Seasonal and spatial variations in microplastics abundances in St. Andrew Bay, Florida

Wastewater treatment plants (WWTPs) cause approximately 25 % of microplastics (MPs) in the marine environment. While research on MPs in WWTP effluent has demonstrated that an abundance of particles enter the marine environment, little effort has gone to assessing MP abundances in coastal sediments to determine their seasonal and spatial variability. Here, we assessed MP abundances in sediments at sites of WWTP outflow and at non-polluted sites over six consecutive seasons within the St. Andrew Bay system in Northwestern Florida. We showed that MP abundances were highest at one of the WWTP sites, where they increased with increasing distance away from the input source (3.16 ± 1.59 MP/kg to 34.03 ± 11.69 MP/kg sediment dry weight). We also found that mean MP abundances were highest in the winter (12.41 ± 3.56 MPs/kg sediment dry weight) and lowest in the spring (2.17 ± 0.63 MPs/kg sediment dry weight). Therefore, while WWTPs differentially retain MPs in their removal processes, MP pollution in the St. Andrew Bay system shows seasonal dynamics like other studies. Although average MP abundance in surface sediments (0-5 cm) was higher than in subsurface sediments (5-10 cm) at all sites, this difference was not as substantial as has been found in other studies. Based on mean MP abundance in surface sediments, we estimate that there are 30 billion MPs within the surface layer of sediment in the St. Andrew Bay system, and that the particles export to the Gulf of Mexico because of seasonal flushing between the winter and spring. The distributions of MPs in the system were also likely driven by extreme weather events that occurred in the bay system during 2018 and 2020, which acts as a cautionary tale for coastal urban ecosystems in the face of sea level rise and climate change.