Distributed by MarineDebris.Info
Marine debris, and particularly plastic marine debris, poses a significant global threat to marine life. Growing public awareness of this threat, including the recognition that floating ocean plastics tend to congregate in remote areas of the open ocean (the so-called “garbage patches”), has inspired some individuals and groups to conceive of systems for cleaning up debris at sea. These systems have ranged from simple net-based methods to ambitious giant filtering systems that would stretch hundreds of kilometers across the sea surface, among other ideas.
These ideas are laudable for their goal of removing plastics from the sea. However, the proposed systems typically fail to account for real-world ocean conditions and/or the many ecological and engineering-related challenges that would face any cleanup effort on the open ocean. In fact, the array and scale of challenges involved in any realistic cleanup of remote ocean areas have been cited often by experts as evidence that marine debris management should focus on preventing new litter from entering the ocean, rather than attempting to remove the litter that is already there.1
Still, many seemingly intractable challenges faced by humans over time have been solved by bright ideas. And as long as people are aware of and moved by the problem of ocean plastic debris, there will be attempts to solve it. In this light, we do not want to dissuade potential innovators from examining the issue of marine debris cleanup. Rather, we want to inform and channel their innovation.
This document presents general guidance for the cleanup of floating plastic debris in the open ocean. This guidance was produced and edited in June-July 2013 by a team of experienced marine debris researchers (see “Background on this document”) and was shared in July with the global MarineDebris.Info community of marine debris managers, researchers, and conservationists for additional input. This list is intended to evolve and improve over time as further knowledge is gained. We also strongly recommend that all prospective inventors of cleanup systems consult experienced ocean engineers early in their development processes.
Systems for open ocean cleanup of marine plastics should account for, and overcome, the following challenges:
- The size and depth of the ocean gyres within which floating marine plastics tend to gather. The North Pacific Subtropical Gyre, the best-known location of a “garbage patch” or trash vortex, has a surface area of approximately 20,000,000 km2 — more than twice the size of the U.S. Thus any system that would tow nets through the gyre to capture debris, for example, must either factor that surface area into its design or determine a way to avoid having to transit the whole area. In addition, in even the most densely polluted regions of the subtropical gyres, microplastics (particles smaller than 5 mm in size) are frequently present at concentrations of less than one piece per square meter, requiring extensive areal coverage to recover just one kilogram of plastic. Furthermore, average water depth of the open ocean is 4,000 meters (2.5 miles); therefore, any cleanup system relying upon moored structures must account for this extreme depth.
- Depth and concentration of microplastics. Floating plastic debris, and particularly microplastics, can be mixed below the surface in even light winds. The depth of the mixing depends on the strength of the wind and the physical structure of the ocean in that particular area. It may range from a few meters depth during typical low wind conditions in summer months to 100-150 meters depth during stormier winter months. Any cleanup system, particularly one that relies on surface-floating mechanisms, must account for subsurface mixing of debris.
- Capturing plastics while not harming marine life that is co-located with the plastics. The goal of ocean plastic removal is ultimately to help sea life. If cleanup systems hurt or kill sea life in the process — such as fish or even plankton — they are counterproductive. Most zooplankton, for example, do not survive being caught in a standard net, never mind spun in a centrifuge where they lose critical appendages like their antennae and feeding apparatus. A system that relies on nets or centrifuges will require engineered solutions to avoid or minimize these effects. In addition, indirect environmental impacts of cleanup systems — such as from fuel use by cleanup vessels scouring the ocean, or from incineration of collected plastics at sea — should be considered and mitigated.
- Potential for entanglement of marine life in the systems. Similar to the above guideline, any cleanup system that poses an indirect entanglement threat to wildlife, such as seabirds or cetaceans, must be reengineered. In the U.S., any activity that might harm protected species is illegal without a permit.
- Strength and stability in extreme sea conditions. Any cleanup system is in danger of becoming marine debris itself if it breaks up, such as from storm action, high waves, or ship collisions. A successful system must be able to avoid extreme weather or be sturdy enough to withstand such conditions, and must be detectable by fast-moving vessels.
- Maintenance and fouling. The reality of operating equipment in extreme environments, of which the open ocean is an example, is that equipment breaks down. There must be a cost-effective means to maintain and repair the cleanup system over time, particularly for systems that involve long-term deployment. Furthermore, there must be a way to account for, and address, the rapid biofouling that occurs when any equipment is placed in the ocean.
- The physical properties of ocean-weathered plastic. Floating marine debris is largely composed of polyethylene and polypropylene — common plastic types with a density less than that of seawater. (Polystyrene, which has a density roughly equal to that of seawater, may also be present but is much less abundant in floating debris.) Ultraviolet light degrades these plastic polymers, making them brittle and difficult to recycle. Cleanup schemes that propose to recycle and/or market the plastic they collect should be aware of the technical issues, and of the actual market value of the plastic they collect, which may be low. Recyclers with the technology to “upcycle” mixed ocean plastic into consumer-quality polymers remain very few (Envision Plastics, which partners with home products manufacturer Method on a recycled ocean plastic bottle, is one). As a result, the market for collected ocean plastic is underdeveloped at best.
- Legal issues. There are extensive laws and regulations governing the deployment of equipment at sea. For example, structures cannot be a hazard to navigation or a threat to protected species. In the U.S., multiple permits from state and/or federal agencies may be required for cleanup devices. The permitting process is lengthy, onerous, and expensive, and may require specialized legal consultation. On the high seas — marine areas beyond national jurisdiction — the applicability and relevance of the United Nations Convention on the Law of the Sea must be evaluated in each case.
Background on this document
This guidance was drafted by:
- John Davis, M.M.A., of MARE
- Miriam Goldstein, Ph.D., of California Sea Grant, 2013 Knauss Fellow, and Scripps Institution of Oceanography at UC San Diego
Editors and contributors included:
- Courtney Arthur, M.S., Research Specialist, NOAA Marine Debris Program
- Pete Davison, Ph.D., Postdoctoral Scholar, Scripps Institution of Oceanography
- Kara Lavender Law, Ph.D., Research Professor, Sea Education Association (SEA)
- Chelsea Rochman, Ph.D., Aquatic Health Program, School of Veterinary Medicine, University of California, Davis
The guidance was submitted for additional review to the MarineDebris.Info community.
We welcome additional editors and contributors; this guidance is intended to be a living document. To provide input, please contact John Davis, project supervisor of MarineDebris.Info, at jdavis [at] marineaffairs.org.
For interviews, please contact:
- John Davis, jdavis [at] marineaffairs.org
- Miriam Goldstein, miriam.goldstein [at] gmail.com
MarineDebris.Info is the global knowledge-sharing community for marine debris management and research, with members representing government agencies, research institutions, conservation organizations, industry, and more (www.marinedebris.info). It consists of a listserv, website, and live chat events allowing members of the MarineDebris.Info community to interact with leaders in the field.
MarineDebris.Info is a project of Marine Affairs Research and Education (MARE), a Seattle-based organization that provides a range of knowledge-sharing services to ocean managers and conservationists worldwide. These services include the OpenChannels forum on ocean planning (openchannels.org), the MPA News service on marine protected areas (mpanews.org), and the Marine Ecosystems and Management information service (meam.net). MARE collaborates on several of its projects, including MarineDebris.Info, with the University of Washington.
- 1. Cleanups of beaches and inshore waters near human communities pose fewer challenges than the open ocean, due to their accessibility. In focusing on open ocean cleanup in this document, the authors are in no way downplaying or ignoring the significant value of collecting debris in these other, easier-to-access areas.