PHENOTYPIC PLASTICITY RECONCILES INCONGRUOUS MOLECULAR AND MORPHOLOGICAL TAXONOMIES: THE GIANT KELP,MACROCYSTIS (LAMINARIALES,PHAEOPHYCEAE), IS A MONOSPECIFIC GENUS1

The giant kelp genus Macrocystis C. Agardh (Laminariales, Phaeophyceae) is one of the world’s most ecologically and economically important seaweed taxa, yet its taxonomy remains uncertain. Although the genus currently contains four accepted species based on variable holdfast and blade morphology [M. pyrifera (L.) C. Agardh, M. integrifolia Bory, M. angustifolia Bory, and M. laevis C. H. Hay], numerous recent studies on Macrocystis interfertility, genetic relatedness, and morphological plasticity all suggest that the genus is monospecific. We reviewed this evidence and present an explanation for the extreme phenotypic plasticity that results in morphological variability within Macrocystis, driven by the effects of environmental factors on early development of macroscopic sporophytes. We propose that the genus be collapsed back to a single species, with nomenclatural priority given to M. pyrifera.     

Repopulation techniques for Macrocystis integrifolia (Phaeophyceae: Laminariales) in Atacama,Chile

The giant kelp Macrocystis (integrifolia) has been intensely harvested in northern Chile for several years. In order to prevent a future disaster, we developed two different techniques for restoration of damaged Macrocystis integrifolia beds in the Atacama region of Chile. (1) Explantation: Laboratory-grown juvenile sporophytes were fixed to different substrata (plastic grids, ceramic plates, or boulders) by elastic bands or fast-drying glue (cyanoacrylate). Explants reached 150–200 cm in length within 5 months (relative growth rate?≈?1.3–1.7 % day^?1), and reproductive maturity in 5–7 months. (2) Seeding of spores: Mature sporophylls were placed at 8 m depth on the sea bottom, supported by cotton gauze sleeves attached to boulders of different origin. Sixty percent of clean boulders collected on the beach produced up to seven recruits per boulder. In contrast, 20 % of the boulders from the sea bottom, colonized by epibionts, showed up to two recruits. Relative growth rates, however, were similar (≈2.4–2.6 % day^?1). Practical applications of our findings are: laboratory-produced juvenile sporophytes fixed to various substrata by elastic bands or cyanoacrylate glue can be used to colonize rocks or artificial reefs. In cases, where laboratory-grown seedlings are unavailable, mature sporophylls from nearby Macrocystis beds can be used to establish new recruits on rocky substrata.     

TRACE METALS IN GIANT KELP,MACROCYSTIS

Concentrations of 38 elements present in Macrocystis tissues are listed. Mean concentrations of Cu, Zn, Fe, and Mn were similar for samples of juvenile and adult sporophytes from field sites. Cu in Macrocystis tissues rose above background levels when concentrations of 0.05 to 0.1 μm were added to the culturing medium. Above-normal accumulation for Zn occurred at 0.1 μm and for Fe at 10 μm . Tissue Mn appeared independent of the ambient Mn concentration over the ranges studied. Computations indicated that upwelling is important for N and P renewal to Macrocystis beds but may be less significant for import of trace metals.     

Opportunities and challenges for the development of an integrated seaweed-based aquaculture activity in Chile: determining the physiological capabilities of <Emphasis Type="Italic">Macrocystis</Emphasis> and <Emphasis Type="Italic">Gracilaria</Emphasis> as biofilters

Seaweed production is a reality in Chile. More than ten species are commercially used to produce phycocolloids, fertilizers, plant growth control products, human food or animal fodder and feed additives. These multiple uses of algae offer a number of possibilities for coupling this activity to salmon, abalone and filter-feeder farming. In this context, different experiments carried out in Chile have demonstrated that Gracilaria chilensis and Macrocystis pyrifera have great potential in the development of an integrated aquaculture strategy. The present Integrated Multi-Trophic Aquaculture (IMTA) approach study showed that Gracilaria can be cultured best at 1 m depth whereas Macrocystis has an especially good growth response at 3 m depth. Both species use available nitrogen efficiently. On the other hand, high intensities of solar radiation (UV and PAR) can be critical at low depths of cultivation, and our results indicate that both species show photosynthetic susceptibility mainly at noon during the summer. The demand of Macrocystis for abalone feeding is increasing, thus improving the opportunity for developing an integrated nutrient waste recycling activity in Chile. Although Gracilaria shows a higher nitrogen uptake capacity than Macrocystis, its market value does not yet allow a massive commercial scaling.     

