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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.

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 14 C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of 14 CO 2 . Grazing had little effect on the rates of mineralization of 14 C used as a respiratory substrate, but a larger proportion of 14 C 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.

Question: 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 subhalophytes 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.

This paper aims to investigate the patterns of exchanges of nutrients and suspended sediments between the sea and a polder, after partial tidal restoration, and to assess if these are comparable to those observed in natural salt marshes. The study site, situated in the Bay of Veys, in Northwestern France, was embanked in the 1870s and accidentally reconnected to the sea in 1990. Water now flows in and out of the polder by a single communication point with the sea, which facilitated water sampling and flux calculation for dissolved and particulate elements. The study was carried out for two years, from May 2002 to April 2004. Results showed that for all the months studied the water flowing out of the polder had lower concentrations of nitrates and suspended sediments, which lead to a retention of these elements throughout the year. Nitrates uptakes in the polder were much higher in winter (up to 473.9 g N ha−1 tide−1) than in summer where they were close to zero. The retention of suspended sediment could be over 80% of the import and was mainly composed of organic matter. Finally, the concentrations of dissolved organic carbon were higher in outflow than inflow water, but due to unbalanced water budgets this lead to low quantities imported in summer and higher amounts exported for all other seasons. No interpretable pattern was observed for ammonium. The nature of these fluxes, according to literature, is close to those observed in immature salt marshes, so as far as restoration is concerned, it has been shown that partial tidal restoration can allow the restitution of the salt marsh exchange functions that were studied.

We studied the seasonal exchange of biogenic silica and dissolved silica between a freshwater and a saltwater tidal marsh and the nearby coastal waters. From both tidal marshes export of dissolved Si (DSi) was observed, while concurrently biogenic Si (BSi) was imported along with suspended solids. The export of DSi was highest (23.4 % and 123.8 % in the freshwater and saltwater marsh respectively) in summer when dissolved Si concentrations were low in the nearby coastal waters. Combined data from both marshes suggested a logarithmic decrease in DSi export with increasing DSi concentrations in the inundating waters. BSi import was observed year round in the freshwater marsh, but only in summer in the saltwater marsh. The results show that dissolved silica recycling from tidal marshes, both freshwater and saltwater, contributes significantly to estuarine Si availability in summer and provide new insight in the link between tidal marshes and secondary production in nearby coastal waters.