Shredder Biomass Research Articles

Functional plasticity of benthic macroinvertebrates: implications for trophic dynamics in acid streams

Functional plasticity of benthic macroinvertebrates was investigated over one year in four acid streams in the Vosges Mountains (northeastern France). The trophic position of macroinvertebrate species within the benthic food web was determined using gut content analyses. Diet analyses revealed that only 24–36% of biomass of putative shredders consumed leaf fragments, whereas up to 44% consumed benthic algae and bryophytes. Although most Nemouromorpha stoneflies were generalist consumers, several other taxa (e.g., Brachyptera seticornis, Chaetopterygopsis maclachlani) specialised on benthic algae and bryophytes. Our study showed that acid streams unexpectedly had very few specialised leaf-shredding species (e.g., Chaetopteryx villosa) that could explain the slow leaf detritus processing rates observed in these systems. Primary producers appear to be an alternative resource for shredders, playing an important role in supporting food webs in forested acid streams. The food web built in this study suggests that overlooking species-specific functional plasticity of invertebrates may result in a misconception of invertebrate community structure in acid streams.

CONTRIBUTION OF STREAM DETRIVORES, FUNGI, AND BACTERIA TO LEAF BREAKDOWN BASED ON BIOMASS ESTIMATES

Linking species and ecosystems is currently one of the great challenges in ecology. To this end, we assess here the contributions of bacteria, fungi, and detritivorous invertebrates (shredders) to leaf litter breakdown, a key ecosystem-level process. We enclosed alder (Alnus glutinosa) and willow (Salix fragilis) leaves in coarse-mesh bags (5 g dry mass), placed them in a stream during peak leaf fall, and retrieved them periodically to determine leaf mass remaining and the biomass of leaf-associated organisms. Shredder biomass was derived from numbers and length–mass relationships, bacterial numbers and biomass were determined by epifluorescence microscopy, and fungal biomass was measured as ergosterol. In addition, conidial production of aquatic hyphomycetes was determined. Leaves decomposed rapidly with exponential breakdown coefficients k of 0.035 d−1 (alder) and 0.027 d−1 (willow). Leaves were also quickly colonized within the first 4 wk of decomposition, when shredder biomass reached 263 and 141 mg dry mass/litter bag, respectively. Maximum bacterial numbers (5.6 and 4.8 × 1010 g−1 detrital dry mass) were attained after 8 wk and corresponded to a biomass of 3.6 (alder) and 3.1 (willow) mg dry mass/g, <5% of the maximum fungal biomass (77 and 70 mg dry mass/g, respectively). Aquatic hyphomycetes released up to 2.7 and 1.4 × 106 conidia·g−1·d−1, equivalent to a daily conidial production of 9.4 and 2.9 mg dry mass/g on alder and willow, respectively. Calculations based on the measured decomposer biomass and conidial production, published microbial growth efficiencies, turnover times, and shredder feeding rates indicate that shredders accounted for the largest portion of overall leaf mass loss (64% and 51% on alder and willow leaves, respectively), fungi contributed at least 15% and 18%, and bacterial contribution also was estimated to be substantial (7% and 9%). Analyses of elemental flows from allochthonous leaf litter thus clearly require a quantitative consideration of the complex decomposer consortium responsible for leaf decomposition.

Leaf litter breakdown in Patagonian streams: native versus exotic trees and the effect of invertebrate size

