Dra. Tellier, Florence

Aim: This study examines how rafting on floating bull kelps can shape the biogeographic patterns of raft-associated species, and analyses the spatio-temporal variability of taxonomic richness and co-occurrences of epibionts on beach-cast rafts of Durvillaea antarctica along a latitudinal gradient. Location: Southeast Pacific, along ~1,700 km of coastline. Methods: We examined the epibionts on stranded individuals of D. antarctica on 33 beaches along the continental coast of Chile (28° S–42° S) within four biogeographic districts during the winter and summer of two years (2014/2015–2015/2016). Taxonomic richness and co-occurrences of epibionts within a holdfast were examined. Known geographic ranges and rafting ranges were compared to determine possible range expansions via rafting dispersal. Results: Sessile species were the most frequent epibionts. Taxonomic richness varied among biogeographic zones and seasons, and was higher between 33° S and 42° S than between 28° S and 33° S, particularly in summer compared to winter. Taxonomic richness decreased with floating time (indicated by the presence and size of Lepas spp.). Habitat-forming epibionts such as mytilid mussels, the polychaete Phragmatopoma moerchi and the seaweeds Gelidium lingulatum and Lessonia spicata favoured co-occurrences of other species within a holdfast, suggesting a habitat cascade (i.e. biogenic holdfast—sessile eco-engineers—other epibionts), while the boring isopod Limnoria chilensis and the excavating limpet Scurria scurra were negatively correlated with many other species. Some rafting epibiont species with low dispersal ability were found more than 100–300 km outside of their known geographic ranges, with more extensive ephemeral range extensions at the southern edge of their respective ranges, probably facilitated by higher availability of rafts in those areas. Main Conclusions: These findings confirm that raft-associated species are frequently dispersed outside their known geographic ranges, although these range extensions vary strongly depending on the availability and persistence of rafts, and on the biotic interactions within the rafting assemblage.

Dispersal on floating seaweeds depends on availability, viability, and trajectories of the rafts. In the southern hemisphere, the bull kelp Durvillaea antarctica is one of the most common floating seaweeds, but phylogeographic studies had shown low connectivity between populations from continental Chile, which could be due to limitations in local supply and dispersal of floating kelps. To test this hypothesis, the spatiotemporal dynamics of kelp strandings were examined in four biogeographic districts along the Chilean coast (28°–42°S). We determined the biomass and demography of stranded individuals on 33 beaches for three subsequent years (2013, 2014, 2015) to examine whether rafting is restricted to certain districts and seasons (winter or summer). Stranded kelps were found on all beaches. Most kelps had only one stipe (one individual), although we also frequently found coalesced holdfasts with mature males and females, which would facilitate successful rafting dispersal, gamete release, and reproduction upon arrival. High biomasses of stranded kelps occurred in the northern-central (30°S–33°S) and southernmost districts (37°S–42°S), and lower biomasses in the northernmost (28°S–30°S) and southern-central districts (33°S–37°S). The highest percentages and sizes of epibionts (Lepas spp.), indicative of prolonged floating periods, were found on stranded kelps in the northernmost and southernmost districts. Based on these results, we conclude that rafting dispersal can vary regionally, being more common in the northernmost and southernmost districts, depending on intrinsic (seaweed biology) and extrinsic factors (shore morphology and oceanography) that affect local supply of kelps and regional hydrodynamics.