What is in our seas? Assessing anthropogenic litter on the seafloor of the central Mediterranean Sea

Highlights

• The density of seafloor macro-litter in the Strait of Sicily increases with depth.

• Single-use and generic-use items accumulate along shipping routes.

• Fishing-related items are associated with specific fishing areas.

• The occurrence of litter is higher than in the ‘90s but decreases from 2015 to 2019.

• Baseline information useful for mitigation policies of seafloor macro-litter.

Abstract

Abundance, composition, and distribution of macro-litter found on the seafloor of the Strait of Sicily between 10 and 800 m depth has been studied using data collected by bottom trawl surveys MEDITS from 2015 to 2019. Three waste categories based on the items use were considered: single-use, fishing-related and generic-use. Over 600 sampling sites, just 14% of these were litter-free. The five-years average density of seafloor litter was 79.6 items/km² and ranged between 46.8 in 2019 and 118.1 items/km² in 2015. The predominant waste type was plastic (58% of all items). Regardless of material type, single-use items were a dominant (60% of items) and widespread (79% of hauls) fraction of litter with a mean density of 48.4 items/km². Fishing-related items accounted for 12% of total litter items. Percentage of dirty hauls and litter density increased with depth. Analysis of the relation density-depth indicates a progressive increase of litter density beyond depth values situated within the interval 234–477 m depending on the litter category. A significant decrease in litter density by categories was observed over the period. Patterns of spatial distribution at the higher depths (200–80 0m) resulted stable over the years. Density hotspots of fishing-related items were found where the fishing activity that uses fish aggregating devices (FADs) is practised and in the proximity of rocky banks. Single-use and generic-use objects densities were greater on the seafloor along main maritime routes than other areas. Comparisons between the percentage of hauls littered with anthropic waste from the mid-1990s against those in 2018–19 highlighted an increase of about 10.8% and 15.3% for single-use items and fishing-related items respectively, and a decrease of 18.6% for generic-use items. This study provides a snapshot of the current situation of littering in the central Mediterranean Sea and represents a solid baseline against which the effectiveness of current and future mitigation strategies of the litter impact on marine environment can be measured.

Introduction

Accumulation of anthropogenic solid waste on the seafloor is a ubiquitous and increasing phenomenon that poses a serious threat to marine ecosystems health worldwide. Types and quantities of marine litter vary considerably across marine regions due to different hydrodynamics, geomorphologic and anthropic factors, being the enclosed seas such as the Mediterranean or the Black Sea the most affected areas in the world (UNEP/MAP, 2015). It is estimated that about 0.5 billion litter items are currently lying on the Mediterranean seafloor (UNEP/MAP, 2015) with densities locally reaching over 1000 items/km² (Ioakeimidis et al., 2014; Fortibuoni et al., 2019) and exceptionally exceeding 100,000 items/km² (Galgani et al., 2000, 2015).

Despite the great variety of waste materials (plastics, metals, glass/ceramics, wood, rubber, textiles, paper, etc.) found in the marine environment, plastics are predominant worldwide (Derraik, 2002). The proportion of plastic consistently varies between 60% and 80% of the total marine litter, overcoming 90% in some regions (Derraik, 2002). According to a recent estimate, the annual input of plastic waste to ocean amounts to 4.8–12.7 million tons and could increase by an order of magnitude by 2025 (Jambeck et al., 2015). Metal and glass/ceramic also contribute significantly to marine litter and generally represent the highest percentage of waste after plastics, while other litter typologies contribute cumulatively with minor percentages (Ioakeimidis et al., 2014; Pham et al., 2014; Spedicato et al., 2019).

Solid waste enters the seas mostly from land-based sources (e.g. about the 80% of plastic waste is from terrestrial sources; Andrady, 2011; Bonanno and Orlando-Bonaca, 2018) and originates from accidental or intentional dumping and poor waste management systems in coastal regions. Nevertheless, marine-based sources play an important role too through unintentional or illegal unloading of waste from ships, including fishing boats, and accidental losses of fishing gear (Ryan, 2015; Bonanno and Orlando-Bonaca, 2018). It is estimated that derelict fishing gear on the seafloor accounts for about 10% of all marine litter in the world oceans (UNEP/MAP, 2015). Furthermore, in some areas, aquaculture can also be an important source of marine-based littering, especially given the continuous expansion of the aquaculture industry (Pasquini et al., 2016; Melli et al., 2017) Different factors determine the fate of litter entering the sea (Bonanno and Orlando-Bonaca, 2018 and reference therein) and, in recent years, an increasing number of studies are focusing on the modelling of marine litter transport and its accumulation patterns (e.g. Liubartseva et al., 2018; Mancia et al., 2020). Heavy waste such as glass and metal probably sink very close to the place of discharge, light waste such as plastics can be dispersed over long distances for the effect of winds and currents causing degradation of the marine environment even in areas far away from the pollution sources (e.g. Bonanno and Orlando-Bonaca, 2018).