1. Nothofagus native forest in the Southern Andes is being progressively substituted by forestation with rapid growth of the exotic trees, mainly species of Pinaceae. The effect on stream processing dynamics is explored through in situ experiments. 2. The effects of leaf litter quality and macroinvertebrate size on in situ litter breakdown were analysed. Experiments were run in litter bags which allowed access of macroinvertebrate fauna in streams running through a dense forest of the deciduous Nothofagus pumilio. 3. In Experiment 1, the decay rates of N. pumilio leaves and Pinus ponderosa needles were measured during an autumn-winter period. N. pumilio decayed twice as fast as P. ponderosa (P<0.01). Shredders fed only on N. pumilio leaves. The total abundance of macroinvertebrates colonizing both treatments was similar; however, the biomass was higher in the N. pumilio treatment. Large shredders were only found colonizing N. pumilio leaves. Since no decay due to shredders was observed in P. ponderosa, the presence of macroinvertebrates in these litter bags was related to refuge and feeding on FPOM-biofilm resources. 4. In Experiment 2, N. pumilio leaf litter was exposed in order to allow (open bags) or restrict (closed bags) access of invertebrates. The invertebrate assemblage in open bags showed the similar pattern observed for N. pumilio in the first experiment. Gathering-collectors were generally smaller and dominant in number while shredder biomass was higher in open bags as a result of high individual biomass. N. pumilio decayed faster when the whole size spectrum of macroinvertebrates colonized the bags (P<0.01). However, feeding signs of small shredders were observed in closed bags, therefore their role on leaf litter breakdown in Andean streams should not be neglected. 5. The combined results of both experiments lead to the conclusion that the whole litter processing mechanism would be affected as a consequence of the substitution of native forest by exotic pine forestation. Copyright © 2002 John Wiley & Sons, Ltd.

Shredder guilds in three Danish forest streams contrasting in forest type

We investigated the impact of riparian vegetation type on shredder guilds in three similarly sized Danish forest streams during the period 1992-1994. One stream drained beech forest (Fagus sylvatica L.), one drained mixed, mainly deciduous forest, and one drained conifer plantations. Temperature was lowest in the conifer stream reflecting the high degree of shading during winter. The standing stock of coarse particulate organic matter (CPOM) differed only slightly among streams, although CPOM quality differed more markedly. Leaf litter respiration rates differed significantly among streams, being lowest in the conifer stream. The shredder guild was comprised of 9 species, of which only Nemoura flexuosa AUBERT. was present in all 3 streams. In terms of biomass, the dominant shredders were the amphipod Gammarus pulex L. and the trichopteran Sericostoma personatum SP. in the deciduous streams, and the plecopteran Nemurella picteti KLP. in the conifer stream. Shredder biomass was significantly higher in the two deciduous streams than in the conifer stream. The condition index expressed as mass: head width ratio was significantly lower for Leuctra nigra (OLV.) and N. flexuosa in the conifer stream than in the deciduous streams. Total shredder guild production was 5982 mg m -2 yr -1 in the beech stream, 4681 mg m -2 yr -1 in the mixed deciduous stream and 1310 mg m -2 yr -1 in the conifer stream. Our results thus suggest that forest stream shredder guilds are influenced by forest type, and apparently attributable to a combination of factors including water temperature and chemistry, and the quality of the available food resource. If low secondary production of the shredder guild is a general feature of streams draining conifer plantation forest, it could have important implications for higher trophic levels and ecosystem functioning.

Catchment land‐use, macroinvertebrates and detritus processing in headwater streams: taxonomic richness versus function

1. We used a litter bag technique to assess the effect of catchment land‐use (forest, wetland, agriculture, urban) on the processing of red maple (Acer rubrum L.) litter in 17 streams in Maine, U.S.A. Litter processing by fungi was predicted to increase with nutrient concentrations along a gradient of land use, from relatively unmodified to highly modified. Litter processing by litter‐shredding macroinvertebrates was predicted to decline along this gradient because of a decline in their taxonomic richness and biomass.2. Land use was associated with the anticipated gradient in nutrient and macroinvertebrate attributes, and a significant relationship was found between land use and nitrate concentration. There was, however, no significant relationship between land use and soluble reactive phosphorus (SRP) concentration. Similarly, shredder taxonomic richness was significantly related to land use type, whereas shredder biomass showed no significant relationship to land use.3. Attributes of the shredder assemblage structure and nutrient concentrations were both strong determinants of litter processing. Increasing biomass and taxonomic richness of shredders was significantly related to increasing rates of litter mass loss. Increasing concentrations of nitrate and SRP were significantly related to increasing rates of litter softening below threshold concentrations (approximately 0.20 mg NO3‐N L–1 and 5 μg SRP L–1).4. The potentially additive effects of nitrate and SRP concentrations or shredder richness and biomass on litter processing rates were confounded by the lack of significant correlation between these pairs of variables. Consequently, rates of litter processing (as rates of softening or mass loss) did not vary systematically among different land use regimes.

