SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID-ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid-Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid-Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based stream classification did not correspond to Omernik's ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid-Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse-scale factors, such as land cover/use in watersheds, and by site-specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid-Atlantic streams.     

Impact of a eucalyptus (Eucalyptus globulus Labill.) plantation on the nutrient content and dynamics of coarse particulate organic matter (CPOM) in a small stream

Litterfall inputs, benthic storage and the transport of coarse particulate organic matter (CPOM) were studied in two headwater streams, one flowing through a mixed deciduous forest and one through a plantation of Eucalyptus globulus. Vertical and lateral traps, transported CPOM and benthic CPOM were sampled monthly to biweekly and sorted into four categories: leaves, twigs and bark, fruits and flowers and debris. The litterfall inputs were about 20% lower at the eucalyptus site but this reduction was unevenly distributed among the litter categories. The reduction of the nitrogen and phosphorus inputs was larger (50%) than that of CPOM because of the low nutrient concentration of the CPOM at the eucalyptus site. Transported CPOM was also lower at the eucalyptus site. Although total CPOM inputs to the stream were reduced in the eucalyptus plantation, benthic storage of CPOM was 50% higher due to (1) high inputs of CPOM and low discharge during summer, (2) more twig and bark inputs, (3) eucalyptus leaves being retained more efficiently in the stream than deciduous leaves (4) a lower discharge, which may in part be attributable to eucalyptus-induced changes in the hydrological cycle. Increased retention balanced lower nitrogen and phosphorus content of CPOM, so benthic storage of nitrogen and phosphorus was similar at both sites. This work demonstrates that the timing, quality and quantity of inputs and benthic storage of CPOM in streams changes substantially because of the substitution of natural deciduous forest with eucalyptus plantation. Maintenance of buffer strips of natural vegetation may be the best way to protect ecological functioning of small, forested streams.     

Relationship between Riparian Vegetation and Stream Benthic Communities at Three Spatial Scales

We examined the influence of riparian vegetation on macroinvertebrate community structure in streams of the Upper Thames River watershed in southwestern Ontario. Thirty-three μ-basins (129–1458 ha) were used to identify land cover variables that influenced stream macroinvertebrates. Micro-basins represented the entire drainage area of study streams and were similar in stream order (first, second) and land cover (agricultural or forest; no urban). We described the structure and composition of riparian vegetation and benthic macroinvertebrate communities at the outflow reach. The nature of the land cover was quantified for the stream network buffer (30 m) and the whole μ-basin. The objective of this study was to measure the magnitude and nature of the relationship between the riparian vegetation and benthic macroinvertebrate community at the outflow reach, stream network buffer, and whole μ-basin scales. Taxon richness (including total number of Ephemeroptera, Plecoptera, and Trichoptera taxa) and Simpson’s diversity of the macroinvertebrate community all increased with increased tree cover in the riparian zone at the outflow reach scale. Simpson’s equitability was lower with greater agricultural land cover in the stream network buffer. No relationship between the macroinvertebrate community and land cover was found at the whole μ-basin scale. Analysis of the influence of land cover on stream communities within a spatial hierarchy is important for understanding the interactions of stream ecosystems with their adjacent landscapes.     

Temporal and Spatial Patterns in Inputs and Stock of Organic Matter in Savannah Streams of Central Brazil

Coarse particulate organic matter (CPOM) inputs from riparian vegetation into streams and CPOM benthic stock vary naturally in space and time, but most studies in the tropical savannah (Cerrado) have been done over a small temporal scale (<1 year), which does not allow for the determination of inter-annual patterns. We found that CPOM collected over two years differed temporally and spatially, whereas there was no significant variation between years for the benthic stock, which indicates high stability in the energy balance of streams. The largest monthly inputs occurred between August and October, at the end of the dry season and the onset of the rainy season, which was partially explained by precipitation. Other factors such as photoperiod, which was not studied, could also have important roles in this pattern. Spatial differences in CPOM between streams were attributed to topography and channel morphology. The plant density was lowest in the stream with a more irregular topography and a deeper channel, which results in drier riparian soil. The benthic stock was highest in the stream with a flat channel, where the lower water speed facilitates the accumulation of CPOM in the stream bed. Inter-annual differences in CPOM were attributed more to the differences in the beginning of the dry and wet periods between years than to the average values of precipitation. Longer-term studies are needed to clarify this temporal pattern.     

