Effects of seaweed canopies and adult barnacles on barnacle recruitment: The interplay of positive and negative influences

Barnacles are dominant sessile invertebrates on many rocky shores worldwide. Hence, investigating the factors that affect their recruitment is important. Through field experiments done on the Atlantic coast of Canada, we investigated interspecific and intraspecific relationships affecting intertidal barnacle recruitment. Specifically, we evaluated the effects of seaweed canopies (Ascophyllum nodosum) and adult barnacles (Semibalanus balanoides) on the density of barnacle recruits at the end of the recruitment season. The effects of three canopy treatments on barnacle recruitment and understory environmental conditions allowed us to identify positive and negative effects of canopies. At mid-intertidal elevations subjected to a moderate wave action, we found that, during high tides, the flexible algal fronds damage wire sensors established on the substrate (whiplash effect) and limit barnacle recruitment. However, at low tide, algal canopies limit water loss and temperature extremes and enhance barnacle recruitment in understory habitats. The net effect of algal canopies on barnacle recruitment, however, was neutral, as the positive and negative influences balanced out. By manipulating the abundance of adult barnacles under the seaweed canopies, we found that adult barnacles enhance barnacle recruitment, likely due to the known attraction that adults exert on larvae seeking settlement and to the absence of post-settlement events that could otherwise have blurred such effects by the adults. The presence of adult barnacles, however, did not protect developing recruits from canopy whiplash effects. By understanding the contrasting influences that intertidal algal canopies have on understory abiotic conditions and barnacle recruitment, our ability to predict net canopy effects depending on the relative degree of physiological (e.g., high vs. low intertidal zone) and physical (e.g., sheltered vs. exposed shores) stresses should increase. This study also suggests that recruitment, considered as an important external factor in environmental models of community organization, can also be affected by components of the community itself, potentially triggering local feedbacks.