Effect of sudden flow reduction on the decomposition of alder leaves (Alnus glutinosa [L.] Gaertn.) in a temperate lowland stream: a mesocosm study

Climate change leads to summer low flow conditions and premature litter input in lowland streams in Central Europe. This may cause a sudden reduction of flow and fragmentation into isolated pools of permanently flowing streams, with a simultaneous increase in the food supply for detrivores during summer months. We performed a mesocosm study to investigate shredder and microbial mediated litter decomposition under these conditions. Leaf litter was placed in a lowland stream with a natural flow regime (reference) and in a stream mesocosm with significant flow reduction (FR) and a representative density of macroinvertebrates and detritus. Physicochemical parameters, leaf mass loss, macroinvertebrate abundance and biomass, leaf-associated respiration, fungal sporulation, and biomass were measured at regular intervals for 6 weeks. Coarse and fine-mesh bags were used to include or exclude macroinvertebrate shredders. In the coarse-mesh bags, leaf mass loss was significantly lower in the FR system than in the reference regime. In the fine-mesh bags, leaf respiration, fungal sporulation, and biomass but not leaf mass losses were substantially lower with flow reduction. Chironomid larvae (Micropsectra spp.) appeared to effectively fragment leaf litter in fine-mesh bags. In the FR system, leaf respiration was higher in the coarse-than in the fine-mesh bags. Our results suggest that, in temperate lowland streams, premature litter input during or after a sudden fragmentation into isolated pools and a reduction of stream flow reduces direct shredder-mediated litter decomposition, but shredders may indirectly influence the decomposition process. Handling editor: B. Oertli     

A long-term improvement in Danish stream fauna: Analyses of temporal dynamics and community alignment of a biotic index

Danish streams have for some decades shown a significant improvement in ecological quality. This is based on a time-series of a subset of 247 sites from the nationwide monitoring program with reoccurring annual sampling. The ecological quality is determined using the benthic macroinvertebrate fauna as bio-indicators via the Danish Stream Fauna Index (DSFI), and expressed in categorically ranked scores, with 1 as the lowest, and 7 as the highest quality. We analysed a data set on DSFI scores and the associated taxon lists from 2004 to 2013, totaling to 2411 individual samples or communities. Our main objectives were to 1) examine the temporal multidirectional dynamics underlying the overall net improving trend in the DSFI scores, and 2) to elucidate how the DSFI responds to differences in faunal community composition. Our analysis showed that most sites exhibited unchanged DSFI over the 10 year period (53%), although inter-annual shifts were still observable within these sites. The DSFI interval 1–2 showed the highest proportion of positive shifts, whereas interval 3–4 the highest proportion of negative shifts. Improvement measures should therefore be directed more specifically towards intermediate quality streams. Sites with moderate to high scores (DSFI 4–7) were most stable. Turnover (i.e., replacement of taxa) was the dominating component of overall beta diversity, while the richness change component (i.e., loss/gain of taxa) was negligible. The specific DSFI scores encompassed wide ranges of community composition, and showed a weak, but significant correlation between differences in scores and the community composition. This is to a certain degree advantageous since it gives the categorical scores robustness and plasticity, and thus makes the DSFI capable of handling natural variation in communities. The ideal biotic index should allow for natural variation, but do so while maintaining the capability of separating ecologically different communities. Our findings consequently give rise to concern to whether the DSFI is too sensitive to stochastic variation in samples with insufficient precision to assign sites to correct ecological quality classes.     

Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (δ¹³C and δ¹⁵N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter.
2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach.
3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus . Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin.
4. Seasonal changes in the δ¹³C signature of the macroinvertebrates corresponded with seasonal changes in δ¹³C of the gold alga H. foetidus . This indicated that the energy base remains autochthonous throughout the year.
5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous.
6. The overall enrichment of δ¹⁵N was 2.25‰ ( r ²=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ¹⁵N within trophic levels decreased.     

