Influence d'une réduction de débit sur un torrent de montagne : l'Aston (Ariège)

La diminution et le déséquilibre de la population piscicole (Salmo trutta fario) sont les conséquences les plus évidentes de l'implantation d'un barrage sur un torrent pyrénéen (débit réservé de 100 1/s avec des lâchers d'eau imprévisibles). ABSTRACT : The reduction and disequilibrium of the fish population (Salmo trutta fario) are the most obvious consequences of the construction of a dam on a Pyrenean river (stored flow of 100 L/s coupled with unexpected discharges of water)

Aquatic Hyphomycete Distribution in South-Western France

In a survey of aquatic hyphomycetes in South-Western France, the species composition of conidia in foam samples was analysed at twenty-seven stations located on different streams. Correspondence analysis was used to examine the relationships between general distribution patterns of the hyphomycete species and some external factors (altitude, pH, temperature, and season). With respect to the first two axes determined, a group of five species (Campylospora chaetocladia Ranzoni, Campylospora sp., Heliscus tentaculus Umphlett, Lunulospora curvula Ingold and Triscelophorus monosporus Ingold) emerged and was clearly associated with typical lowland streams with high pH and temperature. In a second analysis, in which these five species and nineteen rare species were excluded, two species (Clavatospora longibrachiata (Ingold) Marvanova and S. Nilsson and Tetrachaetum elegans Ingold) appeared characteristic of acid water (pH<6), low altitude and autumn months. Many of the remaining species were essentially discriminated in relation to altitude. Tetracladium marchalianum de Wildeman, Tricladium angulatum Ingold and Tricladium gracile Ingold were typical of lowland streams, whereas Taeniospora gracilis Marvanova and Tricladium chaetocladium Ingold were generally found in mountain streams. Compared to the effect of altitude, water pH seemed to be of secondary importance in this region

Hyphomycètes aquatiques du sud-ouest de la France

Les 66 espèces d'hyphomycètes aquatiques récoltées dans les rivières du sud-ouest de la France sont décrites et illustrées. La distribution régionale et mondiale de chaque espèce est indiquée. En fin de catalogue, 6 espèces indéterminées sont signalées

Intraspecific variability in leaf traits strongly affects alder leaf decomposition in a stream

This study assessed the intraspecific variability of senescent leaves of alder (Alnus glutinosa Gaertn.) and the effects of this variability on leaf decomposition in streams. Leaves were collected at five geographically distant locations in Europe. We analyzed 10 batches of leaf samples for seven quantitative leaf traits as well as leaf decomposition rate in coarse and fine mesh bags exposed in a single stream. The geographic origin of leaf samples largely explained the observed variation in litter quality and decomposition rate. Phosphorus (0.034–0.187%) and lignin (3.9–18.7%) concentrations in leaves varied widely. Together, these two traits accurately predicted leaf decomposition rate (r2 = 84.1%). Intraspecific variation in leaf decomposition rate was within a range similar to that reported for interspecific variation among co- occurring riparian plant species in Europe. Our study demonstrates extensive intraspecific variability in leaf traits on a continental scale, which can have enormous effects on major ecosystem processes such as leaf decomposition

Breakdown of Leaf Litter in a Neotropical Stream

We investigated the breakdown of 2 leaf species, Croton gossypifolius (Euphorbiaceae) and Clidemia sp. (Melastomataceae), in a 4th-order neotropical stream (Andean Mountains, southwestern Colombia) using leaf bags over a 6-wk period. We determined the initial leaf chemical composition and followed the change in content of organic matter, C, N, and ergosterol, the sporulation activity of aquatic hyphomy cetes, and the structure and composition of leaf-associated aquatic hy phomy cetes and macroinvertebrates. Both leaf species decomposed rapidly ( k 5 0.0651 and 0.0235/d, respec- tively); Croton lost 95% of its initial mass within 4 wk compared to 54% for Clidemia . These high rates were probably related to the stable and moderately high water tempera ture (19 8 C), favoring strong biological a ctivity. Up to 2300 and 1500 invertebrates per leaf bag were found on Croton and Clidemia leaves after 10 and 16 d, respectively. Shredders accounted for , 5% of the total numbers and biomass. F ungal biomass peaked at 8.4 and 9.6% of the detrital mass of the 2 leaf species, suggesting that fungi contributed cons iderably to leaf mass l oss. The difference in breakdown rates between leaf species was consistent with the earlier peaks in ergosterol and sporulation rate in Croton (10 d vs 16 d in Clidemia ) and the faster colonization of Croton by macroinvertebrates. The softer texture, lower tannin content, and hi gher N content were partly responsible for the faster breakdown of Croton leaves. The rapid breakdown of leaf litter, combined with a low infl uence by shredders, is in accordance with previous findings. The high fungal activity associated with rapid leaf breakdown appears to be characteristic of leaf processing in tropical streams

