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 chlorophyll concentrations around colonies on a reef of the northern Red Sea. The chlorophyll concentration downstream of a 0.75-m-deep colony thicket was depleted by 6.4% (±1.4% SE) compared with the water upstream. Neighboring organisms are thus exposed to water that is significantly depleted of phytoplankton, A 0.75 x 0.1 x 0.1-m passage of actively feeding colonies removed 34 mg of carbon per day from the water, equivalent to ~1.3 times their respiratory carbon demand. Rates of algae intake were also estimated by determining the decrease in gut fluorescence in starved colonies. The in situ depletion technique showed a three- to sixfold greater sensitivity compared with the gut fluorescence technique, and should be preferred as a technique for estimating feeding rates.