Kelp Rafts Research Articles

Numerical experiments investigating the influence of drag on trajectory patterns of floating macroalgae

Abstract Ocean currents are a crucial means of dispersing natural and human-made materials on the ocean surface. Macroalgae are among the most conspicuous natural dispersers, often called the ‘tumbleweeds of the ocean.’ Despite numerous studies on the subject, the relative influence of wind and surface currents on the trajectory of macroalgal dispersal remains uncertain. Previous studies have focused on kelp rafts of varying sizes, making it challenging to determine the impact of wind versus currents. These studies have also disregarded the macroalgae’s drag characteristics and surface area, which have been shown to impact the trajectory and accumulation of floating flotsam. This numerical study aims to shed light on the relative influence of wind and currents and the role of drag in determining the course and accumulation of macroalgae. By comparing simulations of virtual kelp ‘particles’ that incorporate drag and those without, this study focused on solitary kelp plants and considered the impact of morphological characteristics, flow-field combinations, and the presence of Stokes drift. Our results show that virtual kelp particles generally followed ocean currents, but the inclusion of drag caused deviations from purely Lagrangian particles’ trajectories and sheds light on the complex interplay of factors affecting macroalgal dispersal in the ocean.

Repeated freezing impacts buoyancy and photosynthesis of a rafting kelp species

Antarctica was once considered biologically isolated, surrounded by oceanic barriers (Fraser et al.2018). However, floating materials such as kelp rafts (Fraser et al.2018, Avila et al.2020), wood (Lewis et al.2005) and plastics (Avila et al.2020) are now known to cross these barriers and reach Antarctic shores. Such incursions might enable non-native species (either rafting species themselves or associated hitchhikers) to colonize Antarctica as the climate warms (Avila et al.2020, Fraser et al.2020), but whether these species will be able to survive and reproduce in the Antarctic is not yet known. Sea ice is a defining characteristic of Antarctic coastlines, and modelled trajectories of kelp rafts (Fraser et al.2018) cross-referenced against sea-ice observations (Parkinson 2019) suggest that collisions between rafts and seasonal sea ice occur frequently (Fig. 1a); thus, rafts are expected to be entrained in, on or under sea ice and experience multiple freeze-and-thaw cycles on their journeys to Antarctica. In addition, kelp rafts that reach the Antarctic intertidal will experience temperatures well below 0°C if exposed to the air at low tide. Freezing can cause severe disruptive stress to seaweeds, and ice crystals growing in intercellular spaces can damage cell membranes and cause cell lysis (Eggert 2012). Such damage could affect the buoyancy of kelp tissue and decrease rafting ability. Although some non-native kelp rafts recovered from Antarctic shores appeared to still be reproductively viable, with mature gametes observed in reproductive tissue (Fraser et al.2018), tissue damage caused by freezing could have widespread effects on the health, function and establishment success of a non-native species traversing the Southern Ocean.

Giant kelp rafts wash ashore 450 km from the nearest populations and against the dominant ocean current.

Giant kelp rafts wash ashore 450 km from the nearest populations and against the dominant ocean current.

Southern Hemisphere coasts are biologically connected by frequent, long-distance rafting events

Globally, species distributions are shifting in response to environmental change,1 and those that cannot disperse risk extinction.2 Many taxa, including marine species, are showing poleward range shifts as the climate warms.3 In the Southern Hemisphere, however, circumpolar oceanic fronts can present barriers to dispersal.4 Although passive, southward movement of species across this barrier has been considered unlikely,5,6 the recent discovery of buoyant kelp rafts on beaches in Antarctica7,8 demonstrates that suchjourneys are possible. Rafting is a key process by which diverse taxa-including terrestrial, e.g., Lindo,9 Godinot,10 and Censky etal.,11 and marine, e.g., Carlton etal.12 and Gillespie etal.13 species-can cross oceans.14 Kelp rafts can carry passengers7,15-17 and thus can act as vectors for long-distance dispersal of coastal organisms. The small numbers of kelp rafts previously found in Antarctica7,8 do not, however, shed much light on the frequency of such dispersal events.18 We use a combination of high-resolution phylogenomic analyses (>220,000 SNPs) and oceanographic modeling to show that long-distance biological dispersal events in Southern Ocean are not rare. We document tens of kelp (Durvillaea antarctica) rafting events of thousands of kilometers each, over several decades (1950-2019), with many kelp rafts apparently still reproductively viable. Modeling of dispersal trajectories from genomically inferred source locations shows that distant landmasses are well connected, for example South Georgia and New Zealand, and the Kerguelen Islands and Tasmania. Our findings illustrate the power of genomic approaches to track, and modeling to show frequencies of, long-distance dispersal events.

