Macroinvertebrate community loss as a result of headwater stream acidification in the Vosges Mountains (N-E France)

The relationships between water chemistry and aquatic macroinvertebrate communities of 41 headwater streams were studied in the Vosges Mountains (N-E of France) in an attempt to assess the impact of acidification on macroinvertebrate diversity. The taxa richness of macroinvertebrates decreased drastically in headwater streams which were characterized by low pH, low calcium and high aluminum content. All taxonomic groups were affected, but Molluscans, Crustaceans and Ephemeroptera disappeared totally from strongly acidified streams. Simple indices based on taxa richness such as the coefficient of community loss may provide accurate tools to quickly assess the impact of acidification on macroinvertebrate communities. Despite the reduction of atmospheric SO₂ emissions, acidification of freshwater in the Vosges Mountains continues to affect streams which were believed in the past to constitute refuge biotopes for numerous species. Consequently, acidification represents a real threat for numerous invertebrates. This study arises the question of the evolution in the future of headwater stream ecosystems. Urgent decisions and interventions are required to preserve non-acidified streams and to restore impacted ecosystems while awaiting spontaneous recovery.     

BIODIVERSITY RESEARCH: Conserving macroinvertebrate diversity in headwater streams: the importance of knowing the relative contributions of α and β diversity

Aim We investigated partitioning of aquatic macroinvertebrate diversity in eight headwater streams to determine the relative contributions of α and β diversity to γ diversity, and the scale dependence of α and β components. Location Great Dividing Range, Victoria, Australia. Methods We used the method of Jost (Ecology, 2007, 88, 2427–2439) to partition γ diversity into its α and β components. We undertook the analyses at both reach and catchment scales to explore whether inferences depended on scale of observation. Results We hypothesized that β diversity would make a large contribution to the γ diversity of macroinvertebrates in our dendritic riverine landscape, particularly at the larger spatial scale (among catchments) because of limited dispersal among sites and especially among catchments. However, reaches each had relatively high taxon richness and high α diversity, while β diversity made only a small contribution to γ diversity at both the reach and catchment scales. Main conclusions Dendritic riverine landscapes have been thought to generate high β diversity as a consequence of limited dispersal and high heterogeneity among individual streams, but this may not hold for all headwater stream systems. Here, α diversity was high and β diversity low, with individual headwater stream reaches each containing a large portion of γ diversity. Thus, each stream could be considered to have low irreplaceability since losing the option to use one of these sites in a representative reserve network does not greatly diminish the options available for completing the reserve network. Where limited information on individual taxonomic distributions is available, or time and money for modelling approaches are limited, diversity partitioning may provide a useful ‘first‐cut’ for obtaining information about the irreplaceability of individual streams or subcatchments when establishing representative freshwater reserves.     

Headwater biodiversity among different levels of stream habitat hierarchy

With the current loss of biodiversity and threats to freshwater ecosystems, it is crucial to identify hot-spots of biodiversity and on which spatial scale they can be resolved. Conservation and management of these important ecosystems needs insight into whether most of the regional biodiversity (i.e. γ-diversity) can be found locally (i.e. high α-diversity) or whether it is distributed across the region (i.e. high β-diversity). Biodiversity patterns of benthic macroinvertebrates and diatoms were studied in 30 headwater streams in five Swedish catchments by comparing the relative contribution of α- and β-diversity to γ-diversity between two levels of stream habitat hierarchy (catchment and region level). The relationship between species community structure and local environmental factors was also assessed. Our results show that both α- and β-diversity made a significant contribution to γ-diversity. β-diversity remained relatively constant between the two levels of habitat hierarchy even though local environmental control of the biota decreased from the catchment to the region level. To capture most of headwater γ-diversity, management should therefore target sites that are locally diverse, but at the same time select sites so that β-diversity is maximized. As environmental control of the biota peaked at the catchment level, the conservation of headwater stream diversity is likely to be most effective when management targets environmental conditions across multiple local sites within relatively small catchments.     

Filters and templates: stonefly (Plecoptera) richness in Ouachita Mountains streams, U.S.A.

