On average, a full garbage truck load of plastic ends up in the world’s oceans every minute and floats in the water. However, a large part of it also sinks into the depths: a total of three to eleven million tonnes of plastic waste is already estimated to be on the bottom of the oceans, according to a study by the Australian science agency Csiro and the Canadian University of Toronto.

Using remotely operated underwater vehicles (ROVs), it has been possible for the first time to roughly quantify how much plastic waste ends up on the seabed and where exactly it accumulates before it is broken down into smaller pieces and mixed with the marine sediment, said Csiro researcher Denise Hardesty.

“We know that millions of tons of plastic waste end up in our oceans every year, but what we didn’t know was how much of this pollution ends up on the seafloor,” she explained. While there have been estimates of microplastics in the past, the new study looks at larger items such as nets, cups or plastic bags.

Seabed as a plastic reservoir

With plastic consumption expected to double by 2040, understanding how and where plastic waste is transported in the ocean is crucial to protecting marine ecosystems and wildlife, said the study, now published in the journal Deep Sea Research Part I: Oceanographic Research Papers” was published.

Plastic pollution on the seafloor is estimated to be up to 100 times greater than the amount of plastic floating on the ocean’s surface, said Alice Zhu, a University of Toronto doctoral student who led the study. The bottom of the ocean has thus become a long-term resting place or a reservoir for much of the plastic pollution. “This is exacerbated by the extremely slow degradation of plastic in cold environments that lack both oxygen and UV radiation,” the study says. According to the results, around half (46 percent) of the estimated plastic mass is located above a water depth of 200 meters, the rest (54 percent) in the following ocean depths of up to 11,000 meters.