Changes In Macroinvertebrate Community Structure Research Articles

Microplastics in freshwater sediments: Effects on benthic invertebrate communities and ecosystem functioning assessed in artificial streams

The high levels of microplastics (MPs) found in freshwaters, particularly in riverine sediments, may impose a threat to the macroinvertebrate communities with possible consequences at ecosystem-level. The present study aimed to assess the effects of a mixture of different sizes of polyethylene microplastics (PE-MPs) on the composition and structure of macroinvertebrate communities and key-functions, such as primary production and leaf litter decomposition. MPs were mixed in the sediment at three different concentrations (0.1, 1, and 10 g kg-1) already found in freshwater sediments to enhance the relevance of the work. After eight days of exposure to PE-MPs, the observed changes in macroinvertebrate community structure were mostly due to the reduction in the abundance of deposit-feeders and grazers that were reduced by ca 31-50% and 34-39%, in the two highest MPs concentrations respectively, in comparison with the control treatment after 8 days of exposure. MPs internal concentrations were detected only in organisms exposed to plastic particles within artificial streams with chironomids and mayflies presenting higher MPs internal levels (average of 115 particles/individual found in chironomids, 166/individual for Baetis sp. and 415 particles/individual for Ephemerella sp.) suggesting higher ingestion of plastic microparticles. Nevertheless, the alterations in the community structure did not translate into impairments in the functional endpoints analysed, leaf litter decomposition and primary production, that were expected due to possible sub-lethal effects (e.g., feeding inhibition) on detritivores and grazers.This study represents one of the few assessments of MPs effects on freshwater benthic macroinvertebrate community structure and the first that simultaneously considered ecosystem-level functional endpoints. Further research combining different microplastics and longer exposure periods are needed to raise knowledge on potential ecological consequences of MPs to freshwaters.

Invasion of floating fern alters freshwater macroinvertebrate community structure with implications for bottom-up processes

The non-native aquatic fern giant salvinia, Salvinia molesta Mitchell (Salviniaceae), poses a risk to freshwater ecosystems through limiting light penetration, decreasing submerged aquatic vegetation (SAV) abundance, altering water quality, and potentially leading to changes in macroinvertebrate community structure. Here, we conducted repeated quarterly field surveys and measured light, nutrients, water quality, and aquatic macroinvertebrate community composition and energetic value to detect effects from giant salvinia invasion. Giant salvinia reduced dissolved oxygen, pH and light availability in the aquatic environment, and increased the concentration of orthophosphate and ammonium. Following initial colonization, macroinvertebrate communities in giant salvinia resembled SAV communities dominated by aquatic insects, however, richness and relative abundance in giant salvinia decreased over time, resulting in a community populated by few taxa, primarily crustaceans. Total macroinvertebrate energetic value in giant salvinia was significantly lower than SAV communities. Giant salvinia invasion changed habitat composition, triggered internal nutrient loading, and reduced macroinvertebrate abundance, diversity, and ecosystem productivity. Our findings demonstrate larger ecological impacts from giant salvinia than previously reported, including potential disruption to the transfer of energy between trophic levels.

Disturbance, nutrients, and antecedent flow conditions affect macroinvertebrate community structure and productivity in an Arctic river

AbstractClimate change is expected to alter disturbance regimes and biogeochemical cycles that underlie the structure and function of ecosystems worldwide. In the Arctic, rapid warming is already affecting these processes via changes in precipitation and thawing permafrost. We assessed how anticipated changes in disturbance regimes and nutrient availability may affect an arctic river ecosystem (Kuparuk River, Alaska) by analyzing temporal patterns of biofilm chlorophyll mass and macroinvertebrate community structure and productivity. Our study incorporated an upstream reach (sampled 2001–2012) and a downstream reach (sampled 2011–2012) to which phosphorus (P) was added to simulate increases in nutrient supply that are anticipated as permafrost thaws. Greater hydrologic disturbance during the open‐water season correlated with reduced algal biomass and invertebrate secondary production (range ∼ 2–7 g DM m−2 yr−1) in the following spring and summer. Bed disturbing flows also altered macroinvertebrate community structure with distinct “high‐flow” and “base‐flow” assemblages documented. Recovery time was shorter for chlorophyll mass and macroinvertebrate production (∼ 1 yr) than community structure (∼ 3 yr). Experimental P‐addition increased algal biomass and invertebrate production, but also resulted in a third macroinvertebrate assemblage dominated by mobile grazers rather than filter‐feeders. Our results suggest that a decrease in the return interval for bed disturbing floods to < 4 yr will result in persistent changes in macroinvertebrate community structure and fundamental alterations to the food web. These results also demonstrate how arctic river communities may be affected by increases in the magnitude and variability of river discharge and nutrient supplies that are anticipated as the climate warms.