1.  We collected adult stoneflies periodically over a 1-year period at 38 sites in two headwater catchments in the Ouachita Mountains, Arkansas, U.S.A. The 43 species collected were a subset of the Ozark-Ouachita fauna and the much larger fauna of the eastern U.S.A. We estimated 78–91% species coverage in the two catchments using jackknife extrapolation of species richness from our survey.
2.  Many streams, especially small ones, lacked surface water for months, but others, both small and large, flowed permanently.
3.  Using published regional presence–absence and coarse ecological data in a discriminant function analysis (DFA), we identified stream size (negative) and regional frequency of occurrence (positive) as predictors of presence in these headwater catchments. For the combined catchments, the extrapolated richness (51 spp.) was similar to an estimate (48 spp.) based on predicted absences from DFA and the Ouachita provincial total of known stonefly species (57 spp.).
4.  Local species richness (1–27 spp. per site) was correlated strongly with stream size (catchment area) but was independent of stream drying. Generic richness was correlated negatively with stream drying and positively, but less strongly, with stream size.
5.  Regionally endemic stoneflies dominated in drying streams, and widely distributed species dominated in more permanent streams. The composition of stonefly assemblages was associated with regional factors (species pools, regional abundance, evolution of tolerant endemic species, regional climate) and local factors (drying, stream size).     

Microbial and macroinvertebrate communities,but not leaf decomposition,change along a mining‐induced salinity gradient

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Invertebrate Metacommunity Structure and Dynamics in an Andean Glacial Stream Network Facing Climate Change

Under the ongoing climate change, understanding the mechanisms structuring the spatial distribution of aquatic species in glacial stream networks is of critical importance to predict the response of aquatic biodiversity in the face of glacier melting. In this study, we propose to use metacommunity theory as a conceptual framework to better understand how river network structure influences the spatial organization of aquatic communities in glacierized catchments. At 51 stream sites in an Andean glacierized catchment (Ecuador), we sampled benthic macroinvertebrates, measured physico-chemical and food resource conditions, and calculated geographical, altitudinal and glaciality distances among all sites. Using partial redundancy analysis, we partitioned community variation to evaluate the relative strength of environmental conditions (e.g., glaciality, food resource) vs. spatial processes (e.g., overland, watercourse, and downstream directional dispersal) in organizing the aquatic metacommunity. Results revealed that both environmental and spatial variables significantly explained community variation among sites. Among all environmental variables, the glacial influence component best explained community variation. Overland spatial variables based on geographical and altitudinal distances significantly affected community variation. Watercourse spatial variables based on glaciality distances had a unique significant effect on community variation. Within alpine catchment, glacial meltwater affects macroinvertebrate metacommunity structure in many ways. Indeed, the harsh environmental conditions characterizing glacial influence not only constitute the primary environmental filter but also, limit water-borne macroinvertebrate dispersal. Therefore, glacier runoff acts as an aquatic dispersal barrier, isolating species in headwater streams, and preventing non-adapted species to colonize throughout the entire stream network. Under a scenario of glacier runoff decrease, we expect a reduction in both environmental filtering and dispersal limitation, inducing a taxonomic homogenization of the aquatic fauna in glacierized catchments as well as the extinction of specialized species in headwater groundwater and glacier-fed streams, and consequently an irreversible reduction in regional diversity.     

Are biological classifications of headwater streams concordant across multiple taxonomic groups?

1. Studies assessing human impacts on freshwater ecosystems are typically based on a single taxonomic group, often macroinvertebrates or fish. Unfortunately, the degree to which such macroinvertebrate or fish‐based surveys can be generalised across other taxonomic groups remains largely unknown. A prerequisite for useful generalisations is that different taxonomic groups exhibit concordant patterns of community structure across sites. 2. We examined the concordance among fish, benthic macroinvertebrates and bryophytes in 32 streams in a boreal catchment in Finland. Our goal was to test how consistently different taxonomic groups classify stream sites; for example, can site groupings based on macroinvertebrates be used as a surrogate for bryophyte or fish assemblage classification? 3. Our results show that community classifications in headwater streams are not concordant across taxonomic groups, at least not at the within‐river system scale. The lack of concordance reflected the fact that all three groups responded to different environmental factors. Macroinvertebrate community structure was mainly correlated with stream size and pH, whereas bryophytes were related to water colour, nutrient content and in‐stream habitat variability. Fish community structure was best described by stream depth, substrate size and water oxygen concentration. 4. Our results suggest that great care should be taken when typologies based on benthic macroinvertebrates, or any other taxonomic group, are extrapolated to other groups in creating typologies of lotic environments.     

