Effects of surface orientation on the cover of epibiota

Much of the understanding of the dynamics of subtidal epibiota (attached plants and animals) is based on work done on either the vertical surfaces of pilings and settlement plates or on the horizontal surfaces (down facing) of settlement plates that are typically attached to a floating structure. The extent to which commensur‐ability can be assumed in the dynamics of epibiota between these orientations is unknown. It is widely suspected that the orientation of a surface can affect assemblages of epibiota, but this hypothesis has not been rigorously tested. The current study assessed the effect of orientation of the surface, i.e. vertical sides and horizontal undersurfaces of pontoons, on the structure of assemblages of epibiota in Sydney Harbour. Assemblages on vertical surfaces were distinct from those on horizontal surfaces and the consistency of this result among pontoons and sites suggests that orientation had a strong effect over and above spatial variation. The effect was largely restricted to algae, which tended to be more diverse and, for one taxon (Cladophorales), more abundant on vertical than on horizontal surfaces. Generally, the cover of invertebrates was similar on both types of surface but the bryozoan Watersipora subtorquata was more abundant on the horizontal surfaces. These results indicate that the dynamics of assemblages between these orientations may be very different. Future understanding of how orientation of the surface affects epibiota would benefit from studies that follow the development of epibiota among treatments that manipulate the orientation of surfaces under different regimes of processes likely to affect abundance.

Key words:

• algae
• bryozoan
• fouling
• pontoon
• shade

Notes

Current address: Department of Environmental Biology, University of Adelaide, South Australia 5005, Australia; fax: + 61 8 8303 4364; e‐mail: [email protected]