Invasive marine species discovered on non\u2013native kelp rafts in the warmest Antarctic island

Antarctic shallow coastal marine communities were long thought to be isolated from their nearest neighbours by hundreds of kilometres of deep ocean and the Antarctic Circumpolar Current. The discovery of non–native kelp washed up on Antarctic beaches led us to question the permeability of these barriers to species dispersal. According to the literature, over 70 million kelp rafts are afloat in the Southern Ocean at any one time. These living, floating islands can play host to a range of passenger species from both their original coastal location and those picked in the open ocean. Driven by winds, currents and storms towards the coast of the continent, these rafts are often cited as theoretical vectors for the introduction of new species into Antarctica and the sub-Antarctic islands. We found non-native kelps, with a wide range of “hitchhiking” passenger organisms, on an Antarctic beach inside the flooded caldera of an active volcanic island. This is the first evidence of non-native species reaching the Antarctic continent alive on kelp rafts. One passenger species, the bryozoan Membranipora membranacea, is found to be an invasive and ecologically harmful species in some cold-water regions, and this is its first record from Antarctica. The caldera of Deception Island provides considerably milder conditions than the frigid surrounding waters and it could be an ideal location for newly introduced species to become established. These findings may help to explain many of the biogeographic patterns and connections we currently see in the Southern Ocean. However, with the impacts of climate change in the region we may see an increase in the range and number of organisms capable of surviving both the long journey and becoming successfully established.

Long-term persistence of the floating bull kelp Durvillaea antarctica from the South-East Pacific: Potential contribution to local and transoceanic connectivity

Current knowledge about the performance of floating seaweeds as dispersal vectors comes mostly from mid latitudes (30°–40°), but phylogeographic studies suggest that long-distance dispersal (LDD) is more common at high latitudes (50°–60°). To test this hypothesis, long-term field experiments with floating southern bull kelp Durvillaea antarctica were conducted along a latitudinal gradient (30°S, 37°S and 54°S) in austral winter and summer. Floating time exceeded 200d in winter at the high latitudes but in summer it dropped to 90d, being still higher than at low latitudes (<45d). Biomass variations were due to loss of buoyant fronds. Reproductive activity diminished during long floating times. Physiological changes included mainly a reduction in photosynthetic (Fv/Fm and pigments) rather than in defence variables (phlorotannins and antioxidant activity). The observed long floating persistence and long-term acclimation responses at 54°S support the hypothesis of LDD by kelp rafts at high latitudes.