Revealing the pathways by which agricultural land‐use affects stream fish communities in South Brazilian grasslands

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Urban catchment hydrology overwhelms reach scale effects of riparian vegetation on organic matter dynamics

1. Urbanisation severely affects stream hydrology, biotic integrity and water quality, but relatively little is known about effects on organic matter dynamics. Coarse particulate organic matter (CPOM) is a source of energy and nutrients in aquatic systems, and its availability has implications for ecosystem productivity and aquatic communities. In undisturbed environments, allochthonous inputs from riparian zones provide critical energy subsidies, but the extent to which this occurs in urbanised streams is poorly understood. 2. We investigated CPOM inputs, standing stocks, retention rates and retention mechanisms in urban and peri‐urban streams in Melbourne, Australia. Six streams were chosen along a gradient of catchment urbanisation, with the presence of reach scale riparian canopy cover as a second factor. CPOM retention was assessed at baseflow via replicate releases of marked Eucalyptus leaves where the retention distance and mechanism were recorded. CPOM and small wood (>1 cm diameter) storage were measured via cores and direct counts, respectively, while lateral and horizontal CPOM inputs were assessed using riparian litter traps. Stream discharge, velocity, depth and width were also measured. 3. CPOM inputs were not correlated with urbanisation, but were significantly higher in ‘closed’ canopy reaches. Urbanisation and riparian cover altered CPOM retention mechanisms, but not retention distances. Urban streams showed greater retention by rocks; while in less urban streams, retention by small wood was considerably higher. CPOM and small wood storage were significantly lower in more urban streams, but we found only a weak effect of riparian cover. 4. These findings suggest that while riparian vegetation increases CPOM inputs and has modest/weak effects on storage, catchment scale urbanisation decreases organic matter availability. Using an organic matter budget approach, it appears likely that the increased frequency and magnitude of high flows associated with catchment urbanisation exerts an overriding influence on organic matter availability. 5. We conclude that to maintain both organic matter inputs and storage, the restoration and protection of streams in urban or rapidly urbanising environments relies on the management of both riparian vegetation and catchment hydrology.     

Land-use effects on structural and functional composition of benthic and leaf-associated macroinvertebrates in four Andean streams

The replacement of native forests by pastures takes place widely in the Andes. The effects of such land-use change on aquatic assemblages are poorly understood. We conducted a comparative analysis of the effects of forest conversion to pastures on the taxonomic, structural, and functional composition of macroinvertebrates (benthic and leaf-associated) in montane and upper montane streams (ecosystem type) of the south Ecuadorian Andes. Taxonomic composition of benthic and leaf-associated macroinvertebrates was different between ecosystem type and land use. Also, major differences in the structural and functional composition of benthic and leaf-associated macroinvertebrates were mainly promoted by land use in both ecosystem types. Forested streams showed higher diversity than pasture streams, sustaining more shredder, scraper, and predatory invertebrates. We also observed differences in the macroinvertebrate communities between benthic and leaf-bag samples. Leaf bags had lower diversity and more collector invertebrates than benthic samples. This study highlights the large effect of riparian forest conversion to pasture land on macroinvertebrate communities, and the importance of using appropriate sampling techniques to characterize aquatic assemblages. We also recommend the maintenance and restoration of riparian vegetation to mitigate the effects of deforestation on stream communities and ecosystem processes.     