Effects of brine contamination from energy development on wetland macroinvertebrate community structure in the Prairie Pothole Region

Wetlands in the Prairie Pothole Region (PPR) of North America support macroinvertebrate communities that are integral to local food webs and important to breeding waterfowl. Macroinvertebrates in PPR wetlands are primarily generalists and well adapted to within and among year changes in water permanence and salinity. The Williston Basin, a major source of U.S. energy production, underlies the southwest portion of the PPR. Development of oil and gas results in the coproduction of large volumes of highly saline, sodium chloride dominated water (brine) and the introduction of brine can alter wetland salinity. To assess potential effects of brine contamination on macroinvertebrate communities, 155 PPR wetlands spanning a range of hydroperiods and salinities were sampled between 2014 and 2016. Brine contamination was documented in 34 wetlands with contaminated wetlands having significantly higher chloride concentrations, specific conductance and percent dominant taxa, and significantly lower taxonomic richness, Shannon diversity, and Pielou evenness scores compared to uncontaminated wetlands. Non-metric multidimensional scaling found significant correlations between several water quality parameters and macroinvertebrate communities. Chloride concentration and specific conductance, which can be elevated in naturally saline wetlands, but are also associated with brine contamination, had the strongest correlations. Five wetland groups were identified from cluster analysis with many of the highly contaminated wetlands located in a single cluster. Low or moderately contaminated wetlands were distributed among the remaining clusters and had macroinvertebrate communities similar to uncontaminated wetlands. While aggregate changes in macroinvertebrate community structure were observed with brine contamination, systematic changes were not evident, likely due to the strong and potentially confounding influence of hydroperiod and natural salinity. Therefore, despite the observed negative response of macroinvertebrate communities to brine contamination, macroinvertebrate community structure alone is likely not the most sensitive indicator of brine contamination in PPR wetlands.

Experimentally disentangling the influence of dispersal and habitat filtering on benthic invertebrate community structure

Abstract Ecological communities are structured by a combination of local processes like habitat filtering and species interactions, and regional forces driven by the dispersal of organisms between localities on a landscape. Previous studies suggest that the position of local communities within a dispersal network can greatly influence the relative influence of these two sets of processes on community assembly. However, the majority of previous investigations have used models or inferences based on observational data to investigate these hypotheses, while experiments directly addressing this question have been rare. We experimentally investigated the relative influence of local and regional processes in structuring benthic invertebrate communities using artificial streams. We manipulated three factors—source pool for the macroinvertebrate community (headwater vs. mainstem) as a surrogate of network location, habitat complexity (high vs. low) in the flume, and dispersal (high vs. low)—and followed changes in macroinvertebrate community structure for 8 weeks. Previous research suggests that because headwater (HW) streams are isolated within river networks, HWs are less influenced by regional processes relative to more well‐connected mainstems (MSs). We therefore predicted (i) that flumes colonised from a HW source community would respond more strongly to our dispersal treatment than those colonised by MS communities because MS were already largely structured through dispersal‐driven processes, and (ii) that both HW and MS communities would respond to manipulations of local habitat, indicating that responses to the dispersal treatment were a direct result of dispersal driven dynamics rather than specific affinity for conditions in the flumes. Both of our predictions were strongly supported by the results of the experiment. For flumes with HW source pools, the high dispersal treatment had significantly higher diversity than low dispersal flumes. However, this difference only occurred in flumes with HW source pools and did not occur in flumes with MS sources. There was also strong evidence of community composition in HW flumes shifting significantly towards the macroinvertebrate composition in our experimental dispersal treatment. The major effect of experimental dispersal was to introduce new species in fairly low abundances as would be expected from dispersal via drift over a relatively short time. Both MS and HW colonised flumes showed highly significant signals of habitat filtering, though the influence of specific habitat differed between the source types. These results support the hypothesis that dispersal driven processes are a more important structuring force in well‐connected areas of networks by experimentally demonstrating the responsiveness of previously isolated communities to experimentally induced dispersal. They also demonstrate that this responsiveness is not due to an inherent difference in habitat affinity since source communities from both HWs and MSs responded to manipulation of habitat variables. This experiment only simulated one type of dispersal process in streams—drifting—and did not include simulated dispersal from other sources, nor did it include population dynamics given the relatively short duration of the experiment. Nevertheless, the sensitivity of previously isolated communities to one type of simulated dispersal is a powerful indication of the mechanisms that structure these systems.