Flow extremes and benthic organic matter shape the metabolism of a headwater Mediterranean stream

1. Single‐station diel oxygen curves were used to monitor the oxygen metabolism of an intermittent, forested third‐order stream (Fuirosos) in the Mediterranean area, over a period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP) were estimated and related to organic matter inputs and photosynthetically active radiation (PAR) in order to understand the effect of the riparian forest on stream metabolism. 2. Annual ER was 1690 g O₂ m^?2 year^?1 and annual GPP was 275 g O₂ m^?2 year^?1. Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16. 3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O₂ m^?2 day^?1. The maximum values of GPP occurred during a few weeks in spring, and ended when the riparian canopy was fully closed. The phenology of the riparian vegetation was an important determinant of light availability, and consequently, of GPP. 4. On a daily scale, light and temperature were the most important factors governing the shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in the P–I relationships. Hysteresis‐type curves were characteristic of late autumn and winter. Light saturation responses (that occurred at irradiances higher than 90 μE m^?2 s^?1) were characteristic of early spring. Linear responses occurred during late spring, summer and early autumn when there was no evidence of light saturation. 5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g O₂ m^?2 day^?1. ER was highest in autumn 2001, when organic matter accumulations on the streambed were extremely high. By contrast, the higher discharge in autumn 2002 prevented these accumulations and caused lower ER. The Mediterranean climate, and in its effect the hydrological regime, were mainly responsible for the temporal variation in benthic organic matter, and consequently of ER.     

Effects of retention site on breakdown of organic matter in a mountain stream

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Differences in patches of retention among leaves,woods and small litter particles in a headwater stream: the importance of particle morphology

The location of retention in the channel can influence invertebrate assemblage and breakdown processes associated with a litter particle in heterogeneous streams. We previously identified four types of litter patches that formed on riffles or different locations in pools (middle, edge, alcove), and demonstrated that middle patches had higher litter processing rates than the other patches. In this study, we examined differences in retention on the four patch types among leaves, woody materials and small litter particles, and among leaves of different sizes, by sampling natural and manipulated litter particles that were newly retained. Proportionally more woody materials, leaf pieces (16–50 mm) and particulate organic matter (1–16 mm) than leaves (>50 mm) were retained on middle patches, while proportionally more leaves than the other litter particles were retained on riffle and edge patches. The retention pattern of leaf species with different leaf sizes and a released experiment of leaf particles revealed that proportionally more leaf particles with smaller sizes were retained on middle patches. The flexibility, shape and dryness of litter particles also seem to affect the location of retention in the channel. These results suggest that the morphology of litter particles have the potential to affect the biological use and breakdown of litter particles through determining the location of retention within the channel. The size of leaves and processes that alter leaf size may have important roles on the breakdown and utilisation by invertebrates in these heterogeneous streams.     

长白山地区溪流冻结初期凋落叶分解与底栖动物定殖的关系

长白山森林源头溪流每年11月至次年4月有约70%的河面被冰覆盖,季节性冻融过程特征明显.为了揭示溪流冻结初期凋落叶分解与底栖动物定殖的关系,在长白山地区1条源头溪流中,利用2种孔径(5和0.3 mm)的尼龙分解袋对色木槭、紫椴、蒙古栎的单一及混合凋落叶进行了为期35 d的分解研究.结果表明: 凋落叶质量损失率在单一树种间差异显著,表现为:色木槭＞紫椴＞蒙古栎,而在4种混合凋落叶间差异不显著;除紫椴和3树种混合凋落叶外,粗、细分解袋间凋落叶质量损失率差异不显著;凋落叶混合效应仅出现在紫椴-蒙古栎混合的粗分解袋内;定殖在不同凋落叶分解袋内的底栖动物群落结构差异较大,但撕食者密度在3种凋落叶间差异不显著,撕食者对凋落叶混合效应的响应也不显著.由结果可知,溪流冻结初期微生物是凋落叶的主要分解者,底栖动物的贡献率较低.虽然撕食者密度较低,但撕食者的活动是凋落叶混合效应出现的必要条件.底栖动物对食物和栖息地有一定的选择性,但由于定殖时间较短,凋落叶对撕食者定殖的影响不显著.本研究对源头溪流生态系统的冬季生态过程研究及生态系统管理具有一定的理论意义.     

Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream

1. Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events.
2. We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live–dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths.
3. The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live–dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters.
4. Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size.
5. Our findings show two important functions of the hyporheic zone during drought: (1) deeper (?50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone.