Rehabilitation of Stream Ecosystem Functions through the Reintroduction of Coarse Particulate Organic Matter

In streams, coarse particulate organic matter (CPOM) acts as a substrate for microbial activity, which promotes nutrient retention. However, in urban areas, increased peak flows within streams lead to decreased retention of CPOM. The aim of this study was to investigate whether the reintroduction of CPOM, in the form of leaf litter, into a degraded urban stream would increase biofilm activity and phosphorus retention, two ecosystem functions that reflect the integrity of the ecosystem. Stream metabolism and nutrient retention were assessed in treated (T) and control (C) channels of the Torrens River Catchment, South Australia, before and after CPOM addition. Gross primary production and community respiration (CR) were measured as oxygen production and consumption within benthic chambers. Phosphorus retention was measured through a series of short-term filterable reactive phosphorus (FRP) addition experiments. Before CPOM addition, there were no differences in CR, but C retained 6.8% more FRP than T. After CPOM addition, CR was greater in T than in C (572 and 276 mg O₂·m⁻²·day⁻¹, respectively), and T retained 7.7% more FRP than C. The increase in FRP retention in T compared to C was attributed to phosphorus limitation of the CPOM and increased demand for phosphorus of the attached microbial heterotrophic community. The reintroduction of CPOM into degraded streams will be an important step in the restoration of stream metabolism and nutrient retention. Maintenance of CPOM may be achieved through restoration of riparian vegetation, a reduction in the increased peak flows, and rehabilitation of stream morphology.     

美国大陆五个山区集水区的河溪结构分析

河溪结构及其关联的空间特征,是研究集水区中能量和物质移动规律所不可缺少的内容,利用美国地质调查局的水文数据库和地理信息系统（ＡＲＣ／ＩＮＦＯ）,对美国大陆５个主要山地中典型集水区河溪的数据量、密度,分布结构以及河溪岸边带的组成,进行了分析和比较。５个山地分别是：西北太平洋沿岸,西部卡斯卡特山,中部落矶山,东部阿巴拉契亚山和南部欧扎克山,发现这５个集水区中的河溪网络结构非常相似,一级河溪大约占河溪总     

Impact of riparian buffer zones on water quality and associated management considerations

Recent attention has focused on riparian forest buffer systems for filtering sediment, nutrients, and pesticides entering from upland agricultural fields. This paper summarizes the results of a field monitoring study done in Tokachikawa watershed in Hokkaido, Japan, Cisadane, Cianten and Citamyang sub-watersheds in Indonesia and Cauvery watershed, India to quantify the impact of riparian buffer zones on changes in stream water quality. A watershed approach was used to compare land use indicators – uplands, forests, riparian forest, livestock areas – to a wide range of surface water physical and chemical properties. Stream water physical property values increased from upstream to the confluence point, influenced by the upland and livestock land use activities. The greatest reduction in impairment of water quality was observed in buffer zones located along higher order streams where the gradient is very low, leading to slow groundwater movement. The lower stream water temperature in riparian buffer zones suggests that the shading effect is most pronounced in this area of the watershed. The results demonstrate the positive impact of forest buffer zones in reducing the influence of agricultural nutrients and chemicals on surface stream waters. Design and management considerations for establishing riparian zone land use are discussed.     

Benthic invertebrate communities in six Danish forest streams: Impact of forest type on structure and function

I investigated the impact of riparian vegetation type on stream invertebrate communities in SIX Danish forest streams during the period 1992–1993 Two of the streams ran through beech forest Fagus sylvatica , two through mixed, mainly deciduous forest, and two through conifer plantations Algal biomass and detritus standing stock differed significantly depending on forest type In mixed deciduous forest streams algal biomass was higher and detritus standing stock lower than in beech and conifer forest streams Benthic invertebrate community composition was functionally similar in all streams, with detritivores comprising >80% by numbers of the community Grazer abundance was low in all streams Shredder abundance in the four deciduous streams was significantly correlated to coarse detritus standing stock When taking into account both quantitative and qualitative POM parameters for all six streams combined, shredder abundance correlated significantly to the amount of CPOM There were significantly more invertebrates with a ≥ 2 yr life cycle in the conifer and beech forest streams than m the mixed forest streams, probably reflecting the larger and more stable food resource For all streams combined there was a significant correlation between predator abundance and the abundance of potential prey My findings suggest that Danish forest streams are regulated by "bottom-up control" at all trophic levels within the invertebrate community, and hence that forest type can structure benthic communities in forest streams     

