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The River Continuum Concept (RCC) assumes that autochthonous primary production in forest streams is limited by light and is insufficient to sustain secondary production by consumers; they must therefore depend on allochthonous carbon from the surrounding forest. Recent studies have, however, questioned the importance of allochthonous carbon in stream food webs. There is a growing body of evidence using stable‐isotope techniques that demonstrate the importance of algae (autochthonous production) in the food webs of tropical streams. The actual contributions of autochthonous and allochthonous resources are rarely evaluated accurately because few studies consider the diet and the trophic efficiencies of the components of the food web or measure primary and secondary production to estimate the energy flow. We estimated the annual net primary productivity of periphytic microalgae (NPP) and the secondary production of macroinvertebrates (SP) from empirical models and we used stable isotopes of carbon and nitrogen to quantify the flow of material in food webs of five forest streams in the Guapi‐Macacu catchment, Rio de Janeiro, Brazil. NPP ranged from 46 g to 173 g dry mass (DM) m−2, whereas SP ranged from 0.90 g DM m−2 to 2.58 g DM m−2. The contribution of allochthonous carbon to the SP was more important than autochthonous sources and varied from 56% to 74% of all basal energy flow assimilated by primary consumers. The annual ingestion rate of basal sources varied from 8.08 g DM m−2 to 26.57 g DM m−2, with the allochthonous material contributing 76% and 87% of this. The annual ingestion rate of autochthonous material varied from 1.2% to 5.5% of the NPP. The present work suggests that the principal energy source for macroinvertebrates in streams of the Guapi‐Macacu catchment came from the riparian forest, as predicted by the RCC. However, this dependence appeared not to be driven by an absolute lack of autochthonous NPP, which seemed more than sufficient to sustain the entirety of macroinvertebrate SP.

Nutrient limitation assessment is important to understand stream ecosystem functioning.Aquatic primary producers are often limited by nitrogen, phosphorus, or both, as assessed by nutrient diffusing substrata (NDS), a common method for assessing nutrient limitation in streams. But little is known regarding how this method relates to patterns of nutrient uptake at the whole-stream scale. We combined two techniques to examine nutrient limitation in a tropical stream. First, we conducted two NDS experiments using ammonium, nitrate, and phosphate alone and combined, to determine nutrient limitation at substrata scale over several weeks. Second, we conducted whole-stream nutrient addition experiments over the course of a year, using nutrients alone and in combination, to test theoretical predictions about uptake characteristics of limiting and nonlimiting nutrients. NDS results consistently indicated N limitation. Ambient uptake length (S W-amb ) suggested either P limitation (shorter S W-amb for P than N) or colimitation (similar S W-amb for both nutrients). The relationship between N uptake and concentration when added alone or with P suggested P limitation, colimitation, or neither, depending on the date. We speculate that the different conclusions arise from differences in the spatial and temporal scale assessed by these techniques and the microbial processes involved, and the potential for physical processes influencing whole-stream uptake estimates. We conclude that nutrient limitation is not as categorical as NDS results often imply, rather habitat, resource, and biotic diversity result in nutrient uptake rates that do not necessarily conform to predictions drawn from fine scale, process-specific bioassays such as chlorophyll-a accrual on NDS.

Programs for evaluating environmental impacts are often carried out with chemical analyses that are inadequate for evaluating ecological aspects of rivers. In recent times there has been a strong movement towards biomonitoring using indices based on the structure of communities of organisms. Less attention has been given to functional parameters (ecosystem level processes such as decomposition, primary production, nutrient cycling, etc.), despite recent work that demonstrates their applicability. The breakdown of leaf material in streams provides a system in which an aspect of ecosystem functioning (decomposition) can be measured along with the community structure of the fauna associated with the leaves.We measured rate of leaf processing and the associated macroinvertebrates in 9 streams in 3 categories of environmental impact: Reference (pristine forest), Intermediate (pasture land-use with intact riparian vegetation) and Impacted (pasture land-use with degraded banks and stream-bed). Leaf processing was fastest in the least impacted, "Reference", streams. The abundance and taxon diversity of the associated fauna were not different among categories of impact. Thus this aspect of ecosystem functioning was more sensitive to perturbation than was the structure of the community in this case. We suggest that leaf processing can be a cheap and indicative parameter for biological assessment and monitoring.

