Recruitment of intertidal kelps Hedophyllum sessile and Alaria marginata (Laminariales) to articulated and crustose coralline algal species

Kelps and coralline algae are important primary producers and habitat-builders in rocky intertidal ecosystems. On wave-exposed shores along the west coast of North America, Hedophyllum sessile and Alaria marginata are two dominant kelp species with juveniles that often occur at a higher density on articulated corallines than other available substrates. Little is known of the mechanisms underlying this interaction. One hypothesized mechanism is that articulated coralline algae enhance kelp spore settlement and germination. This study tested this hypothesis by releasing spores from H. sessile and A. marginata onto multiple genetically identified articulated and crustose coralline species, as well as bare rock, then observing subsequent sporophyte densities. Kelp recruitment was generally higher on articulated corallines than on crustose corallines, although there was variation across coralline species. There was no significant difference between recruitment on bare rock and on articulated corallines, and recruitment was higher on bleached Corallina vancouveriensis than on live C. vancouveriensis, suggesting that this articulated coralline actively inhibits rather than promotes intertidal kelp settlement. Thus, other mechanisms, such as protection from herbivory or wave action, likely explain observed distributions of kelp recruits. This research contributes to understanding how the fine-scale distribution of kelps is linked to that of corallines.