Effects of livestock grazing on benthic invertebrates from a native grassland ecosystem

SUMMARY 1. The effects of cattle grazing on stream bank stability, biomass of riparian vegetation, instream vegetation cover, biomass of coarse particulate organic matter (CPOM) and epilithon and benthic invertebrate community structure were investigated over a 2‐year period using: (i) enclosures containing different cattle grazing treatments and (ii) by comparing streams with different grazing intensities in the Cypress Hills Provincial Park, Alberta, Canada. 2. Livestock enclosure experiments comprised four treatments of: (1) early season cattle grazing (June–August), (2) late season cattle grazing (August–September), (3) all season cattle grazing (June–September) and (4) cattle‐absent controls. All four treatments were replicated in two streams while two treatments (i.e. cattle‐absent controls, early season cattle grazing) were established in a third stream. 3. Bank stability, estimated visually based on sediment inputs to the stream channel, increased significantly in cattle‐absent treatments compared with cattle‐present enclosures over the 2‐year study period. 4. Epilithic chlorophyll a was significantly affected by time, but neither cattle nor the interaction of time and treatment were significant. 5. At the end of the experiment, total invertebrate biomass in the late and all‐season treatment exceeded that in the early and cattle‐absent treatments. However, excluding cattle from the streams, at any of the different treatments, had little clear impact on the total benthic invertebrate abundance or the abundance of the predominant functional feeding groups over the 2‐year study period. 6. In contrast, comparisons of benthic assemblages from streams with different grazing intensities showed that the non‐grazed reach of Storm Creek contained significantly higher biomass of CPOM and shredders compared with the cattle‐absent enclosures in Battle, Graburn and Nine Mile creeks. Redundancy analysis showed that benthic communities from all enclosures and Storm Creek in summer and autumn 2000 were affected primarily by CPOM biomass, pH, nitrate, turbidity and benthic chlorophyll a. Construction of a 99% probability ellipse from enclosure sites showed that invertebrate communities from livestock enclosures differed from that in the non‐grazed Storm Creek. 7. Results from stream‐scale comparisons indicate that current livestock grazing practices in the Cypress Hills significantly impact riparian zones, stream channels and benthic invertebrate community structure and that alternative practices, such as rotational grazing, need to be developed.     

Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.     

Benthic organic matter dynamics in Texas prairie streams

Concentrations of benthic particulate organic matter (POM) in six Texas prairie streams (2nd–4th order, intermittent and perennial) were monitored over a 20 month period to determine temporal and spatial dynamics. Benthic POM mass was highly variable, having coefficients of variation (CV) in excess of 300%. Benthic POM mass in all streams was similar with the exception of the 4th order intermittent stream which had significantly higher concentrations. Benthic POM at all sites was dominated by coarse POM (CPOM), followed by fine POM (FPOM), ultrafine POM (UPOM), and medium POM (MPOM). The dominance by CPOM is especially noteworthy in the 4th order intermittent stream where it accounted for 83% of the annual POM mass. Seasonally, benthic POM was highest in summer and lowest in the fall.     

Impacts of urbanization on stream habitats and macroinvertebrate communities in the tributaries of Qiangtang River,China