Reach-scale stream restoration in agricultural streams of southern Minnesota alters structural and functional responses of macroinvertebrates

Recent studies suggest that stream restoration at the reach scale may not increase stream biodiversity, raising concerns about the utility of this conservation practice. We examined whether reach-scale restoration in disturbed agricultural streams was associated with changes in macroinvertebrate community structure (total macroinvertebrate taxon richness, total macroinvertebrate density, Ephemeroptera, Plecoptera, Trichoptera [EPT] taxon richness, % abundance of EPT taxa) or secondary production (macroinvertebrate biomass over time). We collected macroinvertebrate samples over the course of 1 y from restored and unrestored reaches of 3 streams in southern Minnesota and used generalized least-square (GLS) models to assess whether measures of community structure were related to reach type, stream site, or sampling month. After accounting for effects of stream site and time, we found no significant difference in total taxon richness or % abundance of EPT taxa between restored and unrestored reaches. However, the number of EPT taxa and macroinvertebrate density were significantly higher in restored than in unrestored reaches. We compared secondary production estimates among study reaches based on 95th-percentile confidence intervals generated via bootstrapping. In each study stream, secondary production was significantly (2–3×) higher in the restored than in the unrestored reach. Higher productivity in the restored reaches was largely a result of the disproportionate success of a few dominant, tolerant taxa. Our findings suggest that reach-scale restoration may have ecological effects that are not detected by measures of total taxon richness alone.

Changes in macroinvertebrate community structure provide evidence of neutral mine drainage impacts

Contamination of aquatic environments as a consequence of metal mining is an international issue. Most historic studies have considered the impact of acid mine drainage (pH < 6) on instream communities and comparatively little attention has been given to sites where drainage is typically circum-neutral (6 > pH < 8). Here, the impacts of historic mining activities on the benthic macroinvertebrate community of a circum-neutral river in Central Wales are assessed. Biotic and diversity indices, widely used for biomonitoring purposes, indicate aquatic macroinvertebrate assemblages within the Afon Twymyn to be in a good condition, despite severe metal contamination of bed sediments and river water. However, Canonical Correspondence Analysis identifies differences in community structure between mining impacted and unimpacted reaches of the river associated with chalcophile (Zn, Pb, Cu, Cd) and common (Fe and Mn) metals. Stream pH was not a significant factor structuring the macroinvertebrate community. Widely utilised macroinvertebrate indices failed to identify impacts at the community level because they either seek to identify impacts of a specific contaminant or are dependent on a model community response to a given stress. The nature of metal mine discharges is temporally complex, having highly variable chemical signatures and as a result, care is advised when interpreting and modelling community impacts. The use of standard macroinvertebrate biotic and diversity indices in the context of the EU Water Framework Directive could lead to erroneous classifications of aquatic ecosystem health when used for bio-monitoring rivers affected by neutral mine drainage where other indicators are unavailable.

Responses of macroinvertebrate communities to long‐term flow variability in a Sonoran Desert stream

AbstractCurrent global models predict a hotter and drier climate in the southwestern United States with anticipated increases in drought frequency and severity coupled with changes in flash flood regimes. Such changes would likely have important ecological consequences, particularly for stream and riparian ecosystems already subject to frequent hydrologic disturbance. This study assessed the potential response of aquatic macroinvertebrates to interannual variation in hydrology in a spatially intermittent desert stream (Sycamore Creek, AZ). We compiled data on the recovery of macroinvertebrate communities following spring floods, with successional sequences captured 11 times over a 16‐year period (1983–1999). This period encompassed a transition from perennial to intermittent flow in this system, and included a record drought in 1989–1990. Results show that while the size of floods initiating sequences had little explanatory power, changes in macroinvertebrate community structure during postflood succession were closely associated with antecedent flooding and drought. Year‐to‐year differences in benthic communities integrated taxon‐specific responses to antecedent disturbance, including differential resistance to channel drying, use of hyporheic refugia, and variable rates of recovery once stream flow resumed. The long‐term consequences of drying on community structure were only evident during later stages of postflood succession, illustrating an interaction between flood and drought recovery processes in this system. Our observations highlight the potential for predicted climate changes in this region to have marked and long‐lasting consequences for benthic communities in desert streams.

