Role of phytoplankton community structure in determining the efficiency of the ocean's biological carbon pump

Increasing amounts of CO2 are entering the atmosphere because of human activity. Phytoplankton, tiny plants that live in the ocean, consume CO2, drawing it down from the air into the sea. But for this CO2 drawdown to have a long-term impact on atmospheric levels, the carbon has to be locked away, out of contact with the air, in the deep waters of the ocean. One way carbon is transferred to deep waters is via the sinking bodies of dead plankton. Oceanographers measure this flux of particles with sediment traps moored to the ocean floor. But not all of the carbon produced by plankton at the surface reaches the traps. The percentage that does is referred to as the transfer efficiency. This has been found to vary over time, i.e. in different seasons and years, and at different locations. One possible reason is that the types of plankton sinking from the surface are different. Two types of plankton are thought to play critical roles in altering the transfer efficiency: diatoms and coccolithophores. Diatoms are large plankton made of silica, whilst coccolithophores are mostly calcite. Diatoms are thought to sink more quickly than coccolithophores, but may also dissolve more quickly in seawater, due to the differences in their chemical make-up. The difficulty in studying this process is that measurements of plankton carbon production and species are needed over large areas and long time periods. A combination of measurements using ships, satellites orbiting the Earth and models of the ocean are needed to study this problem. Ships are able to make measurements of the ocean that we cannot get any other way, but cannot sample over large areas or for multiple years. One type of satellite data, ocean colour, measures the amount of plankton living near the sea surface. Because satellites take a snapshot of a large area of ocean around once a day, we can investigate the spatial patterns of plankton distribution, and how they change over months and years. But satellites only see the surface ocean, so to understand what is happening at the sea floor an ocean model is also needed. We can then study the effect that changes in plankton species have on the material reaching deep waters. Combining data from all these different methods will give new insight into how the transfer efficiency has varied over several years, and what impact changing plankton species may have had. As global climate changes, different parts of the world's oceans are expected to respond in different ways, so four contrasting sites in the North Atlantic will be studied to understand how past changes in surface conditions affected the transfer efficiency. This will help us to predict how future climate change may impact the rate of CO2 transfer from the atmosphere to the deep ocean in the North Atlantic.