XVI International Botanical Congess

The light intensity in most marine algal habitats is highly variable due to surface wave-focusing effects, canopy movements and mixing in the water column, among other factors. Each of these sources of variation are characterized by different dynamical cycles with different frequencies of oscillation. Photosynthetic rates of the green seaweed, Ulva lactuca responded to light fluctuations of 0.005 to 0.2 Hz in a nonlinear way. In general, photosynthetic rates exhibited the same frequency as the driving oscillation in photon flux density but specific frequencies of light variation (~0.05 Hz) resonated with a biological oscillation, producing beats in the oxygen evolution rate. The resonant driving frequency was also associated with nonlinearity of the growth response to light variation and corresponded to the dominant environmental frequency of light variation due to swell and canopy movements at our field site in Fife, Scotland.