IN-SITU DEPLETION OF PHYTOPLANKTON BY AN AZOOXANTHELLATE SOFT CORAL

The in situ removal of phytoplankton by the soft coral Dendronephthya hemprichi was investigated by taking small-scale measurements of chlor ophyll concentrations around colonies on a reef of the northern Red Se a. The chlorophyll concentration downstream of a 0.75-m-deep colony th icket was depleted by 6.4% (+/-1.4% SE) compared with the water upstre am. Neighboring organisms are thus exposed to water that is significan tly depleted of phytoplankton. A 0.75 X 0.1 X 0.1-m passage of activel y feeding colonies removed 34 mg of carbon per day from the water, equ ivalent to similar to 1.3 times their respiratory carbon demand. Rates of algae intake were also estimated by determining the decrease in gu t fluorescence in starved colonies. The in situ depletion technique sh owed a three-to sixfold greater sensitivity compared with the gut fluo rescence technique, and should be preferred as a technique for estimat ing feeding rates.