PHYSIOLOGICAL LIMITATION OF PHYTOPLANKTON PHOTOSYNTHESIS IN THE EASTERN EQUATORIAL PACIFIC DETERMINED FROM VARIABILITY IN THE QUANTUM YIELDOF FLUORESCENCE

On a transect study in the eastern equatorial Pacific, from the high-n utrient, low-chlorophyll tropical waters to the oligotrophic subtropic al waters, we determined the variability in the maximum change in the quantum yield of chlorophyll fluorescence (DELTAphi(m)) by means of a fast repetition rate fluorometer. DELTAphi(m) is a quantitative measur e of photochemical energy conversion efficiency in photosystem 2, the variability of which is determined by the functional organization of t he photosynthetic apparatus. The results revealed that DELTAphi(m) was relatively low throughout the nutrient-rich equatorial waters, provid ing unequivocal evidence for physiological limitation of photochemical energy conversion efficiency in the natural phytoplankton populations . Shipboard enrichment studies showed that DELTAphi(m) increased follo wing addition of nanomolar concentrations of inorganic iron as well as aerosol dust added at similar iron concentrations. This response refl ects an iron-induced repair of photosystem 2 function in the enclosed bottle community. Low DELTAphi(m) at the top of the Equatorial Undercu rrent indicated that the amount of iron upwelled to the surface was in sufficient to repair photosystem 2 function. Our results strongly sugg est that iron availability limits photochemical energy conversion effi ciency and is the principal mechanism controlling rates of photosynthe sis and growth in the nutrient-rich equatorial Pacific.