PHOTOACCLIMATION AND GROWTH RATE RESPONSES OF ULVA ROTUNDATA (CHLOROPHYTA) TO INTRADAY VARIATIONS IN GROWTH IRRADIANCE

A vegetative clone of the chlorophyte macroalga Ulva rotundata was maintained in an outdoor continuous flow system under nutrient sufficient conditions and various light regimes. Step changes between 9 and 100% incident irradiance (I sub(o)) were employed to simulate cloud passage. Temporary (1-4 h) midday I sub(o) perturbations evoked net changes in growth rate ( mu ) and chlorophyll (chl) content. Under I sub(o) alternating at various periodicities from 15 min to 7 h, net mu was the average of the mu under steady state 9 and 100% I sub(o), regardless of periodicity. However, the mu in alternating light was considerably less than mu under steady state 55% I sub(o) ( similar to 9% + 100%/2), as expected based on the nonlinear shape of the mu -I relationship.