In the Weser estuary, most of the spatial variability in the distribution of wader assemblages is observed along the salinity gradient, with generally higher density of most of the species in the mesohaline and polyhaline zones (Appendix 3). It is of note that certain variability occurs among the units within each salinity zone, this being particularly evident in the freshwater and oligohaline areas. This is mainly due to a temporal variability of wader assemblages ascribed to general low densities of Black-tailed Godwit, Golden Plover and Lapwing recorded in the periods 1980-1984 and 2005-2009 compared to the other periods. The matching of the assemblage distribution with the salinity gradient in the Weser estuary is also evident for wildfowl, with higher densities of species feeding or grazing on mudflats like Shelduck and Brent Goose characterising the assemblages in the polyhaline areas (although also the freshwater duck Pintail shows higher density in this zone¹). Mallard, Greylag Goose, Bean Goose and Barnacle Goose show higher density in the mesohaline areas, Teal and Wigeon in the oligohaline areas, and freshwater ducks like Shoveler, Gadwall and Tufted Duck showing higher densities in the freshwater areas. A relevant temporal variability of bird assemblages is observed also for wildfowl, particularly in the freshwater and oligohaline zones, and this can be mainly ascribed to general lower densities of species like for example Mallard, Wigeon, Barnacle Goose and Bean Goose recorded in these areas in the period 1980-1984 compared to following periods.
As there is only a very limited temporal overlapping between the habitat and the water quality datasets in the Weser, multivariate multiple regression models were applied separately to these datasets. As also observed in the Humber, a higher portion of the observed variability in the bird data in the Weser estuary is explained by habitat data alone (42% and 36% for waders and wildfowl assemblages, respectively) compared to the water quality variables (<20% of variance explained) (Table 3), although this might be influenced also by the fact that different datasets were analysed (e.g. water quality data are available for the freshwater and oligohaline zones only in this estuary), hence limiting the comparability of these results. The model selection process highlighted that the combination of all the habitat variables is relevant in determining the distribution of waders and wildfowl species in the Weser, whereas, for the water quality data, autumn NH4 and NO2 (for both waders and wildfowl) and BOD (for wildfowl) were excluded from the best model explaining the species distribution in the estuary.
The habitat predictor that can best explain both waders and wildfowl density distribution is the intertidal area, with 19% and 12% respectively of the species density variability explained by this variable alone (Table 3). Larger intertidal areas are present in the mesohaline and polyhaline zones in the estuary and these conditions are associated to wader assemblages with higher density of species feeding on mudflats like Oystercatcher, Dunlin, Curlew and lower density of species like Lapwing and Black-tailed Godwit, and to wildfowl assemblages with higher density of species like Shelduck and Pintail and lower density of Teal and Greylag Goose (Figure 4, Table 4).
When considering water quality variables only as possible predictors, BOD is the best predictor of the distribution of wader species in the oligohaline and freshwater areas of the Weser estuary, although this variable alone explains only 6% of the variance in the data (Table 3). Lower BOD values (indicative of a lower organic and nutrient enrichment) are associated to oligohaline areas of the estuary, where a higher density of most of wader species is observed (compared to the freshwater zone), thus leading to negative correlations between these species densities and BOD (Figure 4, Table 4).
As regards wildfowl, the best water quality predictor of the assemblage distribution in the oligohaline and freshwater areas of the Weser estuary is PO4, this variable affecting mainly the temporal variability of the wildfowl assemblage, with a decrease of PO4 over the periods considered in the analysis (between 1990-1994 and 2005-2009) associated to higher density of most of species in later periods, in particular goose species like Barnacle Goose, Greylag Goose and European White-fronted Goose (Figure 4, Table 4).
¹ It is of note that the allocation of species to guilds was based on the detailed knowledge of bird use in the Humber estuary. However, as the habitat use depends not only on the ecology of the species but also on the availability and distribution of resources within the estuaries, local adaptations might occur leading to possible discrepancies with the above guild allocation in other estuaries. In the specific case of Pintail, it is acknowledged that a classification as estuarine species might be more appropriate.
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