REPRODUCTIVE LONGEVITY OF DRIFTING KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN MONTEREY BAY,USA1

Drifting Macrocystis pyrifera (L.) C. Agardh sporophytes have long been viewed as the primary long‐distance dispersal vector; yet, few data exist that support the ability of reproductive viable sporophytes to actually travel the presumed hundreds to thousands of kilometers. This study addressed the reproductive longevity of experimental and naturally occurring M. pyrifera drifters. Temporal variability in sporophyte size and reproduction was estimated for experimental drifting sporophytes that were tethered to surface buoys and compared with attached plants (controls). Reproductive viability was also studied for beach‐cast drifters (BCD), and naturally drifting sporophytes observed during field surveys in Monterey Bay. Detached drifting sporophytes were tracked with radio transmitters to follow drifter trajectories and to measure drifting speed. Experimental drifters (ED) experienced a 74% reduction in frond length after 35 days, a 76% reduction in average frond number after 70 days, and a reduction in average sorus area by 83% after 28 days. Although zoospore production was reduced following detachment, sporophytes remained fertile with high zoospore germination success as long as sori were present (125 days). Zoospore production and germination success for natural and BCD was similar to ED. The average displacement of radio‐tagged drifters was 7.12 km·day^?1, suggesting that a sporophyte adrift for 125 days disperses viable propagules (zoospores) over 890 km (±363). Dispersal of propagules is important for population restoration, distribution, and genetic diversity. Such dispersal distances are long enough to connect potentially all Northern Hemisphere Macrocystis populations across a generational timescale and may facilitate inter‐hemispheric gene flow.     

Culture studies on early development of Lessonia trabeculata (Phaeophyceae,Laminariales): Seasonality and acclimation to light and temperature

Lessonia trabeculata is an important economic and ecological algal resource of Chile. Due to intense use in abalone and alginate industries, severe problems of over‐harvesting are emerging. We compared sporophyte‐initiation and ‐growth in two populations from northern and southern Chile (Bahía Inglesa and Maicolpué) under laboratory conditions. Irradiance and temperature were the most important factors affecting gametophyte development. Meiospores harvested in spring exhibited maximum reproductive and growth potential, while spores released during autumn were moribund, and died within a few days. In both study sites, we found evidence for acclimation: Meiospores collected in summer required higher levels of irradiance and temperature for maximum development than winter spores. Juvenile sporophytes from both localities responded similarly to temperature and/or irradiance. The best conditions for recruitment of sporophytes from both localities were 15°C and white fluorescent light of 40–70 μmol m^?2 s^?1. Although due to its low growth potential L. trabeculata is not a good candidate for mariculture, our results provide the necessary knowledge for laboratory‐based seedling production, which is needed for restoration and repopulation projects in damaged areas.     

PRESENCE OF SPOROPHYLLS IN FLOATING KELP RAFTS OF MACROCYSTIS SPP. (PHAEOPHYCEAE) ALONG THE CHILEAN PACIFIC COAST1

Some species of macroalgae continue to live for extended periods of time after detachment and may even maintain reproductive structures, yet very little is known about this process. Here, we describe the presence of sporophylls (with sporogenous tissues) on floating kelp rafts of Macrocystis spp. along the coast of Chile. Surveys were conducted at nine sites (18–50° S) during austral summer 2002, and floating kelp rafts were seen and collected at seven of these nine sites (between 22 and 50° S). Fifteen (26.8%) of the 56 samples had sporophylls, indicating maintenance of sporophylls after detachment. Some of the kelp sporophytes with reproductive blades showed signs of having been afloat for long periods (indicated by the large size of attached stalked barnacles). Additionally, experiments showed that floating kelps released viable zoospores. To understand the reproductive dynamics of floating kelps, we compared these results with information from attached populations of Macrocystis spp. at nearby coastal sites. In general, attached kelp had higher proportions of sporophylls than floating rafts, suggesting that detachment may negatively affect reproductive status. Nevertheless, floating kelps remained functionally reproductive, suggesting that zoospores may be dispersed via floating rafts. Published reports on other macroalgae indicate that some species (Lessoniaceae, Fucaceae, and Sargassaceae) are fertile and probably release zoospores or zygotes while floating or drifting in ocean currents. Because dispersal distances achieved by spores of most macroalgae are relatively short, release of spores from floating algae may be an alternative mechanism of long‐distance dispersal.     

