Kelp forest thinning and fragmentation influence the number of kelp recruits but not the genetic composition of populationsReef-associated Biodiversity

David Aguirre¹, Elahe Parvisi¹, Libby Liggins¹, Cayne Layton^2,3, Victor Shelamoff², Masayuki Tatsumi^2,4, Jeffrey Wright², Vanessa Arranz⁵, Craig Johnson²

¹The University of Auckland, New Zealand - ²University of Tasmania, Australia - - ³Blue Carbon Services, Australia - ⁴Seaforest Ltd, Australia - ⁵University of Barcelona, Spain

Although the drivers of kelp forest decline vary regionally, as conditions become increasingly challenging, kelp forests thin and fragment. Reductions in kelp density and patch size alter the physical and biogenic properties of kelp forests, resulting in poorer kelp recruitment in smaller, sparser kelp forests. However, the impact of changes in kelp forest composition on the genetic attributes of kelp populations remains largely unknown. Here, we use a large-scale manipulative experiment to examine the unseen demographic and genetic impacts of kelp forest thinning and fragmentation. Genetic sequencing of kelp recruits and adults on isolated, artificial patch reefs allowed a unique opportunity to gain insight into patterns and scales of dispersal in Ecklonia radiata and the maintenance of genetic diversity across a generation. Contrary to our expectations, self-recruitment does not dominate, and recruits on isolated patch reefs were as likely to be descendants of adults on the same reef as they were descendants of parents on reefs up to 100m away. Furthermore, there was no detected impact of patch size or kelp density on rates of self-recruitment, immigration, or emigration. Interestingly, for recruits where we could confidently assign both parents, we found no case where both parents were resident on the same reef as the recruit, and moreover, cases where the recruit and both parents were all residents of different reefs were as likely as cases where the recruit was resident with at least one of its parents. In summary, at the scale of this experiment ( 1 hectare) it appears that recruits are a well-mixed population and that there are no genetic biases in kelp recruitment to reefs. These results provide hope for conservation and restoration efforts, as although kelp recruitment is strongly influenced by kelp forest thinning and fragmentation, there are no immediate negative consequences for the evolutionary potential of populations.