The impacts of watershed urbanization on streams have been studied worldwide, but are rare in China. We examined relationships among watershed land uses and stream physicochemical and biological attributes, impacts of urbanization on overall stream conditions, and the response pattern of macroinvertebrate assemblage metrics to the percent of impervious area (PIA) of watersheds in the middle section of the Qiantang River, Zhejiang Province, China. Environmental variables and benthic macroinvertebrates of 60 stream sites with varied levels of watershed urban land use were sampled in April, 2010. Spearman correlation analysis showed watershed urbanization levels significantly correlated with increased stream depth, width, and values of conductivity, total nitrogen, ammonia, phosphate, calcium, magnesium, and chemical oxygen demand for the study streams. There was significant difference in total taxa richness, Empheroptera, Plecoptera, and Trichoptera (EPT) taxa richness, and Diptera taxa richness, percentages of individual abundances of EPT, Chironomidae, shredders, filterers, and scrapers, and Shannon–Wiener diversity index between reference streams and urban impacted streams. In contrast, percentages of individual abundances for collectors, oligochaeta, and tolerant taxa, and biotic index were significantly higher in urban impacted than reference streams. All the above metrics were significantly correlated with PIA. The response patterns of total taxa richness, EPT taxa richness, and Shannon–Wiener diversity index followed a drastic decrease at thresholds of 3.6, 3.7, and 5.5% of PIA, respectively. Our findings indicate that stream benthic macroinvertebrate metrics are effective indicators of impacts of watershed urban development, and the PIA-imperviousness thresholds we identified could potentially be used for setting benchmarks for watershed development planning and for prioritizing high valued stream systems for protection and rehabilitation.     

Implications of water column ammonium uptake and regeneration for the nitrogen budget in temperate, eutrophic Missisquoi Bay, Lake Champlain (Canada/USA)

Invasion by exotic trees into riparian areas has the potential to impact aquatic systems. We examined the effects of the exotic Salix fragilis (crack willow) on the structure and functioning of small streams in northern Patagonian Andes via a field survey of benthic invertebrates and leaf litter and an in situ experiment. We compared leaf decomposition of the native Ochetophila trinervis (chacay) and S. fragilis in reaches dominated by native vegetation versus reaches dominated by crack willow. We hypothesized that S. fragilis affects the quality of leaf litter entering the streams, changing the aquatic biota composition and litter decomposition. Our study showed that crack willow leaves decomposed slower than chacay, likely related to leaf properties (i.e., leaf toughness). Benthic leaf litter mass was similar between the two riparian vegetation types, though in stream reaches dominated by crack willow, leaves of this species represented 82% of the total leaf litter. Benthic invertebrate abundance and diversity were similar between reaches but species composition differed. Our study found little evidence for strong impacts of crack willow on those small streams. Further studies on other aspects of ecosystem functioning, such as primary production, would enhance our understanding of the impacts of crack willow on Patagonian streams.     

Changes in habitat structure, benthic invertebrate diversity, trout populations and ecosystem processes in restored forest streams: a boreal perspective

1. Most Finnish streams were channelised during the 19th and 20th century to facilitate timber floating. By the late 1970s, extensive programmes were initiated to restore these degraded streams. The responses of fish populations to restoration have been little studied, however, and monitoring of other stream biota has been negligible. In this paper, we review results from a set of studies on the effects of stream restoration on habitat structure, brown trout populations, benthic macroinvertebrates and leaf retention. 2. In general, restoration greatly increased stream bed heterogeneity. The cover of mosses in channelised streams was close to that of unmodified reference sites, but after restoration moss cover declined to one‐tenth of the pre‐restoration value. 3. In one stream, densities of age‐0 trout were slightly lower after restoration, but the difference to an unmodified reference stream was non‐significant, indicating no effect of restoration. In another stream, trout density increased after restoration, indicating a weakly positive response. The overall weak response of trout to habitat manipulations probably relates to the fact that restoration did not increase the amount of pools, a key winter habitat for salmonids. 4. Benthic invertebrate community composition was more variable in streams restored 4–6 years before sampling than in unmodified reference streams or streams restored 8 years before sampling. Channelised streams supported a distinctive set of indicator species, most of which were filter‐feeders or scrapers, while most of the indicators in streams restored 8 years before sampling were shredders. 5. Leaf retentiveness in reference streams was high, with 60–70% of experimentally released leaves being retained within 50 m. Channelised streams were poorly retentive (c. 10% of leaves retained), and the increase in retention following restoration was modest (+14% on average). Aquatic mosses were a key retentive feature in both channelised and natural streams, but their cover was drastically reduced through restoration. 6. Mitigation of the detrimental impacts of forestry (e.g. removal of mature riparian forests) is a major challenge to the management of boreal streams. This goal cannot be achieved by focusing efforts only on restoration of physical structures in stream channels, but also requires conservation and ecologically sound management of riparian forests.     

