Temperature-mediated outbreak dynamics of the invasive bryozoan Membranipora membranacea in Nova Scotian kelp beds

We used underwater videography to examine seasonal and interannual patterns in the cover (on kelp) of the encrusting epiphytic bryozoan Membranipora membranacea, and associated changes in the structure and abundance of native kelp (Saccharina longicruris) populations, at 2 sites on the Atlantic coast of Nova Scotia and over 4 to 11 yr since initial introduction of this invasive species around 1992. We show that (1) changes in the cover of M. membranacea on kelp, and in the cover of kelp on the seabed, are reciprocal and seasonal; (2) thermal history during the summer/fall period of bryozoan colony growth explains a large proportion (83 %) of the interannual variation in peak cover of M. membranacea on kelp; and (3) annual decreases in kelp cover and blade size are related to the degree of infestation by M. membranacea, and not to wave action alone. Particularly severe outbreaks of M. membranacea, resulting in extensive defoliation of kelp beds, occurred in 1993, 1997 and 1999. Our field observations indicate that recurrent seasonal outbreaks of this invasive bryozoan can have a devastating effect on native kelp populations in Nova Scotia, which, in turn, facilitates the establishment and growth of the invasive green alga Codium fragile ssp. fragile.