MEPS 376:85-101 (2009)  -  DOI: https://doi.org/10.3354/meps07759

ABSTRACT: 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 q_p on PECE also extended to considerably deeper depths than that of q_n. When moving towards the surface, the q_n effect overrode the q_p 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 ¹⁴C-based P–E data. E(F_q’/F_m’) and E(F_v’/F_m’) were the light 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 ¹⁴C-based light-saturated photosynthetic rate P_max.

KEY WORDS: Pacific Ocean · Phytoplankton · FRR · Fluorescence · Quenching · Photochemical