Light saturation (P - I) curves for oxygen production and consumption were constructed in the laboratory for corals collected from depths of 1 to 45 m on the southeastern (seaward) side of Davies Reef, Great Barrier Reef, Australia. This depth range represents a gradient of from 82.6 to 2.9% of surface light, which was measured as photos~thetic photon flux density (PPFD) between 400 and 700 rim. Adaptative changes in photokinetic parameters were observed over the entire range of light intensities. The compensation intensity (Z,), Zk, the intensity at which photosynthesis was 95% light saturated (I,,,,), and the respiration rate ( - R), all decreased with decreasing light intensity. The maximal rate of photosynthesis when normalized on the basis of coral chlorophyll-a content (PgB) tended to decrease with decreasing irradiance but the change was not statistically significant. The P/R ratio (Pifi/ - R), the initial slope of a light saturation curve (a), and the maximum rate of photosynthesis when normalized by coral protein content (P&f, all increased with decreasing light intensity. The logarithms of the values for each variabie parameter were propo~iona1 to the logarithms of the fraction of the surface PPFD (T) tr~s~tted to the depth at which the corals grew. This enabled changes in these parameters to be accurately estimated for corals growing at any depth on the reef. Comparison of experimental data with published values for “saturating” irradiance and P”,,, suggests that the endosymbiotic algae within reef-building corals are photosynthetically intermediate between classical “sun” and “shade”’ plants.
Scleractinia (Hard Corals)
Australia - Great Barrier Reef