Diel quenching of Southern Ocean phytoplankton fluorescence is related to iron limitation

• Published in: Biogeosciences 2020-02, Vol.17 (3), p.793-812
• Main Authors: Schallenberg, Christina, Strzepek, Robert F, Schuback, Nina, Clementson, Lesley A, Boyd, Philip W, Trull, Thomas W
• Format: Article
• Language: English
• Subjects: Analysis; Availability; Carbon fixation; Chlorophyll; Chlorophyll a; Drifters; Efficiency; Floats; Fluorescence; Fluorimeters; Fluorometers; Incubation period; Iron; Irradiance; Mooring; Novels; Ocean models; Oceans; Optical communication; Optical equipment; Parameterization; Parameters; Photochemicals; Photochemistry; Photosynthesis; Physiology; Phytoplankton; Plankton; Primary production; Production management; Productivity; Quenching; Signal processing; Time; Vortices; Water; Water masses; Working conditions
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-17-793-2020

We present data from a research voyage to the Subantarctic Zone south of Australia. Incubation experiments confirmed that resident phytoplankton were Fe-limited, as the maximum quantum yield of primary photochemistry, F.sub.v /F.sub.m, measured with a fast repetition rate fluorometer (FRRf), increased significantly with Fe addition. The NPQ "capacity" of the phytoplankton also showed sensitivity to Fe addition, decreasing with increased Fe availability, confirming previous work. The fortuitous presence of a remnant warm-core eddy in the vicinity of the study area allowed comparison of fluorescence behaviour between two distinct water masses, with the colder water showing significantly lower F.sub.v /F.sub.m than the warmer eddy waters, suggesting a difference in Fe limitation status between the two water masses. Again, NPQ capacity measured with the FRRf mirrored the behaviour observed in F.sub.v /F.sub.m, decreasing as F.sub.v /F.sub.m increased in the warmer water mass. We also analysed the diel quenching of underway fluorescence measured with a standard fluorometer, such as is frequently used to monitor ambient chlorophyll a concentrations, and found a significant difference in behaviour between the two water masses. This difference was quantified by defining an NPQ parameter akin to the Stern-Volmer parameterization of NPQ, exploiting the fluorescence quenching induced by diel fluctuations in incident irradiance. We propose that monitoring of this novel NPQ parameter may enable assessment of phytoplankton physiological status (related to Fe availability) based on measurements made with standard fluorometers, as ubiquitously used on moorings, ships, floats and gliders.