Population dynamics of a nonindigenous epiphytic bryozoan Membranipora membranacea in the western North Atlantic: effects of kelp substrate

In rocky subtidal ecosystems of the western North Atlantic, the nonindigenous epiphytic bryozoan Membranipora membranacea facilitates the introduction of other nonindigenous benthic species by defoliating kelps. Susceptibility to infestation by the bryozoan varies among kelp species, although the causes of this variability are not known. We monitored the percent cover of the bryozoan on Saccharina longicruhs, Laminaria digitata, and Agarum clathratum, at 2 to 3 depths within 3 sites over 15 mo in 2005 to 2006. To infer mechanisms of observed differences in cover, we examined patterns of colony settlement and abundance, size-frequency distributions, and the proportion of plants with overwintering bryozoans on the respective kelp species. We also quantified the distributions and abundances of kelps before and after a major defoliation event in the fall of 2006. During the peak in abundance in fall, the highest percent cover of the bryozoan was consistently observed on L. digitata, which was achieved by colonies attaining larger sizes. The lowest cover and abundance were observed on A. clathratum, a pattern that we attributed to lower recruitment. Following an outbreak of the bryozoan in the fall of 2006, defoliation of the kelp bed was due to losses of individuals of S. longicruris, and to a lesser extent, A. clathratum, but not to losses of L. digitata, which increased in abundance at 1 location. Our results suggest that differences in the abundance of this introduced species among its algal hosts arise as a result of both differential settlement and survival. In turn, differential susceptibility among hosts to infestation may result in alteration of the algal community structure.