Mastobranchus sp. A and Thyasira obseleta are absent from the two 



Sargassum-treatment cores. 



The two Sargassum cores had a mean of 32 species and the 

 three background cores had a mean of 25 species, however, the 

 background density was less than half. More samples would be 

 needed to compare species diversity. 



DISCUSSION 



Experimental design 



The experimental design was planned to include more 

 replicates, samples from the background community when the 

 experimental treatments were recovered, and controls in which the 

 sites were cored without the addition of Sargassum . Although 

 this design was not achieved, alternatives to the explanation 

 that the observed effect is the result of organic enrichment are 

 not likely. Similar increases as a result of natural 

 aggregations of Sargassum were observed at 3600 m depth on the 

 continental rise (Grassle and Morse-Porteous 1987). In hundreds 

 of cores without obvious organic enrichment taken at several 

 upper slope stations south of New England (Maciolek and Grassle 

 in prep.) and off the southeast U.S. (Blake and Grassle, in 

 prep.), increases of this magnitude have not been apparent. 

 Sediment disturbance in the absence of organic enrichment is 

 likely to lead to a reduction rather than an increase in fauna 

 (Smith 1986, Grassle and Morse-Porteous 1987). 



Comparison with other studies of plant material on the sea floor 



Three of the taxa are the same as those responding to 

 natural increases in Sargassum at 3600 m depth south of New 

 England: Ophryotrocha sp. A, Capitella spp. and Hesionidae spp. 

 (Grassle and Morse-Porteous 1987). The predatory gastropod 

 species Cyclostrema nr. valvatoides has been observed by Turner 

 (1977) on an experimental concentration of wood blocks at slope 

 depths in the Tongue of the Ocean, Bahama Islands. Cocculinid 

 gastropods have previously been observed on seagrass blades from 

 abyssal depths in the Caribbean (Wolff 1976). Eurycopid isopods 

 ( Disconectes ) have not previously been observed associated with 

 plant material. 



Community characteristics 



There are too few samples to compare species diversity, 

 however the pattern of relative abundance in the Sargassum 

 samples is characteristic of disturbed environments (Fig. 1). 

 The most common species in deep-sea communities is generally 



287 