The effects of water‐level manipulation on the benthic invertebrates of a managed reservoir

Summary1. Reservoir creation and management can enhance many ecological services provided by freshwater ecosystems, but may alter the natural conditions to which aquatic biota have adapted. Benthic macroinvertebrates often reflect environmental conditions, and this community may be particularly susceptible to water‐level changes that alter sediment exposure, temperature regime, wave‐induced sediment redistribution and basal productivity.2. Using a before–after control–impact experimental design, we assessed changes in macroinvertebrate community structure corresponding with changes in water‐level management in two lentic systems in the Voyageurs National Park, Minnesota, U.S.A. Littoral zone (depths 1–5 m) benthic macroinvertebrate assemblages were sampled in Rainy Lake (control system) and Namakan Reservoir (impact system) in 1984–85, and again in 2004–05 following a change in water‐level management that began in January 2000. The new regime reduced the magnitude of winter drawdown in Namakan Reservoir from 2.5 to 1.5 m, and allowed the reservoir to fill to capacity in late May, a month earlier than under the prior regime. Rainy Lake water levels were not altered substantially.3. We found changes in macroinvertebrate community structure in Namakan Reservoir relative to Rainy Lake at 1–2 m depths but not at 3–5 m depths. These shallower depths would have been most directly affected by changes in sediment exposure and ice formation.4. In 2004–05, Namakan Reservoir benthos showed lower overall abundance, more large‐bodied taxa and an increase in non‐insect invertebrates relative to 1984–85, without corresponding changes in Rainy Lake.5. Changes in the benthic community in Namakan may reflect cooler water in spring and early summer as well as lower resource availability (both autochthonous production and allochthonous inputs) under the new regime.

Changes in macroinvertebrate community structure and function along a

Aquatic macroinvertebrate communities were sampled between 1994 and 1996 at 13 sites downstream of phosphorus (P)-enriched canal inflows in a northern Everglades marsh to determine the effects of nutrient enrichment on community structure and function. Sampling was performed using sweep nets and Hester–Dendy (HD) samplers. Data were analyzed to assess changes in taxa richness and diversity, species composition, and functional group composition along the gradient. Environmental conditions at each site were characterized to interpret spatial changes in these metrics. Mean water-column total P (TP) increased from ≤10μg lminus;1at sites in the marsh interior to as high as 160 μg lminus;1at sites closest to the canal. Vegetation and habitat composition changed dramatically along the gradient, with sawgrass and slough-wet prairie habitats accounting for most vegetative cover in the interior and cattail accounting for nearly 100%of the cover near the canal. These differences in TP concentrations and vegetation were used to classify sites as reference, enriched, and highly enriched. Daytime dissolved oxygen (DO) concentrations averaged ≥3mg lminus;1at reference sites as compared with concentrations ≤2mg lminus;1at enriched and highly enriched sites. Total macroinvertebrate densities were significantly higher in sweep samples and significantly lower in HD samples from highly enriched sites as compared with the reference condition. Taxa richness and diversity in sweep samples did not change significantly along the gradient, but declined with enrichment on the HD samplers. Insects were the dominant organisms at all sites, but declined in percent abundance with enrichment in sweep samples due to decreases in dipterans, trichopterans and odonates and an increase in oligochaetes. Changes in major invertebrate classes were less pronounced on HD samplers, although amphipods showed significant declines with enrichment. Principal components analysis revealed a clear distinction in taxonomic composition between reference sites and both enriched and highly enriched sites for sweep samples as common chironomid taxa at reference sites declined with enrichment while pollution-tolerant chironomid and oligochaete taxa increased. A similar, but less dramatic trend was found for HD samples, with selected amphipod, chironomid, and gastropod taxa declining with enrichment and pollution-tolerant taxa reaching peak abundance at enriched sites. The functional composition in sweep samples showed modest changes with enrichment, including a shift in dominance from epibenthic collector–gatherers/deposit feeders, which were predominantly chironomids, to subsurface taxa, which were predominantly oligochaetes. Shifts in invertebrate functional composition on HD samplers with enrichment were attributable to declines in the dominance of shredders and collector-filterer/suspension-feeders. Portions of the Everglades exposed to P-enriched runoff are showing characteristic shifts in macroinvertebrate taxonomic composition related to eutrophication. This shift has occurred without a change in species diversity and with an increase in total invertebrate abundance indicative of an overall increase in marsh productivity. The transition from an oligotrophic to eutrophic community signals a decline in the biological integrity of the Everglades ecosystem in response to P enrichment.

