Dispersal potential of invasive algae: the determinants of buoyancy in <Emphasis Type="Italic">Codium fragile</Emphasis> ssp. <Emphasis Type="Italic">fragile</Emphasis>

The capacity for long-distance dispersal is an important factor in determining the spread of invasive species. For algae, positive buoyancy generally is correlated with increased dispersal potential, and the light environment has been previously identified as a possible determinant of buoyancy in several species. We examined the effect of light intensity on the buoyancy of fragments of the invasive green alga Codium fragile ssp. fragile. Under natural and controlled conditions, the buoyancy of samples taken from the thallus tip was higher than those from near the holdfast. Both laboratory and field experiments also showed that buoyancy was dynamic and switched from positive to negative under reduced light intensity, but this change required several days. We also observed seasonal changes in buoyancy, presumably due to natural variations in light intensity, with the buoyancy of fragments washed up on the shore highest in mid-summer. These results show that buoyancy is a dynamic property of the C. fragile ssp. fragile thallus and suggest that buoyant fragments contribute to long-range dispersal and accelerated regional spread of this invader. This finding suggests that dispersal is more likely during conditions of high light intensity and illustrates the need to understand how variations in the natural environment can affect the dispersal potential of invasive species.