in streams

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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.     

Interactive community responses to disturbance in streams: disturbance history moderates the influence of disturbance types

Both disturbance history and disturbance type act to structure communities through selecting for particular species traits but they may also interact. For example, flooding selects for species with flood‐resistant traits in streams, but those traits could make communities susceptible to other disturbances and so could cause shifts in community composition due to anthropogenic climate change. To better understand the interactive influences of disturbance history and type on community composition, we investigated the response of macroinvertebrate communities to disturbance using in‐stream channels. Using a split‐plot design, individual channels in five ‘stable’ streams and five ‘frequently disturbed’ streams (disturbance history) were subject to different disturbance type treatments (flooding, drying and a control). Disturbance type independently drove effects on species diversity, but all other effects of disturbance type depended on disturbance history. In particular, the interaction of disturbance type and history determined overall community response. Both disturbance types tested produced similar community responses in frequently disturbed streams, including changes in community composition and alterations to the abundance of less mobile taxa, but low‐flow had a significantly greater effect in stable streams. Macroinvertebrate drift was greatest in the rock‐rolling treatments and significantly less in the low‐flow treatment for both disturbance histories. Therefore, disturbance history moderated the effects of disturbance type and determined the mechanism of community response by determining how well species were adapted to disturbance. This outcome suggests that previous disturbances strongly influence how vulnerable communities are to changes in disturbance, and so should be considered when predicting how changes in disturbance regimes will affect future community composition.     

Effects of physical disturbance and canopy cover on attached diatom community structure in an Idaho stream

A field experiment was conducted to determine the effect of disturbance frequency on diatom communities established on artificial substrates within an open canopy site and a closed canopy site of a 3rd order stream. The open canopy site (OCS) had a total of 80 diatom taxa colonizing the substrates, while the closed canopy site (CCS) had only 55 taxa. Cluster analysis revealed that the two sites had distinct diatom communities, although the most common species were similar between sites. There was no effect of disturbance frequency on species diversity (H') at OCS, however species diversity significantly decreased as disturbance frequency increased at CCS. At OCS, Amphora perpusilla increased in abundance as disturbance frequency increased, while Navicula lanceolata abundance decreased as disturbance increased. At CCS, Cocconeis placentula v. euglypta remained dominant regardless of disturbance frequency. The results suggest that some diatom species may be shade adapted, which may explain the site-specific responses. In addition, diatom growth-forms may explain the within site taxon-specific responses to disturbance. For example, Achnanthes sp. and Cocconeis sp., small horizontal forms, were predominant on the high disturbance substrates. Vertical or large horizontal forms may be mechanically removed by frequent physical disturbance allowing such small horizontal forms to become abundant. Frequent disturbance, by maintaining the community in an early stage of development, directly influences the diatom assemblage on rocks in streams.     

Patchy bed disturbance and fish predation independently influence the distribution of stream invertebrates and algae

1. The identification of factors determining the patchy distribution of organisms in space and time is a central concern of ecology. Predation and abiotic disturbance are both well-known drivers of this patchiness, but their interplay is still poorly understood, especially for communities dominated by mobile organisms in frequently disturbed ecosystems. 2. We investigated the separate and interactive influences of bed disturbance by floods and predation by fish on the benthic community in a flood-prone stream. Electric fields excluded fish predators from half of 48 stream bed patches (area 0·49 m(2) ) with contrasting disturbance treatments. Three types of bed disturbance were created by either scouring or filling patches to a depth of 15-20 cm or by leaving the patches undisturbed, thus mimicking the mosaic of scour and fill caused by a moderate flood. Benthic invertebrates and algae were sampled repeatedly until 57 days after the disturbance. 3. Disturbance influenced all ten investigated biological response variables, whereas predation affected four variables. Averaged across time, invertebrate taxon richness and total abundance were highest in stable patches. Algal biomass and densities of five of the seven most common invertebrate taxa (most of which were highly mobile) were higher in fill than in scour patches, whereas two taxa were more abundant in scour and stable than in fill patches. Furthermore, two common invertebrate grazers were more abundant and algal biomass tended to be reduced in fish exclusion patches, suggesting a patch-scale trophic cascade from fish to algae. 4. Our results highlight the importance of patchy physical disturbance for the microdistribution of mobile stream organisms and indicate a notable, but less prevalent, influence of fish predation at the patch scale in this frequently disturbed environment. Disturbance and predation treatments interacted only once, suggesting that the observed predation effects were largely independent of local bed disturbance patterns.     

Impacts of climatic stability on the structural and functional aspects of macroinvertebrate communities after severe drought

1. Unpredictable, extreme climatic events (e.g. droughts) can potentially destabilize aquatic communities. From 1998 to 2002, southwestern Georgia, U.S.A., experienced the third worst drought of the last 100 years, leading to loss of surface flow in many small streams. We sampled macroinvertebrates, flow and water chemistry in small headwater streams from 2001 to 2007 in two adjacent coastal plain streams of contrasting headwater type (wetland and groundwater‐fed seep) following resumption of flow. 2. Decreasing water temperature, conductivity and nutrient concentrations within the first 2 years of the study indicated flushing of the streambed associated with increased discharge. Invertebrate community composition became less variable over time and during wetter years, reflecting water chemistry, hydrological and climate conditions. 3. A core set of species appeared immediately following breaking of the drought in both streams, reflecting a shared species pool. These species exhibited resilience traits, including short life cycles and resistance to desiccation, which allowed for rapid recovery from disturbance. Such species, which were small‐bodied, sclerotized and abundant in the drift, were then replaced as flows increased by those that were larger, soft‐bodied and rare in drift, suggesting a more stable and less ephemeral habitat. 4. Hydrologic regime and long‐term precipitation indices were strongly correlated with invertebrate community and trait structure. Long‐term data allowed for better interpretation of the effects of infrequent disturbances on aquatic ecosystems. Additionally, long‐term precipitation indices (i.e. 48‐month standardized precipitation index) can indicate the likelihood of a return to drought, allowing for the collection of pre‐disturbance data.