NITROGEN-DEPENDENT AND IRRADIANCE-DEPENDENT VARIATIONS OF THE MAXIMUMQUANTUM YIELD OF CARBON FIXATION IN EUTROPHIC, MESOTROPHIC AND OLIGOTROPHIC MARINE SYSTEMS

Natural variability of the maximum quantum yield of carbon fixation (p hi C-max), as determined from the initial slope of the photosynthesis- irradiance curve and from light absorption measurements, was studied a t three sites in the northeast tropical Atlantic representing typical eutrophic, mesotrophic and oligotrophic regimes. At the eutrophic and mesotrophic sites, where the mixed layer extended deeper than the euph otic layer, all photosynthetic parameters were nearly constant with de pth, and phi C-max averaged between 0.05 and 0.03 mol C (mel quanta ab sorbed)(-1), respectively. At the oligotrophic site, a deep chlorophyl l maximum (DCM) existed and phi C-max varied from ca 0.005 in the uppe r nutrient-depleted mixed layer to 0.063 below the DCM in stratified w aters. Firstly, phi C-max was found roughly to covary with nitrate con centration between sites and with depth at the oligotrophic site, and secondly, it was found to decrease with increasing relative concentrat ions of non-photosynthetic pigments. The extent of phi C-max variation s directly related to nitrate concentration was inferred from variatio ns in the fraction of functional PS2 reaction centers (f), measured us ing fast repetition rate fluorometry. Covariations between f and nitra te concentration indicate that the latter factor may be responsible fo r a 2-fold variation in phi C-max. Moreover, partitioning light absorp tion between photosynthetic and non-photosynthetic pigments suggests t hat the variable contribution of the non-photosynthetic absorption may explain a 3-fold variation in phi C-max, as indicated by variations i n the effective absorption cross-section of photosystem 2 (sigma(PS2)) . Results confirm the role of nitrate in phi C-max variation, and emph asize those of light and vertical mixing. Copyright (C) 1996 Elsevier Science Ltd