Assembly and maintenance of subtidal habitat heterogeneity: synergistic effects of light penetration and sedimentation

Abstract

I experimentally separated the positive and negative effects of light penetration and sedimentation on the assembly and maintenance of 3 subtidal habitats whose heterogeneity charac- terizes much of the world's temperate coastline; encrusting (non-geniculate) coralline algae, articu- lated (geniculate) coralline algae and filamentous, turf-forming algae. The ability of encrusting corallines to monopolize and retain space without overgrowth depended on the presence of shade (positive effect) if sediment deposition was below that observed on coast characterized by high rates of sedimentation (negative effect). In contrast, the growth and persistence of articulated corallines depended on the absence of shade (negative effect) and high levels of sediment accumulation observed on human-dominated coast (positive effect). The recruitment of filamentous-turfs was facil- itated by full light, but was not strongly affected by sedimentation. Instead, filamentous-turfs toler- ated heavy sediment accumulation, a factor thought to explain the concomitant increase in spatial dominance of algal-turfs and loss of canopy-forming algae on reefs with heavy sedimentation. Impor- tantly, different habitats will assemble or be maintained to match the environmental conditions in which they are most extensive, demonstrating the key role of physical factors associated with habi- tat-formers (kelp forests) and human-dominated coast (heavy sedimentation). These results also demonstrate that an appreciation of the integrated roles of physical processes may assist the devel- opment of predictive models about the assembly and maintenance of heterogeneity of natural com- munities, and their potential disruption by humans.