Macrolitter accumulation in tidal marsh wrack deposits in a large estuary

Wrack refers to debris deposited along the upper intertidal zone by waves and tides. In the past, wrack consisted mainly of natural debris, but modern wrack includes anthropogenic macrolitter (AML). When AML is abundant, wracks become pollution hotspots that threaten ecosystem functioning. Here, we assess AML abundance, composition, and sources in wrack deposits from 27 tidal-marsh sites in the Chesapeake Bay estuary (USA) and examine relationships with surrounding urban pressure and marsh conditions. Marsh wrack deposits in the estuary are dominated by wood, Phragmites, and marine macrophyte debris. AML abundance and mass range from 56 to 4481 items km^-1 and 0.9 to 254 kg km^-1 of wrack, with highest values in the inner estuary. Both variables decrease with increasing distance from main population centers. Marshes with higher vegetation density index also have high AML pollution. Most AML (93.5%) is positively buoyant, predominantly made of plastic (90%), and small (2-25 cm: 74.5%). Items are mainly plastic bottles and fragments, with a lifetime of <1-year. Older bottles (4 to >10 years-old) dominate the inner estuary, whereas newer bottles (<1 year-old) dominate the outer estuary. Food-consumption (38.9%) and fisheries (6.2%) related items are most prevalent. Over half of the AML comes from land-based sources, with >60% resulting from poor waste management. Marsh wrack deposits act as important accumulation zones for urban-derived AML, with high potential ecosystem impacts. Therefore, the inner estuary and its wrack deposits must be given priority for further research on AML transport, deposition, and carbon-related effects, and mitigation actions.