Even when reduced to particles smaller than a grain of salt, plastic debris may pose a large threat to marine environments, researchers warn in a Policy Forum published in the 10 July issue of Science.
Microplastics, pieces of plastic millimeters across or smaller, are likely the most abundant type of plastic debris in the sea. "They are dispersed in all of our oceans," explained Policy Forum co-author Kara Lavender Law, research professor of oceanography at Sea Education Association in Massachusetts. "They are found at the sea surface, on the sea bed, and on the shoreline. But only in recent years has there been an acceleration of research activity with respect to understanding their potential impact on the marine environment."
Microplastics enter the ocean through runoff, wind, and river water. They come from larger plastic pieces like lost cargo from ships at sea or plastic bottles that have deteriorated, or from little bits of plastic called microbeads that are used in cosmetics.
Because they are so small, microplastics pose a threat to a wide range of organisms, which ingest them and any of the industrial additives they contain, as well as harmful chemicals like insecticides that the plastics accumulate from the seawater. This problem can be compounded when chemicals ingested by small animals work their way up the food chain in higher concentrations as the animals themselves are eaten.
Plastic collected using a plankton net in the North Pacific Ocean. The fragments are generally 1-5 millimeters in size. | G. Proskurowski/ Sea Education Association
Though digestion of microplastics is well-documented in mammals, fish, birds, and invertebrates, scientists still don't know just how damaging it is in the wild, largely because most evidence to date comes from laboratory studies. "These studies show emerging evidence of both physical and toxicological effects of ingested plastic," explained Law, "but the extent of any damage, particularly in natural environments, is still uncertain."
In order to more comprehensively assess the risk of these pollutants in the natural world, scientists have to determine how often animals encounter them in their natural settings. But, the size of most microplastic particles makes them hard to sample. "Because they are so small," Law said, "they can't be detected by remote methods like satellites. We must rely on ships in the open ocean."
Scientists have made some progress surveying vast areas of open sea for tiny plastic debris, but studying the deeper water column and seafloor is a significantly more difficult task, Law explained. Adding to the challenge of understanding the impact of these minuscule pollutants, microplastics travel in unpredictable ways, turning up both far away from land and on the coast.
All these factors have left scientists struggling to estimate the ultimate effects of microplastics on natural populations, and on human food safety and public health. In this setting, Law urges more research — to understand the rate at which organisms encounter these tiny contaminants, and to learn more about different polymers from which these plastics are made.
Though substantial removal of microplastics from the ocean might seem like a solution, it is not feasible, Law said. "Even in [places] where the highest concentrations of floating microplastics have been documented, you might only find one particle per square meter. Thus, to remove a substantial amount of material using plankton nets, for example, you would have to cover immense regions at great expense."
"And even if this were logistically and economically feasible," she added, "it is likely that any method that removes floating microplastics is also likely to remove organisms of similar size, such as the plankton that form the base of the food web."
Instead of trying to remove plastic, she advocated reduced consumption of plastics, a strategy that could limit the entry of microplastics into the ocean. Recycling these products at the end of their lifetime could help, too.
"If we design products more effectively for end of life," said co-author Richard Thompson, professor of marine biology at Plymouth University, "then the materials [like plastics] we regard as waste can become a resource. Every ton of waste that is used for new production represents a twofold benefit since it reduces reliance on non-renewable resources and reduces the potential for waste to end up as debris in the environment."