Plastics and Their Impacts in the Marine Environment
Proceedings of the International Marine Debris Conference on Derelict Fishing Gear and the Ocean Environment* 6aug00
August 6-11, 2000 Hawai‘i Convention Center Honolulu, Hawai‘i
Anthony L. Andrady, Program Manager and Senior Research Scientist, Chemistry and Life Sciences Division, Research Triangle Institute, North Carolina
I want to thank the organizers of this conference for providing me with the opportunity to address this gathering. My research specialty is polymer science and engineering, particularly the topic of plastics and the environment. I think it is important to closely monitor the impacts of introducing plastics in to the fragile marine ecosystem and to study the various technical mitigation strategies that are available to minimize any damage due to plastics in the world’s oceans. In this short presentation I plan to achieve two objectives. First, I want to discuss the factors responsible for the breakdown of plastics once they are introduced into the marine environment. Then, I want to consider the various technical options, particularly the technologies for biodegradable and photodegradable plastics that are available, to address the problem of plastics in the marine environment.
TYPES OF PLASTICS
Shown below in table 1 are the major classes of plastics commonly used in fishing gear application. As you know, fishing gear, accidentally lost or intentionally discarded, remain an important component of persistent marine debris. There are hundreds of different types of plastics and plastic compositions, but of this only about four or five types are commonly used in fishing gear. The table also includes the specific gravity of the plastic and as you see some, as indicated, are denser than seawater and will sink rather than float at sea. Out of these plastics it is the nylons and polyethylenes (and also some polypropylenes) that are used most in the construction of fishing gear. This is not surprising as these plastics have the unique combination of properties that make them best suited for the purpose. For instance, they have very good strength, good elasticity, and have low perceptibility in the water column and contribute to the high efficiency and catchability of the fishing gear.
With all these strengths, plastics as a class of material have a significant drawback from an environmental standpoint in that they biodegrade at an extremely slow rate compared to other organic materials. All organic materials, including plastics, do biodegrade, but they biodegrade at such a slow rate that they are of little practical consequence. This bioinertness of plastics is both a drawback and also an asset because the biggest shortcoming of the natural fiber fishing gear that we had a long time ago was that they were readily biodegradable! They weakened as they biodegraded over time and therefore could be used for only a limited duration. However, in cases of loss or abandonment of the natural fiber gear, the environmental consequences were limited as the gear biodegraded readily without posing significant ghost fishing, entanglement, or other hazards.
Table 1. Types of Plastics Used in Fishing Gear Applications.
Type Density (g/cm3)* Buoyancy Gear type Polyethylene 0.96 float Trawls Polypropylene 0.90 float Trawls Nylon 6 or 66 1.14 sink Trawl sections, gill nets Saran fiber 1.70 sink Seine nets * A nominal density is given. Each class of polymers display a range of densities.
Figure 1 (not shown) is representative of the consequences of poor biodegradability of synthetic fishing gear. It shows skeletons of marine mammals entangled in a submerged section of netting, probably nylon gillnet. What are the factors that govern the breakdown of plastics in the ocean environment, or for that matter, in any environment? The primary factor is the solar actinic radiation, or the part of the solar spectrum that spans from about 290 nm to about 315 nm. This ultraviolet radiation, called UV-B radiation, readily photodegrades all plastics commonly used in fishing gear. However, the effectiveness of this factor depends on whether an efficient light-stabilizer is compounded into the plastic. Understandably, manufacturers routinely incorporate efficient light-stabilizers into most plastic products, certainly including fishing gear, in an effort to obtain long service lifetimes. Therefore, in practice, the solar UV radiation does not have that much effect in breaking down most plastic compositions exposed to sunlight. In addition to sunlight, the slow oxidation of the plastic, where oxygen in the air oxidizes the plastics slowly and facilitates the breakdown, can be a contributing factor. This process too is very slow and with some plastics can be comparable to the rate of biodegradation. Hydrolysis (or chemical breakdown by water) is available with certain and very special types of plastics. But these types of plastics are not used in the fishing industry. The conclusion here is that there are no effective reliable mechanisms to breakdown a well photstabilized plastic product in a reasonable time scale when exposed to the marine environment.
