WIDESPREAD MIXOTROPHY IN REEF-INHABITING SOFT CORALS - THE INFLUENCE OF DEPTH, AND COLONY EXPANSION AND CONTRACTION ON PHOTOSYNTHESIS

Primary production, respiration, and bathymetric distribution were inv estigated in the 10 most common zooxanthellate soft coral genera of mi d-shelf reefs of the central Great Barrier Reef, Australia. Oxygen flu x of corals, collected from 5 to 20 m depth, was measured at 6 m water depth over 24 h using data-logging respirometers. In most taxa, photo synthetic carbon acquisition was lower than respiratory carbon loss un der irradiance conditions of cloudless summer days at 20 m depth [mean rate of net photosynthesis P-n for all taxa: -1.0 +/- 0.5 (SE) mg C g (-1) ash-free dry weight (AFDW) d(-1)], and only slightly higher than respiration at 5 m depth (mean P-n = 1.2 +/- 0.7 mg C g(-1) AFDW d(-1) ). Hence, most if not all zooxanthellate taxa were also dependent upon heterotrophic nutrition to cover their carbon demand. Colony contract ion significantly reduced photosynthetic productivity in soft coral ta xa possessing large polyps. Levels of irradiance required to achieve p hotosynthetic compensation (I-c) and saturation (I-k) were higher in c ontracted compared with expanded colonies. The moderate dependence of soft corals on phototrophy was reflected in their bathymetric distribu tion at Davies Reef. Highest soft coral densities were found at 20 m d epth where maximum irradiance was attenuated to about 180 mu E m(-2) s (-1), equivalent to 16% of surface irradiance. At the deeper limits to the distribution of the zooxanthellate taxa, irradiance was sufficien t for photosynthesis to compensate respiration for 3 to 6 h d(-1), but was insufficient to saturate photosynthesis.