Coupling beach ecology and macroplastics litter studies: Current trends and the way ahead

Highlights

• Keywords related to sandy beach ecology and plastics litter studies were selected.

• Co-occurrence analysis was applied to literature on beach ecology and plastics litter.

• All keywords were found in use, though with co-occurrence across compounds still low.

• To overcome limits highlighted, approaches were suggested for forthcoming research.

• Unequivocation of terms, focus on different scales and on dynamics were recommended.

Abstract

As sites of floating marine material deposition, sandy beaches accumulate marine litter. While research and assessment on beach litter is increasing and involves various actors (scientists, society and NGOs), there is the need to assess current and future dominant trends, directions and priorities in that research. As such, a textural co-occurrence analysis was applied to published scientific literature. Words were considered both singly and as part of compound terms related to concepts relevant to sandy beach ecology: morphodynamic state; Littoral Active Zone; indicator fauna. Litter as a compound term was also included. The main co-occurrences were found within compounds, with scarce interaction of “morphodynamic state” with the others, indicating the need for further integration of beach ecology paradigms into beached plastics studies. Three approaches are proposed to overcome the research limits highlighted: the unequivocation of terms, the consideration of adequate scales, and the attention to dynamics rather than just patterns.

Introduction

It is widely accepted that marine litter is a global phenomenon, recognized of concern at international levels therefore included in the UNEP initiatives such as the Sustainable Development Goals (SDG) or in G7 and G20 statements (Borja and Elliott, 2019). Indeed, SDG14 (Life below water) specifically has an extremely ambitious target to reduce or remove this source of pollution by 2025 (UN, 2015) although without further development that target has been criticized as being inaccurate and unattainable (Cormier and Elliott, 2017). Marine Litter has been defined by UNEP (UNEP, 2005) as “any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment”. Macroplastics are a component of plastic litter, defined as plastic pieces above 25 mm size (Galgani et al., 2013), and further detailed as size-classes in the new guideline about macrolitter monitoring (Fleet et al., 2021). They include a broad range of materials and shapes, due to production, mechanical alterations or differential weathering and other degradation conditions of a complex of different polymers (Frigione et al., 2021). Macroplastics litter is often the source of secondary microplastic contamination (Andrady, 2011; Lambert et al., 2014; GESAMP, 2015). Although connected, research related to macroplastics litter differs greatly from that of microplastics in terms of study design, protocols, and analyses (Fleet et al., 2021). Addressing macroplastics contamination and pollution is likely to identify paths from source of littering to the access to food webs via breakdown.

Sandy beaches are an ecosystem exposed to and under threat from many global environmental problems, notable those termed the triple whammy of increased urbanization and industrialization, increased use of resources and decreased resistance and resilience to external threats such as climate change (Defeo and Elliott, 2021). The relatively young discipline (established in the 1980s, McLachlan, 1983) of sandy shore ecology began by identifying features shaping those physically-driven environments, and then proceeded by overlapping morphodynamic characterization with biotic data layers, finally superimposed on by human pressures (Defeo et al., in press). Current paradigms define the morphodynamic type of a beach as the interaction between sand particle size and exposure to tidal range and wave conditions: as such, dissipative beaches are characterized by gentle slopes, wide beach width and fine grain sizes as relevant features. By contrast, the reflective end of the scale occurs when the sediment is coarse and stored on the intertidal beach and backshore, and where there is no surf zone and waves surge directly up the beach face (McLachlan and Defeo, 2018). The macrofauna inhabiting beach environments reflects these variations: an increasing number of species is found toward dissipative beaches, which are more benign as less exposed to substrate tumbling. With a progression through the morphodynamic spectrum through intermediate beaches, most species become less successful, and few can colonize reflective beaches due to the harsher environment given by the saltation of coarse substratum particles subjected to the high energy of incoming waves. The morphodynamic state is hence relevant to beach functioning, with direct repercussion on the quality and quantity of ecosystem services (McLachlan et al., 2013; McLachlan and Defeo, 2018). Consequently, the occurrence of beached plastic could also be affected by the different exposure to and interaction with energy, matter and biota. The co-occurrence of environmental features and beached plastics data could reveal potential interactions occurring on matching spatial and/or temporal scales. It is hence timely to propose tools and standards quantifying beached plastic and beach ecological processes. For instance, the average specific gravity of plastics and polymers is 1.275 ± 0.303 g·cm⁻³ (calculated from AmesWeb, 2021) whereas that of substratum particles such as quartz grains is 2.65 g·cm⁻³ and that of marine mollusc shells 2.68–2.72 g·cm⁻³. Therefore, plastic and polymer accumulate, are buried and re-suspended (Williams and Tudor, 2001). Density, shape and relative size of macroplastics and substratum particles are important when considering these dynamics, occurring along the land-sea axis (Lebreton and Andrady, 2019; Rangel-Buitrago et al., 2017; Moreira et al., 2016; Cresta and Battisti, 2021). Given the high relevance of the local level of beaches (Fanini et al., 2020), the variability in substratum and exposure will likely require tailored approaches depending on morphophysical and landscape features (Ryan and Perold, 2021) together with the application of standard protocols, essential to achieve data interoperability.

