Natasha Christman: The Phytoplankton Census

With such a great, fundamental role in the biological structure of the ocean, plankton are always sought to be understood. While they account for less than 1% of the Earth's plant biomass, phytoplankton are responsible for over 50% of the world's plant primary production and 95% of the ocean's primary production. Consequently, when monitoring water quality and investigating hypoxia along the way, knowing the quantity of phytoplankton present is important to get a better understanding of the delicate biological and chemical relationships found underneath the surface. But phytoplankton are microscopic, and unfortunately they are not capable of performing a reliable population census themselves. There can be an enormous number of individuals in one drop of water. So how do we measure the concentrations of plankton present?

Various phytoplankton in a water sample from our research station
near Eliza Island. 

The answer begins with a simple component of all plant life- the photosynthetic pigment chlorophyll a. Chlorophyll a is the primary pigment of photosynthesis, and it absorbs blue wavelengths of light while emitting (fluorescing) red light. Using this property to our advantage, we can filter a sample of water at a fixed volume and isolate the cell particles on a filter pad in the field. The filter is then dropped into a tube of 10ml of 90% acetone, and this solvent extracts the chlorophyll pigment from the cells with minimum alteration of pigment. After a bit more preparation back at the lab, this samples can be put through a device called a fluorometer. The fluorometer measures the chlorophyll pigments by hitting the acetone and extracted pigments with a beam of blue light. By measuring the amount of red light fluoresced back, the concentration of phytoplankton can be measured by its proportional relationship to emitted light.

The fluorometer in the chemistry lab, complete with
cat stickers (and no, they were already there).

When calculated, this data is a valuable contribution to the overall picture of the bay we are constructing. Comparisons of concentrations across transects can be made and crossed with other data collected, like the water column profiles, to further illuminate the research.

A graph of the chlorophyll concentrations at each station
visited on an early summer cruise.

Analyzing chlorophyll pigments on the fluorometer is a valuable tool and major contributor to our research this summer. It makes efficient a task that would be overwhelming, if not impossible on an accurate scale. Additionally, these measurements over time can describe the cycles of the plankton over generations and how these patterns coincide with other aspects of water quality.