Un-Discovered Islands

Un-Discovered Islands Liz Breazeale (bio) The first island to disappear is barely land at all. My fourteen-year-old daughter discovers the clickbait one evening: They Look For This Island, But What They Find Instead Will Blow Your Mind. I peek over her shoulder as she narrates; apparently, a research vessel on its way to New Zealand noticed Midge Island, a speck of reef, was not where the map said it should be, was, in fact, nowhere at all. My husband scoffs. Don't read that garbage. He is splayed across the loveseat, laptop on his thighs. He continues. This kind of thing used to happen when people drew maps by hand, and these tiny sandbars or whale corpses or kelp rafts, they kept being copied forward because nobody questioned it. I google Midge Island. My daughter scrolls on. Her voice holds a wave, a minor tremble, when she speaks. It's gone, though. That's what it says. Midge Island just disappeared. That doesn't sound real, kiddo. Mom, were you tracking it? My graduate students and I chart islands impacted by climate change, the rate of erosion for each coastline, estimating the amount of time before they are devoured by the sea. I've never heard of it. Don't encourage this, says my husband. He swivels, catches his laptop before it falls. Islands don't just disappear. I zoom in on a satellite map, expecting a grain of sand in a rolling blue Pacific eye. But there's only empty ocean, a patch of skin from which a scar has faded or a birthmark has been removed. My daughter plants herself further back into the couch cushion, tapping at her phone. She has always chosen words carefully, scientifically. When she was younger she saw a spider once, hesitated before stomping it out. She said, I was terrified. No, petrified. She asks, so it's happened before? Sailors used to get confused in the 1800s, my husband says. Drunk. And sometimes they saw things that weren't there. My daughter looks so young in this moment. I want to promise her it's nothing to worry about, that Midge Island has never existed so it couldn't disappear. It's nothing, says my husband. But a thought is resting, a gritty thought, too impossible to speak, a foreboding like being alone in a darkened street. In the morning, my daughter's face is all rocky shore, all treacherous reef and sweeping tide. My husband asks how she is and all she says is, fine, barely looking up from her unfinished geometry [End Page 152] homework. I have coffee with a colleague about a book we are coauthoring and my husband forgets his lunch and I do not think of Midge Island again. Its most recognizable quality is that it is no longer here. ________ Kiribati vanishes one week later, a periwinkle and golden paradise, a flower petal archipelago strung across the sea. We were tracking this one, I tell my husband. Kiribati is number one on internet lists of Top Ten Most Beautiful Remote Islands and 10 Gorgeous Islands You Need To Visit ASAP Before Climate Change Destroys Them. So it was erosion, he says, leaning over my shoulder, hard plastic of his glasses against my temple. Not this quickly. I was interviewed once for a well-known science podcast, a live show, alongside a climatologist from Kiribati, a woman who answered questions in front of an audience with more ease than I ever can. I spoke about ocean levels, the ways in which we are always being eroded, and the scientist from Kiribati described the situation for her country, its atolls always under siege, each tide rising higher than the last. The male host nodded as we spoke, interrupting and questioning and scooping the ends of our sentences under the beginnings of his own, our thoughts and words becoming only sand swept out to sea. He did it while the other woman was discussing her peoples' likely future evacuation, started to ask what could be done, but she cut him off. She said, we won't let ourselves be eroded. You can't disappear us. My...

An integrated ecological, genetic and geological assessment of long-distance dispersal by invertebrates on kelp rafts

Author(s): Waters, Jonathan M.; King, Tania M.; Fraser, Ceridwen I.; Craw, Dave | Abstract: Long-distance dispersal (LDD) is thought to be a key driver of biogeographic processes, yet few direct natural observations have been made of this process. Several studies have characterised diverse benthic epibiotic communities associated with buoyant macroalgae and have proposed that kelp rafting may be an important LDD mechanism for such coastal species. We test for LDD by combining biological, genetic, and geological data from rafted bull-kelp (Durvillaea antarctica) specimens collected in southern New Zealand following a March 2018 storm. Genetic and ecological data strongly indicate that three of 29 detached kelp specimens sequenced (and their associated live epifaunal taxa) had rafted from the sub-Antarctic to mainland New Zealand, traversing both oceanographic and phylogeographic barriers, over the course of an approximately 4-week journey. Numerous additional epifaunal taxa were detected from rafts that had geologically-distant mainland origins. The successful trans-oceanic rafting documented for sub-Antarctic brooding sea-star, chiton and sea-slug taxa presents a mechanism for their broad but phylogeographically disjunct Southern Hemisphere distributions. Moreover, the detection of several such LDD events over the last decade suggests that such journeys are very common over evolutionary timeframes. Although geological and genetic data were informative over different scales, we detected no conflict between the inferences from these distinct data sets, a finding that reinforces the value of integrative approaches to marine biogeography.

Cyclone-driven marine rafting: storms drive rapid dispersal of buoyant kelp rafts

