Oceanography | Vol. 38, No. 2
66
DIY OCEANOGRAPHY
THE PIXIE
A LOW-COST, OPEN-SOURCE, MULTICHANNEL IN SITU
FLUOROMETER APPLIED TO DYE-TRACING IN HALIFAX HARBOR
By Kyle Park, Dariia Atamanchuk, Aaron MacNeill, and Vincent Sieben
INTRODUCTION
Submersible, or in situ, fluorometers are devices used in fresh
water and marine environments to measure the presence of
compounds (fluorophores) that fluoresce when exposed to spe
cific wavelengths of light. These measurements can be used,
for example, as indicators of water quality, contamination, and
flow dynamics. The earliest submersible fluorometers (Wheaton
et al., 1979) were designed with a single channel (i.e., measur
ing fluorescence at a specific wavelength while rejecting the
rest of the spectrum). However, the presence of multiple fluo
rescent species in natural waters makes it sometimes challeng
ing to attribute the measured signal to a single compound with
certainty due to spectral overlap, so multichannel fluorometers
have been employed more recently.
Climate change and its associated impacts are increasing
the demand for high-resolution monitoring of the environ
ment using optical sensors that enable fast detection and are
small enough to be integrated into mobile platforms. For exam
ple, harmful algal blooms (HABs) can cause billions of dol
lars in direct damages to fisheries (Davidson et al., 2020) and
fishery-dependent communities (Weir et al., 2022), and reduce
the socioeconomic value of recreational areas (Mardones et al.,
2020). Preventative and mitigative actions can be taken if
warning signs, such as the concentrations of the fluorophores
chlorophyll a (Chl-a) and phycocyanin (PC) (Shen et al., 2012),
are monitored and detected early (Davidson et al., 2020).
In another example, the assessment of marine carbon diox
ide removal strategies, such as point-source oceanic alkalinity
enhancement, requires a careful understanding of the near-field
dynamics that are studied using dye tracer experiments (Fennel
et al., 2023). These experiments use fluorescent rhodamine
water tracer (RWT) dye to make spatio-temporal measure
ments of dye plume dispersion. In another example, petroleum-
derived contaminants such as crude oil can be detected using
ultraviolet fluorometry. Overall, the scope and scale of human
activity put enormous pressure on the global ocean and water
ways, thus warranting the development and improvement of
autonomous sensors, including fluorometers, for improved
monitoring and response.
Access to this technology as well as to the education required
to take advantage of it, both currently dominated by high-
income countries, is a challenge recognized by the United
Nations Decade of Ocean Science for Sustainable Development
(2021–2030) (Harden-Davies et al., 2022). The current price
of relevant industrial, single channel, in situ fluorometers is
$3,400–$7,800 USD (Park et al., 2023). Industrial multichannel
systems such the three-channel RBRtridente (RBR Ltd., Ottawa,
Canada), Turner C3 (Turner Designs, San Jose, CA), or ECO
Puck (Sea-bird Scientific, Bellevue, WA) have price and perfor
mance characteristics comparable to the single channel devices
on a per-channel basis. To improve access and the use of sensor
technology, the documentation on some oceanographic devices,
their construction, use, and handling have been released to
the public as open source (Butler and Pagniello, 2021; see also
https://tos.org/diy-oceanography for additional open-source
instrument projects published in Oceanography).
ABSTRACT. Fluorometers are ubiquitous tools in the fields of oceanography, limnology, and water quality assessment. Fluorescent
species in our waters range from in vivo chlorophyll, contaminants like crude oil, or intentionally added agents like rhodamine.
Submersible in situ fluorometers can collect real-time data at scales that cannot be matched by discrete bottle samples with lab/
shore-side analysis. However, accessibility of sensors remains a problem recognized by the United Nations Sustainable Development
Goals. Here, we introduce the PIXIE, an open-source, multichannel, in situ fluorometer that performs high-quality fluorometry
at a low cost. The PIXIE is assembled by simple means from almost entirely off-the-shelf components. The few necessary custom
parts are either easily outsourced or printed by consumer-grade 3D printers. The PIXIE draws an average of 225 mW during mea
surement and has been tested to depths of 45 m. It has been calibrated to demonstrate a limit of detection 0.01 ppb rhodamine WT
(a fluorescent dye) in a range up to 60 ppb, and a limit of detection of 0.02 ppb chlorophyll a. The PIXIE has been deployed as part of
a dye-tracer experiment in Halifax Harbor, Canada, demonstrating its performance in a quasi-simultaneous profiling of rhodamine
WT dye and chlorophyll a.