Dominique Lefebvre

Filling of a wetland (Seine estuary, France): natural eutrophication or anthropogenic process? A sedimentological and geochemical study of wetland organic sediments

For over a century the Seine estuary has been highly affected by human activities, resulting in a reduction of the surface of wetland habitat. Several ponds of the Vernier Marsh, one of the main wetlands of the estuary, are being filled in by sediment. The St. Aubin canal, the only inlet from the Seine to the wetland, has regularly been opened in summer to maintain a sufficiently high water level in the system. The purpose of this study was to identify the origin of filling: natural evolution of eutrophication or human intervention by the opening of the St. Aubin canal? Sediment cores were sampled at 19 stations randomly distributed in the ponds of the wetland. Granulo-metric, micro-granulometric and geochemical analyses (ignition loss, C/N/P, chemical fractionation of particulate phosphate) were performed on the top 10 cm of sediment. Moreover nutrient concentrations (ammonium and phosphate) were measured in the interstitial waters using diffusion samplers. The results revealed that sediments sampled from the north are weakly organic (ignition loss: 19%, 15% C, 0.9% N, 40% organic P of total P) whereas those sampled from the southern stations are highly organic (ignition loss: 50–93%, 35% C, 1% N: 59–81% organic P of total P). In summary, sediments accumulating in the north of the ecosystem mainly consist of minerogenic particles from the Seine transported by the St. Aubin canal and thus are a result of the human interference. Interestingly the accumulation of sediments in the southern part is the result of a natural evolution of eutrophication: a peat bog. These sediments are highly organic, enriched with organic particulate phosphate, specifically a humic-acid-bound-phosphorus, a refractory form of phosphorus.