OBSERVATIONS ON COASTAL EROSION IN BERMUDA AND MEASUREMENTS OF THE BORING RATE OF THE SPONGE, CLIONA LAMPA1,2

Abstract

Measured profiles of the submerged portion of the steep cliffs that ring Harrington Sound Bermuda, show that they are undercut as much as 4 to 5 m by a notch whose flat rooi coincides closely with the level of extreme low tides. Nips, intertidal indentations with sloping roofs, are not present.Because of the largely subtidal position of the notch, the morphology of the undercut region, and the restricted wave energy, wave cutting is ruled out and hioerosion is proposed as the mechanism of undercutting,Biocrosion is suggested as a term for the removal of consolidated mineral or lithic substrate by the direct action of organisms.Observations of the emerged and submerged carbonate coastline of Bermuda reveal that organisms of many types alter the exposed rock surface from the upper reaches of the spray zone to depths well below sea level by processes that vary with degree of exposure, tidal range, rock type, organic community, and probably other ecological variables.A series of expcrimcnts with common rock-destroying organisms has been initiated. Experiments in the laboratory and under natural conditions show that Cliona Zumpa, a boring sponge, is capable of removing as much as 6 to 7 kg of material from 1 m2 of carbonate substrate in 100 clays. This corresponds to an erosion rate of calcarenite of more than 1 cm/year, If previous estimates are correct (that 90% of the boring by C. Zampa is mechanical and not chemical), then 5 to 6 kg of fine-grained carbonate detritus can be supplied from 1 m2 of sponge-infected substrate in I.00 days.The present investigation of rock destruction and coastal erosion by biological agencies is an outgrowth of earlier studies on the origin of fine-grained carbonate sediments in Harrington Sound, Bermuda (Neumann 1963(Neumann , 1966. As much as 30% of this sedimcnt is composed of calcite containing very little magnesium in solid solution. The only common local source of this mineral phase is the eolian limestone that is cxposcd at the short steep cliffs ringing the small enclosed lagoon. It is unlikely that the fine calcitic detritus is produced by mechanical grinding ' bccausc there is littlc wave energy, and there are hardly any beach sites for abrasion, Furthermore, sand would be the common product of the disaggrcgation of the calcareous eolianite and, except close to shore, this size class is not volumetrically important in the largely silt and clay-size sediments of the sound. A likely origin was found when it was realized that many rockboring organisms bore wholly or in part by mechanical means. Rock-browsing organisms also contribute fine detritus by rasping rock surfaces. Rock-boring and browsing organisms of several types arc abundant in the sound, and the observations indicate the significance of their activity.Rock-destroying organisms of many descriptions play an important role in the erosion of reefs and coastlines as well as being producers of both fine detritus and coarse rubble. As a general term, the destruction and removal of consolidated mincral or Ethi...

The role of burrowing sponges in bioerosion

“…I calculated comparably high penetration rates but was puzzled by the fact that labeled and mCll.8ured "old" CliQna~inrested substrata (mollusk shells, small coral heads and limestone rock) had not shown significant shape or size changes during 16 months of in situ observation. My conclusion, that a stimulation effect would cause exceptionally high rates during the invasion of new substrata had also been suggested by Neumann (1966).…”

“…crusto8e coralline algae). Neumann's (1966) observation that the substrate mineralogy has little effect on burrowing rate could be confirmed by comparing rates in Iceland spar with those in Chama shclls. Porosity of the S\l bBtrnte aids quicker and decper penetration but no more material is removed per unit time.…”

“…Rates of biocrosioD caused by Cliona tampa were determined by Neumann (1966) by measuring weight loss of limestone blocks which had been attached to sponge bearing material for up to 100 days. From this data he calculated that.…”

“…Large coral specimens, mainly Acropora paimala can be entirely covered by CliOM aprlca near the Carrie Bow Cay reef crest. Neumann (1966). The area of cont-act (attachment) with the "mother" sponge is the only measure of biomass available for the cxpcriment-s up to 4-5 months.…”

Biological Erosion of Marine Habitats and Structures by Burrowing Crustaceans

“…This process also may damage structures or facilities of human value. Bioerosion is defined by Neumann (1966) as the destruction or removal of consolidated mineral or lithic substrata by the direct action of organisms. For this research, however, I define bioerosion as biologically-induced removal, destruction, or deterioration of lithic and non-lithic substrata including consolidated organic soils (marsh banks composed of compacted mud, clay and/or peat), artificial substrata (Styrofoam floats and docks), and persistent organic substrata such as wood (including debris, mangroves, and man-made structures).…”