Laursen, K., Hounisen, J.P., Rasmussen, L.M., Frikke, J., Pihl, S., Kahlert, J., Bak, M. & Amstrup, O. 2009: Rastende vandfugle i Margrethe Kog og på forlandet vest for Tøndermarsken, 1984-2007. Danmarks Miljøundersøgelser, Aarhus Universitet. 78 s. – Faglig rapport fra DMU nr. 702.

 

Summary

In 1982 a sea wall was built in the southern part of the Danish Wadden Sea west of the Tønder Marsh, reclaiming an area of 1,151 ha, named Margrethe Kog. Thereby the salt marsh and part of the mudflats west of the former sea wall ceased to serve as a high tide roost site for thousands of waterbirds. To compensate for this loss of nature the Danish Parliament decided to establish the artificial Saltwater Lagoon in the south western part of Margrethe Kog, and to monitor future biological development of the area. As a basis for this decision a prognosis was made for the bird numbers and species that could be expected to use the Saltwater Lagoon. The Lagoon was established in 1984 and the construction was financially supported by the EEC. The Lagoon is supplied by salt water pumped in from the Wadden Sea during high tide. Excess water runs off to the north through a fresh water reservoir connected to the river Vidå (Fig. 1). The size of the Lagoon is 246 ha with a depth of 0-40 cm. This report presents the results of the monitoring program during 1984-2007 for staging waterbirds in Margrethe Kog and the new salt marshes to the west of the sea wall along the Wadden Sea. In addition comparisons are made 1) between the prognoses for the expected numbers of staging waterbirds before the Saltwater Lagoon was constructed and the actual numbers recorded afterwards, 2) between the bird numbers counted and the criterion for areas of international importance, and 3) between the bird numbers recorded before the reclamation and the numbers counted afterwards.

 

In this report the area is divided into 6 sub-sites: the Saltwater Lagoon, the Freshwater Reservoir, the Eastern Margrethe Kog (permanent grass land grazed by sheep and cattle), Northern Margrethe Kog (arable land), the Southern Saltmarsh and Northern Saltmarsh (the two last mentioned sub-sites are situated west of the new sea wall, Fig. 1). This report focuses on 29 commonly occurring waterbird species with average yearly peak numbers > 250 individuals in at least one of the sub-sites (Appendix 1). In the following presentation species counted in highest numbers are mentioned first (see Appendices 2-9) and the results of the Trendspotter analyses are shown in Appendices 10-17.

 

The Saltwater Lagoon

22 species were recorded as common. The annual peak numbers of dunlin were 39,000-56,000 and the red knot 9,500-27,000 individuals. After 1987, when the technical function of the Saltwater Lagoon was considered stable, and up to 2007 the following 6 species increased significantly: dunlin, red knot, Eurasian curlew, greylag goose, common redshank and greenshank. Two species were stable in numbers: shelduck and grey plover; 12 species were decreasing in numbers: golden plover, wigeon, bar-tailed godwit, lapwing, black-headed gull, avocet, common gull, spotted redshank, mallard, oystercatcher, brent goose and pintail, while the trend of 2 species was uncertain: barnacle goose and teal. All together most of the species in the Saltwater Lagoon showed decreasing and uncertain trends.

 

The Fresh Water Reservoir

16 species were recorded as common, of these the yearly peak numbers of wigeon and common gull were between 2,200-3,600 individuals followed by common gull, with 460-3,400 individuals. Only 2 species increased in numbers: greylag goose and spotted redshank; 5 species had stable numbers: wigeon, teal, oystercatcher, northern lapwing and dunlin; 6 species decreased in numbers: common gull, black-headed gull, mallard, golden plover, herring gull and shelduck, and 3 species had uncertain trends: pintail, red knot and avocet. In total the results show that most of the species in the Fresh Water Reservoir showed stable or decreasing trends.