<Emphasis Type="Italic">Macrocystis</Emphasis> mariculture in Chile: growth performance of heterosis genotype constructs under field conditions

Recent progress in Macrocystis mariculture is based on clonal stock cultures of gametophyte parents. Batches of up to 10⁵ genetically identical sporophyte seedlings can be produced at any time in the laboratory and explanted in the field for production of biomass. Sexual crosses of selected Macrocystis pyrifera gametophyte parents of different geographic origin along the coast of Chile showed heterosis and produced sporophyte batches with superior growth performance. Starting from zygotes, after 10 weeks in the laboratory and 5 months in the sea, our best hybrid genotypes grew up to 11 kg fresh weight per frond, which corresponds to 66 kg m⁻¹ of line in a commercial mariculture installation. In contrast, average yields of 14.4 and 22 kg m⁻¹ are reported in the literature for traditional methods. Additional experiments, including inter-specific crosses M. pyrifera × M. integrifolia and their performance in different climate zones of Chile, confirm that heterosis is a powerful tool for crop improvement in Macrocystis. It opens the possibility to construct tailor-made heterosis genotypes with maximum productivity and/or other desired properties for any given locality.     

Giant kelp vegetative propagation: Adventitious holdfast elements rejuvenate senescent individuals of the Macrocystis pyrifera “integrifolia” ecomorph

Recent findings on holdfast development in the giant kelp highlighted its key importance for Macrocystis vegetative propagation. We report here for the first time the development of adventitious holdfasts from Macrocystis stipes. Swellings emerge spontaneously from different areas of the stipes, especially in senescent or creeping individuals. After being manually fastened to solid substrata, these swellings elongated into haptera, which became strongly attached after 1 month. Within 4 months, new thalli increased in size and vitality, and developed reproductive fronds. Our results suggest the usage of these structures for auxiliary attachment techniques. These could act as a backup, when primary holdfasts are weak, and thus improve the survival rate of the giant kelp in natural beds.     

NORTHERN AND SOUTHERN HEMISPHERE HYBRIDS OF MACROCYSTIS (PHAEOPHYCEAE)1

All combinations of individuals of Macrocystis pyrifera (L.) C. Agardh, M. integrifolia Bory, and M. angustifolia Bory hybridized. Gametophyte isolates obtained from 18 individuals were used, including M. pyrifera and M. integrifolia from the extremes of their Northern Hemisphere ranges along the Pacific Coast of North America and M. pyrifera and M. angustifolia from Tasmania, Australia. All combinations of gametophytes produced sporophytes of normal morphology, with the exception of crosses involving three gametophyte isolates. One female (M. integrifolia) and two male (M. pyrifera and M. angustifolia)gametophyte isolates were unable to produce normal sporophytes in combination with gametophytes of the opposite sex. Some cultures of female gametophytes produced abnormally shaped parthenogenetic sporophytes. Gametophytes and sporangia of M. pyrifera had n= 16 chromosomes. The M. integrifolia female gametophyte that was unable to produce normal sporophytes had n = ca. 32 chromosomes. These results show that these species of Macrocystis have not become reproductively isolated. Although these species may be considered conspecific according to the biological species concept, we recommend that they continue to be recognized as separate species based on morphological differences.     

Variations of chemical composition and energy content in natural and genetically defined cultivars of Macrocystis from Chile

Seasonal and intra-thallus variations of energy content and chemical composition were assessed in an intertidal population of Macrocystis in southern Chile. Phylloid protein and lipid from cultured material were compared with seasonal variation in native Macrocystis. Furthermore, populations in northern and southern Chile and Falkland Islands were compared with various intra-/inter-cultivar genotypes of Chilean Macrocystis. Energetic values did not show seasonal or intra-thallus variations, with the exception of pneumatocysts, which had high levels of ash (49.9% DW) and low values of total energy (8.3% DW). Seasonal patterns were detected in protein and carbohydrate composition, with opposite trends. Likewise, holdfasts contained high amounts of protein (21.0% DW), and phylloids were high in soluble carbohydrates (4.5% DW). Lipids instead showed two peaks per year in an intertidal population and reached up to 0.4% DW. Alginic acid was the major organic compound in intertidal Macrocystis (46.8% DW), with differences on seasonal and intra-thallus levels. Mannitol content, in contrast, was erratic and lower than in other Laminariales (<5% DW). In general, protein and lipid content in our cultivars were 20% higher than in natural populations. Our experimental results indicate the possibility to manipulate the chemical composition of Macrocystis thalli through inter-/intra-specific crosses. This will be a basis, upon which selected genotypes can open new perspectives to Macrocystis mariculture industries in Chile.     

