Spatial distribution patterns and relationship between recent vegetation and diaspore bank of a brackish coastal lagoon on the southern Baltic Sea

Submerged vegetation and the diaspore bank were investigated in the shallow brackish lagoon Salzhaff (southern Baltic Sea). The distribution, density and coverage of vegetation and diaspores were analysed at seven water depths along five transects. The submerged vegetation was dominated by the angiosperms Zostera marina, Zannichellia palustris and Potamogeton pectinatus and its depth limit varied between 2.5 and 4 m. The vegetation varied along the nutrient and Secchi depth gradient in the lagoon: charophytes were absent in the inner part of the lagoon and the depth limit of the submerged vegetation was lower than in the outer part. In contrast to the vegetation, oospores of charophytes dominated the diaspore bank independent of location and water depth on all transects. Diaspores of all recent macrophytes, except Zostera noltii, were found in the sediment surface layer. The species composition of the diaspore bank (BrayeCurtis similarity) showed differences both between wave-exposed and sheltered parts of the lagoon as well as along the nutrient and Secchi depth gradient. Oospore density in the sediment and the vegetation cover was correlated in Tolypella nidifica, Zostera marina, Ruppia cirrhosa and Zannichellia palustris. The distribution of charophyte oospores is characterised by translocation to deeper areas, whereas seeds of Z. marina and R. cirrhosa were accumulated in vegetation stands. Furthermore, diaspore deposition of Z. palustris, R. cirrhosa, Chara baltica and T. nidifica depended on wave exposure. Zannichellia palustris showed the most wide-ranging generative recolonisation potential of all macrophytes in the lagoon.