Origin and Community Structure of the Harrington Sound Notch, Bermuda
The Harrington Sound notch, a deep bioerosional cleft lying just below mean low tide level, is a structure unique to Harrington Sound in Bermuda. The origin of the notch has been ascribed to the boring action of sponges of the genus Cliona, and bivalved mollusks (Lithophaga). The notch supports a rich epibiota; in addition to algae, epifaunal sponges and mollusks are prominent. A variety of burrowing biota penetrate the limestone substratum. Two hundred and forty-six cores from three locations at each of 41 stations were analyzed to determine the structure of both the epibiotic and endolithic communities. Seventy-five species were identified, 9 of which were endolithic borers, 13 species of polychaetes and mollusks occupied cavities in the rock and the remainder were epibionts. The dominant organism in terms of biomass (32 g˙m-2) was the boring bivalve, Lithophaga nigra whose burrows averaged over 4,000˙m-2 of which 22% were occupied by living individuals. By contrast the total mean biomass of all Cliona spp. was 3.0 g˙m-2, and their abundance was very low. The only other common endolithic boring organism was the cyanobacterium, Entophysalis deusta present in just over 50% of samples, with a mean biomass of 6.3 g˙m-2, no other endolithic species had a frequency of occurrence greater than 19.5%. It is concluded that the notch results primarily from the boring actions of L. nigra and E. deusta, with a small contribution by boring sponges and sipunculids. Wave action at reduced tidal levels probably augments bioerosion and at normal tidal levels appears to result in an upward movement of the notch top at many stations.
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Document Type: Research Article
Publication date: May 1, 1996
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