River and Urban-Related Microplastic Pollution in the Gulf of America: A Modeling Study

The Gulf of America (also known as Gulf of Mexico or the Gulf), faces increasing threats from microplastic (MP) pollution especially in its northern region where it receives substantial MP input from the Mississippi–Atchafalaya River system and direct discharges from wastewater treatment plants in coastal areas. The Gulf supports critical habitats for diverse marine species, making it vulnerable to MP contamination. In this study, a Lagrangian particle-tracking model coupled with a high-resolution (1 km) three-dimensional regional ocean model is used to investigate the short-term transport of MPs in the northern Gulf. MPs were released daily over a three-year period (2014–2016) and tracked for 30 days following their release. To assess ecological risks, we linked simulated MP dispersal patterns with habitat and marine protected area data compiled from previous studies and reports. The particle-tracking model accounts for individual MP characteristics, including size and density, and incorporates the influence of Stokes drift on floating MPs. Results show that particle density is key in determining the distribution of settled MPs, while its effect on non-settled MPs is limited. The impact of Stokes drift on the transport of floating MP is also very small. River-sourced MPs emerged as the dominant contributor to pollution in this region. Simulations revealed a prominent accumulation zone west of the Mississippi River Delta, which overlaps with several ecologically and economically important marine species habitats, including Kemp’s ridley sea turtles, red snapper, and bottlenose dolphins.