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The conversion of forests to agriculture in tropical areas profoundly changes adjacent streams by modifying hydrological conditions, altering light regimes, and increasing nutrient concentrations. In this study, we used an integrative approach to examine how transformations of intact forests affected the physical, chemical, and biological properties of periphyton, in three Brazilian Atlantic rainforest streams. We found that riparian land use change affected the stream periphyton in a variety of ways that were linked to the availability of light and nutrients. Periphyton standing stocks and accrual rates of new periphyton biomass on tiles were higher in deforested reaches than forested reaches. Linear mixed‐model analyses showed that the increase of chlorophyll‐a in the periphyton was explained by the increase in deforestation and soluble reactive phosphorus concentration. Deforestation also altered periphyton stoichiometry as deforested streams exhibited lower C:P, whereas C:N ratios decreased with increasing NH4+ concentration that was higher in some deforested reaches. Periphyton productivity appeared to be limited by light in forested reaches and by nutrients in deforested reaches. There was differential availability of nitrogen and phosphorus in the deforested reaches, depending on land use type, and this resulted in different nutrient limitation. Periphyton community structure shifted from taxa less tolerant to high nutrients and light found in forested sites, to species tolerant to these conditions dominating periphyton assemblages in deforested sites. The loss of canopy cover was the strongest predictor of community composition for all sites, whereas phosphorus concentration was the best predictor of algal abundance in deforested reaches. This study highlights the complex effects of forest clearing on stream periphyton, ranging from changes to biomass accrual, nutrient limitation, stoichiometry, and community structure. We show the importance of using a comprehensive approach to help determine and predict how deforestation impacts stream ecosystems.

Variations in physical characteristics along the course of a river influence habitat availability which reflects in species distribution. Knowledge of ecology and diversity of lotic species is important for evaluating how river ecosystems will respond to environmental impacts. Freshwater decapods are a group of high ecological and economic importance, but the knowledge about factors influencing their distribution is scarce in Brazil. We performed a survey of decapods to describe their abundance and distribution as well as to study their relationships with stream physical variables and especially their association with different substrates types. We studied 23 sites located in 15 tributaries of Guapiaçú River, RJ, where we collected decapods in different substrates types and measured a set of physical variables. We found five decapods species, including amphidromous and non-amphidromous shrimps and crabs. Decapods were strongly associated with leaf-litter substrates and their abundance was related to a multivariate axis describing longitudinal changes in stream characteristics. We concluded that decapods occurring in the Guapiaçú catchment inhabit mainly small streams with preserved riparian forests where they find shelter and potential prey of invertebrates. The ongoing project to build a dam on the Guapiaçú River will have negative consequences to migrating shrimps and we strongly recommend that mitigating actions, such the construction of structures to allow the passage of migrating fauna, should be taken.Keywords: shrimps, environmental factors, leaf litter, tropical stream, damming. Distribuição e abundância de decápodes de água doce e sua associação com variáveis ambientais em uma bacia hidrográfica de Mata Atlântica que será represada ResumoVariações nas características físicas de um rio ao longo do seu curso influenciam a disponibilidade de habitas e dessa forma a distribuição de espécies. O conhecimento sobre a ecologia e diversidade de espécies lóticas é importante para avaliar como esses ecossistemas responderão a impactos ambientais. Decápodes de água doce são um grupo de grande importância ecológica e econômica, contudo o conhecimento sobre fatores influenciando sua distribuição ainda é escasso no Brasil. Nós realizamos um levantamento de decápodes para descrever sua abundância e distribuição, bem como estudar sua relação com características físicas dos rios, especialmente sua associação com tipos diferentes de substrato. Nós estudamos 23 localidades distribuídas em 15 rios tributários do Rio Guapiaçú, RJ, onde coletamos decápodas em diferentes tipos de substratos e medimos um conjunto de variáveis físicas. Nós encontramos cinco espécies de decápodas incluindo camarões anfídromos e não anfídromos e caranguejos. Esses organismos foram fortemente associados a substratos de folhiço alóctone e sua abundância foi relacionada com um eixo PCA que descreve as mudanças nas características físicas longo do curso do rio. Nós concluímos que os decápodas que ocorrem na bacia do Rio Guapiaçú habitam princi...