A new spin on ocean eddies and marine primary production
Phytoplankton are the ocean’s primary producers. The microscopic algae take up carbon dioxide from the atmosphere and convert it into organic matter through photosynthesis. This organic matter is the basis of marine food webs, fuelling everything from tiny zooplankton to fish, seabirds, sharks and whales.
Phytoplankton are a critical component of the ocean’s biological carbon pump. When they die, they take the carbon they have converted into organic matter with them to the ocean depths. Some of this may reach the seabed and be sequestered there. As the amount of phytoplankton growth changes, so too does the amount of carbon sequestered.
More nutrients, more phytoplankton
“Increased nutrient availability in the ocean can fuel the growth of phytoplankton,” explains Dr Nick Hardman-Mountford, a biogeochemical oceanographer at CSIRO.
“There are more nutrients found in the deep-water, and when it is mixed with the ocean’s upper layers, hungry phytoplankton that live in the light-soaked top 100 metres consume it, accelerating photosynthesis, leading to more algal growth, much of which sinks deeper in the ocean.”
Swirling currents of water called eddies are responsible for this mixing. Just like weather systems, oceanic eddies can flow in both a clockwise and anticlockwise fashion. And like in weather systems, these are described as cyclonic and anti-cyclonic systems (which rotate in opposite directions in each hemisphere).
Mixing could pump more carbon into the ocean
Previous studies have suggested that, for most parts of the ocean, only cyclonic eddies show increased primary productivity.
However, a team of researchers from CSIRO and the University of Queensland (including Nick Hardman-Mountford) has shown that in winter, when the subtropical ocean (between 30° North and 30° South) is most productive, anti-cyclonic eddies undergo more mixing. This can increase inputs of nutrient, overturning an earlier understanding that these eddies were the ocean equivalent of ‘barren deserts’.
This study has implications for our understanding of the ocean’s biological pump and the capacity of the ocean to sequester carbon dioxide.
“Oceanic eddies could play a role in global climate change mitigation, especially if human induced climate forcing enhances eddy activity as other studies have suggested”, said Dr Richard Matear, another member of the research team.
To test this new found understanding Richard, Nick and colleagues will now use data from BioArgo ocean robots that they have deployed in subtropical eddies to record nutrient changes and phytoplankton growth as they happen.
This research was published in Science Advances on 20 May 2016.