BENTHIC COMMUNITY STRUCTURE AT A SUBTIDAL ROCK PINNACLE IN 



THE CENTRAL GULF OF MAINE 



Jon D. Witman and Kenneth P. Sebens 



Marine Science Center 



Northeastern University, Nahant MA 01908 



ABSTRACT 



Patterns of benthic invertebrate distribution, abundance, 

 and body size were quantified by conducting photo transects from 

 submersibles along a 30-90 m depth gradient, and by SCUBA diving 

 to 30 m at Ammen Rock Pinnacle (ARP) on Cashes Ledge. Three 

 major faunal zones were identified. The shallowest zone is a 

 sparse kelp forest characterized by high densities (800 ■ m Zi ) of 

 sea anemones, Metridium senile , with abundant encrusting sponges 

 Hymedesmia sp.3 and Halichondria panicea . A striking increase in 

 the abundance of suspension-feeding invertebrates at 38 to 40 m 

 depth marks the beginning of zone 2, comprising mounding sponges, 

 Mycale spp., anemones Urticina crassicornis , and ascidians. The 

 brachiopod Terebratulina septentrional is is patchily distributed 

 in zone 2, attaining densities > 500 * m - ^ in some aggregations. 

 A change in the substratum from sloping rock ledge to cobble- 

 gravel bottom at 65 to 68 m is accompanied by a shift in species 

 composition as the cerianthid anemone, Cerianthus borealis , and 

 the polychaete, Myxicola infundibulum , dominate the deepest 

 habitats. The extent to which predation, recruitment, 

 competition and natural mortality can determine patterns of adult 

 epifaunal distribution between and within faunal zones was 

 investigated in controlled field experiments and by photographic 

 monitoring of permanent quadrats and rock walls. We report here 

 on predation and mussel recruitment experiments. The hypothesis 

 that predation by sea stars sets the lower distributional limits 

 of the mussel Modiolus modiolus at ~50 m depth was rejected in 

 a one year predator exclusion experiment. Sea star predation may 

 account for the scarcity of Metridium senile at 50 m. The 

 Metridium experiment was inconclusive however, because predator- 

 caused mortality could not be separated from emigration of 

 anemones from the control cages. The lack of mussel spat on 

 plastic filament scrubbing pads set down at 30, 50, 65 and 80 m 

 for one year suggests that deep Modiolus populations are 

 recruitment -limited. 



INTRODUCTION 



Recent investigations of rocky subtidal communities in the 

 Gulf of Maine have begun to provide us with an understanding of 

 the composition and dynamics of nearshore benthic communities 

 (Sebens 1982, 1985, 1986, Steneck 1982, Logan et al . 1984, Witman 

 1985, 1987, Vadas et al . 1986), yet little is known about the 



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