Variability of nutrient and particulate matter fluxes between the sea and a polder after partial tidal restoration, Northwestern France

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. (c) 2005 Elsevier Ltd. All rights reserved.