This leads to perhaps the most popular question posed to scientists working in this area— “How long will the plastics last at sea?” Typically, scientists respond to this question somewhat vaguely. The lifetime of a plastic material in the marine environment is quite variable and depends upon the intensity of the different factors contributing to the breakdown available at that location of interest. It depends, for instance, on the temperature of the water column, on the amount of solar UV-B insolation, the biotic potential of the environment, and more importantly on how one defines the “lifetime” of plastic at sea. The term can have different meanings. Does it mean how long does the material persist in a geometry (such as webbing) strong enough to cause entanglement? In which case you have a certain time period within which the strength of the extensibility of the plastic is decreased and an animal caught in the plastic netting can free itself without any problem. Alternatively does it mean in a stricter environmental sense that total mineralization or total conversion of the plastic to carbon dioxide and water? The latter process will take hundreds of years because most plastics mineralize at extremely slow rates.
Research over the last decade has clearly established one important factor conclusively. Plastic exposed floating at sea at a given location tends to break down at a much slower rate compared to the same plastic material exposed outdoors on land at the same location. This is reported to be generally true for most plastics, except perhaps for Styrofoam. In the case of Styrofoam, the material does break down into smaller particles faster at sea than on land, perhaps because of the unusual expanded bead structure of the material. This experiment has been carried out in several locations with various types of plastic products including troll webbing, rope used in fishing, six-pack rings, and Styrofoam packaging.
There are two reasons that can explain this finding. The first is that plastics in contact with seawater undergo extensive fouling. Sunlight is often not able to reach the plastic surface partially covered with foulants, unlike with the sample on land. This shielding effect of foulants on floating plastics can reduce the rate of light-induced breakdown. But more importantly, the differences in the temperature of the material exposed floating in seawater and in air on land may also explain the difference in rates of breakdown. The temperature of a piece of plastic exposed to sunlight on land rises up because of the absorption of infrared light, in a process called “heat buildup”. The temperature of the plastic can rise by as much as 20° centigrade higher than that of the ambient air. But when the same plastic is exposed in seawater, the plastic is maintained at the relatively lower temperature of seawater. As the rate of degradation reactions has a positive temperature coefficient, the samples exposed in water degrade at a slower late.
This was recently illustrated in an experiment of weathering plastics in the desert environment where two sets of polyethylene film samples were used. One was exposed in air and the other placed in an UV-transparent box that was air-conditioned and kept at 25° centigrade. The tensile extensibility is a particularly sensitive indicator of photodegradation for film samples and was used to monitor the degradation process over a period of 10 months. While the experiment is still ongoing, the data collected to date illustrates the dramatic effect of temperature on the degradation process. The sample exposed in air disintegrated within a few months while the sample maintained at lower temperatures maintained its integrity and had significant residual extensibility even after 10 months of exposure.
Latex rubber balloons are an important category of product in the marine environment. Promotional releases of balloons that descend into the sea pose a serious ingestion and/or entanglement hazard to marine animals. Based on the fairly rapid disintegration of balloons on exposure to sunlight in air, the expectation is that balloons do not pose a particularly significant problem. In an experiment we carried out in North Carolina we observed that balloons exposed floating in seawater deteriorated much slower than those exposed in air, and even after 12 months of exposure still retained their elasticity.
What technological control options are available to mitigate the problem, if any? I want to briefly discuss four different options here. We have photodegradable plastics and we have a category called biodegradable plastics, which is somewhat of a misnomer in that all plastics are invariably biodegradable. These refer to particular types of organic polymers that biodegrade at a much faster rate than regular plastics. Then we have on-board plastic waste management that can be practiced on fishing vessels as well as on naval vessels. Then finally you have education, because willful discharge of plastics in the ocean is really a behavioral problem and there are no technical options that will completely eliminate that.
