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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	The occurrence of waterfowl on Tipperne 1929-2007 in relation to environmental issues on Ringkobing Fjord
Auteurs :	Hans Meltofte, Auteur ; Preben Clausen, Auteur
Type de document :	Article
Dans :	Dansk Ornitologisk Forenings Tidsskrift (105(1) 2011)
Article en page(s) :	1-120
Langues:	Anglais
Résumé :	Ever since 1929, regular counts of staging and moulting waterbirds have been performed at the Tipperne Reserve in the 285 km(2) shallow brackish lagoon, Ringkobing Fjord, western Denmark. At present, the reserve covers c. 7 km(2) of coastal meadows and marshland and c. 18 km(2) water area primarily of less than 1 m depth. The water level and salinity in the lagoon have been regulated variably through a sluice since 1931, and here we report on the large changes that the lagoon has undergone during these almost 80 years, and the effects this has had on the occurrence and numbers of waterbirds. The most pronounced effects were found when the delta of Skjern angstrom, the biggest river flowing into the fiord was drained in the mid 1960s and euthrophication originating from farming increased in general. Since then, the fiord has been heavily euthrophicated first resulting in mass growth of submerged vegetation, then overgrowing by epiphytic algae resulting in almost total die-off of the submerged vegetation around 1979-1980 followed by extremely poor water transparency caused by a dramatic increase in phytoplancton. To solve these problems, more salt water has been led into the fiord since 1995, whereupon bivalves resettled and the water transparency improved considerably. However, re-growth of submerged vegetation has been extremely slow. These changes have had profound but diverse effects on numbers and composition of the waterbird assembly, with dabbling ducks Anas spp. decreasing from annual maximum numbers generally between 25000 and 50000 up until the 1970s to a few thousand in recent years as the most noteworthy. Other species, such as Mute Swan Cygnus olor and Common Coot Fulica ova occurred in very large numbers during the mass growth of submerged vegetation in the 19705, but have largely disappeared since then. Fish- and benthos-eating species have reacted according to similar large changes in their food resources. Taken together, populations of 12 waterbird species that earlier occurred in internationally important numbers in the fiord, have for many years been far below the 1% threshold of regular occurrence defined for international importance. If this development is to be reversed, it is of decisive importance that the inflow of nutrients to the fiord is further reduced, so that the fiord can recover, and the internationally important waterbird numbers can re-appear.

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