Abstract

Abstract For the fi rst time the subarctic northwestern Atlantic, we examined variation in rhodolith ( Lithothamnion glaciale ) morphology and biogenic potential in two large ( > 500 m 2 ) 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. ). 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 fi sh and invertebrate species. Differences in hydrodynamic conditions within and between beds may have contributed to these patterns.