Meet the plankton

Although most are invisible to the naked eye, the ocean is chock-full of microscopic creatures called plankton. One subset of these is called phytoplankton. Phytoplankton are photosynthesizers, just like plants — they take in carbon dioxide, nutrients, and sunlight and make their own energy. They make up the base of the oceanic food chain but they also have an important role to play in regulating how much carbon dioxide the ocean sucks up from the atmosphere. In total, all the phytoplankton in the ocean produce about as much oxygen as all the plants on land do.

Two of the main groups of phytoplankton in the Southern Ocean are coccolithophores and diatoms. Depending on what region you are in, you will see different concentrations of the different types of plankton — coccolithophores are typically found further north in warmer waters, while diatoms are dominant in the frigid waters around Antarctica. Barney Balch described the mixture we’re currently seeing as a “veritable soup” of phytoplankton. He also called it “a party in the Southern Ocean,” which was a great phrase to hear him say.

An image of a microscope slide, magnified. The background is tan and there are many small dots. On the right side of the frame is a cylindrical shaped plankton with spines coming off it to the left.
This microscope image shows just one example of a diatom (the large shape in the middle with spines coming off it). Diatoms come in a stunning array of shapes and sizes!

Coccolithophores are tiny plankton that build houses around themselves out of a mineral called calcium carbonate. These calcium carbonate plates are one way that carbon dioxide gets moved from the ocean’s surface down into the deep. As coccolithophores grow (or when they die), they shed plates. Some of these plates sink all the way down through the ocean intact, burying carbon on the seafloor.

Diatoms also build casings around themselves. However, they use silica, a clear compound, instead of calcium carbonate. Yes, they basically live in glass houses! They’re usually bigger than coccolithophores and they’re better adapted to the cold waters in the very southern parts of the world. We’ve been seeing them over a lot of the water samples we’ve been taking, but in a day or two when we get near 60 °S, that should be pretty much the only thing in the water. They come in an amazing diversity of shapes and sizes, and they’re truly stunning to look at under the microscope.

Two men are standing at the back of a ship. One has his back to the camera and is wearing khaki pants, tall black rubber boots, an orange raincoat, an orange life vest, and a blue hard hat. The other is in profile and is wearing a similar outfit but with a camouflage hard hat and a brown hoodie. In front of them, on a piece of plywood, is a large black fish-shaped piece of equipment with yellow fins.
Scripps research technician Matt Durham and Woods Hole senior scientist Dennis McGillicuddy eye the VPR as they prepare for deployment.

There are three ways we can look at plankton in the water from the ship, and each method has its strengths and weaknesses. I’ve written about the Video Plankton Recorder (VPR) before — it gets towed behind the ship and goes up and down in the water, so we get a nice, continuous look at plankton throughout the upper ocean. One downside to the VPR is that it can’t image the smallest things we’re interested in — it can capture larger diatoms but not coccolithophores. Sometimes we even see fish larvae and parts of jellyfish in it!

Another way to see phytoplankton is called the FlowCam. Instead of towing this through the water, we use the FlowCam to look at samples of water collected from different depths in the ocean. We filter out the bigger plankton and other critters and this allows us to focus on (and count) the small phytoplankton. However, it isn’t always easy to tell what is what in the FlowCam pictures, and analyzing that data will take a lot of time after the cruise is done.

Two women are looking at what appears to be a black suitcase full of electronics. The woman in the center has red hair, a black mask, and a flannel shirt, and she is pointing into the casing. The woman on the left is wearing a purple floral mask, a black t-shirt, and has glasses and dark hair. She is looking on intently.
Back in the days of masks, Bigelow research technician Sunny Pinkham walked Drew University undergrad Hannah Primiano through the FlowCam basics.

The last way that we look at the critters in the ocean is by using a good old-fashioned microscope. This method provides the best look at individual tiny creatures living in the water. By using different types of light in the microscope (such as green light or polarized light), we can see different properties of the phytoplankton, like which ones are photosynthesizing and which ones are made of calcium carbonate.

Two side-by-side microscopy images show a variety of plankton, including a spherical coccolithophore. In the normally lit image, the coccolithophore appears as a dark circle. Under polarized light (in the right-hand image), it glows brightly.
Two microscopy images showing a coccolithophore (the round object on the left-hand side of the frame). The left image is normal light; the right is polarized light, which reflects off of the calcium carbonate in the coccolithophore’s casing.

As we’ve gone further south, the types of phytoplankton we’re seeing — and where we’re seeing plankton in the water column — have been changing. In the subtropics, we saw generally low concentrations of chlorophyll, peaking deep beneath the surface — sometimes as far down as 150 meters. In the subpolar waters we’re currently traveling through, chlorophyll concentrations are overall higher, and the maximum chlorophyll concentrations are found much closer to the surface.

During our non-science time, much of the ship has gotten involved in a game called “Hide the Pig”. Someone made a cardboard cutout of a pig wearing a Santa hat and hid it on the ship. If you find the pig, you get a point and then you get to move the pig to a new location. Many members of the crew are playing, which makes it quite difficult as they know the ship much better than the scientists do!

Giuliana Viglione is a journalist and science communicator who has joined Senior Research Scientist Barney Balch’s research cruise to study the impact of coccolithophores in the Southern Ocean. On board the ship, she’ll be helping the team carry out experiments, document the research cruise, and conduct educational outreach with students across the country. She can be reached at

Meet the plankton
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