Leaf breakdown, detrital resources, and food webs in streams affected by mine drainage

Breakdown of leaf litter is essential for providing detrital resources for food webs but can be impaired by anthropogenic activities, which may disrupt energy flow to consumers. We investigated the relationship between leaf breakdown and food web structure in 12 streams with or without mining impacts on South Island, New Zealand. Six streams received inputs of acid mine drainage (pH 2.5–4.9), three were naturally acidic (pH ~5.0), and three were circumneutral (pH ~6.8). Streams affected by mining either had highly acidic water (pH <3) or iron precipitates present on substrata. Breakdown rates of leaves were significantly lower in mining-affected streams than circumneutral (by almost 50%) but not naturally acidic streams and were driven primarily by microbial activity, as shredding invertebrates were often absent. Mining-affected stream webs were simplified structures with fewer species and links than those in other streams. With few species to process leaf litter and transfer detrital resources, inputs of AMD disrupted both the mechanisms responsible for breakdown and links for energy flow. While faster breakdown rates were associated with larger food webs, limited function maintained in mining-affected streams was sufficient to support primary consumers and small food webs.     

Diatom community response to an acid mine drainage gradient

Acid mine drainage (AMD) is a significant environmental issue worldwide. On the West Coast of the South Island, New Zealand, many AMD-affected streams occur within close proximity to naturally acidic streams, enabling us to compare the response of communities in naturally and anthropogenic acidified systems. We investigated epiphytic diatom communities in 39 streams along an AMD gradient that included naturally acidic and circum-neutral reference streams. There was a wide range in taxonomic richness in reference streams and those moderately impacted by AMD (8–33 taxa). Taxonomic richness was greatly reduced in severely impacted streams (1–5 taxa) at a threshold of pH 3.4 and was dominated by Pinnularia cf. acidophila (69–100% relative abundance). Community composition differed between circum-neutral reference streams and moderately and severely impacted streams. However, naturally acidic and moderately impacted streams had similar diatom communities primarily composed of Eunotia and Frustulia species. Our results indicate that diatom communities are strongly structured by pH and able to tolerate moderate conductivity and metal concentrations. This is a challenge for researchers and water managers attempting to incorporate diatoms into AMD monitoring in regions with naturally acidic streams.     

Impact of Headwater Stream Acidification on the Trophic Structure of Macroinvertebrate Communities

In order to investigate the impact of freshwater acidification on the trophic structure of macroinvertebrate communities, we performed a study on 22 forested headwater streams characterised by different degrees of acidification (mean pH = 4.49 to 6.98). Results showed that in acidic streams all functional feeding groups were affected in terms of taxonomic richness. As far as the population density was concerned, only a few acid-tolerant taxa of shredders and predators showed an increasing abundance under acidic conditions. Trophic structure of acidified streams appeared to be deeply impacted with a large contribution of shredders and a complete disappearance of scrapers. In contrast, in circumneutral streams, we found that each functional feeding group had an almost equal share of the trophic web.     

Does freshwater macroinvertebrate diversity along a pH‐gradient reflect adaptation to low pH?

1. The impacts of anthropogenic surface water acidification are much better known than those of natural acidity. Recent studies have indicated biodiversity is not degraded and species composition unaltered in naturally acidic compared to circumneutral watercourses.
2. Here, we use a geographically extensive dataset comprising sites in more than 200 Swedish streams to test whether the lack of effects on macroinvertebrate species diversity is due to exaptation and adaptation to natural acidity.
3. To this end, we modelled pH associated with spring flood episodes, which inflict the most challenging hydrochemical conditions to the biota. We compared taxonomic richness and species composition along the modelled pH gradient in northern Sweden, where acidity is largely natural, with southern Sweden, a region influenced by significant anthropogenic acidification.
4. We found Plecoptera richness did not respond to varying pH either in northern or southern Sweden. Ephemeroptera richness was sensitive to pH in both regions, while that of Trichoptera increased with increasing pH in southern Sweden, but decreased in the north. The taxonomic composition of Plecoptera changed along the pH gradient in both regions, whereas that of Ephemeroptera and Trichoptera changed more strongly with pH in southern Sweden.
5. Our results support the hypothesis that stream invertebrates are able to tolerate low pH through exaptation or adaptation, but that this capability varies among taxonomic groups.     