EFFECTS OF MINE DRAINAGE ON BREAKDOWN OF ASPEN LITTER IN MOUNTAIN STREAMS

Rates of aspen litter breakdown were measured at 40 sites in streams of the Rocky Mountains of Colorado, U.S.A. The sites encompassed a range of effects of mine drainage, from pristine (no effects) to highly stressed. The pH, concentrations of dissolved zinc, and deposition rates of metal oxides (the three main stresses from mine drainage) were measured in each stream. Rates of litter breakdown were estimated from changes in mass of aspen leaves in litterbags. The biological communities associated with litter breakdown also were evaluated by measuring the biomass of shredding invertebrates in litterbags and the rate of microbial respiration on litter. Of the stresses from mine drainage, concentration of zinc and deposition rate of metal oxides were most closely related (negatively) to rate of litter breakdown. Biomass of shredding invertebrates was also negatively related to concentration of dissolved zinc and deposition of metal oxides. Microbial respiration was negatively related to deposition rate of metal oxides and positively related to concentration of nutrients. Both shredder biomass and microbial respiration were positively related to litter breakdown rate and, together, accounted for 79% of its variation. Recovery of litter breakdown in streams affected by mine drainage requires remediation that limits both dissolved and deposited metals.

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores.2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn.3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass.4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values.5. Even at low density (approximately 2 m–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.

Linking Shredders and Leaf Litter Processing: Insights from an Acidic Stream Study

We performed a seven-month study on beech leaf breakdown and associated fauna along a forested stream showing a gradient in acidification level (4 stations with mean pH ranging from 5.3 to 7.2). We completed this approach by comparing in situ the shredding efficiency of the main invertebrate species present at each station, the acid-tolerant Protonemura sp. (Plecoptera: Nemouridae) and the acid-sensitive Gammarus fossarum (Amphipoda: Gammaridae). Results showed that beech leaf breakdown rate was severely depressed under acidic conditions. Not only did the acidic station show lower biomass of shredders, but Protonemura sp., the main shredder presented significantly lower feeding activity than G. fossarum in the non-acidic stations. At the other three stations, feeding activities of G. fossarum were similar and differences in litter breakdown rates appeared to be related to G. fossarum densities. By combining both fine-resolution studies of individual species and ecosystem level studies, our results suggest that the functional role played by G. fossarum in the studied system seems to be unique and not fulfilled by other species in the acidified upper part of the stream.

LITTER BREAKDOWN IN MOUNTAIN STREAMS AFFECTED BY MINE DRAINAGE: BIOTIC MEDIATION OF ABIOTIC CONTROLS

Breakdown of plant litter in streams was studied as an example of a major ecological process subject to change through multiple stresses associated with mine drainage. Rates of litter breakdown were measured at 27 sites in streams of the Rocky Mountains of Colorado, USA. Eight of the sites were pristine, and 19 were affected to varying degrees by mine drainage. The pH, concentrations of dissolved zinc, and deposition rates of metal oxides were measured in each stream. Rates of litter breakdown were estimated from changes in mass of willow leaves in litterbags. Biomass of shredding invertebrates in litterbags was monitored at each site, as was microbial respiration on litter. Of the abiotic variables, increased concentrations of zinc and increased deposition rates of metal oxides were most closely related to decreased rates of litter breakdown. Biomass of shredding invertebrates was negatively related to concentration of dissolved zinc and deposition of metal oxides and was more closely related to breakdown rates than was microbial respiration. Microbial respiration was related negatively to deposition rates of metal oxides and positively to nutrient concentrations. Shredder biomass and microbial respiration together accounted for 76% of the variation in breakdown rates. Remediation schemes for streams affected by mine drainage should take into account the distinct ecological effects of the multiple stresses caused by mine drainage (pH, high concentrations of dissolved metals, deposition of metal oxides); remediation of a single stress is likely to be ineffective.

