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Human activities enhance the atmospheric C[O.sub.2] concentration and modify climatic patterns, which could change the growth and quality of plant materials, as well as their subsequent decomposition, thus altering stream ecosystem functioning. These effects could be modulated by interactions between plant species differing in biological traits and competitive capacity. We cultivated the forb Trifolium pratense and the grass Agrostis capillaris under different C[O.sub.2] concentration (ambient or elevated), water availability (control or drought), and competition (monoculture or mixture). The material thus grown was conditioned in a stream for microbial colonization, and its subsequent decomposition was measured in laboratory microcosms. Elevated C[O.sub.2] reduced the quality of T. pratense but not that of A. capillaris. Water shortage limited plant quality but did not interact with C[O.sub.2] concentration. Interspecific competition only affected nitrogen concentration in A. capillaris. Elevated C[O.sub.2] did not affect decomposition rate, and A. capillaris, the species richer in nutrients, decomposed more slowly. Our results showed that the decomposition rates of plant materials grown under elevated C[O.sub.2] are difficult to predict due to species-specific responses and interactions with other factors. Les activites humaines accroissent les concentrations atmospheriques de C[O.sub.2] et modifient le climat, ce qui peut influencer la croissance et la qualite du materiel vegetal, ainsi que sa decomposition subsequente, alterant ainsi le fonctionnement des ecosystemes lotiques. Ces effets pourraient etre modules par les interactions entre des especes de plantes presentant des caracteres biologiques et capacites concurrentielles differents. Nous avons cultive l'herbe non gramineenne Trifolium pratense et la graminee Agrostis capillaris dans differentes conditions de concentration de C[O.sub.2] (ambiante ou elevee), de disponibilite d'eau (temoin ou secheresse) et de concurrence (monoculture ou melange). Le materiel ainsi produit a ete conditionne dans un cours d'eau pour produire une colonisation microbienne, et sa decomposition subsequente a ete mesuree dans des microcosmes en laboratoire. Une concentration elevee de C[O.sub.2] entrainait une reduction de la qualite de T. pratense, mais non d'A. capillaris. La penurie d'eau limitait la qualite des plantes, mais n'interagissait pas avec la concentration de C[O.sub.2]. La concurrence entre especes avait une incidence sur la concentration d'azote dans A. capillaris. Une concentration elevee de C[O.sub.2] n'avait pas d'incidence sur le taux de decomposition, et A. capillaris, l'espece la plus riche en nutriments, se decomposait plus lentement. Nos resultats demontrent que les taux de decomposition de materiels vegetaux cultives a des concentrations de C[O.sub.2] elevees sont difficiles a predire en raison de reactions qui varient selon l'espece et d'interactions avec d'autres facteurs. [Traduit par la Redaction], Introduction Future scenarios of global environmental change forecast a doubling of the atmospheric C[O.sub.2] concentration by the end of the 21st century (Intergovernmental Panel on Climate Change (IPCC) 2013). Increased [...]

Amphibians are declining worldwide due to a combination of stressors such as climate change, invasive species, habitat loss, pollution and emergent diseases. Although their losses are likely to have important ecological consequences on the structure and functioning of freshwater ecosystems, this issue has been scarcely explored.We conducted an experiment in three montane streams—where primary production is the main source of energy and carbon—to assess the effects of amphibian disappearance (i.e. presence or absence of the common midwife toad Alytes obstetricans, a common species found in pools of these streams) on several aspects of ecosystem functioning and structure: periphyton biomass and chlorophyll a concentration, algal assemblage structure, and growth of macroinvertebrate grazers.We compared four types of experimental enclosures: (i) without macroinvertebrates or amphibians; (ii) with larvae of the caddisfly Allogamus laureatus; (iii) with A. obstetricans tadpoles; and (iv) with both A. laureatus larvae and A. obstetricans tadpoles.The absence of tadpoles increased periphyton biomass, but did not cause differences on inorganic sediment accrual. The algal assemblage had a higher diversity in the absence of tadpoles, and their characteristic taxa differed from the assemblages in presence of tadpoles. A. laureatus presented higher mass in presence of tadpoles; however, tadpole length was not affected by presence of macroinvertebrates.Our results suggest that presence of tadpoles is a driver of periphyton accrual and assemblage structure, acting as top‐down control and with key potential consequences on the functioning of montane stream ecosystems. [ABSTRACT FROM AUTHOR]

Climate change and anthropogenic disturbances are expected to lead to more intense and frequent droughts, with potentially severe effects on structure and function of perennial temperate streams. However, more information is required on whether streams flowing through basins already affected by exotic plantations will respond to droughts in the same way as streams under native forests. The recolonisation dynamics of benthic macroinvertebrate communities and leaf litter decomposition rates were examined in nine streams of oceanic-temperate climate that differed in catchment vegetation (three streams draining native deciduous forest, three in pine plantations and three in eucalypt plantations) after a marked drought. In each stream, five benthic samples were collected three times (ca. 1.5 months between sampling dates) after flow recovery, and the taxonomic and functional trait compositions of the macroinvertebrate communities were analysed. The decomposition rate of Alnus glutinosa was measured in fine- and coarse-mesh litter bags. Benthic macroinvertebrate density, richness and diversity increased with time after flow recovery but only richness and diversity differed among stream types, with eucalypt streams showing the lowest values. Both the taxonomic and functional compositions of the macroinvertebrate community were dependent on vegetation type and time, with the differences among stream types diminishing over time. While leaf-litter decomposition rate did not depend on catchment vegetation after drought, detritivore activity was the lowest under eucalypt streams and it was positively correlated to benthic shredder density. Our results indicated that in these perennial temperate streams the catchment vegetation influenced the recovery of benthic macroinvertebrate communities after a period of drought, although the decomposition rate of leaf litter was not strongly affected. Greater understanding of the structural and functional responses of stream ecosystems to different stressors is required before the effects of expected more intense and frequent hydrological changes caused by climate change can be adequately forecast. [ABSTRACT FROM AUTHOR]