     

Aligning indicators of community composition and biogeochemical function in stream monitoring and ecological assessments

Reliable and inexpensive indicators of ecosystem function are essential for accurately monitoring and describing ecosystem integrity. Currently, most state and federal assessments of aquatic ecological integrity rely on structural indicators and assume tight coupling of structure and function. We used fluorescent composition of dissolved organic matter as a metric for certain ecosystem functions and compared the resulting index of autochthonous microbial dissolved organic matter (DOM) to macroinvertebrate indicators and classifications of water quality attainment reported by the Maine Department of Environmental Protection (Maine DEP) at 142 stream sites. We observed that metrics of sensitive insect orders such as relative Plecoptera generic richness, relative Ephemeroptera abundance, and generic richness of EPT (Ephemeroptera, Plecoptera, and Trichoptera) were negatively correlated with higher values of metrics based on autochthonous microbial DOM sources. At the same time we observed an increase in the Hilsenhoff Biotic Index with increasing microbial DOM. We compared the abundance of this microbial DOM component to Maine DEP measured attainment classes and found that microbial DOM generally separated sites with high biological integrity from sites where the biotic community was highly degraded. This highlights that measures of biogeochemical ecosystem function complement measures of structure in biological assessments.     

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K ₂) determined from seven empirical depth-velocity equations. Because the predicted values of K ₂ showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.     

Kinetics and mechanisms of dissolved organic carbon retention in a headwater stream

Freeze-dried aqueous extracts of autumn-shed maple leaves, birch leaves, and spruce needles were added to a third-order reach of Bear Brook, New Hampshire at concentrations similar to those predicted to occur during peak leaf fall. Leachate from each species was rapidly removed from solution. With initial concentrations of added leachate of approximately 5 mgl^–1, dissolved organics (DOC) uptake ranged from 73 to 130 mg m^–2 h^–1 for the first five hours of travel downstream from the point of addition. There was no preferential removal of DOC of low molecular weight, or of monomeric carbohydrates relative to phenolics or unidentified DOC.Stream sediments and organic debris rapidly removed DOC from solution in laboratory experiments. No significant flocculation or microbial assimilation of sugar maple leachate occurred in stream water alone. Stream sediments showed small increases in respiration with addition of leaf leachate, but no increase in respiration occurred upon addition of leachate to organic debris. Abiotic adsorption due to the high concentrations of exchangeable iron and aluminium in stream sediments may be responsible for much of the rapid removal of leaf leachate observed in field experiments. Abiotic processes appear to retain DOC within the stream, thereby allowing subsequent metabolism of dissolved organic carbon by stream microflora.     

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

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

Dynamics of C, N, P and microbial community composition in particulate soil organic matter during residue decomposition

Decomposing residues can be an important source of nutrients for plants, especially of N and P, but the relationship between N and P release and microbial community dynamics have rarely been studied. Two pea (Pisum sativum L.) residues with contrasting chemical composition, shoots from flowering pea (Pea-Y) with 2.9 mg P and 36 mg N kg⁻¹ and from mature pea (Pea-M) with 0.3 mg P and 13 mg N kg⁻¹, were added at a rate of 20 g kg soil⁻¹ to a sandy soil low in nutrients. Particulate organic matter (POM) was isolated on days (d) 0, 5, 15, 28, 42 and 61 after residue addition and analysed for C, N, P and microbial community structure (fatty acid methyl ester analysis). The recovery of POM from residue-amended soils decreased over time to 30–40% of added amounts for both residues. Apart from d 0, the N concentration in POM was lower in residue-amended soil than in the control. Due to a rapid decrease in P concentration during the first 5 days in Pea-Y and a slow increase over the whole experiment in Pea-M, P concentrations in POM on d 61 were similar in all treatments. In Pea-Y, the dynamics of C, N and P were coupled, with amounts of C, N and P decreasing during the first 15 days and remaining stable thereafter. In Pea-M, a steady loss of C from POM was contrasted by a slight increase in P. As a result, the C/P ratio decreased from 1,330 on d 0 to 390 on d 61. The C/N ratio of Pea-M decreased only during the second phase of decomposition. The different nutrient dynamics in Pea-Y and Pea-M led to similar amounts of N and P in POM towards the end of the incubation. Microbial community composition in the POM in Pea-Y and Pea-M remained distinct from the control, even though it changed over time. POM was shown to be an important source of potentially available nutrients after addition of plant residues. In the unamended soil, stable nutrient amounts in POM suggested very low net nutrient release from native POM compared to POM after residue addition.     

