Even when the sea looks clean, its surface can be flecked with tiny fragments of paint and fiberglass. That’s the finding from a study that looked for plastic pollution in the uppermost millimeter of ocean. The microscopic fragments come from the decks and hulls of boats, and they could pose a threat to tiny creatures called zooplankton, which are an important part of the marine food web.

The discovery is “continuing to open our eyes to how many small synthetic particles are in the environment,” says Kara Law, an oceanographer who studies plastic pollution at the Sea Education Association in Woods Hole, Massachusetts, and wasn’t involved in the study.

The millimeter-thick skin of the ocean, called the sea surface microlayer, differs from the underlying water. Surface tension and sticky secretions from microbes keep tiny particles within this layer. Previously, scientists scanning the ocean for plastic pollution hadn’t looked specifically at this layer. Instead, they’d take bulk samples from the surface microlayer and below. Earlier studies also tended to use coarser nets that don’t capture the tiniest particles.

Researchers led by Won Joon Shim, an environmental chemist at the Korea Institute of Ocean Science and Technology in Geoje-shi, South Korea, focused on the microlayer itself. They collected water samples along the southern coast of Korea, up to 16 kilometers offshore. When they gently touched a sieve with 2-millimeter-wide holes to the surface, water from the microlayer gloms on. Examining the samples in the lab, the researchers found well-known kinds of plastics: polyethylene, polypropylene, and expanded polystyrene. But, to their surprise, these made up just 4% of the particles.

Eighty-one percent of the synthetic particles in the microlayer consisted of alkyds, a binder in paints, the team reported online ahead of print in Environmental Science & Technology. Another 11% were polyester resins used in paint and fiberglass. On average, a liter of water from the microlayer contained 195 particles—this concentration is 10 to 100 times higher than microplastic particles in water collected by other methods. Shim’s group has found similar abundances of paint particles along another part of the Korean coast. There will probably be fewer paint particles in the open ocean and coastal regions with less shipping and fishing, Shim says.

The paint and fiberglass particles are coming from the more than 17,000 small fishing boats that ply these waters, the researchers determined. Under the microscope, the fragments were dark green and dark blue, typical colors for the boats. In addition, a laboratory test using infrared spectroscopy matched the particles with paint chips that the researchers collected from local shipyards. Alkyd paints are used above water, so these coatings (and the fiberglass) end up in the water after being abraded by nets, ropes, or anchors, or when the boats are scraped and repainted.

Fortunately, the group did not find particles of antifouling paint, which is used on hulls and contains toxic chemicals. These chips are dense and likely sink; previous studies have found them in bottom sediments in maintenance areas. It’s not clear how much danger the floating alkyd particles could be to marine life, Shim tells Science in an e-mail. Some kinds of alkyd paints contain heavy metals; in addition, they and fiberglass resin particles can absorb other toxic chemicals. Furthermore, they are smaller and more abundant than plastics, so small creatures, such as zooplankton, might eat them in the surface microlayer, Law says.

Shim and his colleagues now plan to study the metals and organic chemicals on the paint particles and determine whether they can harm marine life.