, Volume 373-374, Issue 0, pp 181-191

A wetland managed for agriculture as an interface between the Rhône river and the Vaccarès lagoon (Camargue, France): transfers of water and nutrients

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Abstract

With the development of rice cultivation in the 50's, a great part of the Isle of Camargue, the central area of the Rhône river delta, was polderized. The Vaccarès lagoon system (110 km2, 65 km2 for the Vaccarès lagoon itself) represents the main part of the 'Réserve Nationale de Camargue'. Without human intervention, it is hydraulically isolated from the Rhône river and the Mediterranean sea. It receives drainage water from precipitation and flooded ricefields from its relict catchment.

Our hydrological study concerned the Fumemorte catchment (65 km2, with 20 km2 of ricefields). Irrigation for rice, in 1993 and 1994, was estimated at 56.2 and 52.5 Mm3 respectively. There is great variability in water consumption from one sector to another: 1400 mm to 5500 mm. The drainage volume was 45 Mm3 in 1993 and 53 Mm3 in 1994, a wetter year. Annual fluxes of 400 tons of nitrogen (N) and 150 tons of phosphorus (P) entered the catchment. With a scenario of maximum transfer, only 5.5% of entering N and 4% of P were exported from the Fumemorte catchment to the Vaccarès lagoon. Organic matter exported by the drainage network represents 5 to 6 times the quantity introduced with Rhône water. We attempted to quantify the processes of consumption, storage and loss which occur in the catchment. From preliminary results on the production of a seagrass bed in the Vaccarès, we found that, at an annual scale, nutrient fluxes from the catchment seem minor compared to other sources (lagoon sediments, atmosphere).