Monitoring the abundance of plastic debris in the marine environment

Abstract

Plastic debris has significant environmental and economic impacts in marine systems. Monitoring is crucial to assess the efficacy of measures implemented to reduce the abundance of plastic debris, but it is complicated by large spatial and temporal heterogeneity in the amounts of plastic debris and by our limited understanding of the pathways followed by plastic debris and its long-term fate. To date, most monitoring has focused on beach surveys of stranded plastics and other litter. Infrequent surveys of the standing stock of litter on beaches provide crude estimates of debris types and abundance, but are biased by differential removal of litter items by beachcombing, cleanups and beach dynamics. Monitoring the accumulation of stranded debris provides an index of debris trends in adjacent waters, but is costly to undertake. At-sea sampling requires large sample sizes for statistical power to detect changes in abundance, given the high spatial and temporal heterogeneity. Another approach is to monitor the impacts of plastics. Seabirds and other marine organisms that accumulate plastics in their stomachs offer a cost-effective way to monitor the abundance and composition of small plastic litter. Changes in entanglement rates are harder to interpret, as they are sensitive to changes in population sizes of affected species. Monitoring waste disposal on ships and plastic debris levels in rivers and storm-water runoff is useful because it identifies the main sources of plastic debris entering the sea and can direct mitigation efforts. Different monitoring approaches are required to answer different questions, but attempts should be made to standardize approaches internationally.

Figure 1.

Schematic diagram showing the main sources and movement pathways for plastics in the marine environment, with sinks occurring (1) on beaches, (2) in coastal waters and their sediments and (3) in the open ocean. Curved arrows depict wind-blown litter, grey arrows water-borne litter, stippled arrows vertical movement through the water column (including burial in sediments) and black arrows ingestion by marine organisms.

Figure 2.

Scoring litter collected from a 50 m stretch of beach (top) and trends in the abundance of plastic bottles and lids (mean and s.e.) at South African beaches sampled in 1984, 1989, 1994 and 2005. Light grey bars, 36 beaches with regular, municipal cleaning programmes; dark grey bars, 14 beaches with no formal cleaning programmes (P. G. Ryan & C. L. Moloney, unpublished data).

Figure 3.

A manta trawl (a) being deployed from a research vessel and (b) being towed at sea to sample floating plastics.

Figure 4.

Long-term trends in the impacts of plastics on marine animals: (a) numbers of entangled seals recorded annually on SE Farallon Island, California, 1976–1998 (reproduced with permission from Hanni & Pyle 2000); (b) proportions of prions Pachyptila spp. stranded on New Zealand beaches that had plastics in their stomachs, 1958–1977 (reproduced with permission from Harper & Fowler 1987).

Figure 5.

Typical plastics from a Northern Fulmar stomach (top panel) and trends in the average mass (±s.e.) of user plastic and virgin plastic pellets in Northern Fulmars stranded on Dutch beaches in 1982–1990 (n = 69) and 1997–2005 (n = 580).