PROP J, OUDMAN L, BOER H DE, GERDES K, UBELS R & WOLTERS E (2012) Shorebirds in the Dollard: recovery of numbers or degradation of a natural system?. LIMOSA 85 (1): 1-12.

The Dollard, on the border of the Netherlands and Germany, is part of the Ems-Dollard estuary in the Wadden Sea (Fig. 1). The estuary is a brackish water tidal area, a habitat which is under severe pressure in NW-Europe. The Dollard is affected by a variety of human disturbances, of which eutrophication is most striking. During the first years of this study, heavy pollution by organic industrial waste discharges was reduced to virtually zero as the potato starch industry took measures to reduce nutrient loads. This paper aimed to map the changes of 16 of the most abundant bird species that are dependent on intertidal benthic prey (mainly waders) or seeds (ducks; Fig. 2), during the 20-year period of decreasing eutrophication (1976/77-1995/96) and the subsequent 15-year period without waste discharge (1996/97-2010/11). During the first period, numbers of most species dropped (Tab. 1), and the estimated food consumption declined by 3.7% annually (Fig. 4). During the subsequent period of zero discharge, numbers of most species increased - which was unexpected -, and the calculated consumption by benthivorous waders grew by 2.7% each year. The strongest increases in the second period were found in species exhibiting strongest decline in the first (Fig. 3). In contrast, dabbling ducks showed a persistent decline in numbers. Literature data showed clear trends in the biomass of the main food species, ragworms Nereis spp. Changes in numbers of benthivorous birds (excluding bivalve feeders) corresponded well with these trends. This suggests that food availability was driving the fluctuations in bird populations. Changes in nutrient input into the estuary may well have been the major cause for both the drop and the subsequent increase in productivity of the benthic prey. During the second period, concentrations of anorganic nutrients (nitrogen and phosphorous compounds) increased, as evidenced by literature data. These nutrients were thought to originate from a rapidly expanding agricultural industry (pigs and dairy cattle) in the wide surroundings of the Dollard. In all, fluctuations by benthivorous bird populations in the Dollard estuary were largely ruled by local variations in food supply. Although the nature of eutrophication differed between the two study periods, possibly affecting the ecosystem in dissimilar ways, effects on bird populations seemed similar. Eutrophication seemed to boost total bird numbers, though effects on individual species were highly variable.

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