Measuring phytoplankton from space

Satellites can now more accurately measure phytoplankton populations in the Southern Ocean from space, thanks to three improved ‘satellite chlorophyll algorithms’ that account for some unique Southern Ocean characteristics.

The Southern Ocean is different from other oceans. It’s windy, it’s cold, it’s covered in ice and clouds most of the time. When the sun does come out, it remains low in the sky and distant. These factors make it a difficult environment for organisms like phytoplankton (microscopic marine plants) that use light to photosynthesise, and they make it difficult for us to measure these phytoplankton from space. So difficult in fact that NASA’s ocean colour satellites have missed more than 50% of Southern Ocean phytoplankton over the last two decades.

Phytoplankton are the lynchpin of the Southern Ocean ecosystem. They take up the carbon we emit into the atmosphere, they produce the oxygen we breathe, and they form the foundation of the food chain, by feeding everything from bacteria to krill. So it is really important that we find out more about them and how they flourish in this hostile domain.

As a PhD student at the University of Tasmania’s Institute for Marine and Antarctic Studies, under Associate Professor Peter Strutton, I am investigating how the Southern Ocean is bio-optically different and how phytoplankton have adapted to deal with these differences.

My most recent work has been to correct NASA’s satellite algorithms for the Southern Ocean. We discovered that the standard NASA ocean colour algorithms, which were developed largely for measurements in the Pacific Ocean, performed poorly in the Southern Ocean and underestimated phytoplankton by more than 50% and sometimes up to 100%.

This is because high winds create huge waves with long-lived white caps that tend to disperse the light we’re trying to measure. The sea ice and clouds also reflect so much sunlight that it blinds the satellite as it flies over. In addition, cold water, a shortage of vital micronutrients, huge seasonality, and extreme isolation, change the way phytoplankton photosynthesise and the relationship between light and biomass that we use to measure them from space.

To correct these algorithms we used more than 1000 samples of chlorophyll (a photosynthetic pigment) from Southern Ocean phytoplankton, collected over 10 years, and compared them with NASA’s satellite-derived chlorophyll measured at the same place and time. The majority of the samples were collected for a long-term monitoring program through a collaborative effort between the CSIRO, the Australian Antarctic Division, and the French Antarctic Program.

Our improved satellite chlorophyll algorithms can now be used to produce high-accuracy observations of phytoplankton, and will go a long way towards improving our understanding of how the Southern Ocean works and how the movement of carbon is changing in these remote waters.

The improved data are freely available through the Integrated Marine Observing System. You can read more about this work in the Journal of Geophysical Research, ‘Three improved satellite chlorophyll algorithms for the Southern Ocean’ (doi:10.1002/jgrc.20270).

Robert Johnson
Institute for Marine and Antarctic Studies