Exponentially increasing microplastic accumulation in an urban estuary: insights from the Narragansett Bay, Rhode Island seafloor.

Narragansett Bay, RI, has experienced decades of anthropogenic impacts and as a result has been a hotspot for pollution research. Most recently, microplastic pollution is of increasing concern, with data showing high levels of microplastic contamination on shorelines and in surface sediments. Plastic production worldwide has been increasing exponentially since the 1950s, and this system-wide chronologic study uses seabed and marsh sediment cores to show that plastic pollution in our coastal systems reflects the same exponential trend. Using ten sediment cores, 20-42 cm in length, ranging from outside the city of Providence in the northern estuary to 30 km offshore in Rhode Island Sound, we find exponentially increasing microplastic (63 µm-5 mm) concentrations from the 1940s to the present day. All sites show increasing diversity in plastic polymer types, colors, and morphologies over time. Marsh sediments contained microplastic concentrations ten to fifty times higher than the nearby seabed and have been accumulating microplastics at four times the rate over the past century, raising concern for the benthic and aquatic biota that rely on these important habitats. Today, nearly all sediment sites (9 out of 10) have exceeded thresholds of harm for benthic organisms. Model results suggest anywhere between approximately 2300 and 3000 tonnes of microplastics have accumulated in the Narragansett Bay seabed sediments over the past century. With few wide-scale mitigation measures in sight, these results should serve as a warning that without action, the pollution of our coastal systems will continue to increase exponentially.