Secchi Disk

I am very excited to have teamed up with the Secchi Disk Foundation. A charity aimed at sailors and seafarers to gather data using a secchi disk on phytoplankton levels in the water.

The phytoplankton in the sea account for approximately 50% of all photosynthesis on Earth and, through the food web they support, they underpin the marine food chain.

Living at the surface of the sea the phytoplankton are particularly sensitive to changes in sea surface temperature. A recent study of global phytoplankton abundance over the last century suggested that global phytoplankton concentrations had declined by 40% over the last 50 years due to rising sea surface temperatures as a consequence of current climate change.

We need to know much more about these changes and you can help by making a simple piece of scientific equipment called a Secchi Disk and using the free Secchi app.

The Secchi Disk is a 30cm diameter, plain white disk that is attached to a tape measure and weighted from below with enough weight to make the disk sink vertically. It is one of the oldest and simplest marine scientific tools. You can make a Secchi Disk from any material e.g. a white bucket lid. A good material is 3mm or 5mm Foamex as it is easy to cut and you can often obtain remnants from printing/display companies.

When a Secchi Disk is lowered vertically into the water the depth below the surface at which it just disappears from sight is called the Secchi Depth. The Secchi Depth measures the clarity of the seawater, which away from estuaries and coasts indicates the amount of phytoplankton at the sea surface.

By measuring the Secchi Depth you can therefore help map the ocean's phytoplankton. You can record Secchi Depths on your travels or perhaps, repeatedly from a local sampling site you establish.

This is a long-term study that began on February 22nd 2013. The study has no end date or geographic boundaries in the sea.

Since getting involved with this study I have learnt so much about the oceans and where they are at today. On my recent passage from Antigua to the Azores I saw the most dramatic change in water colour I think Ive ever experienced. From the very blue water of the Sargasso Sea where the only thing I would see was sargasso weed, no fish or dolphins, the secchi readings were very deep at about 50-60 meters. Approximately half way this changed to below 25 meters, the sea changed to a very dark green and from then on I had dolphins with me every day and saw plenty of fish.

To learn more about this fantastic project head over to