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.

Rafting on floating seaweeds facilitates dispersal of associated organisms, but there is little information on how rafting affects the genetic structure of epiphytic seaweeds. Previous studies indicate a high presence of seaweeds from the genus Gelidium attached to floating bull kelp Durvillaea antarctica (Chamisso) Hariot. Herein, we analyzed the phylogeographic patterns of Gelidium lingulatum (Kützing 1868) and G. rex (Santelices and Abbott 1985), species that are partially co-distributed along the Chilean coast (28°S–42°S). A total of 319 individuals from G. lingulatum and 179 from G. rex (20 and 11 benthic localities, respectively) were characterized using a mitochondrial marker (COI) and, for a subset, using a chloroplastic marker (rbcL). Gelidium lingulatum had higher genetic diversity, but its genetic structure did not follow a clear geographic pattern, while G. rex had less genetic diversity with a shallow genetic structure and a phylogeographic break coinciding with the phylogeographic discontinuity described for this region (29°S–33°S). In G. lingulatum, no isolation-by-distance was observed, in contrast to G. rex. The phylogeographic pattern of G. lingulatum could be explained mainly by rafting dispersal as an epiphyte of D. antarctica, although other mechanisms cannot be completely ruled out (e.g., human-mediated dispersal). The contrasting pattern observed in G. rex could be attributed to other factors such as intertidal distribution (i.e., G. rex occurs in the lower zone compared to G. lingulatum) or differential efficiency of recruitment after long-distance dispersal. This study indicates that rafting dispersal, in conjunction with the intertidal distribution, can modulate the phylogeographic patterns of seaweeds.

Lessonia nigrescens used to be an abundant kelp species along the Chilean coast, but recent molecular studies revealed the existence of a L. nigrescens species complex, which includes the two cryptic species Lessonia berteroana and Lessonia spicata. Since these species have different distributions (16°S–30°S for L. berteroana and 29°S–42°S for L. spicata), they experience differences in environmental conditions, such as solar irradiance, seawater temperature and air exposure during low tide. This study tested to what extent the genetic distinctness of each of the two species [identified by a mitochondrial marker (atp8/trnS)] is reflected by ecophysiological traits (total lipids, fatty acid composition, phlorotannins, pigments and variable chlorophyll a fluorescence of PSII) in response to the respective environmental conditions, prevailing along the latitudinal gradient. We studied algal individuals from eight populations (27°S–32°S, including the species overlapping zone). Phlorotannins, pigments and Chl a fluorescence of PSII were most crucial for species-specific adaptations at the respective growth sites, whereas changes in total lipids and fatty acid compositions were negligible. Hence, species differentiation within the L. nigrescens complex is also manifested at the ecophysiological level. These findings may help to predict kelp responses towards future environmental changes.