Searching for Phytoplankton - Sailors for the Sea

Searching for Phytoplankton

By: Dr. Richard Kirby, Plankton scientist & leader of the global Secchi Disk project | April 1, 2015

Sailors become scientists creating the world’s biggest marine plankton survey

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.


These microscopic phytoplankton begin the marine food chain.

What are Phytoplankton?

They are microscopic, plant-like cells that live at the sunlit sea surface and they begin the marine food chain. Like plants on land, phytoplankton use the sun’s energy to combine carbon dioxide and water to create sugar and oxygen in the process known as 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. Therefore, phytoplankton are an important influence upon the Earth’s climate because they consume carbon dioxide. Although the phytoplankton are too small to be visible to the naked eye, their presence colors the sea giving it a green hue (and sometimes other colours), and their abundance affects its clarity. 

Phytoplankton science

In 2010 a group of marine scientists, based in Canada reported that phytoplankton had declined globally by 40% since the 1950s. These scientists suggested that a warming of the ocean surface due to climatic change might have reduced the vertical mixing of the water column, reducing the supply of nutrients from deeper waters, nutrients that are essential for phytoplankton growth. However, this report provoked debate; other marine scientists reported no change or even an increase in phytoplankton. Part of the controversy stems from a lack of continuous, long-term data on the phytoplankton collected in the same manner. Scientists have changed how they measure phytoplankton abundance over the years as new technology has developed. Other difficulties in assessing the phytoplankton include the vastness of the ocean, and the lack of research being done. Because of the important role played by the phytoplankton at the base of the marine food chain and in global ecology, we need to know if, how and why they are changing.


Dr. Richard Kirby the project leader with a Secchi Disk.

The Secchi Disk study

The Secchi Disk study was launched in 2013 by plankton scientist Dr. Richard Kirby. The study involves arguably, the simplest piece of marine scientific equipment ever conceived – a Secchi Disk – paired with modern mobile technology in the form of an app called Secchi. Since the project’s launch in February 2013 sailors have made the Secchi Disk study the largest marine citizen science study.


The welcome screen for the Secchi App

Want to get involved? Here’s how:

First, download and install the free Secchi app on your mobile device. When the app is launched for the first time it guides you through an introduction to the project that describes how to make and use a Secchi Disk to take depth measurements. 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 can easily be made at home.

The Secchi disk was invented by the Pope’s astronomer in 1865, initially to measure the clarity of the Mediterranean Sea and to use this data to help determine the currents. The first disk was lowered into the sea from the Papal yacht l’Immacolata Concezion on April 20, 1865. Away from estuaries and shallows the major determinant of water clarity is the phytoplankton, and creating 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.


A Secchi Disk being lowered into the water to measure the phytoplankton.

How to make and use a Secchi Disk

A Secchi disk can be made from many materials, 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 inventive 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 at which the Secchi disk just disappears from sight. Then, you use the Secchi app to obtain the GPS location and to enter the depth when the disk disappeared – a network connection isn’t required for this. 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.

An App for All!

The Secchi app works on all mobile devices that can obtain a GPS signal from satellites without requiring a network connection. For iOS devices this means the iPhone models 3GS and later, or 3G and 4G iPads. There are over 2000 compatible Android devices. The iOS and Android Secchi apps will only install on a compatible device.

Where and when to measure?

There are no geographic barriers to this project. Although 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 82 feet deep and more than a half-mile from land where the phytoplankton are the major determinant of water clarity. Readings taken from shallower locations and closer to shore are also interesting to help chart local, long-term changes. Just so long as you cannot see the seafloor. 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.


The Secchi Disk project map in February 2015. View the current data by clicking here.

What will the data be used for?

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 a long-term project that carries on indefinitely. This will provide scientists with a unique insight into the phytoplankton.  The new measurements will also be used in conjunction with historical Secchi Depth data to help us better understand long-term trends of phytoplankton.

We urge you to join in the largest citizen science project of our time when you go sailing and leave a legacy that will help scienctist and our understanding of the ocean’s biology for future generations!

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