Effect of water column light gradient on phytoplankton fluorescence transients

The light management of phytoplankton can be assessed in situ on time scales shorter than those of the non-photochemical fluorescence quenching (q(n)) mechanisms. We adopted fast repetition rate fluorometry to water column studies in the northwest Pacific Ocean and its adjacent shelf seas. Near-surface depression of the photochemical energy conversion efficiency (PECE) (effective F-q'/F-m' and maximum F-v'/F-m'; F-q'= F-m' - F', F-v'= F-m'- F-o'; F-q'and F-v'are variable fluorescence yields in the light, F-v' is that with maximum photochemical quenching; fluorescence yields in the light: F-m' maximum, F' steady-state, and F-o' minimum) defined specific zones according to whether PECE was depressed by photochemistry (relaxation of photochemical quenching, q(p)) or photoprotection/photoinhibition (increase of q(n)). q(p) seemed to be the main factor depressing PECE, and the depressing effect of qp on PECE also extended to considerably deeper depths than that of q(n). When moving towards the surface, the qn effect overrode the qp effect on the PECE decrease at depths of 8 to 13 m, depending on the station. The vertical trends of F-q'/F-m' and F-v'/F-m' were modelled according to a typical P-E (photosynthesis -irradiance) dependence to supplement the C-14-based P-E data. E(F-q'/F-m') and E(F-v'/F-m') were the fight levels at which the vertical trends of F-q'/F-m' and F-v'/F-m', respectively, started to decrease. Although the nutrient regime is the main controller of primary photochemistry in general, the ambient light also becomes the controlling factor on PECE whenever the light level rises above E(F-q'/F-m'). At E(F-v'/F-m'), light ultimately overrides the effect of the nutrient status on PECE. E(F-v'/F-m') also marks the light level at which the photoprotective measures first become necessary. This level is close to the onset of the plateau phase producing the C-14-based light-saturated photosynthetic rate P,,,,X.