Focus - Mise en place d’un réseau expérimental sur la réhabilitation des zones humides dans le Finistère

Pour juger de l’efficacité des actions menées et des gains apportés par les travaux de réhabilitation des zones humides, la mise en place d’un suivi le plus pertinent possible est nécessaire. Focus sur l'action du réseau expérimental de réhabilitation des zones humides du Finistère mis en place pour développer un suivi scientifique pluridisciplinaire ambitieux sur des sites pilotes faisant l’objet de travaux de réhabilitation

Dynamique de la végétation et des flux inter-systèmes d'un polder reconnecté à la mer.

Thèse de doctorat de l'Université de Rennes

Dynamique de la végétation et des flux inter-systèmes d'un polder reconnecté à la mer

Cette étude a pour but de déterminer s'il est possible de restaurer un marais salé et ses fonctions dans un polder après reconnection à la mer. Elle se focalise sur la structure de la végétation et sur les échanges inter-systèmes, comme indicateurs du succès de la restauration. L'inondation restreinte du polder n'a pas permis le maintien d'une végétation halophile étendue. Elle se développe uniquement dans les zones inondées, où elle présente une dynamique de zones pionnières de marais salé. Sa production est similaire à celle d'un marais salé peu productif et les flux de matières observés entre le polder et la baie sont de nature comparable à ceux d'un marais salé jeune. La faible ouverture ne permet pas toutefois des échanges inter-systèmes de même ampleur que ceux observés dans les marais salés. Des modifications de gestion sont proposées pour augmenter les chances de succès de la restauration d'un marais salé sur le site d'étude.RENNES1-BU Sciences Philo (352382102) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Fluxes of organic and mineral matter between a polder and the estuary after partial tidal restoration, northwestern France.

communication oral

Fluxes of organic and mineral matter between a polder and the estuary after partial tidal restoration, northwestern France.

communication oral

Seed dispersal in a polder partial tidal restoration : Implications for salt-marsh restoration.

International audienceQuestion: The vegetation in a polder after partial tidal restoration does not resemble the targeted salt-marsh vegetation. Is this difference in vegetation due to lack of dispersal or unsuitable abiotic conditions ? What could be done for a better restoration of the site? Location: Northwestern France. Methods: Seeds were trapped at the single inlet of the polder with a 200-μm mesh net to estimate inputs of seeds from the bay. In parallel, seed dispersal was studied in the polder by placing Astroturf® seed traps on the surface of the sediment at three different elevations in three distinct areas. Abiotic conditions such as flooding frequency, water table level and soil salinity were monitored. Results: All but one species from the adjacent salt marshes were trapped at the inlet. Not all of these species were on the seed traps inside the polder. Seed dispersal was not homogeneous in the polder and seed trap content mostly discriminated in function of their elevation. Salinity and water logging at the bottom of the slope were very high compared to tolerance of most halophytes but decreased rapidly higher up the slope. Conclusions: The development of salt marsh target species is highly restricted by limited hydrochory inside the polder but also by unfavourable soil conditions induced by the actual hydrological regime. Halophytes are excluded at the bottom of the slope by abiotic conditions and out-competed by sub-halophytes higher up. In order to restore salt marsh vegetation inside the polder, a larger opening should be induced in order to increase the flooded surface, and diminish water logging and flooding frequencies

Cattle grazing drives nitrogen and carbon cycling in a temperate salt marsh

We examined the impact of long-term cattle grazing on soil processes and microbial activity in a temperate salt marsh. Soil conditions, microbial biomass and respiration, mineralization and denitrification rates were measured in upper salt marsh that had been ungrazed or cattle grazed for several decades. Increased microbial biomass and soil respiration were observed in grazed marsh, most likely stimulated by enhanced rates of root turnover and root exudation. We found a significant positive effect of grazing on potential N mineralization rates measured in the laboratory, but this difference did not translate to in situ net mineralization measured monthly from May to September. Rates of denitrification were lowest in the grazed marsh and appeared to be limited by nitrate availability, possibly due to more anoxic conditions and lower rates of nitrification. The major effect of grazing on N cycling therefore appeared to be in limiting losses of N through denitrification, which may lead to enhanced nutrient availability to saltmarsh plants, but a reduced ability of the marsh to act as a buffer for land-derived nutrients to adjacent coastal areas. Additionally, we investigated if grazing influences the rates of turnover of labile and refractory C in saltmarsh soils by adding 14C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of 14CO2. Grazing had little effect on the rates of mineralization of 14C used as a respiratory substrate, but a larger proportion of 14C was partitioned into microbial biomass and immobilized in long- and medium-term storage pools in the grazed treatment. Grazing slowed down the turnover of the microbial biomass, which resulted in longer turnover times for both leaf litter and root exudates. Grazing may therefore affect the longevity of C in the soil and alter C storage and utilization pathways in the microbial community

Biogeochemical functioning of grazed estuarine tidal marshes along a salinity gradient

Depending on their location along an estuarine salinity gradient, tidal marshes are thought to have different impacts on the chemical composition of the water during flooding. However, there is a lack of direct measurements of fluxes occurring between the marsh surface and the water column to corroborate this hypothesis. This study compared fluxes of nutrient and dissolved organic carbon (DOC) between the water column and the marsh surface and the emission of greenhouse gases, at four sites located along the salinity gradient of an estuary of north-western Wales (UK). The exchanges of dissolved organic carbon and nitrogen (DON), dissolved inorganic nitrogen (DIN), soluble reactive phosphorus and silicic acid, carbon dioxide, methane and nitrous oxide were measured during tidal inundation in May and September 2008 using in situ incubation chambers. There was no linear pattern along the salinity gradient and the concentration of nutrients and DOC in the flood water did not appear to control the biogeochemical processes driving the marsh surface/water column exchanges in the studied marshes. Multivariate analysis showed a clear discrimination in functioning between sites with the fresh and brackish marshes having a more similar functioning compared to low and middle marshes. The main differences between these two groupings were in the fluxes of DOC and nitrate. The phenology of plant species, soil organic matter content and soil oxygenation appear be the dominant factors explaining the observed fluxes