June 2025 | Oceanography
77
(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).