June 2025

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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.

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