Stream inflow and predation risk affect littoral habitat selection by benthic fish

1. We examined small, fishless headwater streams to determine whether transport of macroinvertebrates into the littoral zone of an oligotrophic lake augmented food availability for Cottus asper, an abundant predatory fish in our study system. We sampled fish and macroinvertebrates during the recruitment and growth season of 2 years, either monthly (2004) or bi‐monthly (2005), to observe whether stream inputs increased prey availability and whether variation in total macroinvertebrate biomass was tracked by fish. 2. Observations from eight headwater streams indicated that streams did not increase the total macroinvertebrate biomass in the shallow littoral zone at stream inflows, relative to adjacent plots without stream inputs (controls). The taxonomic composition of stream macroinvertebrates drifting toward the lake differed from that in the littoral lake benthos itself, although there was no evidence of any species change in the composition of the littoral benthos brought about by stream inputs. 3. Although streams made no measurable contribution to the biomass or taxonomic composition of the littoral macroinvertebrate benthos, there was substantial temporal variation in biomass among the eight sites for each of the (n = 7) sample periods during which observations were made. Variation in total biomass was primarily a function of bottom slope and benthic substrata in the lake habitats. Dominant taxonomic groups were Baetidae, Ephemerellidae (two genera), Leptophlebiidae, Chironomidae (three subfamilies) and Perlodidae, although we did not determine the specific substratum affinities of each taxon. 4. Mixed effects linear models identified a significant interaction between macroinvertebrate biomass and plot type (stream inflow vs. control) associated with fish abundance. Across the observed range of macroinvertebrate biomass, fish showed a significant preference for stream inflows, but more closely tracked food availability in the controls. For young‐of‐the‐year (YOY), a negative effect of temperature was also included in the model, and we observed lower temperatures at stream inflows. However, abundance of predatory adults affected habitat selection for YOY. Lake‐bottom slope also accounted for variation in abundance in both fish models. 5. Our results suggest that the effect of fishless headwater streams on downstream fish may not always be through direct delivery of food. In this study system, fish preferred stream inflow plots, but this preference interacted with macroinvertebrate biomass in a manner that was difficult to explain. For YOY, predation risk was related to the preference for stream inflows, although the specific factor that mitigates predation risk remains poorly understood.     

长白山源头溪流底栖动物群落结构季节动态

源头溪流是河流生态系统物质循环和能量流动的重要区域,对底栖动物的生物多样性维持具有重要意义。目前,针对我国源头溪流底栖动物群落结构的研究薄弱,对长白山源头溪流底栖动物季节动态的研究尚未见报道。采用野外原位定量取样的方法,力图阐释长白山源头溪流底栖动物群落结构的季节动态特征及其主要环境驱动因子。研究结果表明:(1)共计采集底栖动物90个分类单元,隶属于3纲9目38科。其中,水生昆虫85属,占绝对优势。底栖动物群落结构的季节动态明显,底栖动物密度及多样性在夏、秋季显著高于冬季和春季,并在冬季达到最低值。(2)底栖动物功能摄食类群以收集者占优势,其次为撕食者、捕食者和刮食者,滤食者相对丰度最低。不同功能摄食类群的季节动态不一致,但密度和物种丰富度整体表现为秋季最高。(3)水温、凋落叶分布和流速是长白山源头溪流底栖动物群落结构季节动态的主要环境驱动因子。本研究可为长白山源头溪流后续相关研究及长白山松花江水系生态修复提供基础数据支持及参考。     

Climate change effects on upland stream macroinvertebrates over a 25-year period

Climate change effects on some ecosystems are still poorly known, particularly where they interact with other climatic phenomena or stressors. We used data spanning 25 years (1981–2005) from temperate headwaters at Llyn Brianne (UK) to test three hypotheses: (1) stream macroinvertebrates vary with winter climate; (2) ecological effects attributable to directional climate change and the North Atlantic Oscillation (NAO) are distinguishable and (3) climatic effects on macroinvertebrates depend on whether streams are impacted by acidification. Positive (i.e. warmer, wetter) NAO phases were accompanied by reduced interannual stability (=similarity) in macroinvertebrate assemblage in all streams, but associated variations in composition occurred only in acid moorland. The NAO and directional climate change together explained 70% of interannual variation in temperature, but forest and moorland streams warmed respectively by 1.4 and 1.7°C (P<0.001) between 1981 and 2005 after accounting for NAO effects. Significant responses among macroinvertebrates were confined to circumneutral streams, where future thermal projections (+1, +2, +3°C) suggested considerable change. Spring macroinvertebrate abundance might decline by 21% for every 1°C rise. Although many core species could persist if temperature gain reached 3°C, 4–10 mostly scarce taxa (5–12% of the species pool) would risk local extinction. Temperature increase in Wales approaches this magnitude by the 2050s under the Hadley HadCM3 scenarios. These results support all three hypotheses and illustrate how headwater stream ecosystems are sensitive to climate change. Altered composition and abundance could affect conservation and ecological function, with the NAO compounding climate change effects during positive phases. We suggest that acidification, in impacted streams, overrides climatic effects on macroinvertebrates by simplifying assemblages and reducing richness. Climatic processes might, nevertheless, exacerbate acidification or offset biological recovery.     

