A study by Woods Hole scientists has found that microplastics have been accumulating in the sediments of Waquoit Bay salt marshes at increasing rates since as far back as 1950, when plastics came into widespread use.
Javier Lloret, a researcher at the Marine Biological Laboratory and the lead author of the study, described salt marshes as “depositional environments” that “grow vertically” as cordgrasses and other plant life die off, forming layers of peat sediment.
“If you go deeper into those sediments, you are going back in time; you need to imagine this like the rings in a tree trunk,” Dr. Lloret said. “In the last 25 years only, the number of microplastic particles that we find in the sediments has doubled.”
For decades, plastic production and consumption have proliferated. In 2018, global plastic production reached 360 million metric tons and has only continued to grow. Millions of tons of plastic are estimated to enter the oceans each year.
The new study, published in Environmental Advances, analyzed sediments in salt marshes in six Waquoit Bay estuaries and New Bedford Harbor, with watersheds ranging from undeveloped to more than 80 percent developed. Researchers found microplastic contamination at every location sampled.
“This contamination is very pervasive; it’s everywhere,” Dr. Lloret said. “It doesn’t really matter if you have people [living in the watershed] or not; you’re still going to find microplastics.”
Microplastics—which are particles of plastic smaller than 5 millimeters—are mostly microscopic, but some sediment cores from the study contained microplastics visible to “the naked eye,” Dr. Lloret said.
The researchers found a clear connection between the level of development in a watershed and the amount of microplastics accumulated in the salt marsh sediments. As development of the watershed nears 50 percent, microplastic accumulation begins to grow exponentially, Dr. Lloret said.
“The land, when it is developed pretty sparsely, that is not contributing a lot of plastics to the environment, but once you develop the area and the area becomes urban or suburban, people are doing a lot of stuff in the area,” he said. “Once the land is 50 percent [developed], the number of microplastics grows exponentially—grows very fast.”
The study focused on two types of microplastics: fragments, which break off from larger pieces of plastic; and fibers, which are shed from synthetic materials used in clothing and fishing gear.
“What we found in the study was a very contrasting story and history of this type of contamination for the fragments...and fibers,” Dr. Lloret said.
As urbanization in the watershed increased, the amount of microplastic fragments trapped in the salt marsh sediment increased. Microplastic fibers, however, showed no relation to the level of development in the watershed.
“What it’s telling us is the contamination by fragments is produced by the people in the immediate area surrounding the salt marshes, but the contamination by the microfibers depends on the people that are living in the region as a whole,” Dr. Lloret said.
To recreate a local history of plastics, the study also gathered deeper sediment cores from two locations in Waquoit Bay: Timms Pond, which has no development in its watershed; and Childs River, which is heavily developed.
From the sediment cores in these two estuaries, the researchers were able to date the layers of sediment by decade. Below the sediment layer dated to about 1950, researchers found no microplastic contamination.
“The number of people that were on the Cape [around 1950] was relatively limited and they basically were not using plastics at home or in any kind of activity,” Dr. Lloret said. “As populations started to grow, and as the amount of plastics being used per capita increased, the numbers of microplastics have increased very dramatically.”
The human presence in Waquoit Bay has increased sevenfold since 1960. As the tourism industry transformed the rural environment into a more urbanized environment, plastic production grew exponentially in the United States.
Increases in microplastic accumulations over the decades were evident in both the samples from Timms Pond and the Childs River salt marshes, the researchers found. Microplastic concentrations in the urbanized Childs River watershed, however, increased at a much greater rate.
“Local people here on Cape Cod should pay a lot of attention to this plastic contamination of the environment,” Dr. Lloret said.
Salt marshes are important ecosystems that filter nutrients, sequester carbon, protect against erosion, and foster a wide diversity of plant and animal species. While scientists have not yet assessed the full impact of microplastics on salt marshes, there is cause for concern, Dr. Lloret said.
Other studies have shown that microplastics can accumulate in the digestive systems of filter feeders such as mussels, oysters and other shellfish, causing adverse health effects or even death, he said.
Moreover, microplastics could be passed through the food web as other organisms consume filter feeders, he said. Since shellfish are an important commercial species, humans are also likely consuming microplastics, he said.
“If some animals or all of the animals and plants that live [in the salt marsh] are being affected by this, there is a potential that the functions and services that the salt marshes as a whole provide are being compromised,” Dr. Lloret said.