16 March, 2014

Winter conditions crucial for phytoplankton growth in Southern Ocean

An international team of researchers, including the British Antarctic Survey, has found new evidence that winter conditions in the Southern Ocean play a key role in mixing surface level and deeper waters, and distributing iron. This iron is crucial for the growth of phytoplankton (microscopic plant-like organisms) and ultimately enhances the ocean’s capacity to store carbon dioxide.

Reporting in Nature Geoscience, the team describes the first observation that the mixing of deep and surface ocean waters that occurs during winter, is the central process by which iron reaches the ocean’s surface. This provides the nutrient needed for year-round phytoplankton growth.

Phytoplankton sustains the entire aquatic food web and stores carbon dioxide in the ocean. In order to grow, it requires iron and sunlight but whereas light reaches the surface of the ocean, iron accumulates at greater depth. Phytoplankton is therefore dependent on deep water iron reaching the surface of the ocean.

Co-author, Dr Jean-Baptiste Sallée from the British Antarctic Survey and Centre National de la Recherche Scientifique explains:

“The ocean is layered like an onion, and very little mixing occurs between the surface and the deeper waters. But in winter, when it is cold and windy, these layers do mix and iron from the deep comes up to the surface. This means iron can reach the well-lit layer where photosynthesis occurs and, thereby sustains the biological activity of the largest ocean basin of the world.”

“We are really excited to make this discovery because until now we didn’t know exactly the physical processes allowing iron to reach the ocean surface and maintain the biological activity. The combination of strong winds and intense heat loss in winter strongly mixes the ocean surface and the mixing reaches deep iron reservoir.”

“With this understanding of how iron reaches the ocean surface we are better placed to understand the effects of changing climate and future carbon absorption by the ocean. We know that winter processes are key.”

Dr Alessandro Tagliabue, from the University of Liverpool said:

“This is important because iron limits biological productivity and air to sea CO2 exchange in this region. We found unique aspects to the iron cycle and how it is supplied by physical processes, making it distinct to other nutrients.”

Due to the size and remote location of the Southern Ocean, scientists have only recently been able to explore the workings of the ocean. The scientists painstakingly examined over more than 1000 iron measurements collected from ship-based observations since the 1990s, together with satellite images of the area, and used small robotic probes known as Argo floats, to examine the deep winter mixing.

The research involved the University of Liverpool, Southern Ocean Carbon and Climate Observatory, Sorbonne Universites, CNRS, University of Tasmania, University of Cape Town, University of Otago, University of Tasmania.