Responses of taxonomic distinctness and species diversity indices to anthropogenic impacts and natural environmental gradients in stream macroinvertebrates

1. Many studies have shown traditional species diversity indices to perform poorly in discriminating anthropogenic influences on biodiversity. By contrast, in marine systems, taxonomic distinctness indices that take into account the taxonomic relatedness of species have been shown to discriminate anthropogenic effects. However, few studies have examined the performance of taxonomic distinctness indices in freshwater systems. 2. We studied the performance of four species diversity indices and four taxonomic distinctness indices for detecting anthropogenic effects on stream macroinvertebrate assemblages. Further, we examined the effects of catchment type and area, as well as two variables (pH and total phosphorus) potentially describing anthropogenic perturbation on biodiversity. 3. We found no indications of degraded biodiversity at the putatively disturbed sites. However, species density, rarefied species richness, Shannon's diversity and taxonomic diversity showed higher index values in streams draining mineral as opposed to peatland catchments. 4. Of the major environmental gradients analysed, biodiversity indices showed the strongest relationships with catchment area, lending further support to the importance of stream size for macroinvertebrate biodiversity. Some of the indices also showed weak linear and quadratic relationships to pH and total phosphorus, and residuals from the biodiversity index‐catchment area regressions (i.e. area effect standardized) were more weakly related to pH and total phosphorus than the original index values. 5. There are a number of reasons why the biodiversity indices did not respond to anthropogenic perturbation. First, some natural environmental gradients may mask the effects of perturbation on biodiversity. Secondly, perturbations of riverine ecosystems in our study area may not be strong enough to cause drastic changes in biodiversity. Thirdly, multiple anthropogenic stressors may either increase or decrease biodiversity, and thus the coarse division of sites into reference and altered streams may be an oversimplification. 6. Although neither species diversity nor taxonomic distinctness indices revealed anthropogenic degradation of macroinvertebrate assemblages in this study, the traditional species diversity and taxonomic distinctness indices were very weakly correlated. Therefore, we urge that biodiversity assessment and conservation planning should utilize a number of different indices, as they may provide complementary information about biotic assemblages.     

Louisiana waterthrushes (Seiurus motacilla) and habitat assessments as cost-effective indicators of instream biotic integrity

1. Benthic stream animals, in particular macroinvertebrates, are good indicators of water quality, but sampling can be laborious to obtain accurate indices of biotic integrity. Thus, tools for bioassessment that include measurements other than macroinvertebrates would be valuable additions to volunteer monitoring protocols. 2. We evaluated the usefulness of a stream‐dependent songbird, the Louisiana waterthrush (waterthrush, Seiurus motacilla) and the Environmental Protection Agency Visual Habitat Assessment (EPA VHA) as indicators of the macrobenthos community in headwater streams of the Georgia Piedmont, U.S.A. We sampled macrobenthos, surveyed waterthrushes and measured habitat characteristics along 39 headwater reaches across 17 catchments ranging from forested to heavily urbanised or grazed by cattle. 3. Of the indicators considered, waterthrush occupancy was best for predicting relative abundances of macrobenthic taxa, while the EPA VHA was best for predicting Ephemeroptera–Plecoptera–Trichoptera (EPT) richness. Individual components of EPA VHA scores were much less useful as indicators of EPT richness and % EPT when compared with the total score. Waterthrushes were found along streams with higher % EPT, a lower Family Biotic Index (FBI) values and greater macrobenthos biomass. 4. While macroinvertebrates remain one of the most direct indicators of stream water quality, stream bird surveys and reach‐scale habitat assessments can serve as cost‐effective indicators of benthic macroinvertebrate communities. Using stream‐dependent birds as an early warning signal for degradation of stream biotic integrity could improve the efficacy of catchment monitoring programmes in detecting and identifying perturbations within the catchment.     