There are certain types of products with which photodegradable plastics work very well. I do not want to get into a chemical discussion of the structure and function of these materials because of the lack of time. However the structure of polyethylene, for instance, can be changed chemically during manufacture so that it absorbs UV-B radiation from sunlight and breaks down into a very brittle material in a fairly short period of time. As polyethylene is the most used commodity plastic, this is a very useful technology. A common product, such as a six-pack yoke, when discarded outdoors may last a fairly long period of time. If the same item were made of this modified, enhanced photodegradable polyethylene, it would deteriorate in sunlight in a faster time frame, minimizing the chances of entanglement hazards. This type of technology is also useful in litter reduction to improve aesthetic appeal of beach or even urban areas.
An important consideration, therefore, is if this technology will perform adequately under marine exposure conditions as well as it does under land exposure. A test procedure employing a floating rig in the Biscayne Bay in Miami, FL was used in an effort to answer this question (the experiment was subsequently repeated in Seattle, WA). Essentially, a set of plastic samples of interest, for instance sections of trawl webbing, were attached to the PVC pipes that made up the rig and were exposed to sunlight while the samples were floating in sea water. The mechanical properties of these materials were monitored weekly over a period of time. This exposure procedure is now an ASDM standard protocol [ASTM D54 37]. We studied two of the commercially available photodegradable polyethylenes and found that the rate of degradation of the samples accelerated considerably even when the samples were exposed floating in seawater. The advantage in terms of preventing entanglement from at least the polyethylene products in marine environment will be significantly reduced by the use of this technology. We also found, as expected, the rate of deterioration was slower than that of the samples exposed in seawater, compared to those that were exposed in air.
For instance, an unstabilized polyethylene film material, such as a section of a thick plastic sheet, exposed outdoors in Miami, would gradually loose its tensile strength over a period of several months. Typically, the point of embrittlement (the level of degradation where the material has virtually no strength and breaks down into little small pieces on handling) was reached in about three to three-and-a-half months. The period is short because the material has no light-stabilizer in it. But if you were to expose the same sample floating in seawater at the same location, after three to four months no significant decrease in the strength can be found. The test results on tensile strength will be about the same as the unexposed samples. In repeating the same experiment with photodegradable materials we found that the samples exposed in both air and seawater photodegraded and lost strength much faster compared to the regular unstabilized polyethylene material. The point of embrittlement for the samples exposed in water was reached in four to five months of exposure. Samples exposed on land embrittled in several weeks under these exposure conditions. However it is important to recognize that while convincing studies of this nature have been carried out on samples of different plastics, no data is available for plastic fishing gear made out of photodegradable materials.
With fishing gear using this type of technology an important question is the nature of trade off between catchability of the gear and its degradability in the ocean. This issue has not been addressed in the literature. In the early-’90s we carried out some studies on the fouling of fishing gear in both Biscayne Bay and the Seattle, Washington area. We do not have the time to examine all the findings in detail, but an important possibility emerged from that study. The study included measurement of the density of fouled trawl web segments at different durations of floating exposure. Based on the data, we were able to surmise that a floating piece of fishing gear in the ocean would initially increase in density because of copious fouling. The density would be high enough for the material to be negatively buoyant. This is hardly surprising because of the high levels of fouling obtained at the locations where the tests were carried out. Upon submerging it to a level in the water column that is determined by the density, the algal fraction of the foulant colony is likely to die because of the lack of sunlight. The density could change again and become low enough for the sample to float again. This was postulated based on density data for samples exposed floating and submerged in seawater. Recent experimental observations by Murray Gregory are consistent with this notion. In relying on solar exposure to bring about faster degradation of derelict gear at sea, the possibility of foulant-induced sinking and subsequent, possibly intermittent, disruption of the exposure needs to be taken into account.
With controlled lifetime fishing gear that employed photodegradable technology, it would of course be crucial to keep it shielded from light when not in use. The technology allows one to build an approximate timer into the gear that would allow it to be exposed to some predetermined level of exposure to sunlight before enhanced degradation sets in. Typically the transitions in strength of the gear, once the enhanced degradation has commenced, would be fairly rapid. At least in theory, it is possible to set this timer for controlled lifetime at a pre-selected duration of use longer than the anticipated period of use for the gear. This would not work for all gear, it would only work for floating gear, floating plastics, and for gear that is expected to last for a certain fixed period of time (not for gear that is continuously repaired and reused).
