ISA Fact-check 2025/1 –Addressing Plastics in the Deep Sea: Scientific Evidence and the Potential Contributions of the International Seabed Authority.

1/Plastics in a global context

Plastic pollution represents a growing global challenge with potential consequences for the sustainable use of oceans. In 2019, the plastics industry produced over 450 million tons of plastic, a figure expected to rise in the coming decades.[1] This continued growth is likely to also increase pressure on marine environments and species.[2] Global mass balance models derived from monitoring studies, indicate that a portion of plastics entering the oceans remains unaccounted for.[3] This phenomenon, known as the “missing plastics paradox,” may be partially explained by the accumulation of plastics in the open oceans. Some researchers suggest that the deep sea may act as a sink for plastic debris, where their prolonged persistence could pose risks to these environments.[4]^,[5] Recent studies have also shown that turbidity currents can transport microplastic to the deep sea.[6]

The draft text of the landmark internationally legally binding instrument will be further discussed next August 2025 in Geneva, Switzerland.[7] Momentum towards its adoption in 2025 is building, with the United Nations Secretary-General António Guterres urging countries to reach an agreement before the end of the year during his opening remarks at the United Nations Ocean Conference in Nice on 9 June 2025.[8] The current draft text includes provisions for Parties to assess plastic accumulation zones, including in the marine environment, to map existing plastic pollution.^[9] Considering the deep sea may act as sink for plastic debris, improving our understanding of deep sea plastic abundance and impacts will strengthen the science-policy interface and inform responses across national and international waters.[10] As custodian of the deep sea bed in the areas beyond national jurisdiction, the ISA may contribute to advancing research in this domain.

[3] Ritchie, Hannah, Veronika Samborska, and Max Roser. 2023. “Plastic Pollution.” Our World in Data. Available at: https://ourworldindata.org/plastic-pollution.

 [4] Secretariat of the Convention on Biological Diversity (2016). Marine Debris: Understanding, preventing and mitigating the significant adverse impacts on marine and coastal biodiversity. CBD Technical Series: Vol. No. 83 (p. 78).

[5] Woodall, Lucy C., Anna Sanchez-Vidal, Miquel Canals, Gordon L.J. Paterson, Rachel Coppock, Victoria Sleight, Antonio Calafat, Alex D. Rogers, Bhavani E. Narayanaswamy, and Richard C. Thompson. 2014. “The Deep Sea Is a Major Sink for Microplastic Debris.” Royal Society Open Science 1 (4): 140317.

[6] Barnes, David K. A., Francois Galgani, Richard C. Thompson, and Morton Barlaz. 2009. “Accumulation and Fragmentation of Plastic Debris in Global Environments.” Philosophical Transactions of the Royal Society B Biological Sciences 364 (1526): 1985–98. https://doi.org/10.1098/rstb.2008.0205

[7] Thompson, Richard C. 2016. “Sources, Distribution, and Fate of Microscopic Plastics in Marine Environments.” In The handbook of Environmental Chemistry, 121–33. https://doi.org/10.1007/698_2016_10

[8] Chen, Peng, Ian A. Kane, Michael A. Clare, Euan L. Soutter, Furu Mienis, Roy A. Wogelius, and Edward Keavney. 2025. “Direct Evidence That Microplastics Are Transported to the Deep Sea by Turbidity Currents.” Environmental Science & Technology, April. https://doi.org/10.1021/acs.est.4c12007.

[9] Intergovernmental Negotiating Committee on Plastic Pollution | UNEP – UN Environment Programme

[10] Opening Remark Guterres UNOC June 2025

[11] UNEP/PP/INC.5/4 Section 11 para 1 (a)

[12] Cózar, Andrés, Fidel Echevarría, J. Ignacio González-Gordillo, Xabier Irigoien, Bárbara Úbeda, Santiago Hernández-León, Álvaro T. Palma, et al. 2014. “Plastic Debris in the Open Ocean.” Proceedings of the National Academy of Sciences 111 (28). Proceedings of the National Academy of Sciences: 10239–10244. doi:10.1073/pnas.1314705111.