Naturally acid freshwater ecosystems are diverse and functional: evidence from boreal streams

Acidification has strongly affected natural ecosystems due to the deposition of acidifying pollutants in geographical areas with low buffering capacity. Here we show both that taxonomic richness of benthic invertebrates (as a measure of stream health) and breakdown rate of leaf litter (a major process in stream ecosystems) in three naturally acid streams were not significantly different from richness and breakdown rates of three circumneutral control streams in the same area in northern Sweden. In contrast, a comparative data-set representing a gradient from circumneutral to acidified streams in France showed decreasing richness and litter breakdown rates (by 60% and 70% between pH 6.5 and 4.5, respectively) typical of acidified streams. The strong negative effects found in many freshwater systems may be a consequence of low capacity to adjust to rapidly changed conditions. In contrast, organisms in naturally acidic systems, such as streams of the boreal region in catchments with accumulating organic material and a geology poor in buffering substances, seem better adapted as a consequence of protracted exposure to low pH over evolutionary time. These results have implications for the management of streams and rivers in northern Sweden, where considerable efforts are spent on remediation without consideration of the fact that the natural state of many of these systems is acidic.     

Acidic episodes retard the biological recovery of upland British streams from chronic acidification

We tested two predictions required to support the hypothesis that anthropogenic acidic episodes might explain the poor biological response of upland British streams otherwise recovering from acidification: (i) that invertebrate assemblages should differ between episodic and well-buffered streams and (ii) these effects should differentiate between sites with episodes caused by anthropogenic acidification as opposed to base-cation dilution or sea-salt deposition. Chronic and episodically acidic streams were widespread, and episodes reflected acid titration more than dilution. Nonmarine sulphate (16–18% vs. 5–9%), and nitrate (4–6% vs. 1–2%) contributed more to anion loading during episodes in Wales than Scotland, and Welsh streams also had a larger proportion of total stream sulphate from nonmarine sources (64–66% vs. 35–46%). Sea-salts were rarely a major cause of episodic ANC or pH reduction during the events sampled. By contrast, streams with episodes driven by strong anthropogenic acids had lower pH (5.0±0.6) and more dissolved aluminium (288±271 μg L⁻¹) during events than where episodes were caused by dilution (pH 5.4±0.6; 116±110 μg Al L⁻¹) or where streams remained circumneutral (pH 6.7±1.0; 50±45 μg Al L⁻¹). Both biological predictions were supported: invertebrate assemblages differed among sites with different episode chemistry while several acid-sensitive species were absent only where episodes reflected anthropogenic acidification. We conclude that strong acid anions – dominantly nonmarine sulphate – still cause significant episodic acidification in acid-sensitive areas of Britain and may be a sufficient explanation for slow biological recovery in many locations.     

Detrital processing in streams exposed to acidic precipitation in the Central Appalachian Mountains

Continuing high rates of acidic deposition in the eastern United States may lead to long-term effects on stream communities, because sensitive catchments are continuing to lose anions and cations. We conducted a two-year study of the effects of pH and associated water chemistry variables on detrital processing in three streams with different bedrock geology in the Monongahela National Forest, West Virginia. We compared leaf pack processing rates and macroinvertebrate colonization and microbial biomass (ATP concentration) on the packs in the three streams. Breakdown rates of red maple and white oak leaf packs were significantly lower in the most acidic stream. The acidic stream also had significantly lower microbial and shredder biomass than two more circumneutral streams. Shredder composition differed among streams; large-particle detritivores dominated the shredder assemblages of the two circumneutral streams, and smaller shredders dominated in the acidic stream. Within streams, processing rates for three leaf species were not significantly different between the two years of the study even though invertebrate and microbial communities were different in the two years. Thus, macroinvertebrate and microbial communities differed both among streams that differed in their capacity to buffer the effects of acidic precipitation and among years in the same stream; these differences in biotic communities were not large enough to affect rates of leaf processing between the two years of the study, but they did significantly affect processing rates between acidic and circumneutral streams.The Unit is jointly sponsored by the National Biological Service, the West Virginian Division of Natural Resources, West Virginia University, and the Wildlife Management Institute.The Unit is jointly sponsored by the National Biological Service, the West Virginian Division of Natural Resources, West Virginia University, and the Wildlife Management Institute.     