Compensatory growth of the kelp Macrocystis integrifolia (Phaeophyceae, Laminariales) against grazing of Peramphithoe femorata (Amphipoda, Ampithoidae) in northern-central Chile

Compensation of tissue loss has been considered an alternative strategy for seaweeds that have no or only minor chemical or structural defense against herbivory. Compensatory responses are facilitated by resource transfer among different tissues and have been suggested for large kelps. Macrocystis integrifolia (Bory) is a common kelp species from northern-central Chile, which is characterized by high growth rates and the absence of lipophilic chemical defenses against herbivore grazing. Herein, we used the giant kelp M. integrifolia to test for compensatory growth in response to grazing by the nest-dwelling amphipod Peramphithoe femorata (Krøyer). Amphipods were allowed to graze inside nests on subapical blades of M. integrifolia sporophytes for 14 days. We measured growth and chemical composition (C, N, laminaran and mannitol) of apical and subapical blades of grazed and ungrazed (control) sporophytes. Our results revealed the capability of M. integrifolia to maintain elongation rates in grazed subapical blades, which were similar to those of subapical blades from ungrazed sporophytes. Apical blades grew slower in grazed than in ungrazed sporophytes indicating a trade-off between apical and subapical blades when herbivores were present. Thus, compensation occurs in blades directly attacked by grazers and is probably mediated by vertical resource allocation within sporophytes to subapical blades, a suggestion supported by the fact that stipe internodes in these regions grew more on grazed sporophytes. In general, our study indicates that M. integrifolia exhibits compensatory growth against the herbivore amphipod P. femorata, and we suggest that this could be an important strategy of large kelp species to tolerate moderate grazing intensities.     

Production, use and fate of Chilean brown seaweeds: re-sources for a sustainable fishery

Chile is an important producer of brown seaweeds representing 10% of world supply. Landings of Chilean kelp fluctuated between 40,000 t.year⁻¹ in the early ‘80s to 250,000 t.year⁻¹ more recently. Commercialized algae come from natural populations and no mass-cultures of involved species have been established. Four species of brown algae are commercially collected in the country: Lessonia trabeculata, L. nigrescens, Macrocystis pyrifera and M. integrifolia. Since 2000, the demand of alginate sources and food for cultivated abalones dramatically raised the harvesting of these species. Direct evaluations of Lessonia spp. and Macrocystis standing-stocks were made along 700 km of coast in northern Chile. Lessonia spp. estimated populations exceed 900,000 t, whereas M. integrifolia does not exceed 300 t. The insights provided indicate that sustainability of Chilean kelp subjected to intense harvesting would require management programs including the following bio-ecological recommendations: (1) to harvest the entire plant including the holdfast; (2) to harvest plants larger than 20 cm in diameter; (3) to harvest plants sparsely, selecting mayor specimens; (4) rotation of harvesting areas; and (5) for Macrocystis, to cut the canopy 1–2 m from the surface. They must be implemented in a National Program of Kelp Management, elaborated by government, scientists, fisherman, and industry.     

Expression of the<Emphasis Type="Italic"> lacZ</Emphasis> reporter gene in sporophytes of the seaweed<Emphasis Type="Italic"> Laminaria japonica</Emphasis> (Phaeophyceae) by gametophyte-targeted transformation

The seaweed Laminaria japonica (Phaeophyceae) has a two-generation life cycle consisting of haploid gametophytes and diploid sporophytes. Female and/or male gametophytes were transformed using particle bombardment and the histological LacZ assay was performed on sporophytes generated by either parthenogenesis or inbreeding. Female gametophyte-targeted transformation resulted in similar lower efficiencies in both parthenogenetic and zygotic sporophytes, and only a chimeric expression pattern was observed. Male gametophyte-targeted transformation led to a higher efficiency, with 3.5% of the zygotic sporophytes stained completely blue (all-blue), implying the integration of lacZ at the one-cell stage. Polymerase chain reaction analysis using primers specific for a lacZ-vector juncture fragment and subsequent blotting indicated the presence of the introduced gene in the sporophytes. The method reported here has a potential for seaweed transformation using spore-based bombardment followed by the developmental process.Abbreviations DPR Detected positive rate - ER Expression rateCommunicated by F. Sato     

