Evidence for persistent patch structure on temperate reefs and multiple hypotheses for their creation and maintenance

Abstract

Mosaics of habitat dominated by canopy-forming macroalgae and canopy-free (open-gap) habitat are prominent features of temperate subtidal reefs. However, the persistence and mechanisms underlying the arrangement of these patterns are not well understood. We described patterns in the proportion of reef covered by each of these habitats, and the length of patches of each habitat, at 20 sites encompassing a gradient in wave exposure in south-western Australia. Our aims were to characterise patterns in habitat distribution, and the strength of associations with potential influences, in order to develop models of habitat mosaic generation and maintenance. Modelled seabed orbital velocities explained approximately 35% of the variation in the length of open-gap patches, with longer patch lengths at sites that experienced highest water velocities. This observation is consistent with the hypothesis that waves create open-gaps by dislodging canopy algae. Herbivorous damselfish ( Parma spp.) were 5.6 times more likely, and the sea urchin Heliocidaris erythrogramma was 20 times more likely, to be encountered at sheltered inshore sites than at exposed sites further from shore. Parma were 8.2 times more likely to be found in open-gap habitat. However, there was no relationship between the occurrence of either herbivore and the proportion of open-gap habitat among sites. These observations do not support a hypothesis that grazing by herbivores creates open-gaps, but grazing might still be a process that influences the persistence of some open-gaps. Massive sponges were three times as likely to be found in open-gap habitat, and hard corals were 91 times more likely to be encountered in open-gap habitat. The strength of these associations suggests that canopy algae negatively influence sessile invertebrates. Further, the large size and likely old age of sessile invertebrates, particularly hard corals, indicates that patches of open-gap habitat can persist for decades. The patterns observed suggest that wave-induced disturbances create open-gaps and that these gaps are persistent features of temperate reefs.