Persistence and Transport of Fauna on Drifting Kelp (Macrocystis Pyrifera (L.) C
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Journal of Experimental Marine Biology and Ecology 253 (2000) 75±96 www.elsevier.nl/locate/jembe Persistence and transport of fauna on drifting kelp (Macrocystis pyrifera (L.) C. Agardh) rafts in the Southern California Bight Alistair J. Hobday1 Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA Received 9 April 1999; received in revised form 19 June 2000; accepted 29 June 2000 Abstract Drifting rafts of Macrocystis pyrifera may connect isolated kelp forests in the Southern California Bight. To determine which species might utilize this dispersal mechanism, faunal samples from natural drifting rafts and attached M. pyrifera plants were collected during ®ve cruises between March 1995 and December 1997. These rafts, which can be considered as ¯oating islands, were aged and the macroinvertebrate assemblage enumerated. There was no signi®cant relationship between raft age and species richness, or between species richness and distance offshore, which contrasts with predictions based on island biogeography. Species richness, however, was related to raft weight. Patterns of species presence and density were investigated relative to raft age for the species most frequently associated with rafts. Only one species, the isopod Idotea resecata, was found on all sampled rafts. Some species increased in frequency with raft age and others decreased, but only one relationship, a decline in the frequency of the anemone Epiactis prolifera with raft age was signi®cant. When species density was examined over all cruises, only I. resecata had a signi®cant change in density (an increase) with raft age, but additional signi®cant relationships were found when species density patterns were considered by cruise. The results of all the tests were combined to provide a measure of ``raft success''. Nine of the most frequent 19 species had a positive score, indicating a favorable response to rafting, seven were unaffected, and two species had negative responses to rafting. Extinction times were calculated using species abundance and raft age relationships. Two species (E. prolifera and Paracerceis cordata), were predicted to persist on rafts for only about 100 days, which is the maximum estimated raft lifetime. All other species were predicted to persist for longer periods if the rafts ¯oated longer. Kelp fauna that begin rafting appear to be largely unaffected by rafting, and hence dispersal on kelp rafts is possible for many members of the kelp forest community. 2000 Elsevier Science B.V. All rights reserved. Keywords: Algal rafts; Extinction rates; Faunal dispersal; Island biogeography E-mail address: [email protected] (A.J. Hobday). 1Present address: CSIRO Marine Research, P.O. Box 1538, Hobart, Tasmania 7001, Australia. 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00250-1 76 A.J. Hobday / J. Exp. Mar. Biol. Ecol. 253 (2000) 75 ±96 1. Introduction Attached algae provide habitat for many invertebrates (e.g. Mukai, 1971; Gunnill, 1982; Edgar, 1983; Smith et al., 1996). These habitats and their invertebrate assemblage are often isolated from similar patches by varying distances of non-algal habitat. Alga that detaches from the substrate, if it ¯oats, may drift at the ocean surface for a period of time before beaching or sinking (Hobday, 1998, 2000b). The possibility that algal rafts act as a faunal dispersal mechanism and connect isolated populations has been recognized for a long time (Vallentin, 1895; Fell, 1962; Benech, 1978; Highsmith, 1985; Johannesson, 1988; Helmuth et al., 1994; Worcester, 1994). Because movements between algal habitats can occur by active movements or free dispersal in particular life stages (e.g. planktonic larvae), animals with completely non-planktonic life cycles may rely more heavily on rafts for dispersal. While rafts have been studied for a long time, the majority of studies have simply documented the ®sh and invertebrate fauna (Senta, 1966; Gooding and Magnuson, 1967; Hunter and Mitchell, 1967; Ida et al., 1967; Mitchell and Hunter, 1970; Weis, 1968; Fine, 1970; Dooley, 1972; Kingsford and Choat, 1985; Tully and Ceidigh, 1986; Nakata et al., 1988; Fedoryoka, 1989; Kingsford, 1992; Davenport and Rees, 1993; Ingolfsson, 1995), however, changes in community composition are likely as the raft ages and drifts farther from the point of origin (Helmuth et al., 1994; Hobday, 1998; Ingolfsson, 1998). Macrocystis pyrifera is a large brown alga growing in temperate waters in both the northern and southern hemisphere and forms large forests inhabited by a rich ®sh and invertebrate fauna (e.g. Jones, 1971; Ebeling et al., 1980; Coyer, 1984; North, 1994). Attachment to the substrate is with a root-like holdfast and a canopy composed of stipes and blades extends to the surface (Clendenning, 1971; North, 1971). The holdfast- associated