Increase in Cliona delitrix Infestation of Montastrea cavernosa Heads on an Organically Polluted Portion of the Grand Cayman Fringing Reef

Abstract. A marked increase in the biomass of Cliona delitrix infesting Montastrea cavernosa substrate occurred in a portion of the Grand Cayman fringing reef affected by the discharge of untreated fecal sewage. It is suggested that the six‐fold increase in bacteria biomass (both “coliforms” and natural marine bacterioplankton) in reef waters receiving the sewage effluent is linked to a five‐fold increase in sponge biomass at the polluted site relative to a control site. The elevated density of C. delitrix biomass signifies a similar increase in the amount of M. cavernosa skeleton that has been eroded by this sponge and reduced to silt‐sized sediment. Thus, the proliferation of a bioeroding organism in the sewage‐stressed environment has caused a shift in the carbonate balance on the reef.