Vertical distribution of kelp zoospores

The widely accepted theory of kelp propagule dispersal is that reproductive zoospores are released near the benthos and passively dispersed horizontally via advective and diffusive transport. However, it is possible that these zoospores can also be dispersed vertically away from the benthos and into greater unidirectional flow, thus increasing their dispersal potential. To investigate this, we developed a vertical spore profiler to collect settled zoospores at 2, 4, 6, 8, and 10 m above the substrate within and adjacent to the Point Loma Kelp Forest, California. Concurrently, we deployed a Nortek Aquadopp (TM) acoustic Doppler current profiler (ADCP) to measure current velocities at different depths within and outside of the forest. Our results indicate that kelp zoospores are vertically dispersed throughout the water column, reaching at least 10 m off the benthos, but that zoospore density exponentially decreases with increasing height off the substrate. Furthermore, large differences in overall zoospore abundance were observed among different areas within and outside of the kelp forest and among different times of the year. Variability in zoospore densities coincided with differences in the physical environment (e.g. wave activity, horizontal flow, irradiance). Our ADCP data revealed that alongshore currents increased with increasing distance above the benthos, thereby increasing dispersal potential at these depths. However, exposure of the zoospores to irradiances characteristic of shallower water resulted in reduced settlement and postsettlement competency. Although the majority of released zoospores remained near the benthos, our data support the idea that vertical transport of kelp zoospores into shallower portions of the water column, where unidirectional currents are greater, may provide an alternate vector for long-distance dispersal.