GROWTH-RATES AND PHOTON YIELD OF GROWTH IN NATURAL-POPULATIONS OF A MARINE MACROALGA ULVA-LACTUCA

We examined the photon yield of growth of free-floating Ulva lactuca L. in a eutrophic Danish estuary by exposing discs in mesh cages to 10, 40, and 100% of ambient light in shallow water. Growth per incident or absorbed photon was much higher at 10 than at 100% of ambient light, Growth was linearly related to photon flux density in low light and U. lactuca adapted to the shade by increasing the chlorophyll concentration and thus the photon absorptance in the tissue. Maximum yields at high temperatures in August-September ranged from 16.0 to 21.7 mmol C mol-1 incident photon and 40 to 61 mmol C mol-1 absorbed photon at 10% of ambient light. These values show that light-limited growth of U. lactuca proceeds at the maximum attainable photon yields when in situ temperature and nutrient supply are suitable. Such high photon yields for growth are perhaps common among fast-growing species resembling Ulva,