June 2025

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Oceanography | Vol. 38, No. 2

70

To compensate measurements for the temperature of the

sample volume, the slopes of each fit line are plotted, and a best-

fit line is calculated. From the parameters of this fit, the all-cause

temperature exponent can be approximated. The PIXIE achieved

an approximate temperature exponent of –0.019°C–1 for RWT,

consistent with previous results (Park et al, 2023). For Chl-a, the

approximate temperature exponent obtained was –0.008°C–1,

which is consistent with equivalent temperature parameters for

Chl-a from the literature (Watras et al., 2017). The coefficient of

determination for these curves exceeded 0.99.

The standard deviation of each 0 ppb (blank) data point was

calculated. Applying the calibration curves to these blank stan­

dard deviations, the limit of detection was taken as three times

the worst-case standard deviation across blanks. The limits of

detection were found to be 0.01 ppb for RWT and 0.02 ppb for

Chl-a. The upper limit of the PIXIE’s RWT detection range was

found to be 58.9 ppb at 5°C, whereas it was 87.7 ppb at 20°C.

For Chl-a, the upper limit of the detection range was found to

be 78.8 ppb at 5°C, whereas it was 90.2 ppb at 20°C. Further

details about these results can be found in the PIXIE Complete

User Guide on at the GitHub link. Bubbles were observed during

two of the 40 ppb Chl-a calibration trials, resulting in anoma­

lously high standard deviations in those data sets as the bubble

periodically stirred through the sample volume. The inset plot of

Figure 3c illustrates 1-sigma error bars on the 40 ppb, 5°C data

point. This contrasts with the 10-sigma error bars on the 30 ppb,

5°C RWT data point illustrated in the inset plot of Figure 3a.

To directly compare the cross-sensitivity of the two chan­

nels to the opposite fluorophore, the PIXIE’s RWT channel mea­

sured a standard of Chl-a and vice-versa. A complete descrip­

tion of the comparison and its results (Figure S1) is available in

the online supplementary materials.

Field Deployment

Figure 4 depicts the first deployment of the PIXIE in August 2023

during the dye tracer release experiment. Three key stations’

worth of collected data are presented. Pre- and post-​release data

are provided to illustrate the PIXIE’s multichannel functional­

ity. The pre-release station was chosen based on depth and on

the availability of historical Chl-a profile data (Giesbrecht and

Scrutton, 2018), whereas the post-release stations were chosen

for their strong near-surface RWT concentrations as sampled by

the Niskin bottle, so that the measured profile and bottle samples

can be meaningfully compared. The PIXIE data were downsam­

pled to 1 sample per second to parallel the calibration results and

to align with the logger’s depth sensor data. With default gain

settings, the device saturation limits at 18°C are 82.3 ppb and

88.5 ppb for RWT and Chl-a, respectively. The channels’ data

are calibrated assuming a uniform temperature of 18°C. This

temperature is chosen to match the recorded in situ tempera­

ture of the surface Niskin bottle samples, rounded to the near­

est degree. The August surface bottle samples fell within a range

of 16° to 20°C, so the theoretical error in measurement is no

more than 5.1% (Smart and Laidlaw, 1976) for RWT and even

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b

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c

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d

FIGURE 4. (a) GPS paths of Eastcom on August 8, 2023 (violet path) and August 10 (indigo path). Stations 1, 2, and 3 indicate locations of the vertical

profiles in (b), (c), and (d), respectively. Inset: Station 2, magnified, indicates Tufts Cove Power Generation Station, the release site of the RWT. Red text in

the following indicates UTC time of profile start. (b) August 8 (afternoon) pre-release profiles show zero RWT response (top) and a Chl-a profile (bottom).

Solid curves indicate downcast, and dot-dashed curves indicate upcast. (c) August 10 (morning) post-release profiles stationed next to the RWT outflow;

RWT channel saturates near the surface, decently agrees with Niskin bottle sample (black asterisks). (d) August 10 (afternoon) post-release profiles were

made further along the anticipated path of the dye plume. Note modest underestimation of Niskin bottle RWT sample.

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