2/What Science Tells Us

The ISA Secretariat commissioned a targeted literature review of the scientific publications from 2000 to 2023 on plastic occurrence and effect concentrations in the deep sea, both water column and sediment. Using three search engines, 92 peer-reviewed scientific papers were retrieved- a relatively small number compared to the extensive literature on plastics in coastal zones.[13] Methods used to assess plastics varied across studies. Only half employed spectroscopic techniques – essential for confirming plastic particles and identifying polymer types – while the rest relied on visual methods such as human eye detection, video analysis and several types of microscope detection, which are less reliable. Sampling locations were widely dispersed, with about 60% of the studies examining plastic abundance in sediment at single stations. Only one study covered multiple locations in the northwestern Pacific Ocean, where ISA contracts are located.[14] Reported plastic concentration varied greatly and comparison of the concentration proved difficult, due to differences in methodologies and inconsistent reporting practices, including unit measurements.

None of the studies analyzed reported plastic effect concentrations for deep-sea species. These concentrations – measured in laboratory setting – indicate levels at which concentration of plastic particles cause observable effects in organisms. Their absence in the retrieved literature reflects technical challenges of culturing deep-sea species and conducting experiments under high pressure conditions, which require specialized pressure tanks.[15]^,[16] Their unique physiological and ecological traits may result in different sensitivities to plastic compared to surface dwelling species, underscoring the importance of conducting meaningful ecotoxicological tests.

Apart from deep-sea organisms, there is a broader lack of robust data on the chronic effects of plastics across marine species. Reliable long-term toxicological data are essential to understand  sustained impacts, conduct meaningful environmental risk assessments, and determine “Predicted No Effect Concentrations” (PNECs) – benchmarks indicating concentration below which no adverse effects on living communities are expected.[3] Establishing PNECs requires sufficient data across multiple species to build a Species Sensitivity Distribution (SSD), a probabilistic model that captures the range of species responses across trophic levels and feeding strategies.

[13] Search terms used were “allintitle: deep sea plastic OR  microplastic” in Google scholar; for Web of science: TI=((plastic* OR microplastic*) AND “deep sea” OR AB=((plastic* OR  microplastic*) AND “deep sea”) AND PY=2000-2023 and in Pubmed: deep sea”[Title/Abstract] AND (plastic*[Title/Abstract] OR microplastic*[Title/Abstract]) with extra filters for years

[14] Zhang, Dongdong, Xidan Liu, Wei Huang, Jingjing Li, Chunsheng Wang, Dongsheng Zhang, and Chunfang Zhang. 2020. “Microplastic Pollution in Deep-sea Sediments and Organisms of the Western Pacific Ocean.” Environmental Pollution 259 (January): 113948. https://doi.org/10.1016/j.envpol.2020.113948.

 Horton, Alice A., and David K.A. Barnes. 2020

[16] Horton, Alice A., and David K.A. Barnes. 2020. “Microplastic Pollution in a Rapidly Changing World: Implications for Remote and Vulnerable Marine Ecosystems.” The Science of the Total Environment 738 (June): 140349. https://doi.org/10.1016/j.scitotenv.2020.140349.

[17] OSPAR Commission. 2000. OSPAR Background Document concerning the Elaboration of Programmes and Measures relating to Whole Effluent Assessment. Available at: https://www.ospar.org/documents?v=6903.

4/Recommendations to Advance Knowledge of Plastics in the Deep-sea

The following four steps could help strengthen global understanding plastic occurrence in the deep-sea through practical, equitable, and science-based approaches.

1. Standardize sampling, extraction, and analysis protocols using a globally recognized methodology to improve data comparability and reliability.

• Standardized guidelines for sampling: ensure that sampling protocols systematically include key parameters such as location, time, depth, volume, number of replicates, instruments used, storage methods, and transportation conditions.
• Extraction and analysis parameters: define clear protocols for the solutions and instruments employed for separation, instruments used for processing and step-by-step analysis protocol including the software applied for data processing and its calibration.
• Include detailed identification techniques that confirm that particles counted are plastics and that provide information about the polymer type.

2. Promote the creation of sound baselines on plastic concentrations in deep-sea environments, as they are crucial for assessing trends and guiding future research and management strategies.

3. Focus on ecological effects by increasing research on the effects of plastics on deep-sea organisms, populations and communities, to support ecological risk assessments. This requires the collaboration of institutes equipped with pressure tanks capable of simulating deep-sea conditions.

4. Enhance scientific capacity-building efforts on plastics in the deep sea to enable stronger participation of institutions and agencies from developing countries, fostering inclusivity and equity in this important research domain.