Ecosystem Effects and Magnitude of Microplastics Pollution in St. Andrew Bay, Florida

Microplastic (MP) pollution is an ongoing problem in coastal systems, where wastewater treatment plants (WWTPs) and other sources deposit particles daily. While research regarding MP pollution worldwide is increasing, especially surrounding the role of WWTPs in facilitating MP inputs to marine environments, two major gaps exist within the MP literature: (1) MP pollution data within United States coastal areas, especially in marine sediments; and (2) MP effects on animals that may be most at risk. MPs are bioavailable to a range of marine fauna, including fish, benthic invertebrates, and marine mammals, and they can transport invasive species, alter biogeochemical cycles, and they can cause physical stress to fauna. These negative effects are potentially most pronounced at the base of food webs and in fauna that perform microecosystem functions, due to their small sizes and high likelihood for interaction with MPs. I posed five questions in my dissertation: (1) What is the spatial and temporal distribution of MPs in St. Andrew Bay, Florida? (2) How are spatial and temporal factors correlated with the identities of MPs in St. Andrew Bay? (3) What spatial distinctness exists within nematode communities in the St. Andrew Bay? (4) How do microbe-MP associations alter marine nematode feeding behaviors? And (5) How do MPs alter interstitial marine ecosystem functions? I used a combination of field collections and experimental manipulations to address these questions, as microplastics and interstitial communities are difficult to manipulate in natural environments. To assess spatiotemporal variation in MP pollution within St. Andrew Bay, I conducted a seasonal sampling from October 2020 to February 2022 along 1-km transects across five separate parts of the bay system. Sediments in areas of WWTP outflow contained more MP pollution than areas without direct outflow, and concentrations of MPs were highest in winter months. Additionally, MP concentrations at the Millville WWTP were nearly an order of magnitude higher than anywhere else in the bay system, suggesting that damages to and inefficiencies at the plant are exacerbating the magnitude of MPs imported into this system, though other factors that help explain MP loading patterns, including road runoff, tidal and wave action, and microplastic degradation, were not evaluated. Extrapolation of mean MP concentrations in the bay system suggests that there may be approximately 30 billion MPs in the bay at any time, which are likely exported out to the Gulf of Mexico during heavy rains or storms. To assess spatiotemporal variation in MP compositions within the bay system, I evaluated the MP polymer types, particle shapes, and sizes. Unexpectedly, fragments were the dominant particle shape at WWTP sites, while sediments from other sites were mostly fragments and fibers. Mean MP sizes were significantly smaller at WWTP outflow sites than elsewhere, which suggests that waste processing and filtration retains larger particles or that particle collisions during processing and export lead to smaller MP sizes. In the winter, WWTP sediments were dominated by polyester and polyethylene terephthalate, which are indicative of the role that laundering of synthetic textiles plays in coastal MP pollution. To evaluate spatial distinctness within nematode communities in the St. Andrew Bay system, I extracted nematodes from sediments at the two WWTP sites and two reference sites at two separate times: winter and summer. Nematode community diversity was strongly associated with the distance to the nearest WWTP, and communities were dominated by tolerant, opportunistic genera. These findings suggest that the system is possibly stressed by organic enrichment, potentially linked to the WWTPs, but has not reached an ecological tipping point evidenced by an abundance of pollution sensitive nematode genera. To assess the effect of microbe-MP associations on marine nematode feeding behaviors, I set up a factorial experiment varying microbe presence, MP concentrations, and MP exposure time. Microbe-MP associations did not encourage nematodes or other meiofauna to consume MPs, and while nematodes did consume MPs, consumption was rare, suggesting that accidental ingestion may be a more plausible reason. Finally, to assess the effect of MPs on interstitial marine ecosystem functions, I conducted a mesocosm experiment with a control and three MP concentrations (low, medium, high), and luminophores—glow in the dark particle tracers—to assess bioturbation and oxygen microelectrodes to track oxygen turnover. All MP concentrations resulted in reduced bioturbation depths but only the high treatment resulted in increased oxygen penetration depth compared to the control treatment. Discrepancies in bioturbation and oxygen penetration trends under the influence of MP pollution suggests that MPs may be differentially affecting size classes of fauna. Decreased bioturbation because of MP contamination supports that MPs are reducing this behavior in macrofauna and meiofauna members in the sediments. However, increases in oxygen penetration depth that do not align with bioturbation depth suggest that macrofauna and meiofauna may not be responsible for these oxygen trends. Instead, deeper oxygen penetration depth may be linked to altered microbial activities, changes in microbial community compositions where less effective oxygen-consuming bacteria are present in abundance, or changes in sediment physical structures that allow for deeper oxygen penetration at high MP concentrations.