Rafting dispersal constrained by an oceanographic boundary

The role of oceanographic processes in mediating passive marine dispersal events is poorly understood. We conducted transect surveys across the subtropical front (STF), a major oceanographic boundary, to test the hypothesis that this physical feature constrains the rafting of detached macroalgae. Specifically, we used genetic analysis to assess the geographic origins of floating bull-kelp Durvillaea antarctica specimens in southern New Zealand. Nearly half of the floating specimens sampled immediately beyond the STF (30 to 60 km offshore) were shown to have distant subantarctic origins, despite their proximity to the New Zealand mainland at the time of sampling. These results suggest that the biogeographic composition of rafting communities is often predictable based on oceanographic features, but also imply that strong wind events have the potential to modify such patterns.