The variable routes of rafting: stranding dynamics of floating bull kelp Durvillaea antarctica (Fucales, Phaeophyceae) on beaches in the SE Pacific

Abstract

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.

Gall-forming protistan parasites infect southern bull kelp across the Southern Ocean, with prevalence increasing to the south

“…D. antarctica in Chile experiences competition from other algal species including Lessonia spp. towards its northern latitudinal range limit, reducing its density and prevalence, but is more abundant toward the south (Santelices et al 1980), where abundant rafting populations of large adult plants have also been observed (López et al 2017a). In this research, we sampled southward from the northern range limits of Durvillaea in Chile and Australia, such that more southern populations also had higher host densities, which could have led to increased pathogen transmission, and thus infection levels, at high latitudes.…”

“…For each complete kelp individual (including holdfast and fronds), the presence of galls of Maullinia was determined, and the frequency of infected specimens was calculated. The details of the sampling protocols used for collection and measurement of stranded specimens of D. antarctica are described in López et al (2017a).…”

Epibiont communities on stranded kelp rafts of Durvillaea antarctica (Fucales, Phaeophyceae)—Do positive interactions facilitate range extensions?

“…This study was conducted on 33 sandy and pebble/cobble beaches (28° S–42° S) across the benthic and pelagic geographic range of D. antarctica (Figure ). Distances between adjacent beaches varied from 30 to 100 km and the extension of the stretches that were surveyed on each beach ranged from 0.28 to 11.08 km, depending on beach length and/or amounts of stranded bull kelps (for details see López et al., ). Beaches were distributed across the four biogeographic districts, based on Camus () and a previous study (López et al., ): the southern edge of the Peruvian Province, 28° S–30° S, hereafter termed Coquimbo‐Choros District (CCD); Septentrional District (SED), 30° S–33° S; Mediterranean District (MED), 33° S–37° S; Meridional District (MD), 37° S–42° S (Figure , see also Supporting Information Tables S1.1 and S1.2).…”

“…Rafts of D. antarctica frequently wash up on sandy beaches within its geographic range (Duarte, Jaramillo, Contreras, Acuña, & Navarro, ), and stranded biomass tends to vary strongly among biogeographic districts (López, Macaya, Tala, Tellier, & Thiel, ), being highest in the northern‐central (30° S–33° S) and southernmost districts (37° S–42° S). Stranded specimens are also frequently found outside their northern geographic limit (<30° S), showing indications of prolonged floating times (López et al., ).…”

“…Rafts of D. antarctica frequently wash up on sandy beaches within its geographic range (Duarte, Jaramillo, Contreras, Acuña, & Navarro, ), and stranded biomass tends to vary strongly among biogeographic districts (López, Macaya, Tala, Tellier, & Thiel, ), being highest in the northern‐central (30° S–33° S) and southernmost districts (37° S–42° S). Stranded specimens are also frequently found outside their northern geographic limit (<30° S), showing indications of prolonged floating times (López et al., ). This suggests that the connectivity of D. antarctica populations (Fraser et al., ) and eventually also of associated epibionts is higher in some areas than in others, particularly in zones with high availability of rafts and less stressful conditions at the sea surface.…”