DISTRIBUTION OF ALGAL EPIPHYTES ACROSS ENVIRONMENTAL GRADIENTS AT DIFFERENT SCALES: INTERTIDAL ELEVATION, HOST CANOPIES, AND HOST FRONDS 1 Caroline M. Longtin, Ricardo A. Scrosati 2 , Gillian B. Whalen, and David J. Garbary Saint Francis Xavier University, Department of Biology, Antigonish, Nova Scotia B2G 2W5, Canada Understanding epiphyte distribution in coastal communities is important because these organisms affect many others directly or indirectly. Yet, their distribution has been considerably less studied than that of their hosts and other primary-space holders. Identifying major sources of variation in epiphyte abundance is thus still a need. Environmental gradi- ents help predict species distribution and are perva- sive on marine shores. In this study, we test the notion that environmental gradients across intertidal elevation, throughout host canopies, and along host fronds explain a large variation in the abundance of sympatric epiphytes. Our model system was the assemblage of Ascophyllum nodosum (L.) Le Jol. and its epiphytes Vertebrata lanosa (L.) T. A. Chr. [= Polysiphonia lanosa (L.) Tandy], Elachista fucicola (Velley) Aresch., and Pylaiella littoralis (L.) Kjellm. On the coast of Nova Scotia (Canada), we found evidence of a spatial segregation among these species at almost all scales. While the red epiphyte V. lanosa was more common at high- and midinter- tidal elevations (peaking at midelevations) and on middle segments of host fronds, the brown epiphytes E. fucicola and P. littoralis were more common at low elevations and restricted to distal segments of host fronds. Canopy habitat affected abundance only for V. lanosa, which was more com- mon within the host canopy than on its periphery at midelevations. Since the studied gradients are related to predictable changes in abiotic factors, the identification of likely causes behind the observed patterns was facilitated. Our study ends by propos- ing abiotic and biotic factors that deserve priority in the experimental testing of the forces structuring this assemblage. Key index words: Ascophyllum nodosum; Elachista fucicola; environmental gradient; epiphyte; interti- dal zone; Pylaiella littoralis ; spatial scale; Vertebrata lanosa Algal epiphytes play important roles in coastal benthic communities. They have positive effects, such as providing food and habitat for animals (Pavia et al. 1999, Karez et al. 2000, Viejo and A ˚ berg 2003), and negative effects, such as decreasing the performance of macroalgal hosts (Buschmann and Go ´mez 1993, Kraberg and Norton 2007). Thus, understanding epiphyte distribution is important, but these organisms have been considerably less studied in this respect than their hosts and other primary-space holders (Menge and Branch 2001, Bertness 2007). In particular, environmental gradi- ents are known to help predict species distribution (Bruno et al. 2003) and are pervasive on marine shores, suggesting that focusing on such gradients might identify major sources of variation affecting epiphyte distribution. In rocky intertidal habitats, the physiological stresses related to desiccation, irradiance, and tem- perature increase vertically with elevation because of tide dynamics (Davison and Pearson 1996, Garbary 2007). Intertidal environmental gradients also occur at smaller scales, such as throughout host canopies (differences between the periphery and interior due to self-shading) and along host fronds in dense stands (differences from holdfasts to frond tips due to increasing biomass packing toward holdfasts). Thus, investigating epiphyte distribution at those three scales is a potentially productive approach. It is also important to understand the abundance pat- terns of all sympatric epiphyte species across those gradients, as factors explaining their distribution might include interspecific interactions. In the pres- ent study, we test the notion that environmental gra- dients across intertidal elevation, throughout host canopies, and along host fronds explain a large vari- ation in the abundance of coexisting epiphytes on marine rocky shores. We predict that patterns will not necessarily be the same among the epiphyte spe- cies because of their likely ecological differences. Our model study system was the assemblage of A. nodosum (Phaeophyceae; Ascophyllum hereafter) and its epiphytes V. lanosa (Rhodophyta; Vertebrata hereafter), E. fucicola (Phaeophyceae; Elachista here- after), and P. littoralis (Phaeophyceae; Pylaiella here- after). This host-epiphyte system is common at the intertidal zone on relatively sheltered rocky shores in the northwest Atlantic (Lobban and Baxter 1983, Garbary et al. 1991, Levin and Mathieson 1991, Cardinal and Lesage 1992, Garbary and Deckert 2001). All three epiphytes occur across the vertical 1 Received 10 July 2008. Accepted 16 March 2009. 2 Author for correspondence: e-mail rscrosat@stfx.ca. J. Phycol. 45, 820–827 (2009) Ó 2009 Phycological Society of America DOI: 10.1111/j.1529-8817.2009.00710.x 820