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 602:77-85 (2018) - DOI: https://doi.org/10.3354/meps12695 Cyclone-driven marine rafting: storms drive rapid dispersal of buoyant kelp rafts Jonathan M. Waters1,*, Dave Craw2 1Department of Zoology, and 2Department of Geology, University of Otago, 340 Great King St., Dunedin 9016, New Zealand *Corresponding author: jon.waters@otago.ac.nz ABSTRACT: Storm events can have strong effects on marine biological dispersal. Here, we used geological evidence to assess the speed of biological rafting driven by a major cyclonic storm (Cyclone Gita) in southern New Zealand during February 2018. We undertook a comparative analysis of coastal rock types to assign beach-cast macroalgal specimens to their geological source regions. Approximately 10% (23 of 250) of Lepas-encrusted beach-cast kelp (Durvillaea) specimens sampled from Dunedin beaches in the immediate aftermath of Cyclone Gita (21 and 23 February 2018) bore holdfast-clasts consistent with origins in the distant Fiordland region of southwestern South Island. Based on Lepas barnacle growth data, we estimated that this distinctive ‘pulse’ of rafted material achieved the 350 to 500 km journey from Fiordland to Dunedin in as little as 10 d, with many of these rafts thus inferred to have travelled 30 to 50 km d-1. These rapid rafting events were facilitated by westerly gales in Foveaux Strait (12 to 17 February), followed by cyclonic southeasterly winds off eastern South Island (19 to 21 February). The rafting-speed estimates from this study are substantially faster than mean current-flow estimates for the same coastal region (12 to 20 km d-1), and also exceed the speeds of previously documented kelp-rafting events in this region (estimated 10 to 15 km d-1) and elsewhere. Together, these data highlight the key role of storms in driving ‘wind slippage’ of buoyant material, facilitating pulses of dispersal at speeds far exceeding the predictions of oceanographic models. KEY WORDS: Dispersal · Geology · Oceanography · Rafting · Storm · Wind slippage Full text in pdf format PreviousNextCite this article as: Waters JM, Craw D (2018) Cyclone-driven marine rafting: storms drive rapid dispersal of buoyant kelp rafts. Mar Ecol Prog Ser 602:77-85. https://doi.org/10.3354/meps12695 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 602. Online publication date: August 23, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Long distance kelp-rafting of rocks around southern New Zealand

ABSTRACTAbundant rafts of kelp Durvillaea antarctica float northeast past the southeast coast of the South Island of New Zealand, commonly with attached rocks. Most rafts pass offshore unseen, but strong easterly winds during a series of five cyclonic storm events in 2017–2018 forced rafts carrying exotic rocks onto Dunedin city beaches. The kelp-rafted rocks ranged from boulders weighing several kilograms to centimetre-scale veneers. Kelp genetics confirm a southern New Zealand source and colonising barnacles indicate 10–20 days’ travel time. Distinctive limestones, granite-derived calcareous sandstones and amphibolite facies gneisses were derived from southwestern Fiordland 300–400 km from Dunedin. Most of the exotic rocks were granitoids derived from southwestern Fiordland and/or Stewart and Snares islands. Mafic volcanogenic breccia and pyroxenite were derived from the coast of Foveaux Strait. Kelp rafting is not a major source of coastal sediment but may have been responsible for some exotic clasts in the late Cenozoic geological record in inland Otago.

Crossing the front: contrasting storm-forced dispersal dynamics revealed by biological, geological and genetic analysis of beach-cast kelp.

The subtropical front (STF) generally represents a substantial oceanographic barrier to dispersal between cold-sub-Antarctic and warm-temperate water masses. Recent studies have suggested that storm events can drastically influence marine dispersal and patterns. Here we analyse biological and geological dispersal driven by two major, contrasting storm events in southern New Zealand, 2017. We integrate biological and physical data to show that a severe southerly system in July 2017 disrupted this barrier by promoting movement of substantial numbers of southern sub-Antarctic Durvillaea kelp rafts across the STF, to make landfall in mainland NZ. By contrast, a less intense easterly storm (Cyclone Cook, April 2017) resulted in more moderate dispersal distances, with minimal dispersal between the sub-Antarctic and mainland New Zealand. These quantitative analyses of approximately 200 freshly beach-cast kelp specimens indicate that storm intensity and wind direction can strongly influence marine dispersal and landfall outcomes.

Large kelp-rafted rocks as potential dropstones in the Southern Ocean

Cyclonic winds in April 2017 forced oceanic kelp rafts onshore in southeastern New Zealand, providing a ‘snapshot’ of ongoing kelp-rock transport. About 10% of observed southern bull kelp (Durvillaea antarctica) holdfasts contain substantial remnants of their original rocky substrates. The largest rock weighs ~7kg, and was derived from the Mesozoic sedimentary Murihiku Terrane of southern New Zealand, >100km from the beach where it was deposited. Several other rafted rocks were derived from the same general source area, including a conglomerate clast with 10cm igneous cobbles. Genetic sequencing of the kelp specimens confirmed their southern New Zealand derivation, which is distinct from sub-Antarctic or northern New Zealand clades. The floating rocks had been colonised by goose barnacles (Lepas australis), the largest of which were approximately 5mm long, implying that the rocks had been floating for approximately 16days before being blown ashore. The rocks may have travelled up to 200km in that time in a northeasterly directed ocean current that flows at 10–15km/day. If these rock-bearing kelp rafts had not been blown ashore in the cyclonic weather event, they would likely have continued to travel northeast in the Subtropical Front zone where they would likely have been deposited as dropstones. Abundant dropstones noted on the nearby Chatham Rise and Campbell Plateau, have previously been assumed to have been ice-rafted. A component of these dropstones could have been derived by kelp rafting, especially on the Chatham Rise, where the Subtropical Front is deflected east. A thick weathering rind, and remnant organic veneer, if present, along with clast provenance, may be the best distinguishing features of kelp-rafted dropstones compared to ice-rafted dropstones.