Macroplastics is also the most common subject of beach clean-up activities or citizen observation-based initiatives and monitoring actions. There is a common top-down approach to the topic, engaging society as citizen scientists and monitors (see the definition by ECSA, European Citizen Science Association http://ecsa.citizen-science.net/:). NGOs, private sectors and national agencies and departments are conducting surveys, campaigns and projects supporting data collections and evidence-based policies (Hidalgo-Ruz and Thiel, 2015; GESAMP, 2019; Syberg et al., 2020). Despite this, there are still challenges in the definition of the role of citizen science and data that it can provide (Haklay et al., 2021). However, it is through these activities that relevant evidence can be built, enabling macroscale patterns to be determined and finally be mainstreamed into policies. Indeed, it is through these campaigns that plastics were identified as the most common material composing human litter on the beach (Addamo et al., 2017). Also, bans on single use plastic items (SUP) were generally based on the top items found as beached macroplastics litter, on data collected by citizens and mediated by NGOs in their mainstream to policy making. Country implementation of international strategies such as the Programme of Measures for the European Marine Strategy Framework Directive (MSFD) - of which marine litter is Descriptor number 10 for determining Good Environmental Status - are also based on volunteer-led data collection. For example, the main marine litter monitoring in the UK has been the annual volunteer-led beach clean of the Marine Conservation Society (MCSUK) involving many thousand volunteers every September since 1994; this was recognized as part of the UK contribution to implementing the MSFD.

While these studies are powerful in depicting patterns and they strongly support governance via evidence-based information, studies tackling dynamics remain limited. Such studies would require the consideration of marine litter across temporal scales and disciplines, but also would need to be based on shared and quality-assured protocols, and shared data, which are a frequent constraint in large-scale studies (but see Morales-Caselles et al., 2021). The temporal dimension in particular reveals notable gaps, especially related to long-term designs and/or before-after impacts such as floods, typhoons, and bans of specific items e.g. single use plastic bags. Again, the relevance of the single beach unit in both social and ecological perspective would require attention since the very planning of actions.

Reviews about methodologies for marine litter monitoring started in the 1990s (Rees and Pond, 1995) and standard methodologies are proposed by the Regional Seas Convention guidelines within their action plans such as Cheshire et al., 2009 (UNEP/IOC), Helsinki Convention (HELCOM, 2008), OSPAR Commission (2010) and Schulz et al. (2017). Furthermore, monitoring guidelines have been outlined for programmes such as the MSFD (Galgani et al., 2013), to support marine litter baselines (Hanke et al., 2019), threshold values (Van Loon et al., 2020) or providing harmonized list of items (Fleet et al., 2021). They mainly address: 1) Quantification (database – number, weight or volume); 2) characterization (composition - master lists); and 3) evidence-based policies for production consumption systems (e.g. brand auditing, target items campaigns, or littering sources).

Selection criteria for beaches to be monitored are also given, both in the framework of national programmes (Opfer et al., 2012), or international regulations such as MSFD (EC 2008 2008/56/EC), where marine litter represents an indicator of the environmental quality status of the ecosystem. As a general approach, a set of desirable characteristics is provided for identifying the sampling area to design monitoring and assessment programmes as well as for beach cleanup initiatives with volunteers (OSPAR, 2010; Galgani et al., 2013; GESAMP, 2019; WIOMSA manual -Western Indian Ocean Marine Science Association (Barnardo and Ribbink, 2020)), and UNEP/IOC manual (Cheshire et al., 2009).

In order to create robust and comparable quality-assured data, the monitoring methods have to be standardized, agreed and implemented consistently. When this relates to the areas that are monitored, the general indications about site selection include: accessibility of the site, and avoidance of steep slopes (15°-45°); areas not subjected to cleaning activities; avoid nesting sites for threatened species or presence of endangered or protected species; avoid streams, and natural or artificial elements likely to interfere with currents. In particular, the WIOMSA manual suggests a random selection of sites, and if this is not possible, a site selection guided by a pre-defined criterion, without previous investigation, in essence having a random sampling design. In all cases, the surveys for marine plastic macrolitter standing stock should be carried out along a predetermined length of 100 m running parallel to the shoreline (Barnardo and Ribbink, 2020).

There are a few protocols adapted to beach morphology, such as considering whether the area is macro or microtidal, and has reflective or dissipative conditions; fine sand or coarse sand or pebbles, presence/absence of organic litter (GESAMP, 2019). Turra et al. (2014) called for protocols relevant to sandy beach ecology (see also Moreira et al., 2016). However, to date, the integration of relevant sandy beach variables is left to single initiatives rather than embedded in protocols. However, beach structural features are intrinsically connected to functional processes occurring around sandy shores, from physical and biotic (faunal) conditions to socio-economic dimensions (see McLachlan and Defeo, 2018 for a recent comprehensive summary). For this reason, a greater connection of beach ecology with plastic studies would increase the relevance of research and enhance the support to policy and citizens. Given the high attention on the topic and the response by scientists which produce much literature about marine plastics litter, it was urgent to detect and communicate trends for future research. On this background, and with explicit focus on the macroplastics fraction, the aim here is to show the integration of ecological features of sandy beach systems into beached plastics litter studies. As such, the analysis of word co-occurrence in scientific publications was identified as suitable first step in this process.