SPORE SUPPLY AND HABITAT AVAILABILITY AS SOURCES OF RECRUITMENT LIMITATION IN THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) 1 Daniel C. Reed, 2 Stephen C. Schroeter Marine Science Institute, University of California, Santa Barbara, California 93111, USA and Peter T. Raimondi Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California 95060, USA The causes of spatial variation in the recruitment of benthic marine algae are frequently misunder- stood 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 Artifi- cial 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 consid- ered 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), suggest- ing 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. Key index words: artificial reef; colonization; dis- persal; establishment; fecundity; kelp; Macrocystis pyrifera; recruitment limitation; spore Abbreviation: SCAR, Scan Clemente Artificial Reef Recruitment limitation occurs when the size of a population is constrained by the supply and/or estab- lishment of propagules. It has been invoked to explain spatial and temporal variability in the local abundance of terrestrial plants (Tilman 1997, Hubbell et al. 1999, Connell and Green 2000) and benthic freshwater and marine organisms (Underwood and Fairweather 1989, Downes 1995, Caley et al. 1996). Rates of supply depend on the dispersal potential of propagules and on the size and fecundity of their source populations, whereas the availability of suitable habitat, favorable environmental conditions, and biotic interactions de- termine whether propagules survive long enough to become established once they have settled. These different constraints on recruitment have been termed dispersal limitation, source density and source strength limitation, and establishment limitation (Clarke et al. 1998). Recruitment from propagules is the only means of colonization for sedentary marine organisms, and the degree to which constraints on propagule supply and establishment interact to limit recruitment is particu- larly important in systems subjected to frequent disturbance. Shallow temperate reef communities dominated by large brown seaweeds known as kelps (order Laminariales) are one such system. The high productivity, large size, and three-dimensional archi- tecture of kelps lead them to have an overwhelming influence on community structure and ecosystem function in areas where they occur (Mann 1982, Dayton 1985, Foster and Schiel 1985). The shallow reefs on which kelps live are subjected to a variety of physical and biological disturbances, causing their populations in many areas to fluctuate greatly in space and time. Intensive grazing, large waves, and unfavor- able growing conditions such as those during El Nin ˜o- Southern Oscillation events have all been identified as the cause of widespread kelp loss (Schiel and Foster 1986, Harrold and Pearse 1987, Dayton and Tegner 1989). Acting singly or collectively, these disturbances can create bare patches that range in size from several square meters to many square kilometers, which, depending on the size of a reef, can lead to the complete loss of a local kelp population (Mann 1977, Ebeling et al. 1985, Seymour et al. 1989, Camus 1994, 1 Received 10 July 2003. Accepted 6 January 2004. 2 Author for correspondence: e-mail reed@lifesci.ucsb.edu. 275 J. Phycol. 40, 275–284 (2004) r 2004 Phycological Society of America DOI: 10.1046/j.1529-8817.2004.03119.x