Alpert, Sina (2014) Underwater Habitat Mapping in the Nature Reserve “Schleimündung” with Special Focus on the Blue Mussel (Mytilus edulis) Stock and Assessment of its Nutritional Quality for Wintering Waterbirds. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, iv, 53 pp.

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Abstract

The nature reserve Schleimündung has a long history of waterbird protection. It hosts up to 140 bird species, mainly seabirds, which profit from good feeding conditions in the rare habitat types the reserve offers, such as wind flats. In the winter season up to several thousand common eiders (Somateria mollissima) visit the area to winter and rest. These seaducks mainly feed on blue mussels. The population of common eiders has declined Europe-wide over the last decades along with other mussel feeding species. Therefore the protection of the existing blue mussel beds in the nature reserve is crucial. However, a former mapping of mussel beds in 2007 indicated a decline in the Schlei lagoon over the previous 12 years compared to a prior mapping in 1995. In 2011, the nature reserve was extended to include the Olpenitz peninsula and an adjacent area in the Baltic Sea, together making up 317 ha. The underwater habitats included in the reserve “Schleimündung” are mostly unknown, so that potential changes in the community would remain unnoticed and no appropriate protection measures could be applied. To assess recent developments of blue mussel beds in the Schlei lagoon, an afoot mapping was conducted with a hand-held GPS device. Furthermore, video footage of the seafloor of the to-date unmonitored area in the Baltic Sea was taken employing a motored vessel. Extent and location of the different submerged habitat types were evaluated and presented in a map. As eiders swallow mussels whole, they have to handle large amounts of shell material during their digestion, which is time and energy consuming. Therefore, it is important that the total energy content of their prey does not get too low, especially in winter when mussels have a reduced soft body mass. The decline of benthivores has repeatedly been related to a reduction of land based nutrient input of the Baltic Sea. Therefore, as a second part of this work, I investigated whether the fat content of winter mussels differed along a nutrient gradient. Mussel samples were taken from within the Schlei, at Schleimünde and along the adjacent Baltic Sea coast, and their fatty acid composition was analyzed with gas chromatography. Furthermore, population trends of wintering eiders in the nature reserve Schleimündung were collated from Jordsand bird counting reports and long-term nutrient data from three sampling stations in the Schlei and in the Baltic Sea close to Schleimünde were analyzed. The mapping of the Schlei lagoon revealed that the mussel bed area remained similar compared with a mapping in 1995 (Marahrens), but a decline of mussel-covered area has occurred, which corresponds to only 21% of the former area. Despite some indications of previously present mussel beds none of such habitat was found on the Baltic Sea area off Schleimünde. The long-term nutrient data from the Schlei revealed an up to five-fold decrease of nutrient load over the last 24 years which could partly be responsible for the decline of mussel stock in the reserve. Instead of mussel beds other protectable habitats were discovered, such as seagrass meadows and stone fields overgrown by red algae communities and sponges, corresponding to 233 ha “reef” and 251 ha “seagrass meadows and other marine macrophytes” according to national law. Eider numbers since 1990 first appeared to increase then subsequently decreased, with a peak in 1998-2000. A decreased nutrient input can therefore not be the only causal factor explaining the long-term patterns of eider numbers in the reserve. The fatty acid analysis of blue mussels revealed a decrease of fat content with increasing distance from the Schlei. The difference amounted to 27% higher fat content of samples from Kappeln, located in the outer Schlei, compared to samples from the Baltic Sea. Data of total nitrogen and phosphorous from the three sampling stations confirmed the nutrient gradient from the inner Schlei towards the Baltic Sea. These findings demonstrate that mussels from nutrient rich areas, mostly in the vicinity of estuaries, can serve as important energy-rich food for wintering bird species.

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