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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	[Long-term changes of abundance and spatial distribution of moulting waterbirds following changes of trophic state, fish stock and water level in the Ramsar site "Ismaninger Speichersee mit Fischteichen"]
Auteurs :	Eberhard von Krosigk, Auteur ; Peter Koehler, Auteur
Type de document :	Article
Dans :	Ornithologischer Anzeiger (39(2-3) September, 2000)
Article en page(s) :	135-158
Résumé :	Following a significant reduction of nutrients, the trophic state of the Ramsar site "Ismaninger Speichersee mit Fischteichen" turned between 1988 and 1999 from polytrophic to eutrophic in the reservoir and from hypertrophic to polytrophic in the fishponds. Accordingly, aquatic flora and both invertebrate and fish fauna changed. In addition, in parts of the site fish stocks were lowered and water levels raised between 1996 and 1999. Consequently, abundances, spatial distribution and dominance of the eight most common waterbirds have changed markedly. Their former totals of daily maxima during wing moult exceeded 50,000 on 8 km2 water surface. These totals were reduced to a mean of 29,000 in the years 1994-1999. Unparalleled to regional trends, mean daily maxima of the years 1994-1999 decreased greatly in Pochard Aythya ferina (-76 %), Black-necked Grebe Podiceps nigricollis (-50 %) and in Tufted Duck A. fuligula (-41 %). This was accompanied by massive decreases of great chironomid midges and other invertebrates. Mute Swan Cygnus olor, Mallard A. platyrhynchos, and Coot Fulica atra decreased between 21 % and 12 %. Numbers of Gadwall A. strepera remained high. This species compensated the decreasing mass and availability of macrophytic algae (Cladophorales and Hydrodictyon) in the reservoir by shifting to ponds stocked with low or no carp Cyprinus carpio. Red-crested Pochard Netta rufina increased between 1994 and 1999 by 31 %. It was benefitted by the spreading of stonewort Chara and pondweeds Zannichellia and Potamogeton, which it could reach even with raised water levels. Comparing the dominance in the years 1988 and 1999, Black-necked Grebe, Pochard and Tufted Duck, prefering animal food, together decreased from 73 % to 30 %. The relative proportions of the herbivorous species Mute Swan, Gadwall and Red-crested Pochard together increased from 15 % to 51 %.

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