Elevated aluminium concentration in acidified headwater streams lowers aquatic hyphomycete diversity and impairs leaf-litter breakdown.

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

Global Patterns and Controls of Nutrient Immobilization On Decomposing Cellulose In Riverine Ecosystems

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature

Towards a simple global-standard bioassay for a key ecosystem process: organic-matter decomposition using cotton strips

Cotton-strip bioassays are increasingly used to assess ecosystem integrity because they provide a standardized measure of organic-matter decomposition – a fundamental ecosystem process. However, several different cotton- strip assays are routinely used, complicating the interpretation of results across studies, and hindering broader synthesis. Here, we compare the decay rates and assemblages of bacteria and fungi colonizing the three most commonly used cotton materials: Artist’s canvas, Calico cloth, and Empa fabric. Cotton strips from each material type were incubated in 10 streams that span a wide range of physicochemical properties across five ecoregions. Additionally, to evaluate responses to environmental stress without potentially confounding biogeographical effects, we deployed identical bioassays in five streams across an acidification gradient within a single ecoregion. Across all streams decomposition rates (as tensile strength loss [TSL]) differed among the three cotton ma- terials; Calico cloth decomposed fastest (time to 50% TSL [T50]=16.7d), followed by the Empa fabric (T50 = 18.3 d) and then Artist’s canvas (T50 = 21.4 d). Despite these differences, rates of TSL of the three cotton materials responded consistently to variation in environmental conditions; TSL of each fabric increased with stream temperature, dissolved-nutrient concentrations and acid-neutralizing capacity, although Artist’s canvas and Calico cloth were more sensitive than Empa fabric. Microbial communities were similar among the mate- rials, and values of community structure (e.g., phylotype richness and diversity) were comparable to those reported for decaying leaves in streams from the same region, the major natural basal carbon resource in forested-stream ecosystems. We present linear calibrations among pairs of assays so that past and future studies can be expressed in a “common currency” (e.g., Artist’s-fabric equivalents) ‘past and future studies’ repeated two times in the sentence. Lastly, given its relatively low within-site variability, and the large number of streams where it has been used (> 700 across the globe), we recommend Artist’s fabric for future work. These results show that cotton provides an effective and realistic standardized substrate for studying heterotrophic microbial assemblages, and acts as a reasonable proxy for more chemically complex forms of detritus. These findings add to growing evidence that cotton-strip bioassays are simple, effective and easily standardized indicators of het- erotrophic microbial activity and the ecosystem processes that result

Acidification des ruisseaux du massif vosgien et contamination métallique (Al, Mn, Pb, Cd): mise en évidence dans I'eau et dans des bryophytes transplantées

Des transplants de mousses aquatiques Hygrohypnum ochraceum ont été utilisés pour mettre en évidence une éventuelle contamination métallique de 13 cours d'eau du massif vosgien présentant des degrés d'acidification différents. Les ruisseaux sont situés en amont de toute activité agricole, industrielle et urbaine. L'aluminium et le manganèse, deux métaux issus de la géologie locale, et le cadmium et le plomb provenant essentiellement des dépôts atmosphériques, ont été recherchés dans I'eau et dans les bryophytes. Les données sont traitées en terme de concentrations dans I'eau et dans les bryophytes, de facteurs d'accumulation et de facteurs de pollution en fonction du pH, et comparées a des valeurs de référence. L'utilisation de cette méthode indirecte de mise en évidence de contamination par les métaux est discutée dans le cas particulier de I'acidification

Liming of acidified forests changes leaf litter traits but does not improve leaf litter decomposability in forest streams

International audienceLiming, the spreading of Ca/MgCO3 on terrestrial ecosystems, is often used as a way to promote the recovery of terrestrial and aquatic ecosystems affected by anthropogenic acidification. When liming is applied to a watershed, surface waters may be improved directly through water chemistry changes but also indirectly through possible changes in the quality of terrestrial organic matter inputs to the stream. The aim of this study was to evaluate the effect of liming on leaf litter chemical and structural traits and their consequences on another trait, leaf litter decomposability, giving the potential for a leaf litter to be decomposed.Beech (Fagus sylvatica, L.) leaf litter was collected in forested sites on acid bedrock (granite and sandstone), limed (9 or 21 years ago) or not (controls), as well as on two calcareous reference sites. Several leaf litter traits were determined for each collection site: carbon (C), nitrogen (N), phosphorous (P), calcium (Ca), magnesium (Mg), potassium (K), lignin and cellulose content, and leaf mass area and toughness. To compare leaf litter decomposability in aquatic ecosystem, leaf litters from all sites were placed in a reference forested headwater stream over a 128-day period, in fine and coarse mesh litter bags, and leaf mass loss dynamic as well as fungal biomass were evaluated.Liming significantly increased leaf litter Ca and Mg contents and decreased K content while other measured traits remained unchanged. Leaf litters collected in limed sites exhibited significantly lower decomposition rates in coarse mesh bags but similar decomposition rates in fine mesh bags. Liming had no significant effect on fungal biomass. In contrast, leaf litter collected on the calcareous, reference sites, showed significantly different trait profiles (e.g. higher Ca, Mg, K and P contents), leading to up to twice higher decomposition rates and fungal development.Our study shows that under our experimental conditions, increases in Ca and Mg leaf litter contents arising from acidic forests liming did not improve leaf litter microbial decomposition and even tended to reduce macroinvertebrate mediated decomposability. Concomitant changes in other leaf litter traits might be necessary to stimulate leaf litter decomposition. Liming with Ca/MgCO3 might thus not improve functional recovery of stream ecosystems in forests affected by acidification through an increase of litter quality

