Monitoring Plastic Beach Litter by Number or by Weight: The Implications of Fragmentation

Abstract

Eighty surveys of ten Scottish beaches recorded litter sizes and weights. A simple model of fragmentation explains the distribution of plastic beach litter weights, producing a logarithmic cascade in weight-frequencies having a power law exponent of 1.6. Implications of fragmentation are numerous. Heavy litter is rare, light fragments are common. Monitoring by number is sensitive to minimum observable fragment size, age of the litter, and energy of the foreshore. Mean litter item weights should be used to calculate beach plastic loadings. Presence/absence of mega litter can distort monitoring by weight. Multiple surveys are needed to estimate mega litter statistics. Monitoring by weight can change the perception of the importance of litter sources (e.g., in our surveys, contribution from fishing was 6% by number, 41% by weight). In order to introduce consistency between beach surveys using visual methods by number, a standard minimum plastic fragment size should be introduced.

Simulating the distribution of beached litter on the northwest coast of Scotland

“…Originating from sources both on land and at sea, beached litter can comprise a wide range of materials, including plastic, metal, wood, rubber, glass, and paper (Nelms et al, 2017;Turrell, 2019;Morales-Caselles et al, 2021). Many studies (e.g., Derraik, 2002;Topcu et al, 2013;Nelms et al, 2017;Lebreton and Andrady, 2019;Turrell, 2019;Morales-Caselles et al, 2021;Smith and Turrell, 2021;Damian et al, 2022) have shown that litter composition is dominated by plastic, because of its buoyancy and persistence (Topcu et al, 2013). According to the OSPAR commission (OSPAR, Commission, 2019), up to 90% of the items found on beaches in the OSPAR Maritime Area were plastic.…”

“…Once a particle enters the ocean it is exposed to horizontal and vertical advection, diffusion, (e.g., Critchell and Lambrechts, 2016) beaching, and deposition in coastal and benthic sediments (e.g., Zhang, 2017;Turrell, 2018). The variability of deposition is influenced by fluctuating hydrodynamic factors, e.g., the prevalence of offshore/onshore winds, the suspension and resuspension by rising and falling tides (e.g., Turrell, 2018), as well as the impact of in-situ fragmentation (Smith and Turrell, 2021), driven by physical processes such as, currents, wind, Ekman transport and wave-induced Stokes' drift (e.g., Dobler et al, 2019;Onink et al, 2019;. Wind, in particular, could have a significant impact on floating litter transport, and is represented by the intensity of wind drag on the exposed part of a floating object (Onink et al, 2019).…”

Improving the Source-to-Sea approach for marine litter in Brazil

“…Other studies were based on direct visual analysis in the field (n=9) or a mix of both visual analysis and sample collection (n=7). Although faster, visual analysis tends to underestimate the smaller items in a given area (Smith & Turrell, 2021), being a relevant source of bias in marine litter estimates.…”

“…We verified that 27.1% of the studies reported data as quantities per linear distance (m -1 , km -1 , ha -1 ), which is dependent on the non-steady beach width and thus, comprises a limitation to data comparisons. Furthermore, reporting data in terms of weight.area -1 may demand some special attention, since while lightweight fragments tend to be abundant, heavy and big items are not so common and, when they are present, they can affect total sample weight (Smith & Turrell, 2021).…”

“…However, employing distinct methods and expressing data in different units of measurement turns comparisons among studies complicated (Ryan et al, 2009;GESAMP, 2019;Smith & Turrell, 2021). Comparisons are also obstructed by lack of relevant information on strategies used, such as sampling area dimensions (Smith & Markic, 2013).…”