June 2025

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June 2025 | Oceanography

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(multiple, non-concatenated, .raw files), which are basically text

with separations (e.g., commas, but also tabs) between columns,

and without column headers. As part of the NES-LTER proj­

ect, curated 1-min temporal resolution data, including all nav­

igation and meteorological and oceanographic measurements,

can be accessed through the NES-LTER REST API in a comma-​

separated values (.csv) format that also includes all the column

headers. To facilitate access, we provide the underway data as

supplemental .csv files for several cruises as downloaded from

the NES-LTER REST API at the time this article was written.

The data show that the two fluorometers recorded slightly dif­

ferent values during each cruise but generally followed a sim­

ilar pattern (Figure 2 and in online supplementary materials).

During the winter 2021 (February) EN661 NES-LTER cruise,

the WetStar fluorometer was malfunctioning during the first two

days of the cruise, as indicated by the major differences observed

when comparing the two fluorometer values (Figure 2). A clean­

ing of the WetStar fluorometer was performed during the cruise

once the problem was identified, resulting in a better match

of the sensors afterward. We included these data here because

such technical problems occur frequently and highlight the

importance of cleaning oceanographic instruments before each

deployment, and also the importance of real-time monitoring of

the sensors’ displays during a cruise. The difference between the

two fluorometers appears to follow a diel cycle (Figure 2c and in

online supplementary materials), with a larger difference during

daylight hours, highlighting the fact that instruments measuring

the same parameters can produce different data and that those

deviations can be modified by external influences. This diel pat­

tern might be linked to non-photochemical quenching of Chl-a

molecules during the daytime (Marra, 1998; Xing et al., 2012),

when light intensity is high, with one of the instruments being

more sensitive than the other to this process.

PART 2. DISCRETE DATA FOR EXTRACTED CHL-a

Goal. Access and analyze authentic discrete Chl-a data, followed

by preliminary interpretation. Gain familiarity with the dataset

required for Section 2 of this lab activity.

Expected Outcomes. Build an understanding of discrete Chl-a

data, including how they are collected, the uncertainties associ­

ated with discrete sampling, and the quality control procedures

applied. Conduct basic statistical analyses (e.g., averages, stan­

dard deviations) and interpret the resulting data.

Narrative. Discrete Chl-a data have historically been collected

during oceanographic cruises, primarily from water sampled

throughout the water column using Niskin bottles mounted on

a CTD-Rosette. The general procedure for discrete Chl-a mea­

surements involves filtering a known volume of seawater to

retain all phytoplankton cells on the filter, extracting the Chl-a

retained on the filter with a solvent, and then quantifying the

amount of Chl-a in the solvent by fluorescence. Additionally,

high-performance liquid chromatography (HPLC) can be used

to quantify Chl-a concentration. These methods for sampling,

filtering, extracting, and quantifying are relatively simple and

can be performed as a lab activity, depending on resources avail­

able to students.

During NES-LTER cruises, discrete Chl-a samples for the

calibration of the underway fluorometers were collected from

a spigot connected to the underway system so that the samples

contained water that had just run through the two fluorometers

(Menden-Deuer et al., 2022). Additional discrete Chl-a samples

are routinely collected from the Niskin bottles mounted on the

CTD-Rosette at each sampling station at various depths (Sosik

et al., 2023), including at the surface (3–7 m depth). While these

additional data could be used to post-calibrate the underway

FIGURE 2. Examples of (a) underway raw fluorescence (in volts, V), and

(b) manufacturer calibrated fluorescence (mg Chl-a m–3) recorded by the

WetStar (dark green) and the ECOFl (light green) fluorometers vs. time

during the EN661 Northeast US Shelf Long-Term Ecological Research

(NES-LTER) cruise in winter 2021. (c) Difference of the manufacturer-​

calibrated fluorescence signals between the two fluorometers

(mg Chl-a m–3), with light green shaded area representing nighttime (here

defined from 7 p.m. to 7 a.m. local time).

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