Microplastics in St. John, United States Virgin Islands Sediments: Methodological Approaches and Variability of Microplastics Concentrations and Accumulation Rates

Sediment samples were collected from a near shore and reef site in Coral Bay, St. John, United States Virgin Islands (USVI) from 2007-2016, using tube sediment traps. These samples provide an opportunity to study microplastics (MPs) in an area where no previous MP studies have been performed. Most sedimentary MP studies utilize benthic samples which can only measure MP concentrations (#MP/g or #MP/mL sediment), while sediment traps can provide MP accumulation rates (#MP/cm2/day) as a function of time. However, the methods used for extracting MPs from benthic samples have not been tested using sediment trap samples whose characteristics, such as low volumes and variable sedimentary composition, can lead to a more challenging MP extraction. Our objectives were: 1) to conduct a series of pilot studies in order to determine the best methods for applying density separation to sediment trap samples, with a focus on contamination minimization and maximization of MPs recovery, and 2) to determine the concentration, type, and accumulation rates of microplastics in the reefs of St. John. Based on the methodological tests, we employed a sequential phased (interstitial water, organic matter treatment, zinc chloride separation) density separation method in the extraction of MPs from St. John sediment trap samples which resulted in low blank (contamination) levels. Though variable, measurable MP were recovered from each phase, with 15.5±12% and 25±17% of the MP extracted from the sea-water and hydrogen peroxide phases, respectively. Concentrations ranged from 153-6,622 MPs/100mL sediment and 2.8-102 MPs/g sediment. MP accumulation rates ranged from 180-4,800 MPs/day/m2, with the variability driven primarily by the location and depth. Microplastic concentrations were found to be far greater than found in similar tropical reef environments, including heavily urbanized areas, and future research should examine the how field sampling practices affect measurements of MP concentrations and accumulation and how MP vary with sediment composition, sedimentation rates and MPs flux through the water column.