Epifaunal composition and fractal dimensions of intertidal marine macroalgae in relation to emersion

Abstract

The effect of tidal emersion on the epifauna of three common British intertidal macroalgae, Cladophora rupestris (Chlorophyceae), Laminaria digitata (Phaeophyceae) and Fucus serratus (Phaeophyceae) was investigated. Tidally-induced migration of intertidal fauna is well documented, but the aim of this study was to determine the effect of algal complexity on the degree of change in epifaunal community structure between tidal states. The structural complexity of each algal species was determined by measuring the fractal dimension (D) of algal outlines (1.76, 1.23 and 1.11 respectively for the three species). In the case of L. digitata, a weighted value for D was used to take account of the varying morphologies of the holdfast, stipe and blade. The hypotheses tested were: (i) that increased algal fractal complexity is associated with increased abundance and diversity of associated epifaunal communities; (ii) that community composition is significantly reduced during emersion in intertidal algae (due to faunal migration); and (iii) that the degree of migration due to the receding tide is significantly reduced in more geometrically complex algae. Overall, faunal communities associated with C. rupestris were significantly more abundant and diverse than those associated with the other algal species investigated. No significant migration away from seaweeds was observed for any faunal taxon from any of the algal species studied during emersion. However, harpacticoid copepod abundance increased significantly on L. digitata at low tide. It is likely that these copepods were associated with the holdfast or underside of the lamina for protection from desiccation and the elements. This suggested an advantage associated with inhabiting low shore macroalgae during emersion compared with migration into the subtidal zone.