Information about photosynthesis-irradiance (P-I) relationships is crucial in many primary production studies. This is frequently obtained using incubation experiments, which may not simulate in situ conditions where algae are adjusting their photosynthetic systems to permanent light variations. There has been increasing interest in pulse-amplitude-modulated (PAM) chlorophyll fluorometry-which measures instantaneous photosynthetic response-particularly in the use of rapid light curves (RLCs) to minimize the confounding effects of light acclimation encountered with traditional 'steady-state' light curves (SSLC). However, there is still a lack of information about how oxygen SSLC curves and estimations from fluorescence P-I curves are related. The present study addresses some of these topics, using Laminaria hyperborea. The objectives of this study were to (1) determine the effect of light sequence (increasing or decreasing) in the estimation of P-I parameters, (2) characterize the daily patterns of P-I curve parameters, (3) compare oxygen and fluorescence P-I parameters and (4) evaluate the possibility of converting fluorescence P-I parameters into oxygen production measurements. Results showed that light sequence had no significant effect on P-I parameters (fluorescence and oxygen) and that P-I curves exhibited considerable differences over the day (F-ratio analysis). Fluorescence SSLCs and RLCs were good oxygen production predictors only at sub-saturating irradiances, when significant linear relationships were obtained with results from oxygen incubations. Thus care should be taken when interpreting PAM fluorescence gross production estimations from higher light intensities.