Monitoring marine plastics – will we know if we are making a difference?

Abstract

In the context of marine anthropogenic debris management, monitoring is essential to assess whether mitigation measures to reduce the amounts of waste plastic entering the environment are being effective. In South Africa, baselines against which changes can be assessed include data from the 1970s to the 1990s on microplastics floating at sea, on macro- and microplastic beach debris, and interactions with biota. However, detecting changes in the abundance of microplastics at sea is complicated by high spatial and temporal heterogeneity in net samples. Beach debris data are easier to gather, but their interpretation is complicated by the dynamic nature of debris fluxes on beaches and the increase in beach cleaning effort over time. Sampling plastic ingested by biota is a powerful approach, because animals that retain ingested plastic for protracted periods integrate plastics over space and time, but there are ethical issues to using biota as bioindicators, particularly for species that require destructive sampling (e.g. turtles, seabirds). Bioindicators could be established among fish and invertebrates, but there are technical challenges with sampling microplastics smaller than 1 mm. Fine-scale debris accumulation on beaches provides an index of macroplastic abundance in coastal waters, and offers a practical way to track changes in the amounts and composition of debris in coastal waters. However, upstream flux measures (i.e. in catchments, rivers and storm-water run-off) provide a more direct assessment of mitigation measures for land-based sources. Similarly, monitoring refuse returned to port by vessels is the best way to ensure compliance with legislation prohibiting the dumping of plastics at sea.

TSC Opinion

Cet article souligne clairement le manque de méthode pour quantifier avec précision la pollution plastique. Elles sont essentielles à l’évaluation des mesures préventives. Les auteurs soulignent que pour les sources fluviales, des systèmes d’observation sont assez facile à mettre en œuvre de manière semi-automatique. Pour les déchets marins, l’évaluation des quantités échouées donne des estimations indicatives. Par contre, la quantification des déchets flottants et ayant sédimentés sur les fonds est complexe à mettre en œuvre. Seuls les déchets rapportés au port par les bateaux sont faciles à contrôler et pourraient donc faire l’objet d’un suivi.