Oceanography | Vol. 38, No. 2
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diatom cells was observed along the transect. This bloom was
comprised of nitrogen-fixing bacteria living in symbiosis with
a diatom species (Hemiaulus), providing the necessary nitrogen
that was not available as nitrate (Castillo Cieza et al., 2024).
We demonstrated that post-calibrating Chl-a fluorescence
values are essential for accurate comparison, as the calibration
substantially altered the estimated Chl-a concentrations. In this
study, we used fluorescence values from different cruises, where
fluorometers either underwent manufacturer calibrations or
were replaced by spare instruments of the same model. A similar
approach can be applied when comparing fluorescence values
from the same study area but obtained from different research
vessels or other platforms such as moorings, CTDs, or gliders.
Without proper post-calibration, raw Chl-a fluorescence values
cannot be reliably compared.
FEEDBACK FROM STUDENTS AND
RECOMMENDATION TO INSTRUCTORS
The exercise described here was repeatedly tested with stu
dents in class and in self-paced assignments. The major feed
back from students was that they struggled with obtaining the
data from online repositories in reproducible ways. Different
versions of the same spreadsheet tool interpreted dates and
number formatting differently. To accommodate these chal
lenges—which could not easily be alleviated as students may
have many different software types and settings—we have
developed a more explicit step-by-step guide and provided
standardized files for each intermediary step, so students can
access properly formatted files for each step and can avoid lack
of data accessibility or formatting issues. These elements raise
awareness for students as they will certainly encounter similar
challenges related to data management in their own research
or classes. This requirement for troubleshooting often fosters
learning and confidence in the gained competency, as students
overcome obstacles and find solutions independently. As large-
scale open-access databases become increasingly prevalent, the
skills developed through this activity are essential and founda
tional for many researchers.
STUDENT BENEFITS
Our proposed activity offers students a valuable opportu
nity to better understand the limitations of relying on raw,
manufacturer-calibrated Chl-a values, and more broadly, on
any biogeochemical data obtained from sensors. This serves
as a general example of working with calibrated instruments.
Data users may assume that fluorescence-derived Chl-a con
centrations provided by manufacturers represent accurate and
true measurements of Chl-a and possibly by extension, biomass.
However, as demonstrated in this study, this is not the case.
This exercise shows students critical concepts in data valida
tion and underscores the need for quality control by research
ers. This is exemplified by differences between fluorometers with
varying specifications that can lead to discrepancies between
nighttime and daytime measurements (Figure 2c). These vari
ations suggest that non-photochemical quenching (NPQ) of
Chl-a molecules occurs during daylight hours when light inten
sity is high (Marra, 1998; Xing et al., 2012), with some instru
ments being more sensitive to this process than others. Ideally,
FIGURE 5. (a) Map and bathymetry of summer (in Northern Hemisphere) NES-LTER transect cruises from August 2019 (EN644, green), July 2020 (EN655,
light green), and July 2021 (EN668, yellow) from Narragansett Bay, Rhode Island, to the shelf break. (b) Raw underway fluorescence (mg Chl-a m–3) from
each cruise vs. latitude (°N; note the reverse x-axis from higher latitudes in the north [left] to lower latitude in the south [right]). (c) Post-calibrated under
way fluorescence (mg Chl-a m–3) vs latitude. For clarity, only data from the outbound leg of the transect (north to south) are shown.