Drought, an important environmental factor affecting the functioning of stream ecosystems, is likely to become more prevalent in the Mediterranean region as a consequence of climate change and enhanced water demand. Drought can have profound impacts on leaf litter decomposition, a key ecosystem process in headwater streams, but there is still limited information on its effects at the regional scale. We measured leaf litter decomposition across a gradient of aridity in the Ebro River basin. We deployed coarse- and fine-mesh bags with alder and oak leaves in 11 Mediterranean calcareous streams spanning a range of over 400 km, and determined changes in discharge, water quality, leaf-associated macroinvertebrates, leaf quality and decomposition rates. The study streams were subject to different degrees of drought, specific discharge (L s − 1 km − 2 ) ranging from 0.62 to 9.99. One of the streams dried out during the experiment, another one reached residual flow, whereas the rest registered uninterrupted flow but with different degrees of flow variability. Decomposition rates differed among sites, being lowest in the 2 most water-stressed sites, but showed no general correlation with specific discharge. Microbial decomposition rates were not correlated with final nutrient content of litter nor to fungal biomass. Total decomposition rate of alder was positively correlated to the density and biomass of shredders; that of oak was not. Shredder density in alder bags showed a positive relationship with specific discharge during the decomposition experiment. Overall, the results point to a complex pattern of litter decomposition at the regional scale, as drought affects decomposition directly by emersion of bags and indirectly by affecting the functional composition and density of detritivores. [ABSTRACT FROM AUTHOR]

Human activities enhance the atmospheric CO2 concentration and modify climatic patterns, which could change the growth and quality of plant materials, as well as their subsequent decomposition, thus altering stream ecosystem functioning. These effects could be modulated by interactions between plant species differing in biological traits and competitive capacity. We cultivated the forb Trifolium pratense and the grass Agrostis capillaris under different CO2 concentration (ambient or elevated), water availability (control or drought), and competition (monoculture or mixture). The material thus grown was conditioned in a stream for microbial colonization, and its subsequent decomposition was measured in laboratory microcosms. Elevated CO2 reduced the quality of T. pratense but not that of A. capillaris. Water shortage limited plant quality but did not interact with CO2 concentration. Interspecific competition only affected nitrogen concentration in A. capillaris. Elevated CO2 did not affect decomposition rate, and A. capillaris, the species richer in nutrients, decomposed more slowly. Our results showed that the decomposition rates of plant materials grown under elevated CO2 are difficult to predict due to species-specific responses and interactions with other factors. [ABSTRACT FROM AUTHOR]

Aquatic hyphomycetes play a key role in leaf litter decomposition and are mediators of organic matter turnover in streams. Molecular studies have shown that some aquatic fungi are also plant endophytes, however, more evidence is needed to evaluate their multiple ecological abilities. To date, little information is available on fungal lineages that might have undergone convergent evolution to adapt to multiple ecological modes. We examined the phylogenetic relationships and evolutionary divergences of aquatic hyphomycetes, endophytic aquatic hyphomycetes and other fungal endophytes of riparian/terrestrial plants by analyzing ITS1-5.8S-ITS2 sequences retrieved from the National Center for Biotechnology Information (NCBI). Sequences with close phylogenetic affinity to aquatic fungi can occur as endophytes of terrestrial plants or in soil far from streams. To fully understand the ecological impact of aquatic hyphomycetes, we need to document and interpret their niches more broadly. [ABSTRACT FROM AUTHOR]

River ecosystems are subject to multiple stressors that affect their structure and functioning. Ecosystem structure refers to characteristics such as channel form, water quality or the composition of biological communities, whereas ecosystem functioning refers to processes such as metabolism, organic matter decomposition or secondary production. Structure and functioning respond in contrasting and complementary ways to environmental stressors. Moreover, assessing the response of ecosystem functioning to stressors is critical to understand the effects on the ecosystem services that produce direct benefits to humans. Yet, there is more information on structural than on functional parameters, and despite the many approaches available to measure river ecosystem processes, structural approaches are more widely used, especially in management. One reason for this discrepancy is the lack of synthetic studies analyzing river ecosystem functioning in a way that is useful for both scientists and managers. Here, we present a synthesis of key river ecosystem processes, which provides a description of the main characteristics of each process, including criteria guiding their measurement as well as their respective sensitivity to stressors. We also discuss the current limitations, potential improvements and future steps that the use of functional measures in rivers needs to face. [ABSTRACT FROM AUTHOR]