WRACK AND RUIN: CHARACTERIZING PLASTIC AND MICROPLASTIC OCCURRENCES ON SOUTHEASTERN LAKE MICHIGAN BEACHES

This study examines several Lake Michigan beaches in southwestern Michigan to assess possible sources, transport paths, and depositional fates of plastic litter and microplastic. Hypothesized sources of microplastic on beaches include in situ degradation of beach litter left by visitors, stranding of floating lake plastic/microplastic brought by waves and longshore currents, and river transport of plastic from inland locations.

Sites include Oval Beach (OB), a public, groomed beach, the adjacent Saugatuck Harbor Natural Area (SH), an ungroomed nature preserve (respectively 1.5 and 0.9 km south of the Kalamazoo River outlet in Saugatuck, MI), and Macatawa Park (MP), a private, groomed beach 600 m south of the Lake Macatawa outlet in Holland, MI. All sites are currently undergoing erosive shoreline retreat but vary in the presence and elevation of foredunes. We sampled visible plastic litter along 7 1-m wide transects running perpendicular to the shoreline and collected ~500 ml sediment samples from swash zone, lower beach, wrack zone, upper beach, and/or foredune locations to process by elutriation for microplastics.

Shotgun cartridge wads at SH and OB and industrial nurdles at all sites indicate non-local sources for at least some plastic. Between 93 and 99% of plastic and microplastic pieces at all sites have densities less than 1. Beach wrack, both in-place and recovered off-site from OB beach raking equipment, is associated with accumulations of macro- and microplastics. Wrack zone litter transects averaged 64 pieces/m² at MP and 128/m² at SH; other beach zones had no more than 1.8/m² at MP, 3.3/m² at SH, and 3.9/m² at OB. Foredune litter was 0.5 pieces/m² at MP and 1.1 at SH. Wrack zone microplastic samples averaged 5.7 pieces/kg at MP and 31.9/kg at SH, compared with 10 pieces/kg on the lower groomed beach at OB and no more than 0.8/kg at MP and 5.1/kg at SH for other beach zones. Foredune microplastic at MP and SH was 2.5 and 7.2 pieces/kg. Beach wrack may aid in deposition of plastics that otherwise would blow or be washed lakeward or inland. Variable availability of wrack for deposition, as well as variable wave energy conditions that disturb beach sand to different heights above the water line, complicate and may confound comparisons based on number of visitors, proximity to river mouths, or other site characteristics.