Epibiont load causes sinking of viable kelp rafts: seasonal variation in floating persistence of giant kelp Macrocystis pyrifera

Floating seaweeds serve as dispersal agents for various organisms, but their survival at the sea surface may be compromised by physiological stress and epibiont overgrowth. Most previous experiments have been conducted in laboratory mesocosms where epibiont colonization is limited, but in their natural environment floating seaweeds are frequently overgrown by epibionts, which might negatively affect seaweeds or even cause their sinking. To test this hypothesis, we conducted field experiments in northern-central Chile (30°S) with floating giant kelps Macrocystis pyrifera to determine the time until sinking, epibiotic bryozoan load, and their physiological status across different seasons. Floating sporophytes persisted for at least 4 weeks at the sea surface and sank in all seasons after bryozoan loads exceeded 40 % of the raft biomass. At the time of sinking, the kelp rafts were physiologically viable and biomass losses were relatively minor. In autumn, kelp rafts stayed afloat for an average of 41 days (maximum: 52 days) during moderate environmental conditions (cool water temperature and moderate solar radiation) and slow growth of epibionts. However, higher water temperatures in summer seemed to enhance the growth of epibiotic bryozoans but not the growth of M. pyrifera, causing earlier sinking. The results indicate that the high growth rates of encrusting bryozoans provoke sinking of the kelp rafts, representing the first demonstrated case of epibiont-induced sinking of otherwise healthy floating seaweeds. Increasing global temperatures may enhance epibiont growth and thereby suppress the dispersal potential of floating seaweeds, even of species known for their high acclimation potential to the conditions at the sea surface.

Latitudinal and seasonal effects on short-term acclimation of floating kelp species from the South-East Pacific

Floating seaweeds are important dispersal vehicles for many organisms in cold-temperate waters between latitudes 30° and 60°. Molecular studies suggested that long-distance rafting of seaweeds is more prevalent at the polar edges of their distribution but knowledge about their physiological status at high latitudes is limited. Seasonal short-term field experiments were conducted along a latitudinal gradient (30°, 37° and 54°S) with floating kelps Macrocystis pyrifera and Durvillaea antarctica. Kelps were tethered at the sea surface for 14d during austral winter and summer to examine their acclimation potential in floating conditions. Biomass changes, reproductive status and physiological traits (chlorophyll a fluorescence, pigments, phlorotannins and antioxidant capacity) were evaluated after 7d and 14d of floating. Biomass changes (positive or negative) depended on species and latitude: at low latitudes (Coquimbo) both species lost biomass during both seasons while at mid latitudes (Concepción) they grew during summer but not in winter. Species-specific responses were observed at high latitudes (Pta. Arenas) with floating M. pyrifera growing in summer and D. antarctica in winter. Both species were able to maintain reproductive status mainly in winter, but at mid latitudes M. pyrifera also showed high reproductive activity during summer. Physiological adjustment was detected for both species at day 7 and upheld until the end of the experiment (14d). Latitudinal changes in photosynthetic variables (maximum quantum yield and pigments) were more evident in summer conditions, M. pyrifera showing decreases in Fv/Fm and pigment levels at low and high latitudes while in D. antarctica these variables increased with latitude. Phlorotannin contents varied with latitude but showed an opposing pattern in the two kelp species, with high concentrations at low latitudes for M. pyrifera and at high latitudes for D. antarctica. The floating kelps maintained high antioxidant capacity during the experiments. The short-term acclimation responses support the hypothesis of long-distance dispersal at high latitudes for both floating kelp species. The acclimation potential differed between the two species depending on the season, especially at mid and high latitudes. Summer conditions were favorable for floating M. pyrifera at high latitudes and for both species at mid latitudes in which a combination of increasing radiation and moderate temperature can stimulate growth. The results confirm that kelp rafting is favored at mid/high latitudes but suggest that in polar waters (>60°) low temperatures and light limitations might suppress growth of floating seaweeds.

