EARLY COMMUNITY DEVELOPMENT EXPERIMENTS IN ROCKY SUBTIDAL 

 HABITATS (GULF OF MAINE, 30-80 M) 



Kenneth P. Sebens, Jon D. Witman, Robin L. A. Allmon, 



and Edward J. Maney, Jr. 

 Marine Science Center 

 ' Northeastern University 



Nahant, MA 01908 



ABSTRACT 



Sessile invertebrates dominate rocky subtidal communities at 

 depths greater than 30 m, and vertical rock surfaces as shallow 

 as 5 m in the Gulf of Maine. At Cashes Ledge, in the outer Gulf 

 of Maine, sponges, sea anemones, bryozoans and ascidians make up 

 most of the invertebrate community on vertical surfaces at 30 m 

 and become abundant on horizontal and sloping surfaces below that 

 depth. A decrease in water movement and an increase in sediment 

 accumulation on rocks deeper than 50 m may affect both 

 recruitment and growth of sessile species. In 1985 we began a 

 series of experiments to compare colonization of vertical 

 surfaces (roughened plexiglas plates) placed at depths of 30, 50, 

 65 and 80 m on Ammen Rock Pinnacle, part of the Cashes Ledge 

 formation. All settling plate racks had one set of plates 

 oriented outward and another oriented inward, thus experiencing 

 reduced water flow. Panels with settling plates were collected 

 one year later and the composition of the encrusting community 

 on their surface was analyzed. At 30 m plate surfaces were 

 dominated by encrusting bryozoans, inner and outer surfaces had 

 approximately equal cover of encrusting and erect bryozoans, and 

 unoccluded space was less than 10%. At 50 m both inner and outer 

 plate surfaces were colonized primarily by the erect bryozoan 

 species and unoccluded space was approximately 30% on the inner 

 and outer surfaces. Unoccluded space accounted for 45-70% of 

 the area on inner and outer plates at 65 m, and 60-70% at 80 m 

 depths. Bryozoans were by far the majority of species at all 

 depths except at 80 m where the polychaetes Filograna implexa and 

 Spirorbis spp. equalled their abundance. The most common erect 

 bryozoan, the cyclostome Idmidronia atlantica , reached its 

 greatest percent cover at 50 m decreasing above and below that 

 depth. This species alone accounted for much of the distinct 

 pattern of erect bryozoan cover with depth. Our preliminary 

 results show a pattern of decreasing rate of space colonization 

 with increasing depth for all bryozoan species combined, probably 

 as a result of differences in recruitment, survivorship, and/or 

 growth rates over the year. The lesser abundance of erect 

 bryozoans at 30 m, especially on the outer plates, argues that 

 water movement may be too strong at that depth to allow these 

 fragile species to grow well here. The decreasing percent cover 

 of all sessile species with depth may be due to either lower food 



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