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Sailors cruise ahead in a study of the oceans' phytoplankton

by Dr Richard Kirby 6 Dec 19:00 UTC 6 December 2017
Richard Kirby, Secchi Disk Project Leader © Secchi Disk

Around the world, sailors and other seafarers are lowering round, 30 cm white disks into the sea from the side of their boats to study the oceans phytoplankton.

The disk the seafarers are using is called a Secchi Disk and it is named after the Pope's astronomer who invented it in 1865, and the seafarers are all citizen scientists taking part in the Secchi Disk study ( The first data from this global study has just been published in The Public Library of Science ONE (PLOS ONE) ( and the Secchi Disk Seafarers are collective authors. The paper reveals importantly, that seafarers are not only good citizen scientists, but that their measurements of the phytoplankton in the sea may be vital to helping us better understand the oceans' changing biology.

The oceans' phytoplankton the sailors are studying underpins the marine food chain to determine the abundance of other life in the sea from the fish we eat to the number of polar bears on the ice. The phytoplankton also provides the World with oxygen and it plays a central role in the global carbon cycle.

A recent study of a 100-year trend in global phytoplankton primary production revealed a 40% decline since 1950 ( The explanation proposed for the decline was that water column mixing, and so the supply of growth-promoting nutrients to the sea surface, had reduced due to rising sea surface temperatures due to global warming. Because the phytoplankton is so important we need to know much more about how they are changing. Although satellites can now measure phytoplankton at the sea surface remotely from space, direct measurements are essential because of the naturally, uneven distribution of phytoplankton in the water column and this is where any seafarer equipped with a Secchi Disk can help.

A Secchi Disk is arguably, the simplest piece of marine scientific equipment ever invented. When the Secchi Disk is lowered vertically into the water, attached to a tape measure, the depth below the surface when it just disappears from sight is recorded as the Secchi Depth and this reflects the transparency of the water column. Away from estuaries and coasts the phytoplankton are the major influence upon the transparency of the sea and so the Secchi Depth measures the phytoplankton. Scientists have been using Secchi Disks to measure ocean transparency since 1865 and now sailors are joining in around the world. The sailors make their own white Secchi Disk from any material and use the free Smartphone application called Secchi to upload their Secchi depth data to a central database.

Whilst there are more professional marine biologists than ever before, few go to sea and still fewer go far offshore, and open-ocean research cruises rarely return to the same location. In contrast, many of the public go to sea regularly for recreation or work and often to a similar place, and ocean passages sailed by cruisers follow similar routes dictated by the time of year and favourable prevailing winds.

In the paper published in PLOS ONE the Secchi Depths collected by seafarers are compared to satellite ocean colour data to reveal how their data can be used to better interpret satellite colour measurements. So now, sailors around the world are helping us to understand the oceans biology as they cruise along.

Plankton scientist Dr Richard Kirby who founded and runs the Secchi Disk study said: "Secchi Disk data collected by seafarers can help us understand in situ versus satellite observations of phytoplankton. As participation by seafarers using Secchi Disks increases their data will contribute more and more towards understanding how the phytoplankton are changing over long-term timescales to help us understand the essential base of the marine food chain."

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