Nitrogen fixation associated with the decomposition of the giant kelp Macrocystis pyrifera

The seaweed Macrocystis pyrifera plays an important role in the coastal zone as a result of its global distribution, high productivity and detrital export, and the high diversity of the ecological communities that it supports. Growth of the giant kelp Macrocystis, like other seaweeds, is often limited by the availability of fixed nitrogen. Several other macroalgal species have been reported to harbor microbial symbionts that fix ubiquitous dissolved N2, converting it into bioavailable forms, and potentially making a significant contribution to algal nutrition. Our investigation shows that N2 fixation on living Macrocystis (mean 0.17μmol g−1d−1) was more than 10 times greater than that associated with all but one of the 14 other macroalgal species we studied. However, this rate is insufficient to contribute significantly to Macrocystis growth. Instead, we reported much higher rates of N2 fixation (up to 23μmol g−1d−1) associated with detrital or decomposing Macrocystis. Decomposition-related N2 fixation on Macrocystis is stimulated both by anaerobic conditions and amendment with mannitol or phosphate, but is inhibited by light, and is therefore, likely a heterotrophic process in contrast to the cyanobacterial N2 fixation reported for Codium and Sargassum. Using O2 microsensors, we show that anaerobic conditions around the Macrocystis blade are unlikely to develop in attached plants. Nitrogen fixation may be important in kelp rafts and wrack, where it could contribute as much as a 4-fold increase in the availability of nitrogen to support microbial and invertebrate communities.

Genetic analyses of rafted macroalgae reveal regional oceanographic connectivity patterns

AbstractAimThis study examines beach‐cast rafts of Durvillaea antarctica (bull kelp) along New Zealand's east coast to determine spatial variation in beach‐cast haplotype assemblages. By using genetic markers, the geographical origins of rafted D. antarctica samples can be determined, allowing for the examination of dispersal pathways, distances rafted, and potential barriers to dispersal.LocationAn approximately 1000 km stretch of New Zealand's eastern coastline.Methods686 samples of beach‐cast D. antarctica rafts were collected and sequenced for cytochrome c oxidase subunit 1 in order to infer geographical origins and dispersal dynamics of rafted specimens. Results were examined using principal components analysis (PCA) in order to determine spatial groupings. Analysis of molecular variance (AMOVA) was used to determine statistical support for the detected spatial groups.ResultsHierarchical AMOVA revealed three major regional genetic assemblages, which probably reflect broad oceanographic features. In particular, the north–south genetic partitioning in rafted kelp assemblages either side of the Cape Campbell region in central New Zealand closely corresponds to an oceanographically mediated disjunction previously observed in a number of larval‐dispersed marine invertebrate taxa.Main conclusionsFindings from this study suggest that passively drifting kelp rafts and planktonic invertebrate larvae may be similarly constrained by oceanographic features, and demonstrate how the phylogenetic structure present in populations of D. antarctica provide a framework with which to map dispersal patterns.

Morphological, physiological and reproductive conditions of rafting bull kelp (Durvillaea antarctica) in northern-central Chile (30°S)

The bull kelp Durvillaea antarctica is a common floating alga in the southern hemisphere, but despite the ecological and biogeographic importance of kelp rafts, little is known about the responses of detached kelps to the conditions at the sea surface. The morphological, physiological and reproductive performances of D. antarctica rafts in the Coastal System of Coquimbo, Chile (CSC, ~30°S), were examined during winter and summer of two successive years (2010/11 and 2011/12). Epibionts (Lepas spp.) that only attach to floating objects were used as indicator for the floating time of kelp rafts. Photosynthetic efficiency and reproductive maturity of both benthic and floating algae varied seasonally, with a stronger decrease in summer than in winter. Blade size (measured as weight proportion of kelp individuals), phlorotannin concentrations and antioxidant activities were lower in floating than in benthic algae. Environmental conditions and floating time affected the blade tissues, with stronger negative effects during summer. These results confirm that floating persistence of D. antarctica in the CSC is suppressed during the summer months, which indicates that the dispersal potential of this (and other) floating algae varies seasonally.

