June 2025

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June 2025 | Oceanography

71

less for Chl-a. This would slightly overestimate the RWT con­

centrations in deeper/colder water, but no RWT was detected

from the bottom-​depth Niskin bottle samples.

The map provided in Figure 4a illustrates the GPS paths of the

diving vessel Eastcom used for the deployment. The labeled sites

indicate the profile locations: Stations 1, 2, and 3 in Figure 4b,

4c, and 4d, respectively. The bright colored curves indicate mea­

surements while downcasting, whereas the darker dot-dashed

curves indicate upcasting. The results from the Niskin bottle sam­

ples are indicated with black asterisks. The bottom-depth Niskin

bottle samples were found to have no detectable RWT across the

entire data set, in agreement with the PIXIE measurements.

The profiles in Figure 4b were captured on August 8, 2023,

two days prior to dye release. No dye was detected while some

Chl-a was detected at Station 1, the location of the DRDC

(Defence Research and Development Canada) Atlantic Acoustic

Calibration Barge. The vertical Chl-a profile measured by the

PIXIE shows qualitative agreement with historical observations

(Giesbrecht and Scrutton, 2018). The Chl-a concentration maxi­

mized by 10 m depth and returned to zero/background by 15 m

depth on downcast, though the significant difference in time of

day, season, and year confounds their quantitative comparison.

The upcast profile appears in sharp contrast to the presented

and historical downcast profiles. Because this station achieves

the greatest cast depth of 45 m among the dataset, this discrep­

ancy between downcast and upcast may indicate a pressure hys­

teresis effect (Shigemitsu et al., 2020) that is uncharacterized and

warrants future investigation.

The profiles in Figure 4c were captured on August 10, 2023,

during the RWT release at Station 2, directly in front of the

Tufts Cove Power Generating Station effluent where the RWT

was released. The RWT channel saturates immediately below

the surface (>82.3 ppb RWT), confined to an apparent stratum

between 1 m and 5 m depths. This indicates a subduction of the

RWT plume that can be confirmed visually in Figure 1, but the

exact mechanism of this stratification is beyond the scope of

this article. The surface-depth Niskin bottle sample recorded an

RWT concentration of 217.8 ppb, clearly in excess of the PIXIE’s

saturation limit. A second, near-surface bottle sample (3.7 m)

recorded a concentration of 10.7 ppb, in good agreement with

the PIXIE’s measurement of 14.5 ppb. The discrepancy between

bottle and PIXIE measurements at this depth could be attributed

to the difference in interrogated volume at this point. The profile

suggests that the bottle sample was taken at the edge of a steep

RWT gradient. The point sampling of the PIXIE’s measurement

is therefore more sensitive to depth than the ~1 m concentration

gradient over which the Niskin bottle averages.

The profiles in Figure 4d were captured on August10, 2023,

three hours later at Station 3, along the anticipated path of the

RWT plume. The RWT channel detected a weaker but cer­

tainly present signal (10 ppb) in the first 2 m and returned to

zero by 5 m depth. The surface Niskin bottle sample recorded

an RWT concentration of 15.7 ppb, in modest agreement with

the PIXIE’s measurement. The Chl-a channel shows a simi­

lar characteristic to that observed at the previous station, with

no apparent dependence on the presence/absence of the large

(>200 ppb) RWT plume.

To further validate the performance of the PIXIE, the

August 10, 2023, profile at Station 3 can be compared to the near­

est RWT transects captured by the ecoCTD, occurring just after

the Niskin bottle samples were collected. See the online supple­

mentary materials for a summary of the comparison of the two

sets of profiles along with a waterfall plot (see Figure S2).

CONCLUSIONS

The PIXIE is a low-cost, open-source, multichannel fluoro­

meter that demonstrates performance comparable to indus­

try standards. It can be assembled at a cost to the end user of

$741.38  USD per channel on average, and alternate configu­

rations can be even less expensive. While this cost should not

be compared to the internal cost-per-unit of industrial in situ

fluorometers and the end user must consider the value of the

support and quality assurance industrial devices enjoy, the

PIXIE nevertheless represents an open-source option with sim­

ilar performance and a low barrier to entry. The PIXIE’s limit of

detection is 0.01 ppb RWT and 0.02 ppb Chl-a, which is on par

with other in situ fluorometers. The PIXIE was successfully field

deployed and validated as a part of a dye-tracer experiment in

Halifax Harbor. The full availability of the PIXIE’s source files,

from hardware to firmware, allows the end user to customize

the PIXIE as much or as little as desired. The PIXIE makes a

transformative leap in accessibility that can meet the growing

demands for spatio-temporal data from our planet’s waterways,

without sacrificing measurement quality.

POSSIBLE FUTURE DEVELOPMENT

A road map of future work is proposed within the PIXIE

Complete User Guide available on the GitHub project page.

Hysteresis has been identified (Briggs et al., 2011; Cetinić et al.,

2012) as a common problem in fluorometers and similar in situ

devices, and the degrees along which the PIXIE exhibits it should

be studied explicitly. With some modifications to the front end,

the PIXIE could include turbidity and backscattering as poten­

tial channel types along with its current fluorometric channels.

Internal temperature sensing can be integrated through firm­

ware, and external (in situ) temperature sensing could be per­

formed in place of one of the fluorometric channels with only

minor hardware changes. More details toward each of these pro­

posed areas of future work can be found on GitHub.

SUPPLEMENTARY MATERIALS

The supplementary materials are available online at https://doi.org/10.5670/

oceanog.2025.309. To access the PIXIE fluorometer files on GitHub, go to:

https://github.com/KylePark0/PIXIE/tree/main.

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