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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.

For the first time the subarctic northwestern Atlantic, we examined variation in rhodolith (Lithothamnion glaciale) morphology and biogenic potential in two large (>500 m2) rhodolith beds we discovered recently between the depths of 5–25 m off St. Philip’s and Holyrood, Newfoundland and Labrador. Rhodoliths at St. Philip’s were >50% larger and contained 7% more internal space in deep (15–17 m) than shallow (8–10 m) water, whereas shallow rhodoliths were >180% larger at Holyrood than at St. Philip’s. Rhodoliths were predominantly spheroidal and compact at St. Philip’s and platy or bladed at Holyrood. Shallow rhodoliths varied in length from 41.1–114.6 mm at St. Philip’s and 61.3–189.1 mm at Holyrood. Rhodolith density was similar between beds (858.1–938.9 individuals m-2) although biomass was significantly higher at Holyrood than St. Philip’s (25.3 versus 19.4 kg m-2). There was a strong positive relationship (R2>0.93) between rhodolith volume and dry weight in both beds. Invertebrates associated with shallow rhodoliths belonging to the taxonomic groups Asteroidea, Echinoidea, Ophiuroidea, Bivalvia, Gastropoda, Polyplacophora, Crustacea, and Annelida were present at both sites, although they varied in terms of size, density, and biomass. Brittle stars (Ophiopholis aculeata) and chitons (Tonicella marmorea) accounted for at least 82% (up to 2026.7 individuals m-2) of total numbers of invertebrates in each bed. Larger rhodoliths appeared to facilitate reproduction and feeding in dominant fish and invertebrate species. Differences in hydrodynamic conditions within and between beds may have contributed to these patterns.

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