Epiphytic flora on Gelidium corneum (Rhodophyta: Gelidiales) in relation to wave exposure and depth

The canopy-forming macroalga Gelidium corneum (Hudson) J.V. Lamouroux plays a major role in the functioning of the subtidal ecosystem of the Cantabrian Sea (northern Spain). Despite its importance, little is known about the factors that may potentially affect the distribution pattern of its epiphytic flora. Here we examine two indirect factors: coastal orientation (N and NW) and depth (3 and 7 m), as proxies for wave exposure and light availability, respectively. We test their effects on the total epiphytic load, alpha diversity (species richness, Shannon, Simpson and evenness measures) and multivariate structure of the epiphytic flora growing on G. corneum in subtidal waters off the Basque coast. Plocamium cartilagineum, Dictyota dichotoma and Acrosorium ciliolatum were found to be the most common epiphytes. Significant interactive effect of coastal orientation and depth were observed for species composition and abundance of epiphytic flora. Increased wave exposure resulted in a lower epiphyte load and a less diverse community, suggesting that under high hydrodynamic conditions epiphytes were more likely to become dislodged from hosts. However, light availability only had a significant effect on the distribution of epiphytes below a certain threshold of wave action, with the epiphytic load being 30-40% greater on shallow bottoms.