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.

Turning a Ship into a Floating Laboratory

Video by Colleen Durkin, WHOI

Departure

The tugboat Montevideo gently pulled us out of our berth at the port of Montevideo, Uruguay, today. It is a busy port with massive cranes loading cargo ships and the smell of manure filling the air as live cattle are loaded onto ships.

Tugboat pushing R/V Knorr out of its berth and toward the
South Atlantic (Winnifred Johnson, WHOI)

This port is the entry and exit for goods going to and from Uruguay, Paraguay, parts of Brazil, and Argentina. As we wait for our turn to leave the port ,a tugboat pulls a cargo ship, many times the size of the Knorr out of its berthing place. The port is located in the mouth of the Rio de la Plata and the water is full of mud and silt. On the horizon there is a barely visible streak of blue that shows where the river and the ocean meet.

We will be working on the R/V Knorr  for the next 45 days. Our most southerly samples will come from a latitude of 38S. We will then travel north along the western Atlantic, crossing the equator and disembarking in Bridgetown, Barbados (13.2N). This journey will take us from temperate latitudes through the tropics, sampling more nutrient-rich water in the south and entering low-nutrient water as we move north, while also crossing the Amazon River plume. There will be a variety of sampling to look at microbial life and chemistry of this part of the Atlantic Ocean. We will also be sampling for everything from viruses to phytoplankton and chemical analyses ranging from broad snapshots of the small organic molecules present to identify of individual lipids.

A cattleship loading as Knorr leaves the Port of Montevideo
(Winnifred Johnson, WHOI)

The cruise track is being dictated by the presence of north Atlantic deep water. This is water that is cold and salty, which makes it dense and causes it to sink down to around 3000-4000 meters in the North Atlantic near Greenland. This body of water then travels down the western Atlantic, transporting any matter that is associated with it. This is part of a deep ocean transport system that results in carbon and nutrients eventually being moved all the way into the North Pacific.

Radiocarbon dating shows us that some of this material is thousands of years old. Part of the goal of this cruise is to understand how the organic matter that is being transported along this route changes and the processes that are transforming it.

Over the coming weeks we’ll be uploading videos and blog posts highlighting all of the diverse work occurring on this cruise.

DeepDOM: Coming soon!

Tune in soon for updates from the DeepDOM cruise. In the meantime, you can read more about our home for the next five weeks, R/V Knorr (above).