Culture and growth of Lessonia trabeculata (Phaeophyta,Laminariales) juvenile sporophytes in La Herradura de Guayacan Bay,northern Chile

Lessonia trabeculata is one of the major kelps found along the northern coast of Chile. In addition to its ecological and economic importance, L. trabeculata may be severely affected by environmental disturbances such as El Níño, which during 1982–1983 cleared wide areas along the coast of Peru and Chile. The main goal of this work was to mass culture L. trabeculata and to observe the growth of sporophytes obtained in the laboratory and cultured in the sea. Juvenile sporophytes obtained in the laboratory were attached between 1 and 6 m in depth. The linear growth rate, as blade elongation, was recorded weekly for seven months. No significant differences (p < 0.05) were found in sporophyte blade linear growth at different depths. The best elongation growth rate was 7.5 ± 1.6 mm d^–1 at 3 m during March. This preliminary work suggests that L. trabeculata follows an annual growth cycle similar to that of other Laminariales with a high rate of blade elongation during the summer and decreasing towards autumn. This species can be considered a potential candidate for aquaculture to increase the availability of raw material and aid in repopulation of overexploited areas.     

AN EVALUATION OF METHODS USED TO ASSESS INTERGENERIC HYBRIDIZATION IN KELP USING PACIFIC LAMINARIALES (PHAEOPHYCEAE)1

Kelp intergeneric laminarialean hybridizations and hybridization protocol were assessed using seven northeast Pacific kelp species: Alaria marginata Postels and Ruprecht, Costaria costata (C. A. Agardh) Saunders, Eisenia arborea Areschoug, Laminaria saccharina (L) Lamouroux, Lessoniopsis littoralis (Tilden) Reinke, Macrocystis integrifolia Bory, and Nereocystis leutkeana (Mertens) Postels and Ruprecht. Survival and development of sporophyte morphologies derived from selfings, separate males and females, and reciprocal crosses were evaluated over 30 weeks of cultivation. All cultures were initiated from cloned gametophytes. Two closely related species, Laminaria angustata Kjellman and L. japonica Areschoug, demonstrated the efficacy of long‐term (up to 30 years) cloned gametophytes in hybridization studies. Sporophyte morphologies appeared in 34%–69% of control and hybridization trials, and 6%–16% of all trials produced sporophytes in control and hybridization conditions that persisted through 30 weeks of cultivation. Sporophytes in control and hybridization conditions could appear normal or abnormal. Usually, the morphology of sporophytes in hybridizations and female controls resembled the female parent, whereas the sporophytes in male controls often had an abbreviated morphology, lacking definitive generic features. Species‐specific rDNA internal transcribed spacer molecular primers were used to determine the parentage of five putative hybrids. Only the L. japonica♀/L. angustata♂ hybrid bore both parental genomes. That negative controls could produce persistent and normal‐appearing sporophytes negates their value and emphasizes the importance of molecular confirmation in hybridization studies. These findings were applied to critique the only known wild intergeneric hybrid, Pelagophycus/Macrocystis.     

Sexual compatibility and hybrid formation between the giant kelp species <Emphasis Type="Italic">Macrocystis pyrifera</Emphasis> and <Emphasis Type="Italic">M. integrifolia</Emphasis> (Laminariales,Phaeophyceae) in Chile

Two species of giant kelp inhabit the coast of Chile: Macrocystis integrifolia and M. pyrifera, representing important economic resources. As part of our efforts to domesticate these kelps for mariculture, and to obtain superior cultivars, we studied their biological relationship. Hybridization experiments with clonal gametophyte cultures showed reciprocal cross-fertility and produced fertile hybrid sporophytes with intermediate morphological characters. This hybridization potential in the laboratory contrasts with the persistence of two morphologically well-defined sister taxa in natural habitats on the Pacific coast of South America. We conclude that M. integrifolia and M. pyrifera are conspecific and speculate that unknown mechanisms support the co-existence of two morphologically distinct taxa on the subspecific level.     