The main concerns about the accumulation of anthropic waste on the seabed regard its persistence (e.g. plastic only degrades over the centuries) especially in the deep sea, and the potential harmful effects, direct and indirect, on ecosystems and organisms (Thompson et al., 2009; Galgani et al., 2013; Jambeck et al., 2015; Anastasopoulou & Fortibuoni, 2019 and reference therein). Direct damages to marine fauna may be due to the ingestion of large debris, such as plastic bags, or accidental entanglement in fishing gears lost or abandoned on the seafloor (Fossi et al., 2018 and reference therein; Consoli et al., 2019,2020; Mancia et al., 2020). Another aspect is the potential risk associated to the degradation product of plastic items, i.e. microplastics (plastic fragments below 5 mm in size) which are ingested by zooplankton and may be transferred through the entire marine food web up to humans (Thompson et al., 2009; Cole et al., 2011; Lusher et al., 2017). Even social and economic damages are associated to marine litter mainly due to the loss of the aesthetic value and related ecosystem services of marine landscapes littered with anthropic waste (Xanthos & Walker, 2017; Maes et al., 2019). For all these reasons, marine litter and primarily plastic pollution are perceived worldwide as a critical environmental priority and an alarming threat to marine resources and blue growth.

In the last decade, the European Union (EU) has set up many complementing strategies to reduce and prevent the input of waste into the sea, as well as to take remedial action against waste already present in the marine environment. These include the Marine Strategy Framework Directive (MSFD; Council directive 2008/56/EC) which established a framework for the monitoring and achievement of Good Environmental Status (GES) for the marine environment through the assessment of 11 descriptors, with the descriptor 10 focusing specifically on the characteristics of litter in the marine and coastal environment and its impact on marine life. A similar framework was established, at the regional level in the Mediterranean, with the Integrated Monitoring and Assessment Programme of the Mediterranean Sea Coast and Related Assessment Criteria (IMAP) which adopted the Common Indicator 23 “Trends in the amount of litter in the water column including microplastics and on the seafloor” under the Ecological Objective 10 (EO10), i.e. Marine Litter (IMAP Decision IG.22/7 of the 19th Meeting of the Contracting Parties of the Barcelona Convention). Most recently, the European Strategy for Plastics in a Circular Economy (EC, 2018) was adopted, which proposes new EU-wide rules for modernising the plastics economy and modifying the way plastic goods are produced and recycled. In particular, the Council Directive (EU) 2019/904 is aimed at reducing the impact on the environment of single-use plastics, mainly plastic bottles, plates and food containers, and fishing gear containing plastic. According to the European Commission, taken together, these two broad categories constitute 70% of all marine litter items in Europe (https://ec.europa.eu/commission/presscorner/detail/en/IP_18_3927), so the proposed measures aim, among other, to reduce by 2025 the consumption of single-use food and drinks containers by at least 25% and to reach a minimum collection rate of 50% of fishing gear. In addition, a new Council Directive (EU) 2019/883 on port reception facilities (repealing Directive2000/59/EC) introduced some new rules for the delivery of waste from ships, in line with requirements of the International Convention for the Prevention of Pollution from Ships (MARPOL). This new regulation, through a mix of incentive and enforcement measures, such as cost recovery systems based on indirect fee and targeted inspections, intends to ensure that waste generated on ships but also waste collected in nets during fishing operations is delivered to port reception facilities and adequately managed, instead of being discharged at sea. National law of Member States should comply with the adopted rules by 2021. Moreover, the European Maritime and Fisheries Fund (EMFF) provides financial support for the recovery of waste by fishermen from the sea as well as for the development of port facilities for waste collection.

The commitment of the EU to tackle the great challenges facing our oceans has also been strengthened in the new research and innovation programme Horizon Europe 2021–2027, where one of the five mission areas is “Healthy Oceans, Seas, Coastal and Inland Water”. Specific goals of the mission are the development of solutions to prevent, mitigate and remove marine pollution (including plastics), the transition to a circular and blue economy and the development of biodegradable plastic substitutes.

However, according to Bonanno and Orlando-Bonaca (2018), current regulations are still not enough to address the real threat of massive amounts of plastics entering the oceans. The current knowledge gaps on waste distribution and accumulation in specific areas hinder the capacity of making progress towards mitigating marine pollution with particular regard to plastic (i.e. the real quantity, characteristics, sources and distribution of litter released into the sea, as well as the biological and ecological impacts on the marine ecosystems and the harmful effects on human health) and for these reasons, it is necessary to have a clear understanding of this process.

By focused on single-use, generic-use and fishing-related litter, in the present paper we used macro-litter data collected during scientific trawl surveys carried out in the northern part of the Strait of Sicily (central Mediterranean) during the 2015–2019 period to: 1) assess the composition and density of seafloor litter; 2) evaluate how the density varied with depth and time; 3) explore spatial distribution; 4) compare the current level of pollution with that in the mid-1990s. Our study provides a baseline for monitoring and assessing the application of European policies and relevant international initiatives on reducing the amount of litter in the marine environment.