Propagule bank buildup of Chara aspera and its significance for colonization of a shallow lake

Abstract

Several aspects of the propagule bank dynamics including germination, burial and storage characteristics, and the relationship between the distribution of the propagule bank and cover of an expanding Chara vegetation (dominated by C. asperaDeth. ex Willd.) was studied in the shallow lake Veluwemeer. The density of oospores in the sediment was positively correlated with the number of years that Chara was present at that particular site. After six years of Chara presence, at least 1.7 × 106 oospores m−2 had accumulated in the sediment. Oospores in sediment were evenly distributed in the 15 cm top layer, which was very similar to the foraging depth of Bewick's Swans (Cygnus columbianus bewickii Yarr). Burial in the sediment may be an important mechanism by which oospores are stored. On the other hand, at shallow areas about 50% of the biomass was consumed enhancing the potential dispersal of oospores by water birds. In a laboratory experiment, 100% of the tested bulbils of C. aspera emerged and thus may be important for short time survival of established vegetation. In contrast, oospore germination varied between 1 and 15% depending on light level and burial depth. The low germination and the high accumulation of oospores suggest that oospores are adapted to long time survival in a dormant state. Charophytes colonized Veluwemeer step by step in the course of about nine years. Not all the sites with suitable light conditions were colonized at the same speed. C. aspera established a dense vegetation only at sites with high oospores densities (> c. 1 × 104 m−2). The results indicate that the high density of oospores needed for attaining complete vegetation cover may be an important limiting factor during colonization.