High recruitment of the introduced bryozoan Membranipora membranacea is associated with kelp bed defoliation in Nova Scotia, Canada

For many organisms, the magnitude of recruitment of settlers into the juvenile population is a key life-history parameter, which has pronounced effects on population dynamics. However, for benthic colonial organisms, abundance may also be highly influenced by availability of space, growth rate, senescence, and mortality. On the southern shore of Nova Scotia, Canada, outbreaks of the introduced epiphytic bryozoan Membranipora membranacea on kelps have occurred periodically since 1992. We monitored the abundance, size distribution, and percent cover of M. membranacea colonies on blades of the kelp Saccharina longicruris at 2 sites in St. Margarets Bay, Nova Scotia, from July 2005 to November 2006. Following a warm winter, juvenile and adult colonies occurred earlier and were an order of magnitude more abundant in 2006 compared to 2005. The percent cover of M. membranacea on kelp was higher in 2006 compared to 2005 in summer and early autumn, but there was no difference between both years in late autumn. Colonies were larger in summer 2006 than in 2005, but were smaller in late autumn in 2006, suggesting that large colonies had been lost due to kelp blade breakage. By November 2006, there was a 70 % decrease in the canopy cover of kelp on the benthos, despite a similar frequency and magnitude of storms compared to the previous year. We propose that outbreaks of M. membranacea in the northwest Atlantic are the result of periods of early and high settlement and recruitment. By causing kelp defoliation, these outbreaks can have a pronounced ecological effect on kelp beds.