Eat or Be Eaten: The Life and Times of Phytoplankton

Dr. Kevin Arrigo is a Professor in the Department of Environmental Earth System Science at Stanford University. He is the Chief Scientist for NASA’s ICESCAPE (Impact of Climate change on the Eco-Systems and Chemistry of the Arctic Pacific Environment) mission this summer onboard US Coast Guard Cutter HEALY.

We’re surrounded by a green broth of life. The single-celled algae called phytoplankton are so dense that our instruments disappear from sight a mere few feet after we plunk them into the water.

A few days later, we sample the same area and find…nothing. Or almost nothing.

Phytoplankton are the staple food that sustains much of the Arctic marine ecosystem. The bread of the sea. But where did it all go?

That’s what Sharmila Pal, a graduate student of Claudia Benitez-Nelson (University of South Carolina), wants to find out.

In the Arctic, phytoplankton take advantage of the few months of sunshine and ice-free conditions in the spring and summer to grow to extraordinary numbers. So dense are these “blooms” that they completely overwhelm the ability of the grazers, mostly shrimp-like copepods, to consume them (just picture a cow with an entire pasture to itself). The phytoplankton that don’t get eaten will eventually sink after they suck the surface ocean dry of nutrients. Some of this energy-rich salad will decay as it sinks, but because the Arctic Ocean is so shallow, a lot of it reaches the bottom.

We’re just not sure how much.

Its important to know, however, because this food raining down from the surface feeds a lush bottom community of worms, clams, crustaceans, starfish, and lots of littler critters. This bottom – or benthic – buffet is itself supper for much larger and more familiar fauna such as the walrus and California grey whale.

Sharmila uses a clever approach to measure how many phytoplankton exit the surface ocean. It turns out that when a particle, including a phytoplankton cell, sinks to the depths of the ocean, it carries a hitchhiker with it – a radioactive isotope of the element thorium. By measuring how much thorium is missing from different depths of water, Sharmila can tell how many particles must have exited those depths during the preceding few weeks. It’s a lot of work, requiring her to pass gallons of water through a filter for every sample and then put the filter into an instrument that counts radioactive beta particles – kind of a fancy Geiger counter.

But when she’s done, she’ll be able to tell how much of the missing phytoplankton were eaten by grazers and how much has sunk to the bottom. Because the balance between herbivory and sinking is predicted to shift as sea ice in the Arctic continues to decline, understanding that balance today is of critical importance.