This Device Has Been Measuring the Ocean’s Plankton Since the 1930s

Largely unchanged since it was invented, the Continuous Plankton Recorder collects plankton as it is towed behind a ship

Understanding plankton is essential to studying how ocean life is sustained. tonaquatic/iStock
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In the winter of 2013, a mass of warm water began to spread throughout the eastern Pacific Ocean. The Blob wreaked havoc on marine life—sea lion pups starved, seabirds died, and salmon fisheries suffered.

Helping to unravel this mess is the Continuous Plankton Recorder (CPR), an antique device that uses rolls of silk and 18th-century clockmaker’s technology to sample plankton near the surface of the ocean. Invented in England in 1922, the CPR device’s design was refined throughout the 1920s, and has been left more or less unchanged since 1929. Since then, the CPR Survey—a project currently based out of Plymouth, England—has been using these devices to take snapshots of the tiny organisms that make up the ocean’s plankton.

Such long-term records make it possible to track the effects of ocean events like the Blob, says Sonia Batten, who is using data from these devices to understand how plankton in the North Pacific were affected by the marine heatwave. If no one knows what the plankton looked like before a heatwave, it’s impossible to track changes. And to have an accurate comparison across time, the data needs to have been collected using the same method all along. “Your understanding is only as good as the baseline you have,” says Batten.

Each CPR device—of which there have been hundreds over the years—is a meter-long, stainless-steel, torpedo-shaped container that houses long strips of silk and can be towed behind any ship. As the crew deploys the device into the ocean, a propeller begins to spin in the churning water, driving a mechanism that unravels the rolls of silk. Water streams through a tiny hole at the front of the torpedo, and the silk traps the plankton. A second roll of silk then completes a “plankton sandwich” that preserves the sample for analysis.

Each 457-centimeter roll of silk takes 926 kilometers to unspool, capturing a sample of the surface plankton. The original CPR Survey, which began in 1931, followed dozens of shipping routes in the Atlantic. The result is a library of hundreds of thousands of samples, maintained by the CPR Survey in Plymouth. In some cases these records reach as far back as 1946, when samples were first stored for later analysis. Since the CPR Survey began, other projects using the technology have sprung up, covering parts of the Pacific and Indian Oceans, the Mediterranean, and parts of the Southern Ocean around Antarctica and Australia.

Understanding plankton is essential to studying how ocean life is sustained, says Carin Ashjian, a plankton ecologist who is not affiliated with any CPR projects. For instance, she says, big, fat, juicy zooplankton tend to thrive in cool water. So what happens when an ocean heatwave comes along?

Based on the CPR data, Batten can say that the average size of plankton in the northeast Pacific decreased during the Blob years. But, she adds, there’s still a puzzle to solve because shrinking plankton doesn’t seem to explain the dramatic effects observed in the northeast Pacific ecosystem, since there was still plenty of plankton left in the water, just different species.

The Blob’s effects on mammals and fish might be a result of poor-quality food rather than a shortage, she explains: “If you ate nothing but celery all day, you might not feel very full.” The smaller, warmer-water plankton may not pack quite the same nutritional punch as the big, fat cold-water plankton, leaving animals nutritionally stressed even in water full of plankton. More research will clarify whether her hunch is correct.

Despite the importance of having a solid baseline, long-term monitoring is easy to neglect because its value may only become apparent with hindsight, says Batten. “Everyone thinks it’s important, but it’s hard to get it supported until something happens and people want the data.” But its value comes from keeping a finger on the pulse when there is no disaster, because no one knows when that data will suddenly be needed. “I have no idea what next year will bring,” she says. “No one predicted the Blob."

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The oldest CPR device still in operation in the CPR Survey’s fleet was built in 1938. Video via Alaska SeaLife Center.

The history of the long-running CPR Survey is tumultuous. After the survey peaked in 1970 when 5,506 samples were collected, its scope began to contract in the 1980s as UK government funding for long-term oceanographic monitoring projects was slashed because administrators considered environmental monitoring projects “poor science,” wrote a group of CPR researchers in 2005. Ultimately, the survey was temporarily shut down in 1989.

A rescue operation quickly sprang up, and a new charitable foundation—the Sir Alister Hardy Foundation for Ocean Science, named after the CPR’s inventor—was founded to operate the survey. In 1990, the CPR Survey was moved to its current home in Plymouth. But recent years have brought another shock for the charitable foundation. The CPR Survey was forced to suspend some shipping routes and reduce its staff by a third, and in 2018 was assimilated by the Marine Biological Association in Plymouth.

The CPR Survey’s capacity to resurrect might be due in part to its comparatively low running costs. Sending out research ships, or even just putting a researcher on a ship, is pricey, says Batten. But a mechanical device that can be thrown off the back of a ship, with no complicated electronics, is much less expensive. “It’s not pretty when you get it back—all dented and scratched—but it does work, and works well,” she says.

The CPR Survey has a proud scientific record, with its data feeding hundreds of papers on climate change, biodiversity, and sustainability. But there are limitations to the data these antique devices can provide. The CPR device can only gather data from the surface of the ocean, leaving the depths untapped. And the method’s reliance on volunteer ships subjects it to the contingencies of commercial routes. But its long-term record, says Ashjian, is really valuable, and the antique technology still complements modern techniques like rapid photography and acoustic sampling, which study the ocean without capturing any physical plankton. “If you want to know what species something is, or maybe even what life stage it is, you still have to get the bug,” she says.

Although researchers are now attaching more advanced equipment to CPR devices, like temperature sensors, the original design is set to carry on running indefinitely. “We’ve yet to invent something electronic that can do the same thing,” says Batten. “It’s hard to improve on."

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