June 2025 | Oceanography
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Chl-a concentrations were generally higher in inner shelf
waters (northern half of the transect) than in the outer shelf
waters (southern half of the transect). This difference can be
attributed to the shallower depth and greater influence of coastal
inputs in the inner shelf region, which result in more nutrients
for phytoplankton growth. In contrast, the outer shelf waters are
more oligotrophic, similar to some open ocean regions.
During summer, nitrate (an essential nutrient for phyto
plankton growth) is completely depleted in the surface waters
of the NES, indicating that phytoplankton growth is likely based
on remineralized nutrients through the microbial loop, favoring
the growth of small phytoplankton cells (Marrec et al., 2021).
However, in the summer of 2019, an intense bloom of large
FIGURE 4. Discrete Chl-a concentrations (mg m–3) plotted against
the matching fluorescence values from (a) the ECO-Fl fluorome
ter (mg m–3), and (b) the WetStar fluorometer (mg m–3) during the
EN644 summer 2019 cruise (August). The green dashes repre
sent the line of best fit from a model I linear regression, with the
equation, including the slope and intercept, shown as an insert
on each figure. The shaded green area represents the 95% confi
dence interval obtained for the linear regression model.
be used for the post-calibration of the fluorometer. Some basic
statistical concepts such as linear regression will be introduced.
Note that on most oceanographic cruises, only one fluorome
ter is available to record underway fluorescence, meaning that
selection of one of two fluorometers is not possible.
PART 2. POST-CALIBRATION TO ESTIMATE CHL-a
CONCENTRATION FROM IN VIVO FLUORESCENCE
Goals. Post-calibrate the underway fluorescence data by apply
ing the relationships established in Section 2, Part 1, between
the raw fluorescence measurements and the discrete Chl-a con
centration data. Compare the raw fluorescence values with the
post-calibrated data collected during the three summer cruises
and interpret the resulting figures.
Expected Outcomes. Gain insight into the significance of post-
calibrating raw fluorescence data for analyzing the inter-annual
variations in phytoplankton biomass within a highly dynamic
coastal ecosystem.
Narrative. After identifying the best suited fluorometer, the
goal is to apply the relationship obtained from the linear regres
sion to the continuous underway measurements for each cruise
(Figure 2 and in online supplementary materials) and ulti
mately to create a new data package that includes all these post-
calibrated measurements to share with the scientific community.
We also present here an illustration of why post-calibration of
fluorescence data is essential (Figure 5) and invite the students
to interpret the results obtained by comparing post-calibrated
fluorescence among three summer NES-LTER cruises.
When looking at the data from the three summer NES-LTER
cruises together, the first observation is that the fluorescence sig
nal in 2019 has a much higher magnitude and is more variable
and “noisy” compared to the signals from the summers of 2020
and 2021. Based on the raw fluorescence values, the concentra
tion of Chl-a was higher, indicating higher phytoplankton bio
mass in the surface waters of the NES in 2019 than in 2020 and
2021. Additionally, there seemed to be higher concentrations of
Chl-a in surface waters along the 2020 transect than in 2021.
The fluorescence signal in 2019 remains more variable and
higher than during the other two cruises after post-calibration.
Interestingly, while the raw fluorescence data suggested more
Chl-a in 2020 than in 2021, post-calibration revealed that the
Chl-a concentrations were actually very similar. This under
scores the importance of post-calibration when comparing
fluorescence values from different cruises.
Some essential background information about the oceano
graphic context of the NES may be helpful for instructors to inter
pret the data obtained. To support this, we included an introduc
tion to the seasonal dynamics of the phytoplankton community
in NES waters in a dedicated section of the Lab Instructions doc
ument, available in the supplementary materials.