Floating kelps in Patagonian Fjords: an important vehicle for rafting invertebrates and its relevance for biogeography

Floating macroalgae are common dispersal vehicles for associated benthic invertebrates. In order to investigate the importance of kelp rafts for species dispersal in the Patagonian Fjord Region (PFR), the abundance and distribution pattern of floating kelps (Macrocystis pyrifera, Durvillaea antarctica) and of the invertebrate fauna associated with M. pyrifera were evaluated during austral spring of 2002–2005, 2008 and 2010. In the southernmost Magellan Region (MR), benthic M. pyrifera were additionally sampled to compare the community structures in both conditions. Floating kelps were abundant throughout the entire PFR, harbouring a diverse and abundant invertebrate fauna. The density of floating kelps increased towards the south. In the MR, a loss of species was observed between benthic and floating condition (e.g. decapods, echinoderms, several peracarid species), but a high diversity of organisms from all major phyla were observed on rafts. Throughout the PFR, the predominant rafting species belonged to the peracarids, molluscs and annelids, but the community composition differed between floating samples from the northern and southern zones of the PFR. Relative abundances of peracarids were higher in northern zones, whereas molluscs and annelids dominated in the southern areas. Species of the peracarid genera Peramphithoe, Gondogeneia, Bircenna and Limnoria were shared between all areas. The results suggest that kelp rafts not only contribute to local population connectivity in the PFR, but could also be an important dispersal vehicle for rafting species along the PFR in north–south direction, crossing the biogeographic boundary around Taitao Peninsula. Furthermore, the MR appears to be an important stepping stone for species dispersal via kelp rafting in the subantarctic region.

Kelp rafts in the Humboldt Current: Interplay of abiotic and biotic factors limit their floating persistence and dispersal potential

During summer 2009, we conducted a field experiment and a field survey at 30°S in the coastal Humboldt Current to determine how floating time affects the physiological performance of kelp rafts. For the field experiment kelp rafts were tethered in coastal waters and the field survey was specifically designed to collect free‐floating Macrocystis pyrifera across a latitudinal temperature gradient that reflects natural floating time. Experimental kelps were kept under photosynthetic active radiation (PAR) and PAR + ultraviolet (UV; PAR + UV) using filter foils, and tethered at the sea surface in their natural habitat. Ultraviolet radiation (UVR) did not affect kelp physiology, but caused a decrease in kelp biomass. The field survey confirmed that sea‐surface temperature increased with distance from upstream source populations of M. pyrifera. Rafts sampled at increasing distance from sources showed high epibiont cover and reduced blade lengths. Physiological performance declined with increasing size of algal epibionts, which are indicators of floating time. Rafts that were farthest from the southern source populations had lost their sporophylls, suggesting that dispersal potential decreases with increasing floating time. The combined effects of abiotic (UVR and temperature) and biotic factors accelerate degradation of M. pyrifera and, thus, can impede successful dispersal in the Humboldt Current at 30°S. This suggests that floating macroalgae can be important dispersal vectors in areas with moderate environmental stress (i.e., in temperate oceans).

Physiological acclimation of floating Macrocystis pyrifera to temperature and irradiance ensures long-term persistence at the sea surface at mid-latitudes

Large numbers of giant kelp rafts are floating along temperate coasts of the southern hemisphere, carrying a wide diversity of associated organisms with them. During voyages, floating kelps are exposed to strong variations in environmental conditions such as UV-radiation (UVR) and temperature that affect algal physiology, growth, and reproductive output. Consequently, it was predicted that the interactive effects of high temperature and UVR suppress algal persistence and reproductive output at the sea surface. This hypothesis was tested by exposing Macrocystis pyrifera (Linnaeus) C. Agardh sporophytes to two irradiance (PAR, PAR + UV) and three temperature (cool, ambient, warm) conditions. An outdoor-tank experiment with two consecutive runs (1st and 2nd) was conducted in northern-central Chile (30°S) to assess growth and physiological responses (pigment contents and photosynthesis) of floating M. pyrifera. Results showed that after being afloat for 15 days, algae physiologically acclimated efficiently to changing abiotic conditions by a decrease in pigment contents and dynamic photoinhibition. However, in kelps exposed to 20 °C these acclimation processes operated at the expense of growth, resulting in reduced biomass gains, lower blade elongation rates, and diminished reproduction. Overall, floating M. pyrifera responded with high physiological plasticity to the tested UV regimes (UVB 30–100 kJm − 2 ; UVA 300–2000 kJm − 2 ), but under stressful temperature conditions (~ 20 °C) photoacclimation processes are costly and an important fraction of the energy gained via photosynthesis becomes unavailable for algal growth. We suggest that at mid latitudes (25°S–40°S) detached sporophytes of M. pyrifera have the capacity to float for long time periods and over large distances at water temperatures < 20 °C.