The macrobenthic community of a shallow subtidal cobble bed at Eagle Head on the Atlantic coast of Nova Scotia was monitored from ca 4 mo before to 41 mo after mass mortalities of sea urchins Strongylocentrotus droebachiensis in September 1983. Biomass of erect algae on cobbles increased from 0 to between 50 and 75 g (dry weight) m-' after the sea urchin die-off, and consisted mainly of small and sparsely distributed plants of Fucus evanescens and Chondrus crispus. In contrast, algal biomass increased from 0 to ca 2.8 kg m-2 on an adjacent boulder ridge where large kelps (Laminaria longicruris, L. digitata, Saccorhiza dermatodea) formed a dense canopy. The macroflora of cobble beds at 2 other sites was similar in biomass and con~position to the cobble bed at Eagle Head. At a third site, the sea urch~n population had recovered and precluded the establishment of non-calcareous algae on the cobbles. In the absence of sea urchins, herbivorous molluscs, mainly perwinkles Littorina littorea, limpets Notoacmaea testudinalis and chitons Tonicella rubra, dominated the cobble macrofauna, reaching densities of up to ca 150, 400, and 1300 ind. m-2 respectively at Eagle head. However, these grazers (and other organisms) suffered mass mortalities in summer 1986, when thick mats of drift algae inundated the bottom creating anoxic conditions. Suspension feeders consisted mainly of small bivalves of which juvenile mussels (Modiolis modiolis, Myllis edulis) were most abundant. Predators/ scavengers included crabs, lobsters, whelks, sea stars, and polychaetes, many of which were small and seasonally active. With the exception of juvenile mussels, which increased in abundance (but rarely reached adult size), there was no evidence of a numerical response among macrofaunal populations to the elimination of sea urchins.

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We examined the role of molluscan mesograzers (periwinkles Littorina littorea, limpetsTestudinalia testudinalis and chitons Ischnochiton ruber) in mediating macroalgal succession after a mass mortality of sea urchins Strongylocentrotus droebachiensis in a subtidal cobble bed in Nova Scotia, Canada. Experimental exclusions of mesograzers from cobbles encrusted with smooth (Phymatolithon) or rugose (Lithothamnion) coralline algae resulted in the establishment of a variety of ephemeral filamentous algae, suggesting that these small (2 to 5 mm) but abundant herbivores are the dominant agents controlling early succession and community organization in this habitat. In removing inhibitive effects of early successional species, molluscan grazing facilitated the establishment of fleshy perennial species Fucus evanescens and Chondrus crispus, which occasionally escaped grazing at small size to attain a growth refuge on the cobbles. Algal biomass and species richness were greater on Lithothamnion than Phymatolithon, suggesting that rugose crusts provide more favourable microhabitats for recruitment or survival of various macroalgae. The distribution of mesograzers differed between coralline types, likely influencing the rate and intensity of grazing: periwinkles foraged on both types of crust, whereas limpets occurred primarily on Phymatolithon and chitons on Lithothamnion. All non-coralline algae were consumed when urchins were experimentally reintroduced to the cobble bed, returning the assemblage to the urchin barrens state.

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