Utility of biological monitoring for detection of timber harvest effects on streams and evaluation of Best Management Practices: a review

Best Management Practice (BMP) guidelines have been developed to reduce the negative impacts of timber harvest on streams. BMPs are widely implemented, but the effectiveness of timber harvest BMPs has not been evaluated using modern biological monitoring techniques. Most current biological monitoring is based on 1 of 2 main approaches: multimetric monitoring or predictive modeling. These approaches differ considerably, and their respective merits and failings have been debated extensively in the literature. Our review evaluated the ability of these biological monitoring approaches to detect timber harvest effects and to assess the effectiveness of BMPs. Both techniques detect impairment via changes in macroinvertebrate community structure, despite their differences in approach. Most of the negative effects of timber harvest result in changes in the macroinvertebrate community, so we have concluded that both techniques should be effective for the evaluation of timber harvest and BMPs.

An assessment of the health of two rivers within Harare, Zimbabwe, on the basis of macroinvertebrate community structure and selected physicochemical variables

The aim of this study was to assess and compare the water quality of the Gwebi and Mukuvisi Rivers, on the basis of selected physicochemical variables and macroinvertebrate community structure. Five sites where selected on both rivers and these were sampled on three separate occasions between January and July of 1998. The water variables measured were the concentrations of iron, chromium, zinc, lead, copper, manganese, chlorides, fluorides, sulphates, total phosphates, nitrates, ammonia, total dissolved salts, dissolved oxygen, biological oxygen demand, as well as pH, conductivity, temperature, water surface velocity and discharge. The concentration of most of the chemical variables was relatively similar along the course of the Gwebi River, but there were drastic increases in the levels of iron, chromium, copper, zinc, chlorides, fluorides, sulphates, and ammonia along the Mukuvisi River. The two rivers were different with respect to the physicochemical variables, with the exception of the first site on the Mukuvisi, which was similar to sites on the Gwebi River. This was because of the differences in the levels of human activities on the two rivers. Industrial, sewage and domestic pollution has had an adverse effect on the water quality of the Mukuvisi River. There was a sharp decline in the number of macroinvertebrate taxa along the Mukuvisi River. The lower reaches of the river where dominated by oligochaetes and Chironimidae larvae. Sample score classification of water quality based on the South African Scoring System Version 4 (SASS4) showed that most of the Mukuvisi river had poor quality water quality, whilst much of the Gwebi River had fair quality water. The HABS1 habitat assessment index was used to assess habitat quality at each site. Although much of the Mukuvisi recorded fair to good habitat scores and had generally higher habitat scores than sites on the Gwebi, the SASS scores were generally lower compared to those along the Gwebi. The sample scores and average score per taxon (ASPT) of the SASS4 showed that the Mukuvisi River was of much lower quality than the Gwebi. Both the sample score and ASPT were negatively and significantly (p<0.05) correlated to most of the physicochemical variables. The water quality variables accounted for 61.1% and 59.0% of the differences in the sample score and ASPT respectively. There was a marginal decrease in the Margalef and Shannon indices along the Gwebi River, but the Simpson's index remained relatively constant. Along the Mukuvisi River, there was a clear and distinct decrease in the magnitude of all three diversity indices, indicating decreasing macroinvertebrate community structure. The change in water physicochemical variables accounted for 61.3%, 69.2% and 87.2% of the changes in the Margalef, Shannon and Simpson's index respectively. The study provides evidence that the changes in macroinvertebrate community structure along the Mukuvisi River is due to decline in the water quality. On the Gwebi, water quality is not the main factor determining macroinvertebrate community structure.

