marine debris; however, existing information about marine debris and its impacts is sufficient to support immediate action to arrest this global environmental problem.

ABUNDANCE AND FLUX

For many people, the term “marine debris” evokes images of litter strewn on a beach, such as the one shown in Figure 2.1, but marine debris is much more than beach litter. Debris is found throughout the marine environment, from coastal waters to the deep ocean and from the sea surface down to the benthos (Figure 2.2). Monitoring is a primary mechanism for identifying sources; for understanding temporal and spatial trends in debris composition, prevalence, and distribution; and, therefore, for understanding the extent of the problem and the effectiveness of efforts to address it. The vastness of the world’s oceans makes estimating the total amount of marine debris a significant challenge; nonetheless, the number and geographic coverage of studies carried out so far (see Appendix C, Tables IIII) highlight the worldwide pervasiveness of marine debris.

Coastal Environments

Coastal areas have served as the primary focal point for marine debris awareness and mitigation and remain hotspots of marine debris accumulation. Debris of both terrestrial and maritime origins converges and is concentrated at the land–sea interface. Because of the visibility of the problem, more is known about the occurrence and impact of marine debris along coastlines than in any other marine environment.

The prevalence of shoreline debris deposits is summarized in Appendix C, Table I. Marine debris items range from 4 to more than 48,000 items per kilometer (km) of shoreline, while the weight of the items ranges from 31 grams per km to more than 3.8 metric tons per km. Plastic materials dominate coastal marine debris in number, volume, and weight at all debris sizes examined to date, particularly on beaches and areas near population centers (e.g., Ribic et al., 1997; Sheavly, 2007). Because of the variation in methods used (e.g., data collected along transects from the waterline to the “edge” of the beach, along transects parallel to the shoreline, or along a strandline where debris is likely to be highest), straightforward comparisons among studies is problematic.

The majority of studies of coastal marine debris have noted increasing quantities of debris (e.g., Merrell, 1984; Ryan and Moloney, 1993; Walker et al., 1997; Willoughby et al., 1997; Velander and Mocogni, 1998), other studies found no change over time (e.g., Lucas, 1992; Williams and Tudor, 2001; Santos et al., 2005a; Sheavly, 2007), and a few studies have docu-



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