Preliminary diagnosis and prospects for the elimination of phosphorus (P) in excess in Lake Ribou (Cholet, Maine-et-Loire, France)
In wetlands, the accumulation of phosphorus (P) is a consequence of anthropogenic activities. Eutrophication is a serious environmental problem and an important issue in the implementation of the European Water Framework Directive. In our case, cyanobacteria blooms and loss of biodiversity have afflicted Lake Ribou (Cholet town, Maine-et-Loire Department, western France), leading to a production of cyanotoxins higher than acceptable levels. Phosphorus loadings to Lake Ribou are attributable mainly to wastewater treatment processes, which cannot totally remove phosphorus, and to intensive agriculture. The phosphorus concentration in the lake water is frequently above 0.3 mg•L-1 (10 times higher than the acceptable level). Total phosphorus loadings from the watershed have been estimated to be 18.7 T per year. As a first step in the restoration of the lake, an initial diagnosis was established. The present paper details the results of a principal component analysis (PCA) of the physicochemical data bank (52 data over six years). This analysis identified station TR as the most eutrophicated, and thus a priority site for restoration. Floristic inventories were conducted during the months of May and October 2006 at station TR. According to these floristic inventories, 21 species were identified in May and 12 species in October; the predominant species were Juncus effusus, Phalaris arundinacea, Salix sp., Rorippa amphibia, and Lemna minor. The observation of Phalaridaie helophytes, including Rorippa amphibia, Lycopus europaeus, Mentha aquatica, Phalaris arundinacea, Alisma plantago-aquatica and Lythrum salicaria, confirmed the eutrophic state of the reservoir. On the other hand, three species identified as Phalaris arundinacea, Salix sp. and Juncus effusus could be used in order to remove excess phosphorus in wetlands as described recently. This original approach could allow the creation of artificial wetlands throughout a watershed using autochthonous macrophytes.