Leaf litter breakdown rates in boreal streams: does shredder species richness matter?

1. Leaf litter breakdown rates were assessed in 23 boreal streams of varying size (first–seventh order) in central and northern Sweden. 2. Shredders were most abundant in small streams, while shredder species richness showed a hump‐shaped relationship with stream order, with most species in fourth order streams. 3. In a partial least‐squares regression analysis, year, water temperature, shredder species richness and shredder abundance were those factors correlating most strongly with leaf breakdown rates. Shredder species richness was more strongly correlated with leaf litter breakdown rates than shredder abundance, and shredder biomass showed no such correlation. 4. These data suggest that shredder species richness is an important variable in terms of leaf litter dynamics in streams.

Macroinvertebrates of a fluvial system in Patagonia: altitudinal zonation and functional structure

Most investigations of altitudinal zonation of river macrobenthic communities have been conducted in the northern hemisphere. Conversely, South American habitats, and specifically the Argentinean ones, are poorly known. This is the first study of altitudinal zonation of macrobenthic communities in the Patagonian Andes. It was carried out at 14 sampling stations along 51 km of the Esquel-Percy River system over a 1000 m altitudinal gradient. Untreated sewage of Esquel City discharges in the middle section of the system. The main species assemblages of the monitored sites were identified and a functional categorization of the taxa was produced. Population distribution patterns were analyzed throughout a complete annual cycle, and both qualitative and quantitative variations were interpreted as a function of abiotic and biotic variables. The macrobenthic community found along the Esquel-Percy system is rich and diverse, except at middle-stretch stations, which are altered by anthropogenic perturbations. Specific composition, diversity and density were influenced by the topographic gradient, geochemical (conductivity and total alkalinity) and anthropogenic factors (BOD and oxygen saturation), and the seasonality of the system. Similarities with lotic environments of Australia and New Zealand were found in specific composition, density, and community structure. The biomass of shredders and predators decreased, while collectors increased, downstream from the headwaters. Collector-gatherers were the most important group at all sites. Functional organization showed better correspondence with the RCC model if biomass was used instead of macroinvertebrate density. We found that local and regional characteristics produced deviations from the RCC model, but it is still useful as a comprehensive framework for stream ecology in the Patagonian Andes.

A Perspective on Leaf Litter Breakdown in Streams

Leaf litter breakdown, a critical ecosystem level process in streams and other aquatic environments , has been conceptualized using models borrowed from terrestrial systems. We argue that current views of the process in fresh waters need to be conceptually improved. Specifically, we think the idea that breakdown proceeds in three distinct temporal stages (leaching, conditioning, fragmentation) has been over emphasized. Leaching, the massive loss of soluble leaf components within 24 h after immersion, is generally considered to constitute a well-defined first stage. Recent evidence suggests, however, that the initial solute losses are largely an effect of the un natural drying procedures to which experimental leaves are normally subjected. Fresh leaf litter does lose solutes when immersed,but gradually throughout the breakdown process rather than instantly upon wetting. Conditioning, the second breakdown stage, describes the enhancement of leaf palatability for detritivores by microbial colonization, and is thus ultimately targeted towards a group of organisms (which contribute to litter degradation) rather than addressing the breakdown process per se. Furthermore,conditioning implies a key role for detritivorous invertebrates and underrates the established direct degradative activity of microbial decomposers. If, thus, leaching and conditioning are not generally useful operators to describe portions of the litter breakdown process in freshwaters, the traditional concept, which emphasises leaching, conditioning and fragmentation as three sequential stages, loses much of its appeal. Consequently, we propose a new conceptual model, in which the coincidence and interplay of various subprocesses of litter breakdown is more strongly recognized. In this model, we propose to view the process in terms of the products of litter breakdown-as a complement to the usual perspective which focuses on litter mass loss. Six primary breakdown products are considered : bacterial, fungal and shredder biomass; dissolved organic matter; fine-particulate organic matter; and inorganic mineralization products such as CO2, NH+ and PO3-. We present a scheme illustrating the hypothesized formation of these products throughout breakdown. However, to improve understanding of the process, application of the proposed conceptual framework in experimental work is necessary.