Phosphorus availability modulates the toxic effect of silver on aquatic fungi and leaf litter decomposition

International audienceThe functioning of forested headwater streams is intimately linked to the decomposition of leaf litter by decomposers, mainly aquatic hyphomycetes, which enables the transfer of allochthonous carbon to higher trophic levels. Evaluation of this process is being increasingly used as an indicator of ecosystem health and ecological integrity. Yet, even though the individual impacts of contaminants and nutrient availability on decomposition have been well studied, the understanding of their combined effects remains limited. In the current study, we investigated whether the toxic effects of a reemerging contaminant, silver (Ag), on leaf litter decomposition could be partly overcome in situations where microorganisms were benefitting from high phosphorus (P) availability, the latter being a key chemical element that often limits detritus decomposition. We also investigated whether these interactive effects were mediated by changes in the structure of the aquatic hyphomycete community. To verify these hypotheses, leaf litter decomposition by a consortium of ten aquatic hyphomycete species was followed in a microcosm experiment combining five Ag contamination levels and three P concentrations. Indirect effects of Ag and P on the consumption of leaf litter by the detritivorous crustacean, Gammarus fossarum, were also evaluated. Ag significantly reduced decomposition but only at the highest concentration tested, independently of P level. By contrast, P and Ag interactively affected fungal biomass. Both P level and Ag concentrations shaped microbial communities without significantly affecting the overall species richness. Finally, the levels of P and Ag interacted significantly on G. fossarum feeding rates, high [Ag] reducing litter consumption and low P availability tending to intensify the feeding rate. Given the high level of contaminant needed to impair the decomposition process, it is unlikely that a direct effect of Ag on leaf litter decomposition could be observed in situ. However, subtle Ag effects in relation to nutrient levels in ecosystems could be expected. In particular, owing to higher consumption of low P leaf litter, shredding invertebrates could increase the ingestion of contaminated resources, which could, in turn, represent an important threat to headwater stream ecosystems

Behavioural and physiological responses of Gammarus fossarum (Crustacea Amphipoda) exposed to silver

International audienceThe study aims at investigating the effects of silver (Ag), a re-emerging contaminant, on physiological and behavioural responses in Gammarus fossarum. In a first experiment, G. fossarum Ag LC50s were evaluated during 96 h under semi-static mode of exposure. Juveniles appeared to be more sensitive to Ag (LC50(96 h): 1.01 mu g L-1) than ovigerous females (LC50(96 h): 1.9 mu g L-1) and adult males (LC50(96 h): 2.2 mu g L-1). In a second experiment, the physiological (osmo-/ionoregulation; antioxidant enzymes; lipid peroxidation (LPO)) and behavioural (locomotor activity and ventilation) responses of male G. fossarum exposed to Ag (0, 0.5, 1, 2, and 4 mu g L-1) were investigated. The mortality and Ag bioconcentration of gammarids exposed to Ag were significantly higher than controls. Concerning physiological responses, a 48 h-exposure to Ag had no impact on catalase activity but led to a significant decrease of haemolymph osmolality and [Na+]. On the contrary, LPO, Se-GPx and Na+/K+-ATPase activity were significantly increased. Behavioural responses, such as locomotor and ventilatory activities, were also significantly reduced in Ag exposed gammarids. After 96 h-exposure, especially to 0.5 mu g Ag L-1, most responses (ventilation, locomotor activity, haemolymph osmolality and [Na+]) were even more pronounced and haemolymph [Cl-] was significantly decreased but, contrary to observations after 48 h-exposure, Na+/K+-ATPase activity was significantly reduced. Our results demonstrate the drastic effects of realistic [Ag] concentration (0.5 mu g Ag L-1) on an ubiquitous and functionally important freshwater invertebrate (implied in detritus breakdown), but also strongly suggest an energetic reallocation to the detriment of locomotor activity and in favour of maintenance functions (i.e., osmoregulation and detoxification). These results highlight the risk represented by Ag and the need to perform integrated studies (at different scales, from individual to ecosystem)

Functional, behavioral and physiological effects of metal contaminated litters on aquatic ecosystems

International audienceNumerous polluted sites have been left unmanaged since the end of the industrial era in Europe, in the mid 20th century. Metals such as cadmium and zinc are accumulated in leaves, in quantity depending on tree species. At abscission, leaves still contain metals which can be transferred to adjacent ecosystems (terrestrial or aquatic) and are then susceptible to affect organisms and associated functional processes. In the present study, we aimed to assess the effects of contaminated litter entering small streams, highly dependent on allochthonous organic matter for their functioning. We first assessed the leaching of metals from contaminated litter, showing its fast and high release in water. Then, we tested the impact of leachates on the common freshwater shredder Gammarus fossarum following survival, metal bioconcentration, physiological (osmoregulation, digestive enzymes, lipid peroxidation, energy reserve) and behavioral responses (locomotion & ventilation). Finally, in an in situ experiment, we conjointly assessed the effects metal contaminated litters on their decomposition and on their consumption by G. fossarum. Despite behavioral modifications, litter decomposition and consumption did not differ between contaminated and un-contaminated litters suggesting that in spite of detrimental effects of contaminated litters, ecosystem functions can be maintained, at least in the short term