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Two methods of measuring primary production, modulated fluorimetry (PAM) and the traditional carbon incorporation method ( 13 C), were compared in four phytoplankton species, two diatoms ( Pseudo-nitzschia pungens  and  Asterionellopsis glacialis) , and two dinoflagellates ( Heterocapsa sp  and  Karenia mikimotoï ), under N (nitrogen), P (phosphorus) and Si (silicon) limited semi-continuous culture. N and Si-limited cultures showed relatively high quantum efficiency of the PSII (F v /F m ) values, confirming that F v /F m  is not a good proxy for nutrient stress in balanced systems, whereas P limitation had a drastic effect on many physiological parameters. In all species, the physiological capacity of phytoplankton cells to acclimate to nutrient limitations led to changes in the cellular biochemical composition and the structure of the photosynthetic apparatus. The observed physiological responses were species and nutrient specific. The values of the chlorophyll-specific absorption cross section (a*) increased with nutrient limitation due to package effect, while the carbon/Chl  a  ratio was higher under N and P limitations. In diatoms, Si limitation did not affect photosynthesis confirming the uncoupling between Si and carbon metabolisms. In all four species and under all treatments, significant relationships were found between photosynthetic activities, ETR Chl  (electron transport rate) and P Chl  (carbon fixation rate) estimated using PAM measurements and  13 C incorporation, showing that the fluorescence technique can reliably be used to estimate carbon fixation by phytoplankton. The relationship between ETR Chl  and P Chl  can be described by the shape and the slope of the curve (Φ C.e ). Linear relationships were found for dinoflagellates and  P. pungens  under all treatments. A decrease in Φ C.e  was observed under N and P limitation probably due to structural damage to the photosynthetic apparatus.  A. glacialis  showed a logarithmic relationship in N and P limited conditions, due to the alternative electron flow which takes place to optimise photosynthetic performances under high light and/or nutrient stress.

Influence of Nutrient Stress on the Relationships between PAM Measurements and Carbon Incorporation in Four Phytoplankton Species