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There’s no better indicator of the health of the oceans than the amount of phytoplankton that resides in them. That’s not only because this microalgae produces at least 50 percent of the oxygen we breathe, but also because it’s the start of the marine food chain, determining what other creatures live and thrive in any given area.
The changing seasons and the climate crisis may play a big role in the presence of phytoplankton over time, so it’s of the utmost importance for researchers to know what levels look like in oceans around the world. Sailors, boaters, and interested sea-faring travelers can help track and study this microorganism by using one simple tool: the Secchi disk. You can contribute to important citizen science by building one and taking it with you the next time you head to the ocean.
What is a Secchi disk?
A Secchi disk is an impressively low-tech piece of scientific equipment invented in 1865 by Italian astronomer Angelo Secchi to measure water transparency and turbidity. In deep-water ocean environments, these factors are determined by biological material like phytoplankton, explains Verena Meraldi, chief scientist for HX Hurtigruten Expeditions, a cruise line that invites passengers to participate in scientific data collection.
The tool itself is usually a round piece of white plastic with a diameter of 30 centimeters (about 12 inches), that is attached to the end of a tape measure or line marked at 20 centimeters (about 8 inches) and 1-meter intervals (a little more than 1 yard).
We’ll explain in more detail below, but using a Secchi disk is easy: just lower the disk on a line into the water and record the depth at which you lose sight of the contraption. This measurement is called Secchi depth. Deeper measurements mean there’s less phytoplankton in the water, whereas shallow measurements indicate an abundance of the microalgae and therefore, a healthier environment.
Once you have a reading, you can log your findings in the Secchi app (available for iPhone and Android). The platform is part of the Secchi Disk Study citizen science program launched in 2013 by marine biologist Richard Kirby after a controversial 2010 report published in Nature that claimed phytoplankton levels had declined 40 percent between 1950 and 2008. Kirby’s initiative collects data to track the presence of this crucial microalgae worldwide.
Researchers have long collected data on phytoplankton by measuring ocean surface color using satellites. But this information is not enough, so this is where citizen scientists come in.
“You need some means of determining in situ measurements, and the simplest way to do that is to measure the clarity of the water with a Secchi disk,” Kirby explains.
How to make a Secchi disk
There are two kinds of Secchi disks: the ones made to measure clarity in freshwater are painted in black and white, and are smaller than the white-only Secchi disks designed for the ocean. To participate in Kirby’s study, you’ll need the latter.
You can order a Secchi disk online, but you can also make your own, as they are easy to make and much cheaper, too.
[Related: How to become a citizen scientist]
Please note that some of the measurements in this project are in metric units. This is important because the Secchi Disk Study measures depth in centimeters, so the data you provide must be measured accordingly.
Stats
- Time: 30 to 60 minutes
- Cost: about $8
- Difficulty: easy
Materials
- 12-by-12-inch piece of wood, plastic, or styrofoam board, ⅛-inch or ¼-inch thick
- 50 meters (165 feet) of sturdy light-colored cord
- 2-pound fishing weight (or anything heavy enough to pull the disk down)
- (Optional) 4-inch eyebolt
- (Optional) 2 washers
- (Optional) 2 hex nuts
- (Optional) 50-meter (165-foot) fiberglass surveyors tape
- (Optional) White matte paint
- (Optional) Carabiner
Tools
- Heavy-duty scissors, box cutter, or saw
- Measuring tape
- 2 permanent markers of contrasting colors
- Drill
- Ruler
1. Cut a disk with a 30-centimeter diameter. You can craft your Secchi disk from just about any material, including metal or wood, though plastic is most common as it’s often easier to cut to size. A trimmed 5-gallon paint bucket lid, a thick signboard, or even a cutting board will work well. Just make sure that whatever material you choose won’t break easily and end up polluting the waters you’re trying to study and protect.
2. (Optional) Paint your disk matte white. If the material you chose is already matte white, you can skip this step. If it’s not, paint your disk with matte-finish white paint and let it completely dry. You can use whatever you have at hand—just keep in mind that you may need more than one coat to get the required opacity.
3. Drill a small hole in the center of the disk. Use a ruler to find the center and drill a hole that’s just a bit bigger than the width of your cord.
4. Thread your cord. Thread your cord through the hole you just drilled, measure 16 inches down the cord, and make a secure knot there to keep the disk in place. It doesn’t have to be exact—you want enough of a tail below the disk to tie several knots and secure your weight.
- Pro tip: You can also affix a 50-meter (165-foot) or longer fiberglass surveyors tape to the top of the disk by screwing an eye bolt into the center and clipping the tape on with a sturdy carabiner.
5. Securely attach the weight to the bottom side of the disk. The weight can be a 2-pound fishing weight, repurposed link of mooring chain, or anything else that will help the disk sink.
- Pro tip: “Be creative—you just need a lump of heavy metal,” Kirby says.
6. Mark your line. Once everything is knotted securely, use a permanent marker to draw lines on the cord at 20-centimeter intervals. Use the contrasting color to make marks at 1-meter intervals.
How to use a Secchi disk
Once you have your disk, head for the ocean. Make sure it’s at least partly sunny and that you embark ideally between 10 a.m. and 2 p.m., as the angle of the sun will affect light penetration. Don’t set sail unless you’re accustomed to being on a boat, wearing proper safety equipment (like a life jacket), and know how to swim.
If you’re not comfortable on the water or don’t have a way to leave shore, no data is uninteresting, Kirby says. That means you can still join in and if you can only take readings once from a jetty or pier near shore where you live, you can still join in. Although the instructions below require a boat, you should be able to adapt them to wherever you are.
To pick a good reading location, Kirby says to find a spot at least 1 kilometer (0.62 miles) from shore where you can’t see the ocean floor, so around 25 meters deep (82 feet) deep. This depth and distance from shore will help reduce the amount of tannins and sediment obscuring visibility that could alter the measurement.
Take off your sunglasses if you’re wearing them, and drop your clean disk into the water on the shady side of your boat. Keeping a firm grip on your measuring tape or rope, slowly let out the line. If you think it might slip from your fingers, tie it off to a secure surface for extra peace of mind. Watch carefully as your disk descends, and make sure it sinks vertically. If it doesn’t, the sinking weight might be off-balance or the current may be too strong, in which case you may have to make some adjustments and try again later.
Stop when you can no longer make out the disk beneath the surface. Raise and lower the disk a few times to pinpoint exactly the point where you lose sight of it. This will help you get the most accurate reading and make sure your eyes aren’t playing tricks on you. When you’re ready, record your Secchi depth by looking at your measuring tape at the point where it touches the water, or counting the submerged interval markers. You’ll need the average measurement when you use the app. Finish by opening the Secchi app at the drop site—follow the prompts and instructions to record your GPS location and enter your data.
You can repeat this procedure anytime you’re on the ocean. In fact, if you visit far-flung destinations or regularly return to the same spot, all the better: repeated readings from various times of the day, different seasons, and from hard-to-reach locales are extremely valuable for helping scientists understand how phytoplankton levels change over time and around the world.
The Secchi Disk Study has published two research papers on phytoplankton, with more in the works. That’s thanks to citizen science contributions: cruise passengers, avid sailors, recreational kayakers, and anyone who even occasionally takes to open water and wants to contribute to important and quantifiable environmental science. You can add yourself to that list now too.