The effects of catchment liming on the chemistry and biology of upland Welsh streams: testing model predictions

1. The catchments of three acidified streams in mid Wales were limed in 1987/88. Here we assess their chemical and biological response in comparison with unmanipulated reference streams over a period of 5 years post-liming. 2. Stream chemistry was measured weekly/fortnightly between 1985 and 1992, while macroinvertebrates were sampled annually. Colonization by acid-sensitive taxa was assessed and trends in community structure were monitored by TWINSPAN. Real biological responses were compared with those predicted by an empirical model constructed using chemical data. 3. There were marked changes in stream chemistry following liming: calcium concentrations and pH values increased, while aluminium concentrations decreased to levels similar to those in naturally circumneutral streams. These conditions have persisted since liming. 4. Empirical models predicted that stream invertebrates would respond to the altered stream chemistry, with the establishment of communities typical of circumneutral conditions. 5. Following liming, the taxon richness and abundance of acid-sensitive taxa was significantly higher in limed compared with reference streams. Colonization by, and persistence of acid-sensitive taxa was patchy, however, and richness was still substantially lower than in naturally circumneutral streams. Moreover, contrary to the model predictions, there were no wholesale changes in the structure of macroinvertebrate communities. 6. We conclude that liming has created and maintained chemical conditions suitable for macroinvertebrate communities typical of circumneutral streams, but these chemical changes have not been matched by sustained responses among the biota. The views expressed in this article are those of the authors and do not necessarily reflect those of the NRA.     

Contrasting effects of anthropogenic and natural acidity in streams: a meta-analysis

Large-scale human activities including the extensive combustion of fossil fuels have caused acidification of freshwater systems on a continental scale, resulting in reduced species diversity and, in some instances, impaired ecological functioning. In regions where acidity is natural, however, species diversity and functioning seem to be less affected. This contrasting response is likely to have more than one explanation including the possibility of adaptation in organisms exposed to natural acidity over evolutionary time scales and differential toxicity due to dissimilarities in water chemistry other than pH. However, empirical evidence supporting these hypotheses is equivocal. Partly, this is because previous research has mainly been conducted at relatively small geographical scales, and information on ecological functioning in this context is generally scarce. Our goal was to test whether anthropogenic acidity has stronger negative effects on species diversity and ecological functioning than natural acidity. Using a meta-analytic approach based on 60 datasets, we show that macroinvertebrate species richness and the decomposition of leaf litter -- an important process in small streams -- tend to decrease with increasing acidity across regions and across both the acidity categories. Macroinvertebrate species richness, however, declines three times more rapidly with increasing acidity where it is anthropogenic than where it is natural, in agreement with the adaptation hypothesis and the hypothesis of differences in water chemistry. By contrast, the loss in ecological functioning differs little between the categories, probably because increases in the biomass of taxa remaining at low pH compensate for losses in functionality that would otherwise accompany losses of taxa from acidic systems. This example from freshwater acidification illustrates how natural and anthropogenic stressors can differ markedly in their effects on species diversity and one aspect of ecological functioning.     

Impacts of acidification on macroinvertebrate communities in streams of the western Adirondack Mountains,New York,USA

