Erosionsschutzmaßnahmen in Unterwasserdenkmälern am Bodensee-Obersee : Auswirkungen auf die Makrophytenvegetation und Naturschutzverträglichkeit

Abstract

Many underwater monuments in the Alpine lakes are subject to a creeping erosion process, which is counteracted with various erosion protection measures. We compared six monument stations at Überlinger See (Lake Constance, Germany), each with several construction variants aged 3 to 18 years, with nearby reference areas. Compared to the reference plots, the cobble and gravel fill areas were characterised by a lower total cover of submerged vegetation (mainly Chara species), a lower phytomass, a higher species diversity and an altered species composition. This is attributed to the lack of filling of the pore space by fine material (colmatation), which in an experiment was not completed even after more than 8 years. The effects were limited to the fill areas and did not affect the conservation objectives of the sites protected by the Habitats Directive (habitat type codes 3130, 3140). In contrast, an area secured with geotextile showed only reduced submerged vegetation even after 25 years, so that this design variant is less suitable for ecological reasons. We recommend a stronger admixture of fine gravel and sand in future fills in order to accelerate colmatation.