Forest proportion as indicator of ecological integrity in streams using Plecoptera as a proxy

An assessment system suitable to support implementation of the EU Water Framework Directive's local water management plans should build on quantitative knowledge about a suite of well-documented indicator and umbrella species’ requirements for different stream orders. Assuring high communication value for improving local public awareness and participation for restoring ecological integrity in impaired headwater streams is critical. Loss and fragmentation of forests are major threats to ecological integrity. The aquatic macroinvertebrate order Plecoptera is commonly used as an indicator of the ecological integrity of streams. We measured abundance and taxonomic richness of Plecoptera in relation to land cover and water chemistry in second and third order catchments’ in 25 headwater streams in Central Europe's Carpathian Mountains. Plecoptera abundance and Plecoptera taxa richness were positively correlated to each other, as well as to forest proportion in the catchments, but negatively correlated to catchment area, inorganic carbon, alkalinity and conductivity. Segmented linear regression was then used to identify thresholds associated to forest proportion as a surrogate for catchment integrity. No threshold was found for Plecoptera abundance, but for taxa richness a threshold of 54% forest cover was found, below which Plecoptera was affected in second order streams. Using Plecoptera as a proxy for ecological integrity this study indicates that forest cover is an effective bioindicator in headwater catchments for predicting the ecological status of headwater streams. The non-linear relationship between forest cover and Plecoptera can be used as a science-based norm whereby land cover monitoring can be used to assess the ecological status of streams.     

Relationships between land use, spatial scale and stream macroinvertebrate communities

1. The structure of lotic macroinvertebrate communities may be strongly influenced by land‐use practices within catchments. However, the relative magnitude of influence on the benthos may depend upon the spatial arrangement of different land uses in the catchment. 2. We examined the influence of land‐cover patterns on in‐stream physico‐chemical features and macroinvertebrate assemblages in nine southern Appalachian headwater basins characterized by a mixture of land‐use practices. Using a geographical information system (GIS)/remote sensing approach, we quantified land‐cover at five spatial scales; the entire catchment, the riparian corridor, and three riparian ‘sub‐corridors’ extending 200, 1000 and 2000 m upstream of sampling reaches. 3. Stream water chemistry was generally related to features at the catchment scale. Conversely, stream temperature and substratum characteristics were strongly influenced by land‐cover patterns at the riparian corridor and sub‐corridor scales. 4. Macroinvertebrate assemblage structure was quantified using the slope of rank‐abundance plots, and further described using diversity and evenness indices. Taxon richness ranged from 24 to 54 among sites, and the analysis of rank‐abundance curves defined three distinct groups with high, medium and low diversity. In general, other macroinvertebrate indices were in accord with rank‐abundance groups, with richness and evenness decreasing among sites with maximum stream temperature. 5. Macroinvertebrate indices were most closely related to land‐cover patterns evaluated at the 200 m sub‐corridor scale, suggesting that local, streamside development effectively alters assemblage structure. 6. Results suggest that differences in macroinvertebrate assemblage structure can be explained by land‐cover patterns when appropriate spatial scales are employed. In addition, the influence of riparian forest patches on in‐stream habitat features (e.g. the thermal regime) may be critical to the distribution of many taxa in headwater streams draining catchments with mixed land‐use practices.     

The energetic importance of terrestrial arthropod inputs to three warm-water streams

1. Inputs of terrestrial arthropods (number and mass m^–2 d^–1) from riparian corridors to three streams representing different orders were highly variable among seasons and sites, with significantly greater ( P < 0.05) inputs at the headwater stream during summer months, compared with other sites and seasons.
2. No significant differences in estimates of stream retention of terrestrial arthropods (number and mass m^–2 d^–1) were observed among sites; however, retention of terrestrial arthropods at all sites was significantly greater during summer months, compared with other periods.
3. The gravimetric proportion of terrestrial arthropods present in the stomachs of redbreast sunfish ( Lepomis auritus ) and bluegill ( L. macrochirus ) was equivalent among sites. However, estimates of the dietary importance of terrestrial arthropods at all study sites were significantly greater in the summer, compared with other seasons.
4. Estimates of the potential annual energetic contribution (kJ m^–2 d^–1) of terrestrial arthropod inputs to the stream system were comparable with published rates of total annual production of aquatic macroinvertebrates in other Virginia headwater streams.
5. Results of this study supported the hypothesis that terrestrial arthropods represented an important energetic subsidy to stream fish during periods of low aquatic macroinvertebrate availability, and suggest that this component of allochthonous input is a potentially significant, but poorly understood energetic linkage between riparian landscapes and stream ecosystems.     

