Mesoscale dispersal of the introduced kelp Undaria pinnatifida attached to unstable substrata

In areas of Tasmania and New Zealand, the introduced Japanese alga, Undaria pinnatifida, grows attached to unstable substrata such as small cobbles and shells. Observations suggest these algae may disperse by saltation (i.e., a series of jumps) while attached to their substratum. A predictive model was developed to estimate the dispersal rate of U. pinnatifida by this mechanism, depending on drag force (as a function of lamina surface area and water velocity), the buoyant weight of the attached substratum and the depth at which the alga was located. The model was parameterised based on empirical measurements of 89 tagged thalli in the field, and estimates of drag on different sized laminae determined from experiments in a large flume tank. Empirical observations and model predictions suggest that under typical conditions at moderately exposed sites, dispersal via this mechanism is likely to be of the order of at least 10¹–10² m per year. When compared to the likely dispersal of spores from the parent (~10¹ m per year), and the likely dispersal of fertile drift thalli (~10³–10 4 m per year), we suggest that algae on unstable substrata may provide a mechanism for intermediate dispersal over moderate distances, providing sufficient spore density to ensure high fertilisation success. This is likely to enhance the rate of spread of U. pinnatifida in circumstances where habitat suitable for establishment is available.