Leaf breakdown along an altitudinal stream gradient

Breakdown of alder (Alnus glutinosa) leaf litter was followed in fourteen sampling stations along a 1400 m altitudinal gradient of a 1st to 3rd-order undisturbed stream. Altitudinal patterns of leaf mass loss, nitrogen concentration in the leaves, fungal biomass determined as ergosterol content, and numbers and biomass of shredders and total macroinvertebrates associated with the decomposing leaf litter were analysed using rank and linear correlations. The breakdown rate (0.020-0.041d -1 ) almost doubled from the upper to the lower sites and exhibited a significant correlation with altitude. The abundance of shredders, which largely dominated invertebrate assemblages on leaf litter, was negatively correlated both with altitude and percentage of leaf mass remaining after 13 days. Similarly, fungal biomass on leaf litter showed a negative linear correlation with altitude after 37 days. These results, which contrast with those of other studies, suggest that the increasing role of shredders and micro-fungi as one goes downstream can explain the altitudinal pattern of leaf litter breakdown in streams.

Larval Size and Leaf Conditioning in the Breakdown of Nothofagus pumilio Leaves by Klapopteryx kuscheli (Insecta, Plecoptera) in a South Andean Stream

AbstractThe effect of the shredder Klapopteryx kuscheli (Plecoptera, Austroperlidae) on the breakdown of Nothofagus pumilio leaves was studied in field experiments. Decay rates due to leaching and shredder activity were measured on conditioned and unconditioned leaves. Two sizes of shredder were tested in order to establish if the consumption per unit of shredder biomass varied with insect development. In three days weight losses due to leaching were 8.5%, and decay rates due to shredders were significantly higher than in control replicates. There were no differences in decay rates between conditioned and unconditioned leaves. Small larvae consumed more leaf biomass per unit of insect biomass than large ones. Growth efficiencies of K. kuscheli larva were low, but smaller larvae showed higher efficiencies than larger ones.

Shredder–collector interactions in temporary streams of western Oregon

1. Field data and results from laboratory rearing are combined to describe shredder–collector interactions in western Oregon temporary streams.2. The composition of functional feeding groups in the temporary streams conforms to that predicted for permanent headwaters by the River Continuum Concept (RCC).3. A distinct functional feeding group phenology was found, with shredders emerging first and collectors emerging last in the season. Scrapers were intermediate.4. Presence of shredders enhanced collector growth in laboratory experiments (P &lt; 0.01;n = 12). The species reared in the study trials accounted for 67% of the shredder biomass and 76% of the collector biomass in the natural systems.5. Field data and laboratory rearing trials both support the hypothesis of a close shredder–collector interaction in the processing of allochthonous material in the summer‐dry headwaters studied.

Leaf litter decomposition and macroinvertebrate communities in headwater streams draining pine and hardwood catchments