Salamander diversity alters stream macroinvertebrate community structure

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The energetic importance of terrestrial arthropod inputs to three warm-water streams

1. Inputs of terrestrial arthropods (number and mass m^–2 d^–1) from riparian corridors to three streams representing different orders were highly variable among seasons and sites, with significantly greater ( P < 0.05) inputs at the headwater stream during summer months, compared with other sites and seasons.
2. No significant differences in estimates of stream retention of terrestrial arthropods (number and mass m^–2 d^–1) were observed among sites; however, retention of terrestrial arthropods at all sites was significantly greater during summer months, compared with other periods.
3. The gravimetric proportion of terrestrial arthropods present in the stomachs of redbreast sunfish ( Lepomis auritus ) and bluegill ( L. macrochirus ) was equivalent among sites. However, estimates of the dietary importance of terrestrial arthropods at all study sites were significantly greater in the summer, compared with other seasons.
4. Estimates of the potential annual energetic contribution (kJ m^–2 d^–1) of terrestrial arthropod inputs to the stream system were comparable with published rates of total annual production of aquatic macroinvertebrates in other Virginia headwater streams.
5. Results of this study supported the hypothesis that terrestrial arthropods represented an important energetic subsidy to stream fish during periods of low aquatic macroinvertebrate availability, and suggest that this component of allochthonous input is a potentially significant, but poorly understood energetic linkage between riparian landscapes and stream ecosystems.     

Longitudinal extent of acidification effects of plantation forest on benthic macroinvertebrate communities in soft water streams: evidence for localised impact and temporal ecological recovery

Plantation conifer forest can increase the risk of acidification in acid-sensitive catchments with consequences for macroinvertebrates and ecosystem functioning. This study compared headwater streams in forested and non-forested catchments to appraise the distance required for the acid effect on macroinvertebrates to diminish downstream. Strict criteria were followed in the selection of paired streams, including similarities in elevation, aspect, stream order, geographical proximity, geology and soil type, with no inflowing tributaries in the first 2.5 km and no major land-use other than plantation forest and moorland. Consequently two headwater streams (one forested, one non-forested) drained Ordovician sedimentary geology and two headwater streams (one forested, one non-forested) drained Old Red Sandstone (ORS) were selected. All streams drained peaty soils. Up to six sites at 500 m intervals were sampled in triplicate by multihabitat kick sampling. Ecological impact and recovery from acid effects involved the entire macroinvertebrate community, but varied between seasons. Acid-sensitive Ephemeroptera revealed marked effects of episodic acidification, with Baetis rhodani and Rhithrogena semicolorata found to be transient between seasons. The increase in acid-sensitive ephemeropteran species with increasing distance downstream indicates the finite effects of forest mediated acidification on soft water streams in Ireland. Ecological impact appears, therefore, to be localised within the catchment, with ecological recovery occurring a short distance downstream. This is the first study to have reported such results.     

Evidence for consumer regulation of biofilm–nutrient interactions among hardwater streams (Pennsylvania, USA)

Experimental studies evaluating the simultaneous effects of consumers, nutrients, and other biotic/abiotic factors on intact, natural food webs are rare, particularly among ecosystems of varying trophic conditions. We conducted a series of in situ studies that used nutrient-diffusing substrata with nitrogen (N) and phosphorus (P) concentrations in a full factorial design in three temperate, limestone streams in Pennsylvania across a trophic gradient (mesotrophic, eutrophic, and hypereutrophic streams). We assessed differences in algal and macroinvertebrate biomass, taxonomic composition, and functional groups relative to amended nutrients across the trophic gradient; as such, these results facilitated predictions about regulators of food web structure. All factors varied significantly among the streams (e.g., algal biomass P = 0.005, macroinvertebrate biomass P < 0.001, algal diversity P = 0.006, macroinvertebrate diversity P < 0.001, algal group P < 0.001, macroinvertebrate guilds P < 0.001); the streams, however, did not exhibit simple responses to nutrient amendment. Algal and macroinvertebrate biomass and diversity responded greatest in the mesotrophic stream while grazing seemed to be a strong factor preventing algal nutrient response in the eutrophic and hypereutrophic streams. Brillouin’s Evenness Index was most influenced by nutrient amendment (nutrient effect on algae and macroinvertebrates P = 0.021). As such, we concluded that biomass and diversity were mediated by complexity within intermediate trophic levels.