Keywords:
island barrier dune
dune development
herring gull
nutrient input

Contact:  A. Gerlach, University of Oldenburg, FB 7 AG terr. ecology, Germany

Location:  Germany, North Sea, Lower Saxony

The island Mellum
The island of Mellum is located in the southern North Sea east of the Jade mouth. It is situated on the northern spit of a large sandy mud flat area and has developed from a base sandplate around 1880. The area above sea level covers approximately 630 ha, but shape and surface are not stable. They change from year to year depending on erosion by storms and winter gales and by accretion.
Mellum belongs to the Wadden Sea National Park of Lower Saxony as a strictly protected area ‘Ruhe-zone I’ and is not open to the public.The island has not been inhabited since 1945 with exception of a game warden living there in the summer. Between 1990 and 1995 a colony of 12.000 to 14.000 pairs of the herring gull breeded on Mellum between March and August. This is approximately 25 % of the breeding population on the German North Sea coast. Further the black winged gull (Larus fuscus) and the oystercatcher (Haematopus ostralegus) breed on Mellum.

Observation
In 1990, the University of Oldenburg with support from the government of Lower Saxony investigated the introduction of nutrients through Herring gulls, one of the most frequent breeding bird of the Northern Sea. This gull feeds mainly on the eulitoral and sublitoral, e.g. mussels, shrimps and fish. During the breeding period, every week the introduction of excrements, spit balls and mating presents was measured on an area of 1 ha, furthermore biological research about breeding was carried out.

Results
Every pair brought about 2,2 kg excrements, 1,6 kg spitballs, eggs, egg-shells and dead chicks into the breeding territory. From 12.000 pairs on 225 ha resulted in an input of biomass of about 215 kg/ha. About 25 %  (55,6 kg/ha) was due to the most important plant nutrients N, P, Ca, K and Mg. Above all, the fertilising effect for vegetation because of the N and P content may be the most important factor. In the past, the growing breeding colony was correlated with a spread of tall grass communities. Furthermore, a great amount of Ca (39 kg/ha) was introduced. This may slow down or even prevent the acidification of the dune soils.
Because of the great amount of introduced nutrients, breeding colony birds have a great importance for plant growth and even dune formation and island development in the area of the Wadden Sea.
 
 

Location of Mellum in the Southern sea. The different shaded areas show the different elevations above sea level

References

• Exo, K.-M. & A. Gerlach (1997): Eintrag von Bioelementen durch Silbermoewen (Larus argentatus) auf der Nordseeinsel Mellum. Jber. Institut Vogelforschung 3: 24. (31R5So97).
• Gerlach, A. (1999): Winter driftline debris on the wadden island of Mellum, Germany; distribution, quantity and decompositi-on. Abh. Naturwiss. Verein Bremen 44 (2-3): 707-724. (31R5Sp99).