Waterbird communities along the estuarine salinity gradient of the Scheldeestuary, NW-Europe

The zonation of non-breeding waterbirds along the Schelde estuary (The Neth erlands-Belgium), one of the longest estuaries in NW-Europe with still a co mplete salinity gradient, including a large freshwater tidal area, was desc ribed. Numbers of birds were counted monthly over the period October 1991 t o June 1997. Highest numbers of waterbirds were observed in late autumn and winter, with annual peak numbers ranging between 150,000 and 235,000 indiv iduals for the whole estuary. Based on a multivariate analysis different wa terbird communities were observed along the salinity gradient. The polyhali ne areas of the estuary were numerically dominated by the waders Oystercatc her and Dunlin. Due to the presence of a large brackish marsh in the mesoha line zone, the waterbird community in this area was dominated by the herbiv ores Wigeon and Greylag Goose. In the oligohaline and freshwater tidal area s, the waterbird community was dominated by duck species, with Teal and Mal lard being the most important. The international importance of the Schelde estuary for waterbirds was evidenced by the fact that for 21 waterbird spec ies the 1% level criterion, according to the Ramsar convention, was exceede d. The relation of the observed diversity and community patterns with the f unctional and habitat diversity of the Schelde estuary as well as the effec t of recent conservation measures to preserve this habitat were discussed.