Leaf litter decomposition and microbial activity in nutrient-enriched and unaltered reaches of a headwater stream

SUMMARY 1. Decomposition of red maple ( Acer rubrum ) and rhododendron ( Rhododendron maximum ) leaves and activity of associated microorganisms were compared in two reaches of a headwater stream in Coweeta Hydrologic Laboratory, NC, U.S.A. The downstream reach was enriched with ammonium, nitrate, and phosphate whereas the upstream reach was not altered.
2. Decomposition rate, microbial respiration, fungal and bacterial biomass, and the sporulation rate of aquatic hyphomycetes associated with decomposing leaf material were significantly higher for both leaf types in the nutrient-enriched reach. Species richness and community structure of aquatic hyphomycetes also exhibited considerable changes with an increase in the number of fungal codominants in the nutrient-enriched reach.
3. Fungal biomass was one to two orders of magnitude greater than bacterial biomass in both reaches. Changes in microbial respiration rate corresponded to those in fungal biomass and sporulation, suggesting a primary role of fungi in leaf decomposition.
4. Nutrient enrichment increased microbial activity, the proportion of leaf carbon channelled through the microbial compartment and the decomposition rate of leaf litter.     

Epilithic bacterial and algal colonization in a stream run, riffle, and pool: a test of biomass covariation

Epilithic bacterial and algal biomass were compared among a run, riffle, and pool along an open-canopy section of a third-order, temperate stream. Epilithic biofilms were sampled after 3, 7, 14, 21, 28, and 35 days colonization on unglazed ceramic tiles that were attached to plastic trays (n = 3) placed across each of the three habitats (i.e., run, riffle, pool). The diverse habitats and sampling regime were selected to provide a range in algal biomass so that potential covariation between epilithic bacterial and algal biomass could be assessed. There were significant differences among habitats and among trays within each habitat for both chlorophyll a and AFDM. Chlorophyll a and AFDM increased in the run and pool throughout the colonization period. In the riffle, chlorophyll a and AFDM increased rapidly early in colonization, then decreased. Epilithic bacterial biomass increased rapidly with no significant differences among the three habitats throughout colonization. Further, bacterial biomass did not correlate with either chlorophyll a or AFDM in any of the three habitats or on any of the sampling days. These results suggest that epilithic algal and bacterial biomass may be regulated by independent controls in some stream environments.     

Drift patterns of particulate matter and organisms during artificial high flows in a large experimental channel

Using a large experimental channel, five artificial high flows with different flow regimes were launched to reveal the precise temporal sequences of drifting particulate matter and organisms during the high flows. Drifting fine particulate matter and organisms were collected by sampling bottles and a water pump, respectively. The peaks of drift abundance occurred before the peak discharges, and the abundance declined quickly within several minutes during the rising phase of high flows. The major determinant of drift abundance of particulate matter and organisms was periods of stable conditions before each high flow (tested by determination coefficients in a correlation analysis). The drift of macroscopic plant material also accelerated the drifts of epiphytic fauna and infauna, which drifted with plants and bed sediments. The magnitude (discharge and duration of peak flows) of high flows only slightly affected drift abundance. The fauna more resistant to high flows were invertebrates that fasten their cases or retreats by silk threads on the substrata. Invertebrate taxa having sucking apparatus or high swimming activity were also resistant to high flows.     

