The Biological Pump

Greetings from Knorr! Right now we are at the edge of the Sub-Antarctic Gyre and the Subtropical Gyre in the Atlantic Ocean (learn more about gyres). My name is Gwenn Hennon and I’m a graduate student from the University of Washington. I am running an instrument called SeaFlow, a flow cytometer that continuously measures the tiny cells in the ocean that make up the base of the food web: phytoplankton.

Gwen and the SeaFlow (Winifred Johnson, WHOI)

Like plants on land, phytoplankton use sunlight to convert carbon dioxide into oxygen and energy. As a result, they are responsible for about half of the oxygen in Earth's atmosphere. When they die and sink into the deep ocean they carry the carbon in their bodies, which they got from carbon dioxide, down into the ocean far from the atmosphere. The carbon carried by sinking phytoplankton into the deep sea is called the "biological pump" because it pumps carbon from the atmosphere into the interior of the ocean.

I am interested in how different types of phytoplankton change the strength and efficiency of the biological pump. During this cruise we will be crossing into areas of the Atlantic Ocean with different water types and, hence, different types of phytoplankton. Right now we are in the Sub-Antarctic Gyre, which generally contains larger phytoplankton that we think probably sink faster and therefore create a stronger biological pump. In the Subtropical Gyre the phytoplankton are smaller and sink more slowly, taking less carbon into the deep ocean.

With the SeaFlow instrument I can measure the size of the phytoplankton as well as the number of cells in the ocean where we are and the amount of chlorophyll in each cell. Since my measurements happen all day and night I can see how the cell size changes over the course of a day. From this I can calculate how fast the cells are growing and dividing. Evan Howard, another scientist on this cruise, is measuring how much oxygen the cells produce and how much carbon they take up. When we put our data together we will be able to see whether different types of phytoplankton affect how much oxygen is produced and how much carbon is carried to the deep ocean by the biological pump.

This is important because we know that tropical areas of the ocean are likely to warm as the climate changes. As a result, the types of phytoplankton that can live there will probably be smaller cells. We want to understand what types of phytoplankton live in ocean regions like the Subantarctic and Subtropical Atlantic and how those phytoplankton types affect the biological pump. With that information we can start to think about how climate change will affect where phytoplankton groups are found and how much carbon is carried to the deep ocean by the biological pump.