The ecology of rafting in the marine environment - Bedim

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MARTIN THIEL & LARS GUTOW members of the biofilm. Cundell et al. (1978) analysed microbial populations associated with the surface of the brown alga Ascophyllum nodosum and described the antibacterial activity of floating plants. Cyanobacteria also represent an important component of floating detritus matrices (Phillips 1963, Faust & Gulledge 1996). Fungi Various marine fungi growing on wood and algae have been suggested to be transported via rafting (Kohlmeyer 1984, Hyde 1989, Sundari et al. 1995, 1996, Prasannarai & Sridhar 1997) (Table 2). Reports of rafting fungi are not surprising since one third of all described marine fungi occur on algae (Kohlmeyer 1972). Accidental rafting of fungal organisms can also be the result of the intricate symbiotic relationships found in this group. Parasitic and hyperparasitic fungi, which may be dispersed simultaneously, have been described from Sargassum natans by Kohlmeyer (1972). In the tropics many fungal species were collected from stranded Sargassum spp. on which they are often found in empty bryozoan skeletons (Kohlmeyer 1984). Many lignicolous and arenicolous fungi isolated from wooden substrata have a cosmopolitan distribution (Kohlmeyer 1984, Sundari et al. 1996). The distribution pattern of these fungi might be a result of the high buoyancy of their primary substrata. Driftwood might become inoculated with fungi after having been cast ashore on sandy beaches. Spores of various arenicolous fungal species found in marine foam in the tropics (Kohlmeyer 1984) are likely to colonise stranded wood. During storm events with high wave surge, this wood may become waterborne again and be carried with currents to new areas, as has been suggested by Dyke et al. (1997). This process may lead to efficient dispersal of wood-dwelling fungi resulting in their cosmopolitan distribution (e.g., Sundari et al. 1996). Wood-dwelling fungi are often found in empty tubes of teredinid bivalves or limnorid isopods (Kohlmeyer 1984). Marine fungi (and bacteria) have been shown to essentially precondition wood, allowing access of other wood-boring organisms (Kampf et al. 1959 cited in Kohlmeyer et al. 1995). Larvae of teredinids, for example, are incapable of penetrating submerged wood as long as the bark is intact. Lignicolous fungi often require incubation times of several months before they fruit on driftwood making them identifiable (Kohlmeyer et al. 1995, Prasannarai & Sridhar 1997). A surprisingly low number of mangroves have been found to support marine fungi. Even though the plants are in permanent contact with sea water, apparently only a low proportion (
RAFTING OF BENTHIC MARINE ORGANISMS line about 10 m below the surface of the central North Pacific. They assumed that overgrowth with micro-organisms affected the buoyancy of the plastic line and caused it to sink. Benthic microalgae (including diatoms and dinoflagellates) may also form a dense matrix, which entraps gas bubbles during the day and then floats to the surface, carrying with it a wide diversity of microalgae (Phillips 1963, Faust & Gulledge 1996). Rafting dinoflagellates have been primarily reported from floating macroalgae and plastics (Table 2). Non-buoyant stages in the life cycle of marine microalgae may attach to floating substrata (Masó et al. 2003). For example, temporary cysts of dinoflagellates (Alexandrium taylori) have been found from a variety of floating plastic items (Masó et al. 2003). Bomber et al. (1988) reported nine dinoflagellate species as epiphytes of floating macroalgae. Dinoflagellates were collected from about 65% of all floating macroalgae (mainly Sargassum natans and S. fluitans) collected in the Florida Keys region. One of the species found was the benthic dinoflagellate Gambierdiscus toxicus, a ciguatoxin-producing species, for which Besada et al. (1982), based on its poor swimming abilities and non-planktonic habit, concluded that rafting may be its major dispersal mechanism and help explain some of its patchy distributions. Thus, as suggested by Hallegraeff (1992) and Masó et al. (2003), it appears possible that toxic microalgae could also become dispersed via rafting on natural and anthropogenic floating substrata. Macroalgae Some macroalgae are positively buoyant and may float over variable distances (Thiel & Gutow 2004). Most algae, though, are negatively buoyant and may raft as epiphytes on floating macroalgal species (Woelkerling 1975, Hoek 1987) or on other floating items (Winston et al. 1997, Aliani & Molcard 2003) (Table 2). Besides several self-buoyant macroalgae, Stegenga & Mol (1983) mentioned large numbers of allochthonous, non-buoyant algae with southerly origin (Normandy, Brittany, southern England) that were found on beaches of the Netherlands. These algae grew attached to other floating substrata such as wood, cork and plastic. Edgar (1987) listed a large number of macroalgal species associated with detached holdfasts of Macrocystis pyrifera several months after detachment of the latter. Even large species that are also buoyant such as Ecklonia radiata were found growing on algal substrata. Mitchell & Hunter (1970) reported some non-buoyant brown algae entangled among rafts of Macrocystis pyrifera. Large numbers of epiphytic algae had also been found on floating Sargassum off the coast of Brazil (Oliveira et al. 1979), and Ingólfsson (1998) mentioned that the red alga Polysiphonia lanosa is a common epiphyte on Ascophyllum nodosum found floating in Icelandic waters. Kornicker & Squires (1962) observed algae growing on floating corals that had recently been cast ashore but they did not specify the algae. At the Great Barrier Reef mainly crustose coralline and filamentous blue-green algae rafted on floating corals (DeVantier 1992). Very similar observations have been made for macroalgae on floating volcanic pumice (Jokiel 1989). The spores of many macroalgae have a very limited dispersal potential and may be dispersed over only a few metres, while adult plants or parts thereof may float over large distances and either reattach or release spores near new habitats (e.g., Hoffmann 1987, but see also Reed et al. 1988, 1992). Spores of macroalgae may thus raft on the parent plant. Distributional evidence was used by Hoek (1987) to infer that the algal flora of some oceanic islands has arrived via rafting. He demonstrated that long-range dispersal of seaweeds by planktonic propagules such as spores could be neglected since the lifetime of these developmental stages is too short for them to reach distant shores. The importance of floating and rafting for long-distance dispersal of macroalgae is underscored by two facts, namely that (1) island flora in many cases is dominated by algal species that are positively buoyant and that (2) distances from potential sources are far beyond the dispersal range of spores (Hoek 1987). 293
Page 1 and 2: 1 Oceanography and Marine Biology:
Page 3 and 4: RAFTING OF BENTHIC MARINE ORGANISMS
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RAFTING OF BENTHIC MARINE ORGANISMS
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Substratum selectivity Lepadidae RA
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B A Organic Matter Small Predators
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Abundance (number of individuals) 0
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Table 18 Comparison between benthic
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Proportion of Debris Items Colonize
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Fall off (scraped off) Clonal growt
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Species number Proportion Holding o
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The ecology of rafting in the marine environment - Bedim

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