Summary1. Scientific understanding of acidification in aquatic ecosystems relies on effective assessment, which at present is mostly limited to chemical and sometimes structural biological variables. Effects on ecosystem functioning are, in contrast, largely neglected. Litter breakdown is a potentially useful, highly integrative and crucial process that could enhance such assessment programmes. 2. Breakdown rates of beech Fagus sylvatica leaves were determined in 25 woodland headwater streams along an acidification gradient in the Vosges Mountains, France. Additional data relating to micro-organisms (microbial respiration, fungal biomass and degree of conditioning measured as leaf palatability) and macroinvertebrates (shredder diversity, abundance and biomass) associated with decomposing leaves were collected to elucidate the mechanisms underlying leaf breakdown. 3. Breakdown rates varied more than 20-fold between the most acidified and circumneutral sites ( k = 0·0002-0·0055 day). Stream water alkalinity and total Al concentration together accounted for 88% of the variation in litter breakdown rates among streams. Microbial factors associated with decaying leaves, particularly microbial respiration, declined with increasing stream acidity and were significantly related to Ca 2+ and total Al concentrations. 4. Total abundance, biomass and richness of leaf-shredding invertebrates associated with decomposing leaves were not related to stream acidity. However, the abundance and biomass of the amphipod Gammarus fossarum , an acid-sensitive and particularly efficient leaf-shredder, showed a strong positive relationship with leaf breakdown rate. Gammarus abundance and microbial respiration together accounted for 85% of the variation in litter breakdown rates among streams. 5. Synthesis and applications . These results indicate that leaf-litter breakdown responds strongly to stream acidification, with both microbial decomposers and invertebrate detritivores markedly affected. Measuring leaf breakdown rate may be developed into a simple, powerful and low-cost tool for assessing a critical component of ecosystem functioning. We advocate further investigation of this approach for the routine biomonitoring of freshwaters affected by, or recovering from, other anthropogenic stresses.

An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

We removed stream-living macroinvertebrate shredder species in the sequences in which they are predicted to disappear, in response to two common types of anthropogenic disturbances: acidification and organic pollution, and analysed the effects on leaf breakdown rates. The experiment was performed in field microcosms using three shredder species. Species identity significantly affected leaf breakdown rates, while species richness per se was non-significant. The simulated sequential species loss showed large effects on leaf breakdown rates, with observed rates being significantly higher than expected from single-species treatments in two, out of four, two-species, and in all four three-species treatments. The invertebrates used in this study were taxonomically distinct (Insecta: Plecoptera and Trichoptera; Crustacea: Amphipoda), and of different sizes, hence a high degree of complementarity was probably present. A method to study the effects of species loss, characteristic of perturbation type, could be more useful than a random approach when investigating the impact of perturbation. Our results may have general applicability for investigations on the effects of diversity loss on ecosystem functioning in any ecosystem exposed to human perturbations, given that the order of extinction is known or can easily be assessed.

1. We investigated the effect of trophic status on the organic matter budget in freshwater ecosystems. During leaf litter breakdown, the relative contribution of the functional groups and the quantity/quality of organic matter available to higher trophic levels are expected to be modified by the anthropogenic release of nutrients. 2. Carbon budgets were established during the breakdown of alder leaves enclosed in coarse mesh bags and submerged in six streams: two oligotrophic, one mesotrophic, two eutrophic and one hypertrophic streams. Nitrate concentrations were 4.5-6.7 mg L )1 and the trophic status of each stream was defined by the soluble reactive phosphorus concentration ranging from 3.4 (oligotrophic) to 89 lg L )1 (hypertrophic). An ammonium gradient paralleled the phosphate gradient with mean concentrations ranging from 1.4 to 560 lg L )1 NH 4 -N. The corresponding unionised ammonia concentrations ranged from 0.08 to 19 lg L )1 NH 3 -N over the six streams. 3. The dominant shredder taxa were different in the oligo-, meso-and eutrophic streams. No shredders were observed in the hypertrophic stream. These changes may be accounted for by the gradual increase in the concentration of ammonia over the six streams. The shredder biomass dramatically decreased in eu-and hypertrophic streams compared with oligo-and mesotrophic. 4. Fungal biomass increased threefold from the most oligotrophic to the less eutrophic stream and decreased in the most eutrophic and the hypertrophic. Bacterial biomass increased twofold from the most oligotrophic to the hypertrophic stream. Along the trophic gradient, the microbial CO 2 production followed that of microbial biomass whereas the microbial fine particulate organic matter and net dissolved organic carbon (DOC) did not consistently vary. These results indicate that the microorganisms utilised the substrate and the DOC differently in streams of various trophic statuses. 5. In streams receiving various anthropogenic inputs, the relative contribution of the functional groups to leaf mass loss varied extensively as a result of stimulation and the deleterious effects of dissolved inorganic compounds. The quality/quantity of the organic matter produced by microorganisms slightly varied, as they use DOC from

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In many ecosystems, detritus is the dominant source of energy and the driver of ecosystem functioning. In particular, in forested headwater streams, allochthonous detritus (e.g. leaf litter, dead wood) constitute the main energy source for detritivores and living primary producers contribute marginally to ecosystem metabolism and energy flows. We hypothesised that a low consumption of benthic diatoms, a high‐quality resource, could be of major importance for the growth of detritivores. In particular, these resources might represent an essential source of polyunsaturated fatty acids (PUFAs). In a microcosm experiment, three food resources were manipulated: alder (Alnus glutinosa: Betulaceae) leaf litter, fungal mycelium and a common benthic diatom. They were offered to juveniles of Gammarus fossarum (Crustacea: Amphipoda) as food resources, either alone or in combination, with each resource type being enclosed in agarose pellets. Juveniles were fed for 5 weeks in controlled conditions. Survival, feeding and growth rates were monitored. The fatty acids content of food resources and gammarids were also quantified. Our results showed that detritus alone permits survival, but not the significant growth of detritivores. The presence of diatoms in food resources was necessary to ensure a significantly positive mass growth of detritivores over the 5‐week experiment. More importantly, detritivores that did not receive algae in their food were generally unable to maintain their PUFA levels when compared to juveniles collected in the field. Gut‐content analysis of field‐collected G. fossarum showed that low amounts of benthic algae were always visible, indicating that most individuals fed at least for a small part on benthic algae. These minor, but high quality, food sources might therefore be essential for ensuring the growth and survival of detritivores. Our results clearly highlight the need to consider the functional importance of such minor food sources.

Aquatic hyphomycetes play an essential role in the decomposition of allochthonous organic matter which is a fundamental process driving the functioning of forested headwater streams. We studied the effect of anthropogenic acidification on aquatic hyphomycetes associated with decaying leaves of Fagus sylvatica in six forested headwater streams (pH range, 4.3-7.1). Non-metric multidimensional scaling revealed marked differences in aquatic hyphomycete assemblages between acidified and reference streams. We found strong relationships between aquatic hyphomycete richness and mean Al concentration (r = -0.998, p < 0.0001) and mean pH (r = 0.962, p < 0.002), meaning that fungal diversity was severely depleted in acidified streams. By contrast, mean fungal biomass was not related to acidity. Leaf breakdown rate was drastically reduced under acidic conditions raising the issue of whether the functioning of headwater ecosystems could be impaired by a loss of aquatic hyphomycete species.

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