Buoyancy of Avicennia marina Propagules in South-Eastern Australia

Abstract

Buoyancy of propagules of Avicennia marina (Forsk.) Vierh. was observed in the laboratory and in the field. Under constant exposure to seawater in the laboratory, propagules that shed their pericarp sank while those that retained their pericarp floated. The time taken for propagules to shed their pericarps varied greatly between trees in a population and between populations. However, more propagules lost their pericarps and sank faster in 10% seawater than in full seawater. Propagules that had sunk in 10% seawater refloated after several days' immersion. In the field, propagules placed at three tidal levels and exposed to differing periods of tidal inundation all lost their pericarps and sank within 24 h with no differences detected between tidal levels. Field observations were also made where brackish and seawater sites could be compared. Propagules shed their pericarps faster and sank earlier at the brackish site, although after 24 h more than 80% of propagules had sunk at all sites. After 48 h propagules in the brackish site began to refloat. These observations suggest that most A. marina. propagules strand and establish near the parent tree populations and only a few are dispersed more widely.