 

The Eastern Margrethe Kog

15 species were recorded as common and they were dominated by barnacle goose and golden plover that occurred with annual peak numbers between 7,500-19,000 individuals. Four species showed increasing numbers: barnacle goose, northern lapwing, greylag goose and coot; 4 species stable numbers: golden plover, Eurasian curlew, mallard and teal; 4 species decreasing numbers: black-headed gull ruff, red knot and shelduck, and the trend of 3 species were uncertain: wigeon, dunlin and common gull. Thus, the majority of the species in the Eastern Margrethe Kog had an increasing or stable trend.

 

The Northern Margrethe Kog

6 species were common, in particular barnacle goose and golden plover that occurred with annual peak numbers of 470-4.600 individuals and both species were increasing. The trend of 1 species was stable: northern lapwing, and for 3 species it was uncertain: black-headed gull, common gull and dunlin. In total the trend of most species in Margrethe Kog North was stable or uncertain.

 

The Southern Salt Marsh

21 species were common, and the most numerous species were shelduck and dunlin with annual peak numbers of 3,300-9,300 individuals. The numbers of only 3 species increased through the study period: oystercatcher, wigeon and shoveler; the numbers of 7 species were stable: shelduck, dunlin, common gull, Eurasian curlew, pintail, herring gull and ringed plover; 6 species were decreasing: mallard, black-headed gull, common eider, golden plover, grey plover and common tern, and the trend of 5 species was uncertain: bar-tailed godwit, red knot, redshank, barnacle goose and teal. In total the trends for most species in the Southern Salt Marsh were increasing, stable or uncertain.

 

The Northern Salt Marsh

13 species were common, and of these the dunlin and common gull were the most numerous with yearly maximum numbers between 980-4,000 individuals. Only the oystercatcher increased in numbers, 4 species were stable: dunlin, common gull, shelduck and mallard; 4 species had decreasing numbers: black-headed gull, golden plover, common eider and herring gull, and the trend of 4 species were uncertain: pintail, bar-tailed godwit, wigeon and redshank. Thus, most species in the Northern Salt Marsh showed stable, decreasing or uncertain tendencies.

 

Margrethe Kog, in total

The trends of 5 species were increasing through the period 1987-2007, 6 were stable, 6 decreasing, and only 1 species was uncertain. Thus the majority of species were either increasing or stable.

 

The Salt Marshes, in total

The trend of 2 species increased, 6 species were stable, 5 species decreased, and no one showed an uncertain trend. Thus the trends for most species were either stable or decreasing. 

 

In the Saltwater Lagoon dunlin and red knot occurred in international numbers (Table 3), and so did pintail in the Fresh Water Reservoir; in the Eastern Margrethe Kog: barnacle goose and golden plover and in the Southern Salt Marsh: shelduck, pintail and shoveler.

Comparison between the prognoses for the number of staging water birds and the actual recorded numbers showed that geese were recorded about 5 times more than expected (Table 4). The numbers of dabbling ducks were 14% lower compared to the prognosis. Concerning waders the recorded numbers were nearly 6 times higher than anticipated, and nearly all species occurred in higher numbers than expected. Thus it is concluded that the Saltwater Lagoon has more than fulfilled its objectives.

 

After reclamation of the Margrethe Kog, 25 waterbird species increased and 9 decreased. The species showing the largest increase was greylag goose, Northern lapwing, Ruddy turnstone, knot and common tern. Largest decreases occurred for pink-footed goose and dark-bellied brent goose. We assume that the diversity of habitats (saltwater lagoon, fresh water reservoir and permanent grass land) established in the Margrethe Kog after the reclamation caused the increase in the bird numbers.

 

It is concluded that the bird numbers in Margrethe Kog and on the salt marshes are influenced by the water level during high tide in the Wadden Sea, by population trends in the Wadden Sea and changes in species numbers since the Margrethe Kog was established and therefore the bird fauna is dynamic. Supporting maximum numbers of individual species on a yearly basis of more than 100,000 and 30,000 respectively, the Margrethe Kog and the salt marshes west of it are among the most significant sites in the Danish Wadden Sea and for more species of international importance.

 

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