Fungi are involved in the effects of litter mixtures on consumption by shredders

1. Decomposition of litter mixtures in both terrestrial and aquatic ecosystems often shows non- additive diversity effects on decomposition rate, generally interpreted in streams as a result of the feeding activity of macroinvertebrates. The extent to which fungal assemblages on mixed litter may influence consumption by macroinvertebrates remains unknown. 2. We assessed the effect of litter mixing on all possible three-species combinations drawn from four tree species (Alnus glutinosa, Betula pendula, Juglans regia and Quercus robur) on both fungal assemblages and the rate of litter consumption by a common shredder, Gammarus fossarum. After a 9-week inoculation in a stream, batches of leaf discs were taken from all leaf species within litter mixture combinations. Ergosterol, an indicator of fungal biomass, and the composition of fungal assemblages, assessed from the conidia released, were determined, and incubated litter offered to G. fossarum in a laboratory-feeding experiment. 3. Mixing leaf litter species enhanced both the Simpson’s index of the fungal assemblage and the consumption of litter by G. fossarum, but had no clear effect on mycelial biomass. Specifically, consumption rates of J. regia were consistently higher for mixed-species litter packs than for single-species litter. In contrast, the consumption rates of B. pendula were not affected by litter mixing, because of the occurrence of both positive and negative litter-mixing effects in different litter species combinations that counteracted each other. 4. In some litter combinations, the greater development of some fungal species (e.g. Clavariopsis aquatica) as shown by higher sporulation rates coincided with increased leaf consumption, which may have resulted from feeding preferences by G. fossarum for these fungi. 5. Where litter mixture effects on decomposition rate are mediated via shredder feeding, this could be due to indirect effects of the fungal assemblage

Elemental composition and degree of homeostasis of fungi: are aquatic hyphomycetes more like metazoans, bacteria or plants?

Ecological stoichiometry generally assumes that heterotrophs have a higher degree of ele- mental homeostasis than autotrophs. Differences between fixed consumer nutrient requirements and nutrients available in resources allow prediction of the intensity of nutrient recycling ensured by heterotrophs. Despite their fundamental role in detritus decomposition, extremely few data are currently available on fungal elemental composition. In this study, we quantified the degree of elemental homeostasis of aquatic hyphomycetes used as model organisms. Contrary to metazoans, but similar to plants, aquatic hyphomy- cetes exhibited highly plastic elemental compositions. Mycelium also reached far higher C/ nutrient ratios than reported for bacteria. Our results suggest that non-homeostasis of fungi should be explicitly included in stoichiometric models dealing with nutrient recycling, and that the discrepancy in homeostasis between some bacterial strains and fungi should cer- tainly be considered when investigating interactions between both groups of decomposers

Effects of stream acidification on fungal biomass in decaying beech leaves and leaf palatability

We examined the effect of surface water acidification on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and palatability to shredders of common beech leaves (Fagus sylvatica L.) in five mountain streams (pH 4.7-7.1). Leaf decomposition was significantly faster in the circumneutral streams (pH 6.4-7.1; k > or = 0.00175 d(-1)), when compared to acidic streams (pH 4.7-4.9; k < or = 0.00100 d(-1)). Fungal biomass showed no particular trend along the acidification gradient except that it peaked earlier in the stream closest to neutrality. Leaf palatability, measured as the feeding activity of the leaf-shredding amphipod Gammarus fossarum Koch, varied with the exposure time in the streams. Except for the higher palatability of leaves exposed during 6 weeks at the highest pH, patterns among streams were mostly similar. These results suggest that reduced processing rates in the most acidic streams were not related to differences in fungal biomass associated with decomposing leaves and that microbial conditioning was only slightly delayed by acidification. Possible effects of low pH and related variables (Ca, Al) on microbial decomposition and detritivorous macroinvertebrates are discussed to clarify the inhibition of beech leaf decomposition in the studied systems

Biodiversity and litter decomposition: a case study in a Mediterranean stream

The importance of riparian diversity for the functioning of headwater streams has been demonstrated repeatedly. For example, mixing litter from different riparian tree species can influence their decomposition rates, an effect that is contingent on both the litter assemblage and the detritivore community. However, the effects of mixing litter species have been studied mostly in temperate streams, and very few studies have been done in non- temperate streams. Mediterranean streams are often subjected to recurrent flow intermittency, and their riparian tree and stream detritivore communities have unique sets of species and traits, which probably influence the effects of litter mixtures on decomposition. We hypothesized that high dissimilarity in litter traits could promote effects of litter mixtures on decomposition that would be counteracted by the low abundance and small body size of detritivores in Mediterranean streams. We manipulated litter diversity and the size-class and presence/absence of detritivores in a 2nd-order Mediterranean stream in a 46-d experiment and found substantial but contingent effects on litter mixtures. Mixture effects were not significant on average, but both negative and positive effects of litter mixtures occurred. For instance, mixing soft and nutrient-rich litter species led to up to 9.6% increase in leaf mass loss. Microbial activity accounted for 85% of total leaf mass loss, and no effect of litter mixture was observed when detritivores were excluded. In contrast, the presence of detritivores, despite their relatively low abundance and diversity, was a key factor for litter decomposition and promoted effects of litter mixture. These results suggest that the extinction of a few key taxa (riparian tree species or large detritivores) could impair nutrient and C cycling in Mediterranean streams with potential consequences for stream food webs