Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa
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Fast Repetition Rate Fluorometry

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To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Sampling Procedure

Day and night time FRRf profiles were made using a Chelsea FASTtracka Dynamic Photosynthetic Fluorometer. The FRRf was part of an instrument package that also included a Sea-Bird CTD, WET Labs ECO-BBF2 Triplet (measuring Chlorophyll, CDOM & Phycoerythrin), a WET Labs AC-9, a WET Labs AC-S and a Sequoia Laser In-Situ Scattering and Transmissometry (LISST-100X) instrument. The FRRf was oriented horizontally and lowered at a more-or-less constant speed of 10 m/min to a bottom depth of approximately 200m. This speed was empirically determined to better resolve fluorescence response of small scale (~ 0.5m) photoautotrophs assemblages and to allow enough time for the gain switch of the instrument without losing significant vertical resolution (Corno et. al. 2005). The sampling protocol of the FRRf was set to an acquisition sequence of 100 saturation flashes, 20 relaxation flashes and 10 m/sec sleep time between acquisitions. The flash duration was of 0.65 µ sec (4 instrument units). This sampling protocol was found to better characterize the fluorescence response (i.e. saturation curve fitting) of this specific oceanic waters in preliminary tests (G. Corno unpublished data). Depth and in situ irradiance (PAR) were also logged with each profile.

Results

In the North Pacific Subtropical Gyre (NPSG), aperiodic fluctuations in primary productivity are difficult to capture using the in situ 14C-primary production measurements. In order to determine photosynthetic properties controlling variability in primary production, in situ, time-series measurements of Fast Repetition Rate Fluorometry (FRRF) have been conducted. Photosynthetic efficiency, as indicated by Fv/Fm (left panel), was high and constant through the water column (averaging 0.60 ± 0.10), exceeding the theoretical maximum (i.e. > 0.65) below the Deep Chlorophyll Maximum Layer (DCML) (0.75 ± 0.10) and in some discrete layers between 40 and 70m. Averaged Fv/Fm through the mixed layer were linearly related to mixed layer depth (MLD), suggesting the influence on photosynthetic activity by possible nutrient injections.

The FRRF-derived initial slope of the P vs. E curve (α) (center panel) was approximately six times lower at the surface than at the DCML, highlighting the presence of high and low light adapted populations. The derived ΦC (quantum yield of photosynthesis) was low (0.0016 mol C mol quanta-1) throughout the year, with maxima in the DCML region. ΦC was significantly related to changes in functionally reaction centers (linear) and in α (exponential), and to a lesser extent to Fv/Fm (linear). Significant (Δ = ± 100%) daily variations in primary productivity (right panel), driven by changes in α, were also found in a couple cruises.

These results show a high photosynthetic efficiency in this oligotrophic region, highlighting that photoautotrophs may have successfully optimized their photosynthetic apparatus to the low nutrient environment. The absorption of light (α), and not the efficiency of light utilization (Fv/Fm), appears to be the physiological parameter driving ΦC variations, daily productivity and to some extent the observed aperiodic variation in the NPSG. (Corno et. al. 2005)

This study is a collaborative effort between the College of Oceanic and Atmospheric Sciences of Oregon State University and the Laboratory for Microbial Oceanography of the University of Hawai'i at Manoa.