Limited stream chemistry and macroinvertebrate data indicate that acidic deposition has adversely affected benthic macroinvertebrate assemblages in numerous headwater streams of the western Adirondack Mountains of New York. No studies, however, have quantified the effects that acidic deposition and acidification may have had on resident fish and macroinvertebrate communities in streams of the region. As part of the Western Adirondack Stream Survey, water chemistry from 200 streams was sampled five times and macroinvertebrate communities were surveyed once from a subset of 36 streams in the Oswegatchie and Black River Basins during 2003–2005 and evaluated to: (a) document the effects that chronic and episodic acidification have on macroinvertebrate communities across the region, (b) define the relations between acidification and the health of affected species assemblages, and (c) assess indicators and thresholds of biological effects. Concentrations of inorganic Al in 66% of the 200 streams periodically reached concentrations toxic to acid-tolerant biota. A new acid biological assessment profile (acidBAP) index for macroinvertebrates, derived from percent mayfly richness and percent acid-tolerant taxa, was strongly correlated (R² values range from 0.58 to 0.76) with concentrations of inorganic Al, pH, ANC, and base cation surplus (BCS). The BCS and acidBAP index helped remove confounding influences of natural organic acidity and to redefine acidification-effect thresholds and biological-impact categories. AcidBAP scores indicated that macroinvertebrate communities were moderately or severely impacted by acidification in 44–56% of 36 study streams, however, additional data from randomly selected streams is needed to accurately estimate the true percentage of streams in which macroinvertebrate communities are adversely affected in this, or other, regions. As biologically relevant measures of impacts caused by acidification, both BCS and acidBAP may be useful indicators of ecosystem effects and potential recovery at the local and regional scale.     

Organic matter breakdown as a measure of stream health in New Zealand streams affected by acid mine drainage

Functional indicators of stream health have the potential to provide insights into stream condition that cannot be gained by traditional structural indices. We examined breakdown of leaves, wood, and cotton cloth strips at 18 sites along a gradient of effects of drainage from coal mines in New Zealand to determine the usefulness of these methods as functional indicators of stream health. The pH varied from 2.7 to neutral across the streams, and the more acidic streams typically had higher concentrations of aluminum, iron, zinc, and other metal ions. Precipitates of metal (mainly iron) hydroxides were present in most streams affected by mine drainage, especially in those with a pH of 4–5. Breakdown rates of all organic matter types were highest in several reference streams with neutral pH and lowest in sites with high rates of metal hydroxide deposition. Breakdown was relatively fast in the most acidic streams (pH < 3), in some cases as fast as at reference sites; these sites also had elevated nutrient concentrations. Shredding invertebrates were absent in litterbags from acidic streams and common at only 2 reference sites; their presence contributed to fast breakdown of leaves in the field and in lab microcosms. Microbial respiration was closely related to breakdown rates of leaves and wood; it was high at neutral and highly acidic streams, but lower at sites with pH 4–5, where metal hydroxides were precipitating onto solid surfaces. In these metal hydroxide-stressed streams, leaf and wood breakdown was slower, and associated biota, including microbes, were more affected than by water chemistry stressors (pH, dissolved metals) associated with mine drainage. Litter breakdown and microbial respiration provide insight into the functioning of streams, yielding different responses than traditional structural measures based on macroinvertebrates, which did not accurately distinguish impacts from acid mine drainage.     

Humic substances: remediation option for anthropogenically acidified waterways

The acidification of waterways through anthropogenic means is a global problem. Treatment options for anthropogenically acidified waterways are currently limited. This review proposes the use of humic substance (HS) to treat waterways acidified via anthropogenic means such as those affected by acid rain, acid mine drainage and acid sulphate soils. In laboratory settings, HS have been shown to successfully remove metals from solutions. HS also have the added benefit of being present in all natural waters and have been shown to positively influence aquatic organism’s survival to acidification, metals and the combined stressors of low pH and metal mixtures commonly associated with anthropogenically acidified waters. Comparative food webs of waterways affected by natural and anthropogenic acidification are provided along with a third conceptual food web showing the potential benefits of adding HS to an affected system. HS may represent a sustainable remediation option as it may be locally sourced from environmentally friendly options such as green waste. However, if such options are to be fully developed and utilised, the specific requirements for treating anthropogenically acidified waters with different HS sources (e.g. treatment volumes and efficacy) will require further study.     

Microhabitat availability in Welsh moorland and forest streams as a determinant of macroinvertebrate distribution

SUMMARY. 1 Eighteen streams in mid-Wales were sampled for macro-invertebrates in both riffles and margins in April 1985–87. Stream macro-flora, substrata and marginal habitats were surveyed in May 1988.
2. TWINSPAN classification of the macroinvertebrate data indicated three major stream groups. One was distinguished by circumneutral pH and had a flora and fauna typical of such conditions. The other two groups consisted of acidic streams with moorland and conifer afforested catchments respectively. The forest streams were the more acidic but the two groups also differed significantly in the composition of their marginal habitats.
3 The acidic moorland streams had more vegetation ('soft' features) in the margins and supported several invertebrate taxa which were relative more abundant there than in the riffles. These taxa may be excluded from forest streams because the margins are 'hard' due to greater erosiveness and shading.
4. In view of the increasing cover by conifer afforestation in Britain, it is clearly necessary to elucidate all its effects on stream ecosystems, which include changes to the physical environment.     

