A Validated Method to Quantify Microplastic Contamination in Subsurface Seawater: A case study sampling the Sydney nearshore under sail

AIMS, in collaboration with The Clean Ocean Foundation, has completed this proof-of-concept study to facilitate further sampling of microplastics in the Southern Ocean (a joint collaborative initiative involving the Australian Institute of Marine Science (AIMS), the Integrated Marine Observing System (IMOS) and solo yachtswoman Lisa Blair). In this project, AIMS trialled density flotation and chemical digestion separation methods to process subsurface seawater samples and retrieve environmental microplastics. This report presents findings from controlled spike recovery tests as well as opportunistic field sampling events of subsurface waters processed with both separation methods. Opportunistic samples were collected along the NSW coastline in December 2021. Given the location of the sampling transects, this report also provides the first spatial baseline information on microplastic contamination in NSW waters adjacent to estuary outflows and urban outfalls. After trialling both separation methods, microplastics retrieved from the NSW samples were physically and chemically characterized used stereomicroscopy and spectroscopy, respectively. No statistical analysis were performed due to the low number of replicates. Results showed repeated chemical digestion employing potassium hydroxide solutions was most effective at microplastic recovery. Microplastics were retrieved from each of the eight samples collected in NSW subsurface seawaters, with polyethylene and polyester fibres being the most prevalent polymer types detected. The highest numbers of microplastics were recorded adjcent to the estuary outflows and urban outfalls present within the sampling area. Overall, the sampling and processing protocols developed here are allowing for the extension of the spatial coverage of microplastic data in seawaters.