Relevance of large litter bag burial for the study of leaf breakdown in the hyporheic zone

Particulate organic matter is the major source of energy for most low-order streams, but a large part of this litter is buried within bed sediment during floods and thus become poorly available for benthic food webs. The fate of this buried litter is little studied. In most cases, measures of breakdown rates consist of burying a known mass of litter within the stream sediment and following its breakdown over time. We tested this method using large litter bags (15 x 15 cm) and two field experiments. First, we used litter large bags filled with Alnus glutinosa leaves (buried at 20 cm depth with a shovel) in six stations within different land-use contexts and with different sediment grain sizes. Breakdown rates were surprisingly high (0.0011–0.0188 day-1) and neither correlate with most of the physico-chemical characteristics measured in the interstitial habitats nor with the land-use around the stream. In contrast, the rates were negatively correlated with a decrease in oxygen concentrations between surface and buried bags and positively correlated with both the percentage of coarse particles (20–40 mm) in the sediment and benthic macro-invertebrate richness. These results suggest that the vertical exchanges with surface water in the hyporheic zone play a crucial role in litter breakdown. Second, an experimental modification of local sediment (removing fine particles with a shovel to increase vertical exchanges) highlighted the influence of grain size on water and oxygen exchanges, but had no effect on hyporheic breakdown rates. Burying large litter bags within sediments may thus not be a relevant method, especially in clogged conditions, due to changes induced through the burial process in the vertical connectivity between surface and interstitial habitats that modify organic matter processing

Impacts de simulations de restauration des habitats aquatiques sur la biodiversité de ruissaux drainant des bassins versants contrastés

214 p.Thèse de doctorat de l'Université de Rennes I. 27/10/200

Impacts de simulations de restauration des habitats aquatiques sur la biodiversité de ruisseaux drainant des bassins versants contrastés

L intensification de l agriculture affecte fortement les écosystèmes aquatiques continentaux, en induisant une augmentation des flux de particules fines et de nutriments dans le cours d eau, pouvant entraîner un colmatage de ses sédiments, une perte d'habitat et de biodiversité. Pour évaluer l'efficacité de travaux de restauration, nous avons adopté une stratégie d'expérimentations in situ en modifiant les habitats aquatiques de ruisseaux drainant des bassins versants plus ou moins impactés par l'agriculture : décolmatage de substrat, ou ajout de sédiments à différentes échelles, ou encore ajout des touffes de végétation artificielle. Les résultats de ces expériences sont pour la plupart très positifs, mais seulement sur le court terme. Le contexte paysager joue un rôle prépondérant sur l efficacité et la viabilité des différentes simulations de restauration. Il apparaît donc nécessaire de contrôler les flux en provenance des bassins versants.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Effet de travaux de restauration sur la chimie des eaux interstitielles et les peuplements benthiques d'un cours d'eau agricole breton.

communication oral

Hyporheic invertebrate community composition in streams of varying salinity in southwestern Australia: diversity peaks at intermediate thresholds.

International audienceMany streams of southwestern Australia have become secondarily saline through land clearance and other human activities in their catchments. Elevated salinities impact on aquatic biota and ecological processes of surface streams but little is known of the effects on the diversity and community composition of hyporheic (subsurface) invertebrates occupying the saturated sediments where surface and groundwaters exchange. We hypothesized that biodiversity of hyporheic invertebrates would decline with increasing salinity, especially where saline groundwater upwelled into the surface stream. We also predicted changes in community composition associated with salinity and direction of vertical hydrological exchange. Water and hyporheic invertebrates were sampled from downwelling and upwelling zones of 13 streams in southwestern Australia ranging in median surface water salinity from 0.27 to 17.86 g L1. Overall, taxa richness of hyporheic invertebrates was uncorrelated with salinity but, surprisingly, correlated positively with the salinity of upwelling water. However, when the sites were divided into ‘fresh' (3gL1) groups, this relationship became non-significant. Instead, taxa richness and total abundance were correlated positively with salinity of downwelling water in fresh sites and negatively in mesosaline sites, resulting in a peak in richness at intermediate salinities. Community composition was unrelated to direction of hydrological exchange but was strongly associated with hyporheic salinity. Hyporheic assemblages of ‘fresh' rivers were typified by harpacticoid copepods and candoniid ostracods, whereas the amphipod Austrochiltonia and several dipteran groups were more common below ‘mesosaline' rivers. Although many hyporheic taxa collected in this study apparently have broad tolerances to salinity, secondary salinization due to human activities potentially changed community composition, possibly altering rates of ecological processes such as organic matter breakdown occurring within the sediments of streams undergoing salinization

Rôle de l'hétérogénéité des habitats sur les communautés d'invertébrés aquatiques le long d'un gradient d'occupation des sols.

4/07/200

Rôle de l'hétérogénéité des habitats sur les communautés d'invertébrés aquatiques le long d'un gradient d'occupation des sols.

4/07/200

Effects of catchment disturbance on stream invertebrates : comparison of different habitats (vegetation, benthic and interstitial) using bio-ecological groups.

International audienc

Effects of catchment disturbance on stream invertebrates : comparison of different habitats (vegetation, benthic and interstitial) using bio-ecological groups.

International audienc

Effects of bottom sediment restoration on interstitial habitat characteristics and benthic macroinvertebrate assemblages in a headwater stream

International audienceThe restoration of in-stream habitats by structural improvement of stream beds is more and more frequent, but the ecological consequences of such works are still little known.We have examined the influence of the deposit of a 15 cm gravel layer over the stream bottom on the chemical characteristics of the interstitial water, the sediment grain size and the composition of the benthic assemblages. We have compared a restored reach to an upstream control over three years and at three seasons each year. Dissolved oxygen, ammonium, nitrite and nitrate contents were measured in both surface and interstitial (15 cm deep) waters, together with the depth of anoxia estimated using wooden stakes and fine sediment content at the surface. During the same period and seasons, benthic invertebrates were sampled at five points in each reach. The restoration induced an increase in vertical exchanges of water between surface and interstitial habitats, with an increase in the depth of hypoxia. Changes were observed in the composition of invertebrate communities, but not in the density or in the taxonomic richness of assemblages. These changes in assemblages were fragile: a local disturbance (such as a drying period) diminished the beneficial effect of the restoration with the disappearance of several organisms. The viability of such restoration works may be associated with catchment management designed to reduce fine sediment inputs to the river