faunal community has attracted most study (Andrews, 1945; Ghelardi, 1971; Jones, 1971; Ojeda and Santelices, 1984), while the canopy community has achieved less attention, due in part to the dif®culty of sampling the large biomass (but see Coyer, 1984). Rafts of M. pyrifera form when plants become detached from the substratum and ¯oat to the surface (Dayton, 1985; Harrold and Lisin, 1989; Tegner et al., 1995; Hobday, 1998, 2000b). Studies of detached M. pyrifera plants have also focused on the more easily sampled holdfast community and have often considered temporal changes following detachment (Ojeda and Santelices, 1984; Edgar, 1987; Vasquez, 1993). Edgar (1987) noted successional and dominance changes in inhabitants of the kelp holdfasts, perhaps due to the absence of kelp forest predators such as ®sh. Perhaps the most complete study to date of the fauna of drifting kelp rafts is that of Bushing (1994) who sampled 109 drifting brown algal rafts (81 of M. pyrifera), within a few kilometers of Santa Catalina Island, California, between 1969 and 1972. A total of 179 invertebrate and 25 vertebrate species were associated with those drifting rafts. The faunal sampling, however, was not quantitative and age or size of drifting rafts was not measured. Like Highsmith (1985), Bushing postulated that for species with limited larval dispersal, drifting rafts might be the most important dispersal mechanism. Rafting will only be a successful dispersal mechanism for those species that can persist on the ¯oating object. Initially the raft community is likely to be similar to that of A.J. Hobday / J. Exp. Mar. Biol. Ecol. 253 (2000) 75 ±96 77 attached algae. Some losses may result during detachment and mobile animals may subsequently leave the drifting raft. As the raft drifts, changes in predation and competition may also in¯uence species abundance. Members of the pelagic ¯otsam community, colonizing as larvae, juveniles or adults may join the raft community (Helmuth et al., 1994). Pelagic species may primarily use rafts as substrate for completing a portion of their life cycle rather than for transport (Hobday, 1998). Because of the large size of M. pyrifera rafts complete sampling has never before been attempted. In this study patterns of macrofaunal species presence and density on natural rafts of M. pyrifera were examined with regard to raft age. The methodology for aging M. pyrifera rafts is described in Hobday (1998, 2000b). 2. Methods 2.1. Sample collection Drifting M. pyrifera rafts were collected during cruises on ships of opportunity in the Southern California Bight between March 1995 and December 1997. Raft location was recorded with the ship's differential GPS system and the distance to the closest land (island or mainland) calculated. The focus of this study was the macroinvertebrate assemblage associated with kelp rafts, which dictated the sampling style, although juveniles of some ®sh species were also well collected. Two different net designs, using a modi®ed Sigsbee trawl with mesh size of 6 mm, were used to collect rafts. The ®rst design, the Rumsey Net, had a ®xed 3.3 3 3 m rectangular mouth and a tail about 10 m long, and operated as a large neuston net. The second version, the Giant Rumsey Net, was implemented to better capture the associated juvenile ®sh that often school beneath kelp rafts. This net was similar to a large dip net, with the net mouth attached to a rigid 2.5 m diameter circular frame (Hobday, 1998). When a raft of suitable size was observed, the ship slowed and deployed the net using the ship crane and maneuvered alongside the drifting raft which drifted into (design 1) or over (design 2) the net mouth. The mouth was then lifted clear of the surface and net and raft craned aboard. Rafts were transferred through the net cod-end to a large sorting bin (1 m 3 2m3 1 m) with two layers of coarse screens at the bottom (6 cm 3 3cmand1cm3 1cm).A combination of salt and fresh water was used to wash the mobile animals from the kelp, through the coarse screens, out a hole in the bin base and into a 333 micron sieve. Representatives (and the largest) of each of the attached invertebrate species (e.g. Lepas barnacles) were removed by hand as the kelp was removed for weighing. Raft weight was measured in a bin with a spring scale. Stipes with terminal ends (between 1±10 stipes per raft) were set aside to determine the age of each raft according to methods described in detail in Hobday (1998, 2000b). Brie¯y, raft age was calculated using the average blade length on collected rafts, the average blade length on attached plants, and the rate of change in blade length as rafts drift. If raft age was estimated as negative because the raft had longer blades than the average attached plant, raft age was set at 78 A.J. Hobday / J. Exp. Mar. Biol. Ecol. 253 (2000) 75 ±96 zero. A maximum of two to three rafts could be collected at sea per day using these methods.