Sailors turn scientists to create the world’s biggest marine plankton survey

Caption: These microscopic phytoplankton begin the marine food chain. Credit: Richard Kirby

Caption: These microscopic phytoplankton begin the marine food chain. Credit: Richard Kirby

A unique, citizen science study uses a Secchi disk and a free mobile phone app called Secchi to conduct a vital global study of the sea’s phytoplankton.

Within the sunlit surface of the sea begins the marine food chain. Here, microscopic, free-floating, plant-like cells called phytoplankton live and use the energy in sunlight to combine carbon dioxide and water to create sugar and oxygen in the process we call photosynthesis. Despite being tiny (each phytoplankton cell is smaller in diameter than a strand of human hair) they are so numerous that they account for about 50% of all photosynthesis on Earth. Although the phytoplankton are each too small to be visible to the naked eye, you will be aware of them because their presence also colours the sea giving it a green hue (and sometimes other colours), and their abundance affects its clarity.


In 2010 a group of marine scientists reported that the phytoplankton had declined globally in their abundance in the oceans by 40% since the 1950s. The scientists suggested that a warming of the ocean surface due to climatic change may have reduced the vertical mixing of the water column, thereby reducing the supply of nutrients from deeper waters – in effect the input of fertiliser to the surface had lessened with effects upon phytoplankton growth. The Canadian scientists’ results provoked debate however; as other marine scientists thought they saw no change or even an increase in phytoplankton in some places. Because of the important role played by the phytoplankton at the base of the marine food chain a citizen science study has begun to help study their abundance. Called the Secchi Disk study, any sailor, diver or fisherman who goes to sea can take part by using a Secchi Disk (a simple piece of equipment first invented in 1865 by the Pope’s astronomer Pietro Angelo Secchi) and a mobile app called Secchi.

How does the Secchi Disk study work

A Secchi Disk is a round, white disk exactly 30 cm in diameter that is attached either to a fibreglass tape measure, or to a marked length of synthetic (non-stretchy) rope, and weighted from below; the Secchi disk is a self-made component of the study. When the Secchi Disk is lowered into the water column the depth below the surface that it just disappears from sight is noted; this number is called the Secchi Depth and it measures the clarity of the water. Away from estuaries and shallows the major determinant of water clarity is the phytoplankton, and so the Secchi Disk is a very simple tool for measuring the amount of phytoplankton in the sea. It is a tried and tested method that has been used by marine scientists since 1865.

Having recorded the Secchi Depth using the Secchi Disk, you next use the free Secchi App to obtain the GPS location and to enter the Secchi depth – a network connection isn’t required for this. It is very important that you use the Secchi App to obtain the GPS location when you measure the Secchi Depth. The Secchi App stores the data on the phone and the Secchi Disk project receives the data as soon as network connectivity is regained. Once you have submitted your data you can follow it, and the data submitted by others, on the interactive project map that is accessible from the project website.

Secchi DiskILY

A Secchi Disk being lowered into the water to measure the phytoplankton. Credit: Richard Kirby.

How to make and use a Secchi Disk

A Secchi Disk can be made from any material, such as a white plastic bucket lid or a piece of plywood painted white. The only restriction is that it is 30 cm in diameter and plain white. So far, sailors have been very ingenious in the materials they have used. To use a Secchi Disk, you hold the tape measure and lower the disk vertically into the seawater (you need sufficient weight to make the disk sink vertically), and you note the depth below the surface at which the Secchi disk just disappears from sight.

Caption: The Secchi Disk project map in February 2015. You can view the current data form the project website. Credit: Richard Kirby

Caption: The Secchi Disk project map in February 2015. You can view the current data form the project website. Credit: Richard Kirby

The aim of the project is to build a phytoplankton map of the oceans that charts the seasonal and annual changes of the phytoplankton from now and into the future. It is therefore a long-term project that carries on indefinitely. There are two different types of location to take a Secchi Depth reading. While the project is especially interested in Secchi Depth measurements in water more than 25 m deep and 1 km from land where the phytoplankton are the major determinant of water clarity, however, readings taken from shallower locations and closer to shore are also interesting to help chart local, long-term changes in the sea. Just like there is no end point for the project, there are no geographic barriers. You may choose to measure the Secchi depth at the same place regularly (once a week for example), or just occasionally, or you may take measurements from different places as you travel. So why not join in and help leave a legacy that will help science and our understanding of the oceans’ biology for future generations?

Richard KirbyILYRichard Kirby (@planktonpundit) is a plankton scientist and the leader of the Secchi Disk citizen science project. Richard obtained a degree in zoology from Exeter University and a PhD in Genetics from University College, London after which he worked at Hopkins Marine Station, Stanford University, California and at the Belle W Baruch Institute of Marine Science, University of South Carolina. In 2003 Richard was awarded a Royal Society University Research Fellowship to study plankton ecology, which he held at Plymouth University, UK until 2011. Richard’s research laboratories are currently located at the Marine Biological Association of the UK in Plymouth where he is an Associate Fellow. Richard believes that public engagement and communicating science to the public are fundamental to being a modern scientist. His website is


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