Macroalgae on shallow tropical reefs reduce the availability of reflected light for use in coral photosynthesis.
Bulletin of Marine Science
We tested the hypothesis that light reflected from dense growths of macroalgae [i.e., a low reflectance (LR ) surface] vs heavily grazed surfaces [i.e., a high reflectance (HR ) surface] has no effect on the photophysiology of scleractinian corals on the shallow (3-m depth) back reef of Moorea, French Polynesia. Underwater light was measured using a cosine-corrected sensor that detected photosynthetically active radiation (PAR , between 400 and 700 nm), and an underwater spectrophotometer that measured spectral reflectance, R(λ), with nanometer wavelength resolution. The effect of reflected light on corals was assessed by incubating small colonies of massive Porites spp. and Pocillopora verrucosa (Ellis and Solander, 1786) for 27 d on artificial HR and LR surfaces, and measuring the effective photochemical efficiency (ΔF⁄Fm') of Symbiodinium on the sides of colonies adjacent to the benthos. At noon on a cloudless day, upwelling photon irradiance (Eu) from the benthos was reduced 20% over LR vs HR surfaces, and LR surfaces reduced R(λ) 50%–60% for PAR . For massive Porites spp., ΔF⁄Fm' did not differ between HR and LR surfaces, but for P. verrucosa, ΔF⁄Fm' increased 60% on LR compared to HR surfaces. The increase in ΔF⁄Fm' suggests that P. verrucosa modifies its photophysiological performance in tissue adjacent to LR surfaces to increase the efficiency with which light can be utilized in photochemical pathways.