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).