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 investigated in the 10 most common zooxanthellate soft coral genera of mid-shelf reefs of the central Great Barrier Reef, Australia. Oxygen flux 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, photosynthetic carbon acquisition was lower than respiratory carbon loss under 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 contraction significantly reduced photosynthetic productivity in soft coral taxa possessing large polyps. Levels of irradiance required to achieve photosynthetic compensation (I-c) and saturation (I-k) were higher in contracted compared with expanded colonies. The moderate dependence of soft corals on phototrophy was reflected in their bathymetric distribution at Davies Reef. Highest soft coral densities were found at 20 m depth 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 sufficient for photosynthesis to compensate respiration for 3 to 6 h d(-1), but was insufficient to saturate photosynthesis.