GRAZING ON GIANT KELP MICROSCOPIC PHASES AND THE RECRUITMENT SUCCESS OF ANNUAL POPULATIONS OF MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYTA) IN SOUTHERN CHILE1

The giant kelp Macrocystis pyrifera (L.) C. Agardh is widely distributed in the Northern Hemisphere and Southern Hemisphere, yet it exhibits distinct population dynamics at local to regional spatial scales. Giant kelp populations are typically perennial with the potential for year‐round reproduction and recruitment. In southern Chile, however, annual giant kelp populations exist and often persist entirely on secondary substrata (e.g., shells of the slipper limpet Crepipatella fecunda [Gastropoda, Calyptraeidae]) that can cover up to 90% of the rocky bottom. In these populations, the macroscopic sporophyte phase disappears annually during winter and early spring, leaving a 3–4 month period in which a persistent microscopic phase remains to support the subsequent year’s recruitment. We tested the effects of a suite of grazers on the recruitment success of this critical microscopic phase at two sites in southern Chile. Field experiments indicated that the snail Tegula atra negatively impacted M. pyrifera sporophyte recruitment, but that recruitment was highest in the presence of sessile female limpets, C. fecunda. Conversely, small male C. fecunda (biofilm grazers) did not regulate kelp recruitment. Laboratory observations showed that C. fecunda males only grazed on microscopic kelp gametophytes and small (<250 μm) sporophytes, rejecting larger sporophytes, whereas T. atra grazed on all the kelp stages. Recruitment to the C. fecunda treatments far exceeded that to bare rock in the absence of grazers but was not due to the physical presence of C. fecunda shells. We concluded that the key to M. pyrifera recruitment success in southern Chile is its capacity to colonize secondary substrates provided by the slipper limpet C. fecunda.     

Growth of Ecklonia cava (Laminariales, Phaeophyta) sporophytes transplanted to a locality with different temperature conditions

Transplanting experiments were carried out to determine whether the small type sporophytes with short stipe of Ecklonia cava Kjellman (Laminariales, Phaeophyta) growing in a locality with warm temperatures, change into larger type with a long stipe when transplanted to a locality with cooler temperatures. Juvenile E. cava sporophytes, having a stipe shorter than 5 cm long were collected from Tei in Tosa Bay (southern Japan) (seawater temperature 15–29°C) and transplanted to Nabeta Bay (central Japan) (seawater temperature 13–25°C), where larger type E. cava sporophytes characterized by long stipe (ca 1 m) grow. They were attached to artificial reefs at the sea bottom (9 m depth) in Nabeta Bay to monitor their growth. For comparison, juvenile E. cava sporophytes of almost similar size growing in Nabeta Bay were also transplanted in the same way to the same experimental site. Observations of growth of sporophytes from Tei and Nabeta were carried out monthly for 2 years from November 1995 to October 1997. The transplanted Tei and Nabeta sporophytes showed an increase in stipe length and diameter from winter to spring, whereas almost no increase was observed during summer and autumn. At the end of the study period, the stipe of Nabeta sporophytes reached 25.6 cm in length and 17.0 mm in diameter, whereas that of Tei sporophytes reached 11.1 cm in length and 11.2 mm in diameter. The primary blade length was 16.0 cm in Nabeta sporophytes, whereas it was 5.5 cm in Tei sporophytes. Thus, Tei sporophytes still remained smaller than Nabeta sporophytes even under the same environmental conditions.     

SPORE SUPPLY AND HABITAT AVAILABILITY AS SOURCES OF RECRUITMENT LIMITATION IN THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

The causes of spatial variation in the recruitment of benthic marine algae are frequently misunderstood because of difficulties in distinguishing among the many factors that influence the supply and establishment of microscopic propagules. We used the recently constructed San Clemente Artificial Reef (SCAR) experiment to examine the roles of dispersal distance, size of spore source, and habitat availability as sources of variation in the recruitment of the giant kelp Macrocystis pyrifera (L.) C. Ag., a species whose recruitment has often been considered to be dispersal limited. Sparse colonization on SCAR by adult Macrocystis occurred within 6 months after reef construction via drifters (i.e. individuals from neighboring kelp beds that became dislodged and set adrift). The abundance of drifters on SCAR declined exponentially with distance from the nearest source population (San Mateo), suggesting that San Mateo was the likely source of drifters. Dense recruitment of small Macrocystis sporophytes was observed within 8 months of reef construction. The density of recruits on SCAR showed an initial increase with distance from San Mateo before declining exponentially. Nonetheless, substantial recruitment was observed at the most distant locations on SCAR located 3.5 km from San Mateo. In contrast to drifters, the density of recruits was positively correlated to the bottom cover of artificial reef substrate. Importantly, no correlation was found between the local density or fecundity of drifters and the local density of kelp recruits suggesting that recruitment on SCAR resulted from widespread spore dispersal rather than from the local dispersal of spores from drifters.