Effect of Zebra Mussels, Dreissena polymorpha, on Macroinvertebrates in a Lake-outlet Stream

Zebra mussels, Dreissena polymorpha, increase macroinvertebrate abundance on hard substrata in lakes and large rivers because they enhance surface area, substratum heterogeneity and the accumulation of benthic organic matter (BOM). We tested the hypothesis that zebra mussel colonization of hard substrata in a lake-outlet stream would similarly increase macroinvertebrate abundance. Zebra mussels were attached to artificial rocks (0.05 m2) in a crossed factorial design with two states of mussels (live or dead) and three densities [low (200·m−2), medium (500·m−2) or high (1000·m−2)] plus a noncrossed control (no attached mussels). Rocks were placed in an erosional zone of a lake-outlet stream in southwestern Michigan that contained low densities of zebra mussels. After 28 d total macroinvertebrate abundance was significantly higher on rocks with high densities of zebra mussels, but live or dead treatment had no effect on macroinvertebrate abundance. Family richness differed significantly among density treatments (high = medium > low = control), but Simpson's diversity index did not differ among treatments. We conclude that changes in macroinvertebrate community structure probably were related to the increased substrate complexity provided by zebra mussels.

Comparison of summer zoobenthic communities in four habitats of a floodplain impoundment:1975 and 1990

AbstractThe diversity, evenness, similarity, standing crop and abundance of benthic macroinvertebrates in Navigation Pool 8 of the upper Mississippi River were compared in 1975 and 1990. Macroinvertebrates were collected in midsummer from four habitats: marsh, bay, open water and side channel. These habitat types accounted for 64% of the total habitat area in the impoundment. The community structure changed in all four habitats; the percentage similarities between 1975 and 1990 were lowest in open water (19%), intermediate in bays (44%) and side channels (50%) and highest in marshes (62%). Macroinvertebrate diversity and evenness were not significantly different between 1975 and 1990. The standing crop decreased in all habitat types. The abundance decreased in all habitats except bays between 1975 and 1990. The greatest changes in standing crop and community composition occurred in bays and open water. Because the bays and open water habitat comprise 55% of the total area, it is possible that these declines may signal degradation of the aquatic habitat in the entire reservoir. Changes in macroinvertebrate community structure and standing crop may have been due to unstable sediment conditions caused by erosion of islands, resuspension of sediments and loss of aquatic macrophytes and depth.

Assessing river water quality by means of multifactorial methods using macroinvertebrates. A comparative study of main water courses of Biscay

Ninety two taxa of benthic macroinvertebrate were used to ordinate 175 sites from main water courses of Biscay (Basque Country) by means of correspondence analysis (CA). On the plane of the first two axes from CA, changes in macroinvertebrate community structure were observed between rivers due to differences in natural eutrophication and also as a result of the combined effect of industrial and urban sewage. The groups of taxa observed could be interpreted in ecological terms and were representative of headwater reaches, moderately eutrophic waters and polluted waters, respectively. This method of ordination provides more information on differences in water quality among the non-polluted sites than does a biotic index (BMWP score).

Macroinvertebrate Communities and Environment in a Southern African Mountain Stream

In a study of a second-order southern African stream, complementary classification and multidimensional scaling (MDS) techniques revealed longitudinal changes in macroinvertebrate community structure, but no temporal changes. Stepwise discriminant analysis and multiple linear regression were used to identify environmental variables correlated with the community changes but produced conflicting results depending on the information used, possibly because of strong correlations between some of the variables. The MDS plot of biotic samples illustrated that potassium levels correlated most strongly with community distribution. Because of the large number of variables now shown worldwide to correlate with faunal distributions, we suggest ways to choose the variables to suit the kind of study to be undertaken. Attempts to assign the invertebrates to functional feeding groups (FFGs) were unsatisfactory as the relevant categories are poorly defined and often inadequate for classifying the fauna. We suggest that until these categories are more clearly defined, and more uniformly applied, concepts relating to FFGs cannot be tested satisfactorily. The stream community in Langrivier is more similar to communities found in two other mediterranean ecosystems than to those in other southern African rivers, probably because of the greater predictability of flow in the former.

Community Dynamics of Leaf Litter Breakdown in a Utah Alpine Stream

Leaf litter breakdown was examined in a Rocky Mountain stream in Utah during fall 1976 and winter 1977. Leaf mass loss, changes in macroinvertebrate community structure, and patterns in community respiration were measured on pairs of leaf packs removed from the stream every 2-3 d. Colonization of the leaf packs by macroinvertebrates was rapid. The dominant organisms during the initial colonization were shredders. Highest total oxygen consumption occurred during the initial colonization phase. Macroinvertebrate density, invertebrate biomass, and oxygen consumption adjusted for remaining leaf mass all showed a dramatic increase in variance associated with the final phase of decomposition (<20% leaf material remaining). The final phase of breakdown may have the most interesting biological interactions owing to the high variability in leaf biomass and long residence times that directly affect the utility of the leaf pack as habitat and food.