ecology of benthic communites in offshore regions. Patterns of 

 community structure have been quantified at only one offshore 

 site, Pigeon Hill on Jeffreys Ledge (Sears and Cooper 1975, 

 Witman et al. 1980, Hulbert et al . 1982, Witman and Cooper 1983). 

 This is surprising considering that offshore rocky ledges in the 

 Gulf of Maine (GOM) support productive fisheries (Bigelow 1917). 



The objectives of this report are to provide the first 

 quantitative description of spatial variation in the 

 distribution, abundance and population structure of benthic 

 invertebrates along a 30-90 m depth gradient at a subtidal rock 

 pinnacle in the central Gulf of Maine USA. These data form a 

 necessary community-level baseline for ongoing experimental tests 

 of the processes determining the invertebrate distribution 

 patterns. A later paper will describe temporal variation in 

 community dynamics based on 3 years of photographic monitoring of 

 rock walls at 30, 50, and 65 m depth. The hypothesis that 

 predatory sea stars control the lower depth limit of anemones 

 and horse mussels was tested in a manipulative field experiment 

 conducted at 50 m depth. The alternate hypothesis that the lower 

 depth limit of mussel distribution is determined by recruitment 

 failure was tested by a mussel colonization experiment. 



STUDY SITE 



Ammen Rock Pinnacle is a submerged rocky peak rising 

 abruptly from 140 m to 28 m depth on the crest of Cashes Ledge 

 approximately 120 km offshore in the Gulf of Maine (42° 51.25N: 

 68° 57.11W, Fig. 1). Its location far offshore suggests that 

 ARP is partially isolated from the counterclockwise Gulf of Maine 

 gyre (Brooks 1985) that flows along the coast. Because of the 

 steeply sloping bottom topography of Cashes Ledge, the pinnacle 

 site offers an unusual opportunity to study depth variation in 

 epifaunal community structure. 



Substratum Type 



The range of substrata occurring along the 30-90 m depth 

 gradient is shown in Fig 2. A sloping bedrock shelf composed of 

 peralkaline granite (Toulmin 1957) extends to approximately 

 55-68 m depth where it ends in a cobble area resembling a talus 

 slope. The topography of the granite ledge is irregular; large 

 expanses of sloping bedrock are interrupted by numerous small 

 joints, resulting in the formation of vertical or undercut rock 

 walls 1-2 m high. Patches of sand and gravel occur between the 

 cobbles and scattered boulders at 68-75 m depth. The bottom 

 beyond 75 m is a sloping sand and gravel plain. 



Temperature 



Typical vertical profiles comparing temperature at the 

 beginning and end of the summer at ARP are shown in Fig. 3. 

 Comparison of the two profiles indicates that the thermocline is 

 steeper late in the summer than in early summer, due to solar 



68 