Interactive effects of three pervasive marine stressors in a post-disturbance coral reef

Ecosystems are commonly affected by natural, episodic disturbances that can abruptly and drastically alter communities. Although it has been shown that resilient ecosystems can eventually recover to pre-disturbed states, the extent to which communities in early stages of recovery could be affected by multiple anthropogenic stressors is poorly understood. Pervasive and rising anthropogenic stressors in coastal marine systems that could interactively affect the recovery of these systems following natural disturbances include high sedimentation, nutrient enrichment, and overfishing. Using a 6-month field experiment, we examined the effects of all combinations of these three stressors on key functional groups in the benthic community growing on simulated, post-disturbance reef patches within a system recovering from large-scale natural disturbances (corallivorous seastar outbreak and cyclone). Our study revealed that sedimentation, nutrient enrichment, and overfishing (simulated using exclusion cages) interactively affected coral survival and algal growth, with taxon-specific effects at multiple scales. First, our treatments affected corals and algae differently, with sedimentation being more detrimental to macroalgal growth but less detrimental to coral (Porites rus) survival in caged plots, driving significant interactions between sedimentation and caging for both taxa. We also observed distinct responses between coral species and between algal functional groups, with the most extensive responses from algal turf biomass, for which sedimentation suppressed the synergistic (positive) combined effect of nutrient enrichment and caging. Our findings suggest that different combinations of ubiquitous anthropogenic stressors, related to either sea- or land-based activities, interactively influence community recovery from disturbance and may alter species compositions in the resulting community. Our findings further suggest that anthropogenic stressors could promote further degradation of coral reefs following natural disturbances by inhibiting recovery to coral-dominated states that provide vital ecosystem services to coastal populations worldwide.     

Co‐varying impacts of land use and non‐native brown trout on fish communities in small streams

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The impact of regional landscape context on local maladaptive trait divergence: a field test using freshwater copepod acid tolerance

Theoretical work has shown that spatial landscape context can contribute to reducing local adaptation in populations depending on the spatial pattern of environmental heterogeneity, the spatial scale of distances between habitats on landscapes, and the level of habitat connectivity. However, only a handful of empirical studies have addressed the impact of regional landscape context on local trait divergence in natural populations. We tested if local adaptation in abiotic tolerance is diminished in landscapes with strong spatial heterogeneity and habitat proximity. We used a freshwater copepod (Leptodiaptomus minutus) that is known to show local adaptive divergence in acid tolerance as a study system to understand the effects of regional landscape-level spatial heterogeneity in lake/pond pH on local trait divergence. We compared local divergence in copepod acid tolerance from three types of landscapes: (i) a homogeneous pH landscape of exclusively circumneutral lakes (pH ≥ 6.0); (ii) a heterogeneous pH landscape with a mixture of acidic and circumneutral lakes; and (iii) a heterogeneous pH landscape in which relatively infrequent circumneutral ponds are embedded in a predominantly acidic landscape. We found that local adaptation to circumneutral lake/pond pH was most reduced in the pH-heterogeneous landscape dominated by acidic habitats, likely because of gene flow from surrounding nearby acidic ponds. Our study empirically confirms theoretical predictions that spatial landscape context is important for explaining regional differences in population environmental tolerances. These effects may become important for understanding regional differences in population fitness trade-offs when presented in combination with multiple stressors.     

Growth and population size of crayfish in headwater streams: individual- and higher-level consequences of acidification

1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams. 2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity. 3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites. 4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.     

