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Stoneways Marine 2021 - LEADERBOARD

Perfect - the Secchi Disk Study enters its tenth year, thanks to sailors

by John Curnow, Global Editor, 7 Feb 2022 12:00 UTC

Just brilliant. No need to mention the bumbling comedy of Dudley Moore in 10 (which is so hard to do perfectly), or the utter brilliance of Romania's Nadia Comaneci back in 1976 at Montreal. Ooops. I just did. Sorry.

But funnily enough, even further back in grade school I remember learning about whales, and in particular, blue whales, who live way South in the mighty Southern Ocean. They open their enormous mouths, take in oceans of water, and then strained it all out to leave behind up to six tonnes of krill for their day's work.

Now the krill themselves feed on phytoplankton (or more commonly known as just plankton) and these are microscopic, single-celled plants that drift near the ocean's surface, and live off the energy from the sun and CO2. They are the dichotomous end of the food spectrum from the whales and sharks, but oh-so-important to the whole gig, and that includes us!

Recently, Dr Richard Kirby, the project leader of the Secchi Disk Study, reached out to us. This is a citizen science study that will enter its tenth year on February 23, 2022, and with cruising sailors getting back out there now, it seemed perfect to take a closer look at this incredibly important work into such a vital subject.

Kirby explained, "The Secchi Disk study engages sailors and all other seafarers around the world in citizen science enabling the collection of 'hard to get' in situ data on the oceans' phytoplankton. It is the largest marine citizen science study collecting phytoplankton data; it's huge success and longevity is due to the interest and enthusiasm of the seafaring public."

"While the longevity and global extent of the Study is already a remarkable achievement, its continuing success relies totally upon awareness among the seafaring public and their goodwill in participation. We constantly need to both reach new people and show current participants that the study is still running, growing, and has broad industry-wide interest and support."

Now the Secchi Disk study was established in 2013 to engage seafarers in a long-term study of the ocean's phytoplankton, in response to reports that the phytoplankton had declined by 40% in abundance over the preceding 50 years; a decline suggested to be due to ocean surface warming as a result of climate change. Here is a video to watch about it all.

Highlighting the incredibly important value of this work, Kirby detailed that, "The most recent publication from the Secchi Disk study appeared just six months ago in the prestigious journal Nature Scientific Reports. The research paper favourably compared the data collected by seafarers with similar data collected by scientists, and this certainly underscores the value of Secchi Disk citizen science to marine research."

Blue? Green? Or something in between...

Taking us back to the beginning, as it were, the microscopic phytoplankton colour the sea, begin the marine food chain, and are sailors' constant, sea surface companions. "Because the phytoplankton underpin the marine food chain, their abundance determines the abundance of all other life in the sea, from the numbers of fish to how many polar bears on the ice, crabs on the seabed, or seabirds in the sky."

"The phytoplankton are declining in abundance globally due to climate change, and it is vital that we understand by how much and where the changes are greatest. To do so we need to collect in situ measurements, which is where the Secchi Disk study comes into it. The oceans are vast and there are not that many marine biologists to sample them. However, there are many sailors, and so I started the citizen science Secchi Disk study to harness this great resource."

"Marine scientists have used Secchi Disks to measure the phytoplankton since 1865, and the measurements sailors collect in the Secchi Disk study today will help scientists in the future better understand the changing phytoplankton", said Kirby. "Sailors can take measurements wherever they are, and the Secchi application contains instructions on how to take part. It only takes a few minutes to take a measurement."

Just what is it? How do I get involved? What do I do?

A Secchi Disk is a white, 30 cm diameter disk attached either to a fibreglass tape measure, or to a marked length (every 20cm) of non-stretchy rope, and weighted from below. The disk itself can be made from a white bucket lid, a food-chopping board or 5mm Foamex, painted matt white. After that, all you need is the Secchi App on your smartphone or tablet, and away you go.

The Pope's Astronomer, Pietro Angelo Secchi, invented the disk in 1865 to help measure the clarity of the Mediterranean Sea to determine the position of the various currents.

To use your Secchi Disk, lower it vertically into the seawater (remembering you need sufficient weight to make the disk sink vertically), and measure the depth below the surface when it just disappears from sight. This is called the Secchi depth. You then use the Secchi App to obtain your GPS location to record the Secchi depth (a network connection isn't necessary), and then upload your data when ashore.

Because the Secchi Depth measures the clarity of the sea and the phytoplankton is the major determinant of water clarity away from estuaries and shallows, the Secchi Disk is a very simple tool for measuring the amount of phytoplankton in the sea around the world. Scientists have used Secchi Disks for this purpose ever since their invention.

Cruisers make a difference

"A great aspect to the Secchi Disk study is that sailors regularly visit the same areas, whether they are day sailors from their local port, or they are following longer passages determined by the winds, currents and time of year, and in this way the data can build up over time."

"The Secchi Disk study is also a great science activity for all ages when out on the water and especially, for introducing children to the natural world surrounding them."

"Once you start collecting data regularly you will start to learn much more about the sea around you, how the biology changes with the seasons, or from place to place. Taking part brings about a great connection to the natural world, and I hope it will only add to your sailing enjoyment."

"Since the study began it has become the largest citizen science study on the phytoplankton due to the enthusiasm and participation of sailors who have collected data from every ocean, and from pole to pole."

"Already, the Secchi Disk study has published two research articles and both are Open Access, which means they are free to download and read. These two publications show that the data collected by sailors can make a very valuable contribution to marine science are available at and also too.

"The Secchi Disk study is a global study with no end date; as more people join the study and the data increases, it will become more and more useful to helping understand the phytoplankton."

"Please join us in our tenth year, make your Secchi Disk, download the Secchi App, and start collecting your phytoplankton data to leave a lasting legacy of your time on the water for future generations."

Now in closing, Kirby also added this, "There is also Jimmy Cornell's great quote at the beginning of his most recent Ocean Atlas 2nd ed, 2018: Sailors are far more aware of what's happening out on the oceans, and don't question that change is taking place, [they] just simply deal with it. And anyone who doesn't believe it's happening should just go out and sail the Northwest Passage."

From our perspective, it was wonderful to see this kind of activity growing, and Kirby was actually out at sea when we were swapping emails, so this is not a decree from a lab or an office at the end of a rabbit warren in some institution somewhere. We are not only thrilled that Dr Richard Kirby reached out, but that many of you are engaged in this already, and some will be inspired to do so as a result of this editorial.

If you want to write in to us about your experiences with the Secchi Disk, then please email us - we'd love to hear about your experiences.

OK. We have stories, lessons, inspirations and history to regale yourself with. Please use the search window at the top of the home page if you are after something specific, as only the latest news appears on the site as you scroll down. We enjoy bringing you the best stories from all over the globe.

If you want to see what is happening in the other Hemisphere, go to the top of the SailWorldCruising home page and the drag down menu on the right, select the other half of the globe and, voila, it's all there for you. Also, we have just had a significant upgrade to our systems, and trainspotters will have seen that the button next door to 'home' now says 'editorial' which collects the latest from our team and also lets you see what each member has been up to of late.

Finally, stay safe, and ready for all that 2022 will offer.

John Curnow
Global Editor,

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