June 2025

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June 2025 | Oceanography

69

et al., 2020) equipped with a Cyclops-7F rhodamine fluorome­

ter (Turner Designs). Discrete water samples were later analyzed

in the laboratory using a benchtop fluorometer. The Niskin bot­

tle was lowered into the water column and triggered at depths

ranging from 0.5 m to 45 m. The sampling was conducted on a

release day, a pre-release day (1 day prior) and a post-release day

(1–3 days later). The PIXIE captured the vertical RWT/Chl-a con­

centration profiles to demonstrate its multichannel functionality,

while the Niskin bottle provided a ground-truth measurement

of the RWT concentration at the surface and above the seafloor.

Data collected by each method were used to semi-quantitatively

validate the performance of the PIXIE as an in situ fluorometer.

RESULTS AND DISCUSSION

Calibration

A total of 37 data points was collected during the calibration of

the PIXIE’s RWT channel. These consisted of six standard con­

centrations across six temperature setpoints. The 60 ppb mea­

surement at 5°C saturated the device. A replacement 6.9°C tem­

perature set point was also collected. A total of 37 data points

were collected during the calibration of the Chl-a channel. The

80 ppb measurements at 5°C and 8°C saturated the device. A

replacement 9.6°C temperature set point was also collected.

The raw fluorescence data for each data point were collected

at the PIXIE’s maximum sample rate of 16 samples per second.

In line with previous work (Park et al., 2023), the raw data were

downsampled through a moving average of 16 samples, for an

effective sample rate of 1 sample per second. Fifteen minutes

of raw data were collected for each data point. During the last

five minutes, 300 samples were collected, and the mean of these

300 samples was taken as the calibration data point. The prior

10 minutes of data were inspected to ensure an apparent equilib­

rium fluorescence had been reached.

The six standard concentrations for each fluorophore

were used to generate a best-fit line for each temperature (see

Figure 3). In the 60 ppb RWT case, a 5°C data point was first

extrapolated from the six unsaturated temperature set points

(6.9°C as well as the original five). In the 80 ppb Chl-a case,

5°C and 8°C were first extrapolated from the five unsaturated

temperature set points (9.6 °C as well as the original four). The

parameters of the resulting curves do not change significantly

between the inclusion or exclusion of the extrapolated points.

The coefficient of determination (R-squared) for each calibra­

tion curve exceeded 0.99 for RWT and 0.98 for Chl-a.

FIGURE 3. (a) Rhodamine water tracer (RWT) calibration curves, with a dashed line indicating saturation. The extrapolated point is encircled. Inset: Plot

with concentration-equivalent 10-sigma error bars. (b) RWT temperature compensation “slope of slopes” curve. (c) Chl-a calibration curves, with a

dashed line indicating saturation. Extrapolated points are encircled. Inset: Plot with concentration-equivalent 1-sigma error bars, illustrating sensitivity to

bubbles. (d) Chl-a temperature compensation “slope-of-slopes” curve.

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