PARAMETERS OF PHOTOSYNTHESIS - DEFINITIONS, THEORY AND INTERPRETATIONOF RESULTS

A global assessment of carbon flux in the world ocean is one of the ma jor undertakings of the Joint Global Ocean Flux Study (JGOFS). This ha s to be undertaken using historical in situ data of primary productivi ty. As required by the temporal and spatial scales involved in a globa l study, it can be conveniently done by combining, through appropriate models, remotely sensed information (chlorophyll a, temperature) with basic information about the parameters related to the carbon uptake b y phytoplanktonic algae. This requires a better understanding as well as a more extended knowledge of these parameters which govern the radi ative energy absorption and utilization by algae in photosynthesis. Th e measurement of the photosynthetic response of algae [the photosynthe sis (P) versus irradiance (E) curves], besides being less shiptime con suming than in situ primary production experiments, allows the needed parameters to be derived and systematically studied as a function of t he physical, chemical and ecological conditions. The aim of the presen t paper is to review the sig nificance of these parameters, especially in view of their introduction into models, to analyze the causes of t heir variations in the light of physiological considerations, and fina lly to provide methodological recommendations for meaningful determina tions, and interpretation, of the data resulting from P versus E deter minations. Of main concern are the available and usable irradiance, th e chlorophyll a-specific absorption capabilities of the algae, the max imum light utilization coefficient (alpha), the maximum quantum yield (phi(m)), the maximum photosynthetic rate (P-m) and the light saturati on index (E-k) The potential of other, non-intrusive, approaches, such as the stimulated variable fluorescence, or the sun-induced natural f luorescence techniques is also examined.