Bacterial biomass on fine detritus particles in a woodland stream of Hokkaido,Japan

Bacterial biomass on four different size fractions of benthic fine particulate organic matter (FPOM) was measured throughout a year in a woodland stream in Hokkaido, Japan. Bacterial biomass per ash free dry mass (AFDM) of the FPOM was significantly different among the four size fractions, while the biomass per unit surface area of FPOM was not. Annual mean of bacterial biomass per particle mass positively correlated to the surface area per mass in the four size fractions. In addition, negative correlation was found between the C:N ratio and bacterial biomass per AFDM in FPOM when all fractions were pooled. There was also negative correlation between C:N ratio and the surface area per particle mass in the four size fractions. The difference of bacterial biomass on FPOM among the size fractions could be ascribed to surface area and/or C:N ratio of FPOM, although these effects were not separately evaluated in the present study.     

Fungal colonization and decomposition of Castanopsis sieboldii leaves in a subtropical forest

Fungi play a crucial role in the decomposition of lignin in fallen leaves but few studies have examined the functional roles of ligninolytic fungi associated with the decomposition of fallen leaves on tropical forest soils. This study examined fungal populations responsible for lignin decomposition in Castanopsis sieboldii leaves in a subtropical evergreen broad-leaved forest in southern Japan. Fallen leaves of C. sieboldii are characterized by the occurrence of bleached portions attributable to fungal colonization of leaf tissues and decomposition of lignin. The bleached area accounted for 29.7%, on average, of the total area of C. sieboldii fallen leaves in the study site. Leaf mass per unit area (LMA) and lignin content were lower in the bleached area than in the surrounding nonbleached area of the same leaves, indicating that removal of lignin enhanced mass loss from leaf tissues and created small-scale heterogeneity of decomposition within single leaves. An unidentified species of Lachnocladiaceae (Basidiomycetes) was isolated frequently from the bleached area and caused selective decomposition of lignin in leaves under pure culture conditions, indicating that this fungus was responsible for the bleaching. The greater hyphal length of basidiomycetes in the bleached area than in the nonbleached area supported the finding that this Lachnocladiaceae sp. was associated with the bleaching. The relatively rapid decomposition of C. sieboldii leaves on the subtropical forest soil is partly attributable to colonization of the litter by this Lachnocladiaceae sp.     

Litterfall,litter decomposition,and flux of particulate organic material in a coastal plain stream

Import of allochthonous material in terms of litterfall in a 3rd order stream in Mississippi coastal plain was 386g dry wt/m²/yr. Litter materials consisting of deciduous leaves, pine needles, and woody twigs collected during different seasons showed some differences in ash-free dry weight, caloric, carbon, hydrogen, nitrogen and phosphorus contents. In situ decomposition to particulate form of deciduous and pine litter enclosed in nylon litter bags showed 15% and 65%, respectively, of the litter remaining after 334 days. Downstream net transport of suspended particulate organic material in the river averaged 688,290 kg dry wt/yr with a range of 456,061 to 920,518 kg dry wt/yr. There was a tendency for the particulate organic matter load of the water to increase during ebbtide and to decrease during floodtide.Supported by the National Aeronautics and Space Administration (NASA Grant No. NGL-25-001-40) and by the National Science Foundation Biological Oceanography Section (NSF Grant No. GA-35715).     

Changes in mechanisms of summer detritus processing between wooded and agricultural sites in a Michigan headwater stream

The mechanisms of leaf decay, leaf-associated macroinvertebrate community structure, leaf-associated microbial activity and physicochemical stream characteristics were investigated on a mid-Michigan headwater stream in summer. An undisturbed wooded site was compared with two agriculturally perturbed sites. Discharge, total suspended particulates, and nutrients were all higher and more variable throughout the season within the agricultural reaches. Leaf decay rates were higher at the agricultural sites and results suggest discharge abrasion was the major leaf processing mechanism at these sites while microbial decay and macroinvertebrate shredding appear to be the primary mediators of leaf weight loss at the wooded site. Total macroinvertebrate densities on leaf packs at the agricultural sites were 1.9 times the densities at the wooded site. It is suggested that experimentally introduced leaf packs acted as a lure for net-spinning invertebrates limited by stable substratum at the agricultural sites. Species shifts were observed from wooded reaches where Pychnopsyche spp., Gammarus, Ephemeroptera, Bezzia, and Nigronia serricornis were important, to downstream agricultural reaches which were dominated by Cheumatopsyche, Chironomidae, Elmidae, Hydracarina, Hemerodromia, and Caecidotea.