The effects of low pH and palliative liming on beech litter decomposition in acid-sensitive streams

The decomposition of allochthonous leaf litter is retarded by stream acidification, but few studies have evaluated whether this effect can be offset by liming – the palliative addition of calcium carbonate either to streams or their catchments. We assessed the response of litter decomposition to pH and experimental liming in Welsh upland streams. Small-mesh (<335 μm) litter-bags containing common beech (Fagus sylvatica L.) were submerged in main river sites along the River Wye, and in replicate acid, circumneutral and experimentally limed tributaries (all n = 3) for 20 days. Beech decomposition was inhibited in acid tributaries and main river sites compared to circumneutral tributaries. Despite having only moderately increased pH relative to acid streams, limed sites had increased decomposition rates that were indistinguishable from naturally circumneutral streams. Decomposition rates increased highly significantly with pH across all 12 sites studied, and values were near identical to those in more prolonged experiments elsewhere. There were no significant variations in shredder numbers with decomposition rate, and no evidence that sites with faster decomposition had smaller shredder proportions. Although based on short-term observations and leaves from just one tree species, these results are consistent with the well-known retardation at low pH of some aspect microbial decomposition (e.g. by hyphomycete fungi). They are among the first to suggest that stream liming to combat acidification might reverse such impacts of low pH. Further data are required on the microbiological causes and ecological consequences of altered detrital processing in acid-sensitive and limed streams.     

Habitat characteristics and macroinvertebrate assemblages in boreal forest streams: relations to catchment silvicultural activities

We compared the stream habitat characteristics and macroinvertebrate assemblages of boreal headwater streams in both the Finnish and the Russian parts of a single river basin, the Koitajoki River. Over the last 50 years, the Finnish side of the catchment has been managed using modern forestry techniques, whereas Russian side has remained nearly unexploited and is near to its natural state. Differences in silvicultural activities were observed to contribute to differences in habitat structure. The channel habitats were in fairly natural state in the Russian reference streams, whereas the impacted Finnish sites were cleared and straightened. In comparison with the impacted channels, the abundance of coarse woody debris (CWD) was 10–100-fold higher in the reference streams. Implications on the forestry-induced deterioration of water quality were also observed. On the contrary, only small differences in macroinvertebrate assemblages were detected. Despite the lower amount of retentive structures (CWD), significantly higher relative abundance of shredders was observed in the forestry-impacted streams. Otherwise the zoobenthic communities were quite similar in the two subcatchments. We suggest that several mechanisms may explain this similarity: (1) community structure is controlled by naturally acidic conditions, (2) the adverse impacts of forestry on habitat structure and water quality of streams may be compensated by increased input of deciduous litter and organic compounds from drained, structurally young riparian forests and (3) macroinvertebrate species have flexible feeding habits and may thus readily adapt to changing conditions.     

Episodic acidification affects the breakdown and invertebrate colonisation of oak litter

1. Although European streams are now recovering chemically from acidification, biological recovery is limited. One hypothesis is that continuing acid episodes restrict acid‐sensitive species in recovering locations either through direct toxicity or by affecting ecological processes. Here, we test this hypothesis by assessing the effects of episodic acid exposure on the breakdown and macroinvertebrate colonisation of oak (Quercus robur) litter. 2. Over 83 days, acid episodes of 4 days’ duration were simulated by repeatedly transplanting litter bags of contrasting mesh size between replicate acidic and circumneutral streams around Llyn Brianne (Wales, U.K.). Results were compared against controls from circumneutral streams and circumneutral transplants, while invertebrates colonising litter were compared with adjacent assemblages. 3. Breakdown was retarded significantly by repeated acid exposure in comparison with circumneutral transplants, but only in litter to which invertebrates had access. Overall breakdown was also significantly slower in fine‐mesh than in coarse‐mesh bags. 4. Plecopteran shredders were the major invertebrate colonists of litter, along with smaller numbers of grazers and predators. However, acid exposure eliminated or suppressed acid‐sensitive families, resulting in an overall composition converging on that in acid streams. 5. The rapid loss of sensitive invertebrates from acid‐exposed litter supports the hypothesis that acid episodes suppress biological recovery from acidification through direct physiological effects. However, our litter breakdown data indicate that (i) some effects of acid episodes could be mediated through litter processing; and (ii) episodic acidification could disrupt litter breakdown through effects on invertebrate composition or activity. These data suggest that delayed biological recovery from acidification can reflect a combination of direct toxic and indirect ecological effects.