Marine litter is a serious environmental problem worldwide. Reducing it would require implementing a global monitoring system, agreeing on the use of common methods and protocols for data collection, and categorizing all components of marine debris. This involves a tremendous scientific, political, and social effort at the international level — one that cannot be carried out with the same intensity by all countries — given the magnitude of what is still unknown about the pollution of seas and oceans, particularly the deep ocean, where the vast majority of marine litter accumulates.
This is one of the main conclusions of the article published in Marine Pollution Bulletin. The principal authors include Professor Miquel Canals, from the Faculty of Earth Sciences of the University of Barcelona; Georg Hanke, from the European Commission’s Joint Research Centre (JRC); Ryota Nakajima, from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC); Melanie Bergmann, from the Alfred Wegener Institute (AWI) in Germany; François Galgani, from the French Research Institute for Exploitation of the Sea (IFREMER); and Daoji Li, from East China Normal University (ECNU), among other experts.
The study aims to raise awareness and emphasize the need to include marine litter — and particularly that on the seafloors — in any directive, convention, treaty, or agreement established at the national, regional, or global level to minimize marine pollution. Experts warn that the seafloor serves as the ultimate dumping ground where the vast majority of marine litter ends up.
The focus of the study is on macrolitter — items larger than 2.5 cm — that accumulate on the seafloor, the ultimate sink for marine litter. Despite its significant environmental relevance, this fraction has often been overlooked in the flood of studies on smaller-size debris, such as microplastics.
An environmental problem that begins on land
Nearly all seas and oceans are affected by marine litter, much of which reaches them via rivers. The rivers contributing the most waste are located in southern Asia (the Amur, Hai, Yellow, Yangtze, Pearl, Mekong, and the Ganges and Indus systems) and in Africa (the Niger and the Nile).
In addition to riverine inputs, certain extraordinary and catastrophic events — such as the April 2011 tsunami in eastern Japan — carry enormous amounts of debris into the ocean. Fishing and other marine activities are also continuous sources of specific types of litter, such as discarded nets, longlines, buoys, and tires used as boat fenders.
“In developing countries, which lack robust waste management systems, the dumping of waste into the natural environment — and consequently along the coast and into the sea — is common, as shown by some recent studies we have carried out in mangrove ecosystems in Colombia,” explains Professor Miquel Canals, director of the UB Chair on Sustainable Blue Economy.
In more economically developed countries, waste management is far more effective than in developing nations with more limited economic capacity. Although all countries aspire to a healthier environment, the actual ability to achieve this varies greatly, and economic priorities differ in each case.
“For example — the expert notes — in countries with low or very low per capita income, often with large populations, the priority of providing daily food for the population takes precedence over protecting the environment. In more influential countries, although the fight against marine pollution is on the list of priorities, the situation is also diverse, as many variables influence the discharge of waste into the environment — for instance, population density, climatic conditions, social awareness, and cultural factors.”
“In the Mediterranean, a sea surrounded by humanity, large amounts of litter are found at the surface, throughout the water column, and especially on the seafloor. Some areas, such as the Strait of Messina between Sicily and southern Italy, are real underwater dumping grounds. During episodes of heavy rainfall, water carries huge amounts of waste previously dumped into coastal ravines by residents down to the depths of the sea, including the most unlikely objects, from refrigerators to automobiles, and from toys to garbage bags,” says Canals.
A common methodology for comparing data
Having comparable and standardized data across all research is also an essential requirement for making progress in the fight against marine litter pollution. However, the lack of a global monitoring system makes this goal difficult to achieve. To obtain an overall picture of the situation, some models use indicators such as the amount of mismanaged plastic waste by river basin or country, population density, or gross domestic product.
“However, these models often produce figures that differ greatly from one another,” notes Canals. “It is necessary to use homogeneous and harmonized observation and measurement methods, and to prioritize the use of advanced technologies both for observation and for data analysis and management. We also consider local training at different levels and capacity building to be essential elements. In this context, the temporal perspective is fundamental for determining reference or baseline states.”
From changes in consumption to environmental management
Technological progress is an ally in the fight against marine litter. The integration of modern technologies — such as airborne sensors and cameras, autonomous and crewed underwater vehicles, side-scan sonar, very high–resolution multibeam bathymetry, image analysis, among others — has improved the processes used to identify, quantify, and categorize marine litter. It is essential for the data to be accurately georeferenced and for technologies to be non-destructive, that is, they must not cause any impact on the habitats under study.
At the same time, greater synergies should be established between the academic sector and private companies, which possess vast amounts of observations, data, and images of marine litter acquired over recent decades. Data from the private sector could contribute very effectively to the definition of historical baselines, to scientific monitoring programs, and therefore also to the identification of long-term trends.
Eliminating or reducing plastic pollution — the main component of marine litter — requires identifying source emissions, transfer pathways to the sea, and accumulation areas within the marine environment. It is necessary to prevent waste from reaching the ocean in massive and insufficiently controlled — or entirely uncontrolled — ways, as occurs in many regions of the world. For this reason, it is essential to implement upstream measures targeting production, consumption habits, and on-land waste management, both at the individual and collective levels. The application of direct measures in the marine environment — for example, the large-scale removal of litter from the seafloor — should only be carried out under strict technical and environmental criteria, to avoid causing even greater harm to the environment.
A greater political commitment to protect the marine environment
Informing and raising awareness among policymakers about the scale and consequences of marine pollution caused by macro-litter is a critical step in designing and implementing meaningful and effective environmental policies. It is essential to convey the most relevant information to the political sphere in order to generate effective actions throughout the value and use chain (producers and consumers), as well as measures focused on the marine and coastal environment. In this context, macro-litter on the seafloor represents a key element of the evidence base justifying the implementation of mitigation measures at the global level.
In Europe, there are regulations such as the EU Marine Strategy Framework Directive — aimed at achieving good environmental status (GES) of Europe’s marine habitats — which includes a descriptor focused on marine litter; the Directive on port reception facilities for the delivery of ship-generated waste; and the Directive on reducing the impact of certain plastic products on the environment.
It is also important to mention the resolutions and decisions of the United Nations Environment Assembly (UNEA) and the Global Partnership on Plastic and Marine Litter (GPML) under the UN Environment Programme (UNEP), as well as Regional Seas Conventions and Action Plans (RSC), including the Mediterranean Action Plan (MAP) and the Barcelona Convention and Protocols (UNEMAP). “Unfortunately, the negotiations promoted by the UNEP to conclude an international treaty to stop plastic pollution, including in the marine environment, ended without agreement last August in Geneva,” emphasizes Canals.
“Today, almost 70 years after the first deep-ocean dive in 1958, we have images of only 0.001% of the seafloor at depths greater than 200 meters, which accounts for 66% of the total ocean surface. This figure clearly illustrates the magnitude of what we still do not know about marine litter pollution, and the enormous effort that remains necessary to gain an accurate understanding of the presence and impacts of debris in the world’s seas and oceans,” concludes Miquel Canals.