June 2025

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June 2025 | Oceanography

75

PHYTOPLANKTON

Phytoplankton are photosynthetic single-celled microscopic

algae. They are primary producers that form the base of food

webs in aquatic ecosystems and play a key role in the global car­

bon cycle. Although phytoplankton only make up 0.06% of the

global primary producer biomass, they are responsible for nearly

half of Earth’s primary production (Stoer and Fennel, 2024).

Nearly all marine organisms rely directly or indirectly on the

organic matter or oxygen produced by phytoplankton through

photosynthesis. The key roles phytoplankton play in ocean eco­

systems and global biogeochemical cycles make phytoplankton

an essential component of oceanographic studies, from food

web processes to climate change.

HOW TO MEASURE PHYTOPLANKTON BIOMASS

USING A FLUOROMETER

Chl-a is commonly used as a proxy for phytoplankton biomass,

as all photosynthetically active phytoplankton use Chl-a as a pig­

ment to produce organic matter through photosynthesis. While

Chl-a concentration is relatively easy to quantify, Chl-a should

always be considered cautiously as a proxy for phytoplankton

biomass because of:

• Species Variability. Different phytoplankton species have

varying Chl-a concentrations per unit of biomass (often

expressed as C:Chl-a ratio, Geider, 1987; Smyth et al., 2023).

• Environmental Factors. Light availability, nutrient concentra­

tions, and other environmental conditions can influence and

rapidly change the amount of Chl-a phytoplankton cells con­

tain (Graff et al., 2015; Jakobsen and Markager, 2016).

• Phytoplankton Physiology. The growth and physiologi­

cal state of phytoplankton can affect Chl-a concentration

(Geider, 1987).

• Other Pigments. Not all phytoplankton rely solely on Chl-a.

Some species use different pigments for photosynthesis, and

pigments can interfere with fluorescence profiles.

Chl-a molecules fluoresce in the red wavelengths (695 nm)

due to higher absorption of light by Chl-a at the 460–470 nm

(blue) wavelength (Figure 1, Ocean Optics Web Book). Therefore,

Chl-a concentration can be quantified by measuring the emitted

fluorescence, with the intensity of the fluorescence signal being

proportional to the concentration of Chl-a pigment. The fluores­

cence intensity is first measured in volts (V) by the fluorometer

and then converted into Chl-a concentration using a set of coef­

ficients from calibrations performed by the manufacturer. Given

the sensitivity of fluorescence to ambient conditions, (e.g., light;

Graff et al., 2015), fluorescence may not be a reliable indicator of

actual Chl-a concentration.

Digression. A demonstration of these principles can be done

with a blue laser (e.g., pointers <$10 online) and a coastal water

FIGURE 1. (a) Illustration of the chlorophyll a (Chl-a) fluorescence principle and the

functioning of a fluorometer, representing the interactions between the pigment,

the light used for excitation, and the sensor used for detection of the emitted red

fluorescence. The (b) absorption and (c) fluorescence spectra of Chl-a in diethyl

ether (Dixon et al., 2005) are represented. Note the offset between absorption and

fluorescence peak wavelengths (EX: 420 nm and EM: 670 nm) in diethyl ether and

the wavelengths used by the fluorometer to detect Chl-a fluorescence in vivo in sea­

water (EX: 460–470 nm and EM: 695 nm).

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