The impact of habitat complexity on the structure of marine sessile communities and larvae supply

Coastal infrastructure replaces complex and heterogeneous natural habitats with flat, two-dimensional concrete walls, reducing refuges against predation, which modifies the composition and identity of the dominant species in sessile communities. This modification in the community structure can also change the reproductive propagules available in plankton, affecting the recruitment dynamics in communities from natural habitats nearby. Here, we tested the combined effects of the habitat type (simple vs. complex with holes) and predation on the diversity, larval production, and structure of sessile communities from a recreational marina. Complex substrates showed a larger biomass and a greater abundance of solitary organisms, mainly ascidians and bivalves, that benefited from refuges. Barnacles and calcified encrusting bryozoans dominated simple, flat substrates. The difference in dominance affected the pool of larvae produced by the communities. After eight months, communities growing on flat substrates produced more barnacle larvae than those from complex substrates, where larvae of ascidians were more abundant. However, this difference disappeared after 18 months of community development. The difference in the pool of larvae between simple and complex substrates did not affect the structure of the community on flat substrates nearby, which was determined by the predation regime. In the studied region, communities in artificial environments are under intense predation control, suppressing eventual recruitment differences in communities developing in flat substrates. Large interventions that modify habitat topography, creating refuges in the subtidal zone, can change the dynamic of the sessile communities in artificial habitats and, consequently, the larval supply in the vicinities. However, differences in larval supply will only translate in distinct sessile communities when the scale of intervention encompasses large areas, and other processes do not buffer the differences in recruitment.