凉水和帽儿山地区低级溪流生境和水质状况

为研究不同植被景观土地利用下低级溪流的生境状况和水质差异,对凉水国家级自然保护区红松原始林内和帽儿山国家森林公园境内天然次生林植被和农田背景下各3条溪流生境和水质状况进行调查。对溪流生物特性（悬浮藻、附着藻）和理化性质（温度、混浊度、溶解氧（DO）、pH、NH4^＋-N、NO3^-N、PO4^3-P、总氮（TN）和总磷（11P））以及细小颗粒有机物质（FPOM）和粗大有机物质（CPOM））进行测定。研究结果表明,凉水地区原始林溪流的生境状况好于帽儿山地区的次生林,帽儿山地区农田溪流生境最差。原始林溪流具有稳定的溪底生物生活基质,稳水区和急流区均匀分布,稳水区尺度变化大,受淀积物沉降干扰小,河道较弯曲,河岸稳定,河岸植被覆盖度高;次生林溪流以急流区为主,稳定基质相对较差,并受到一定程度的淀积物沉降干扰,河岸尚稳定,有一定程度的人为干扰;农田溪流基质不稳定,受到强烈淀积物沉降影响,渠道化严重,河岸带植被严重破坏。3种景观背景下溪流总磷（11P）、溶解氧（DO）、混浊度、温度、氮磷比值（N／P）（P〈0．05）存在显著差异。原始林溪流NH4^＋、DO、TP、TN、悬浮藻浓度和pH较高、附着藻数量较多,温度较低、FPOM和CPOM的数量较少;次生林溪流的NO3^--N、N／P和TDIN较高。 浊度较低;农田溪流浊度、温度、PO4^3--P较高,DO和pH较低,附着藻数量较少。景观尺度上的土地利用对溪流生境具有深刻的影响,同时决定溪流的水质状况。     

Effect of a leaf-shredding invertebrate on organic matter dynamics and phosphorus spiralling in heterotrophic laboratory streams

Summary The effect of invertebrate shredders on organic matter dynamics and phosphorus spiralling was studied over a 30-week period in laboratory streams. The streams were fed by groundwater, layered with cobble and gravel from a natural stream, covered with opaque material to eliminate algal growth, and initially contained 195 g/m² of autumn-shed leaves. Four weeks after leaf addition, leaf-shredding snails (Goniobasis clavaeformis) were added to each of three streams in densities of 75, 220, and 800/m². A fourth stream contained no snails and served as a control.Presence of snails increased the loss rates of coarse particulate organic matter (CPOM) and total organic matter (TOM), primarily by increasing leaf fragmentation and seston export. Although snail feeding increased specific metabolism of microbes associated with CPOM and cobble surfaces, it was not enough to compensate for reduction in bacterial cell numbers per unit surface area and in stream TOM. Consequently mineralization of detritus and whole stream phosphorus utilization rate were maximum in the stream with no snails and decreased with increasing snail density. From previous simulations of a stream model based on the nutrient spiralling concept, we predicted that there should be an intermediate shredder density which would minimize phosphorus spiralling length (maximize phosphorus utilization) in a natural stream nearby. Our current results conflict with the model-based predictions primarily because the increase in microbial metabolism was less important than reduction in bacterial cell numbers and total benthic organic matter resulting from snail feeding. Although our results indicate macroinvertebrate shredders reduce phosphorus utilization in headwater streams, shreders may increase nutrient utilization downstream where riparian inputs are lower, thus linking low- and high-order streams.Research supported by the National Science Foundation's Ecosystem Studies Program under Interagency Agreement No. BSR-8103181, A02 with the U.S. Department of Energy, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.Publication No. 2394, Environmental Sciences Division, ORNL