In 1925, on a fruitless whale-watching expedition somewhere in the Southern Ocean, Sir Alister Hardy had a bright idea. The British zoologist had been tasked with investigating whale populations while aboard the RSS Discovery but he was having trouble spotting any cetaceans. Hardy soon realized something: The smartest way to find something as big as a whale was to find something as small as plankton.
Hardy needed to find a way to track and evaluate changes in the abundance and distribution of plankton in order to understand anything about what preyed on them. Hardy’s solution, called the Continuous Plankton Recorder, would exceed his expectations over the rest of the 20th century, not just in tracking populations of plankton, but also in archiving the abrupt rise of plastic pollution across the world’s oceans.
For his first prototype, Hardy whipped up a metal instrument that could pick up planktonic stowaways across an entire ocean, all while hanging off the back of a barge. In 1930, he brought a smaller model back on the Discovery to sample Antarctic krill, a tiny crustacean that feeds on phytoplankton. After making minor tweaks to the design, Hardy led the first CPR survey in September 1931, collecting oceanic microorganisms from Hull, England, all the way to Bremen, Germany. Hardy saw this data as a service, one as simple and necessary as weather forecasting, and something that could help the fishing industry operate more economically.
Now operated by the Marine Biological Association in Plymouth, England, the CPR survey is one of the longest running marine biological monitoring programs in the world, according to a 2003 history of the survey. Since 1948, volunteer barges and merchant ships have carried CPRs in their wake in pursuit of an international chronicling of the earth’s most microscopic species. But in recent years, this project has moonlighted as an archivist of something more sinister and considerably less decomposable, according to a study published April 16, 2019, in Nature Communications. In 1957, a CPR recorded the first known example of plastic pollution in the ocean: a plastic piece of trawl twine. In 1965, a plastic bag.
Because the CPR survey’s methods of sampling and analyzing the plankton have remained unchanged since 1948, the survey has accumulated a comprehensive 60-year data set of both plankton patterns and plastic pollution. Over the past few years, researchers had noticed deployed CPRs were getting more and more entanglements, according to Clare Ostle, a marine biochemist at the Marine Biological Association and the study’s lead author. So they decided to pore over the survey’s historic logbooks and count the number of times a CPR had been stopped in its tracks by plastic.
A CPR is essentially a three-foot-long metal torpedo that hangs off the back of a ship, dangling anywhere from 10 to 30 feet underwater, somewhat mimicking the swimming habits and body type of an average seal, Ostle says. Its stomach contains a cassette of two tightly wound rolls of silk. Water enters the CPR through an aperture at its front and passes through the tiny holes in the silk, ensuring that larger particles in the water column, such as plankton and microplastics, will get caught in the netting. While the cassette entraps these organisms and particles, the CPR continuously winds its roll, ensuring scientists can later geolocate where each sample was collected. The silk then enters a storage tank filled with the preservative formalin.
The entire process is similar to the way film is rolled and stored inside a camera, according to the 2003 history. If the scientists didn’t immediately douse the plankton in formalin, Ostle says, “they would be very stinky and hard to identify if they degrade.”
Later, scientists in laboratories unwind the silk scrolls to count and identify all the species, largely consisting of phytoplankton, copepods, and zooplankton. Every four inches of unwound silk corresponds to 10 nautical miles. The researchers cut and store these sheets of silk in boxes in a specialized, temperature-controlled library in the Marine Biological Association, which stores plankton samples going back all the way to the 40s, Ostle says.
Because CPRs spend their lives trailing behind container ships, they’re most frequently hauled up by crew members from those ships who are asked only to record in a form where, geographically, they pick up and drop off the CPR. The form includes a small section for comments, which Ostle’s team realized was a goldmine of recorded plastic pollution (as well as one-off comments about the weather). Whoever hauled up the CPR in 1957 wrote down less than entire sentence: “recorder fouled by trawl twine.” These comments soon arrived in hordes, noting entanglements wrought by plastic fibers, fishing nets, blue twine, and, of course, way more plastic bags.
Aside from uncovering evidence of some of the earliest examples of oceanic manmade pollution, Ostle’s team is the first to begin to quantify the often observed, but never proved, rapid proliferation of plastics in the sea since the 1990s, confirming what scientists had long believed. From the 1950s to the 1970s, fewer than one percent of CPRs experienced any entanglement with synthetic materials—many were often deterred by natural causes, such as a knotted strand of seaweed or a rogue pipefish that wound its way inside the canister. By the 1990s, almost two percent of CPRs were derailed by plastic. “The clear increase kicked up after the ‘90s, where most entanglements were driven by fishing-related items,” Ostle says. The current figure is somewhere around four percent.
Due to its microscopic focus, the CPR isn’t built to account for larger pieces of plastic pollution, such as water bottles. One time when Ostle was taking samples on a ship sailing in the open ocean from the United Kingdom to the Caribbean, she noticed something strange bob up against the CPR. “It wasn’t a bird or a turtle, but a floating toilet seat,” she says. “It just bounced off, as there’s no way the CPR would capture that.” But CPRs are an effective record of plastics that tend to entangle, as well as an archive of microplastics that scientists have recently begun to count.
The survey nearly faced obsoletion in the 1980s, when scientists considered such monitoring to be as weak a science as stamp collecting (a real zinger by British standards), according to the survey’s history. Thanks to government cuts, the survey was officially closed in 1989, four years after Sir Alister Hardy’s death. For several years, it had been operated by a skeletal staff working outside the bounds of an official laboratory. But it reopened soon, thanks to a rescue package from the U.K. Ministry of Agriculture, Fisheries, and Food.
Since then, the survey has gained appreciation from the scientific community for its tireless decades of work, monitoring the world’s plankton populations even when no one quite asked it to. Since the first CPR tow in 1931, the humble device has traveled over four million miles and collected close to 190,000 samples, according to the survey’s history. “It’s such a simple design and doesn’t really have anything flashy or new, so people still suggest we should move on,” Ostle says. “But having a consistent dataset is hugely important if you want to look at big shifts.”