Shallow marine bioerosion: preliminary results of an experimental study

RICHARD G. BROMLEY, NILS-MARTIN HANKEN and ULLA ASGAARD Bromley, R. G., Hanken, N.-M. & Asgaard, U. Shallow marine bioerosion: preliminary results of an experimental study. Bull. geol. Soc. Denmark, vol. 38, pp.85-99, Copenhagen, April 25th, 1990. In order to investigate long-term bioerosional effects on carbonate rock surfaces in shallow marine settings, test blocks were placed off a rocky shore on Rhodes, Greece at —3m to —17m. Retrieval of blocks each year allowed the progress of bioerosion to be monitored. Rapid colonization by endolithic algae was followed by recruitment of browsing chitons and echinoids. The first boring sponges arrived in year 1 but were barely visible until year 2. After six years, stable blocks were deeply bioeroded. The history of community development varies considerably from block to block, largely depending on degree of disturbance. Physical disturbance consists of block tumbling as a function of water turbulance levels and the block's weight and shape; biological disturbance comprises the activity of rasping herbivores. The surfaces reveal a mosaic of successional stages in community development. Equilibrium has not yet been established on stable blocks; unstable blocks remain at a pioneer phase of community succession. The slow progress implies that, in the fossil record, well bioeroded substrates such as skeletons and hard-grounds represent several years rather than months of depositional hiatus. R. G. Bromley & U. Asgaard, Institut for Historisk Geologi og Palaeontologi, Oster Voldgade 10, 1350 Copenhagen K, Denmark. N.-M. Hanken, Instituttfor Biologi og Geologi, P.O. Boks 3085-Guleng, 9001 Tromso, Norway, October 1st 1989. "In 1871 a vessel ladenwith marble was sunk in Long Island sound, and according to Prof. Verrill, the boring sponge has penetrated the exposed parts of the blocks for a depth of two or three inches from the surface" (Ryder 1879, p. 279).