Wetland effects on water quality: input-output studies of suspended particulate matter, nitrogen (N) and phosphorus (P) in Grand-Lieu, a natural plain lake

One of the oldest and largest natural flood-plain lakes in France, the Grand-Lieu, situated 23 km from the Atlantic ocean at a point 25 km upstream from the mouth of the Loire, formed the subject of a detailed study of the fluxes and input/output balances for nutrients and particulate solids under a range of hydrological conditions. The lake, with a total area of 5600 ha, is fed by 2 tributaries, the Boulogne and Ognon, and drains into the Loire via a short (25 km) channel, the flow in which was controlled by a sluice gate which was normally closed from about April until the following December when it was opened in preparation for the flood flows from January to March. The lake has become increasingly eutrophic in character since the 1970s and the open water area of around 4000 ha hosts large growths of floating plants, while bordering areas consist of marsh and reedbeds. The inputs from the 2 tributaries were quantified and compared during the inflow period (October to May) and input-output studies were conducted for 2 hydrologically contrasting periods, a very wet year, 1993-94, with an inflow of 292 x 106 m3 and a dry year, 1995-96, with an inflow of only 76 x 106 m3. Overall, the wetland received important quantities of inorganic nitrogen from the combined catchment areas, trapping or converting most of it (62-85 per cent) to organic matter, while exporting large amounts of dissolved and particulate organic nitrogen, which constituted 1.9 and 2.6 times the quantities received in the 2 years under investigation. Possible mechanisms accounting for these findings are discussed. There are 58 references.