Benthic invertebrates, litter decomposition, andlitterbag invertebrates were examined in streamsdraining pine monoculture and undisturbed hardwoodcatchments at the Coweeta Hydrologic Laboratory in thesouthern Appalachian Mountains, USA. Bimonthlybenthic samples were collected from a stream draininga pine catchment at Coweeta during 1992, and comparedto previously collected (1989–1990) benthic data froma stream draining an adjacent hardwood catchment. Litter decomposition and litterbag invertebrates wereexamined by placing litterbags filled with pine ormaple litter in streams draining pine catchments andhardwood catchments during 1992–1993 and 1993–1994. Total benthic invertebrate abundance and biomass inthe pine stream was ca. 57% and 74% that of thehardwood stream, respectively. Shredder biomass wasalso lower in the pine stream but, as a result ofhigher Leuctra spp. abundance, shredderabundance was higher in the pine stream than thehardwood stream. Decomposition rates of both pine andred maple litter were significantly faster in pinestreams than adjacent hardwood streams (p<0.05). Total shredder abundance, biomass, and production weresimilar in maple bags from pine and hardwood streams. However, trichopteran shredder abundance and biomass,and production of some trichopteran taxa such asLepidostoma spp., were significantly higher in maplelitterbags from pine streams than hardwood streams(p<0.05). In contrast, plecopteran shredders(mainly Tallaperla sp.) were more important inmaple litterbags from hardwood streams. Shredderswere well represented in pine litterbags from pinestreams, but low shredder values were obtained frompine litterbags in hardwood streams. Resultssuggest conversion of hardwood forest to pinemonoculture influences taxonomic composition of streaminvertebrates and litter decomposition dynamics. Although the impact of this landscape-leveldisturbance on invertebrate shredder communitiesappeared somewhat subtle, significant differences indecomposition dynamics indicate vital ecosystem-levelprocesses are altered in streams draining pinecatchments.

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.

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12‐week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day–1) and kd (degree day–1).3. There were no significant differences in processing rates (k day–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.

Environmental patchiness and processing of maple leaf litter in a backwater of a mountain stream: riffle area vs. debris dams

Processing rates of Acer pseudoplatanus leaves were investigated in three types of patches - debris dams, buried and exposed, and a riffle area - of a backwater of the alpine stream Oberer Seebach. The results show a gradient of processing rates from slower, for the leaves buried in the debris dams, to the highest, in the riffle area, with significant differences between these two extremes. Shear stress near leaf bags was significantly higher in the riffle area than in debris dams; however, this probably does not reflect the internal conditions of the bags. It is argued that the accumulation of great amounts of plant debris against the bags in the riffle area, and the relatively hard texture and the uniform perimeter of the leaf discs used in the experiment, might minimize mass losses due to mechanical abrasion by the water current. Nitrogen and phosphorus dynamics of the leaf material suggest that microbial processing did not differ between patches. Likewise, density and biomass of shredders did not help to explain differences in processing rates between patches. Nevertheless, significant taxonomical differences in the shredder assemblages between patches were observed. Differential population dynamics, feeding styles and metabolic capacities of shredders and the different environments in which to obtain food between the studied stream patches may be of great importance in the differences between processing rates.

Macroinvertebrate production in a headwater stream during recovery from anthropogenic disturbance and hydrologic extremes

Recovery of the macroinvertebrate community inhabiting a headwater stream (catchment 54; C54) that received 3 years of seasonal insecticide treatment was investigated. Estimates of abundance, biomass, and production in C54 during 1989 and 1990 were compared with those of a nearby undisturbed reference stream (catchment 55; C55), and those of C54 during a pretreatment year (1985). Total macroinvertebrate abundance was similar throughout pretreatment, treatment, and recovery periods of C54. In contrast, biomass and production, which decreased during treatment, increased to levels similar to those of C54 in the pretreatment year and those of the reference stream during recovery. By 1990, the functional structure of C54 was similar to that of C55 and that of C54 before the treatment. However, taxonomic and developmental stage differences within some functional groups, particularly shredders, persisted. Despite poor recovery of some larger shredder taxa, rapid recovery of a relatively small trichopteran shredder, Lepidostoma spp., contributed significantly to recovery of ecosystem processes associated with shredders. Relationships between shredder biomass and coarse particulate organic matter differed during treatment and recovery periods. Invertebrate taxa with shorter life cycles recolonized rapidly, while those with life cycles &gt; 1 year generally displayed limited recovery. Hydrologic extremes during treatment (drought) and recovery (wet) periods affected organic matter and macroinvertebrate community dynamics in both streams, and may have influenced observed recovery patterns.