The Southern Ocean’s Invisible Forest

In addition to the scientists directly involved with the DIMES project, we welcome two biologists from the University of Oxford on our cruise who were also on the previous GEOTRACES leg (JC068). Earlier today, I asked Dr Heather Bouman to prepare a short piece explaining their activities onboard:

The flora of the open ocean is dominated by microscopic plants called phytoplankton.  These singled-celled algae form the base of the marine food chain and are responsible for half of all the photosynthesis on Earth.  Dr. Heather Bouman and Thomas Browning, from the University of Oxford, are collecting seawater samples to examine the optical signatures of phytoplankton to characterise the species that dominate these waters, and their photosynthetic efficiency.

Knowing the types of phytoplankton present helps us to understand the role of the oceans as a sink for atmospheric carbon dioxide.  By  virtue of their size, some types of phytoplankton, such as diatoms, sink more rapidly than smaller cells, and can therefore transport to the deep ocean more readily the carbon dioxide they fix through the process of photosynthesis  Since size also affects the optical signatures of phytoplankton, the data will be useful to help in the derivation of algorithms to diagnose the community structure of phytoplankton using information collected by optical sensors mounted on earth-orbiting satellites.

The Southern Ocean is not only a challenging place for oceanographers to do their research, but is also a difficult place for phytoplankton to thrive. High winds and cold temperatures cause deep mixing of surface waters, which is unfavourable for phytoplankton growth. Phytoplankton in the Southern Ocean may also be limited by a lack of iron.  All plants need iron to maintain the photosynthetic machinery that converts of light energy to chemical energy.  In most of the global ocean, there is a sufficient supply of iron contained in dust deposited on the surface ocean from land.  The Southern Ocean, however, is not bound by continents, and therefore the surface supply of iron is much less, leading to a reduction in phytoplankton growth.

Tom Browning is conducting incubation experiments on DIMES to investigate the distribution of phytoplankton iron stress across the Atlantic sector of the Southern Ocean.  These data will help us understand how the supply of iron may govern the primary productivity of this important marine ecosystem.

 

Map of surface chlorophyll for the DIMES study area.  Purple and dark blue regions south of 55 degrees latitude represent low chlorophyll regions (< 0.2 mg m³) that are potentially iron limited.

Tom collecting seawater

for his iron incubation

experiments.