In these discussions we have assumed embrittlement to be an adequate end point in the degradation process. From the point of view of minimizing entanglement of marine mammals and perhaps ghost fishing, it is certainly a very pertinent end point. At embrittlement we have essentially converted the plastic six-pack ring, or a piece of netting, from a hazard into a collection of relatively small pieces. But have we removed the plastic material from the marine environment? In a recent experiment we exposed an enhanced photodegradable polyethylene sheet (the same material used in photodegradable six-pack rings) in Florida. The tensile strength was measured every few weeks to monitor the degradation of the sample. In addition, the molecular weight of the same samples used for tensile testing was measured using gel permeation chromatography (GPC). The goal was to find if the dramatic reduction in the strength of the photodegradable plastics was accompanied by a correspondingly large reduction in the molecular weight. At embrittlement, the molecule weight was 11,000! (see table 1) The material still remained a polymer. It is well established in the literature that a molecule rate of 11,000 polyethylene is not biodegraded in any practical time scale. There is no mechanism for biodegradation of that long of a molecule in the marine environment. In defining degradation in terms of embrittlement we may be helping the marine mammal population. The powdery residue, however, remains persistent in the sea environment as debris perhaps affecting the filter feeders. Some evidence of accumulation of small plastic pieces in the ocean environment has appeared in the literature.
A related issue is that of careless handling of virgin resin beads during transport and processing. Plastic resin beads are widely distributed in the world’s oceans and are ingested by marine birds. This is an example of a type of plastic pollution for which there is no technological solution. What is needed here is better shipboard management and an increased general awareness of the fragility of the ocean ecology by all users of the sea. If one stops to think of it, except for the small amount of plastics incinerated, every little bit of plastic manufactured in the world for the last 50 years or so, still remains in the environment somewhere. It’s either in the landfill or it’s somewhere in the ocean because there is no effective mechanism to readily break it down.
While most plastics biodegrade far too slowly to be practically significant, a few have chemical structures that allow rapid biodegradation. These are relatively expensive specialty plastics, rarely used in common applications. Some of the more promising polymers in this category are polylactic acid and poly (hydroxy butyrate valerate) copolymers. In biodegradation, microbial enzyme action converts a plastic material first into small organic molecules and invariably into carbon dioxide and water. This is the ultimate fate of natural materials such as plant and animal debris in the marine environment. The enhanced biodegradable polymers are generally more expensive than the common polymers such as polyethylene and nylon used in fishing gear. Also the effectiveness of the biodegradable plastics as materials of design for fishing gear has not been well studied. Materials for gear application require specific properties. Good elongation is important in gill net applications and good abrasion resistance as well as fast sinking rates are desirable in troll or bottom type gear. Nylons remain the most popular choice for construction of fishing gear because this class of plastics exhibits nearly all of these key properties (except perhaps for sinking speed where polyesters are superior).
While we continue to gain an increasingly better understanding of the fate of plastics in the marine environment, there are no ready-to-apply technical solutions to the problem of marine plastic pollution. Degradable plastics technology may eventually mature into a class of controlled lifetime fishing gear. But much developmental work needs to be undertaken to make this a reality. Other practical options include incentives to encourage the return of waste gear to the shore where collection facilities hold the gear for subsequent recycling. The reduction of shipboard plastics material is also a valuable contribution in this regard. The previous speaker had very elegantly described the Naval efforts in this regard. It is important to continue our efforts in education aimed at increasing the environmental awareness of users of the ocean.
Land-based plastic debris is a significant source of the plastic waste found in the oceans. The photodegradable plastics technology, already used in some products such as six-pack rings, has a valuable role in reducing beach plastic debris. Biodegradable plastics may also be appropriate for some of the products often found in beach debris.
I want to acknowledge the help of Jim Coe and the Entanglement Research Program of the National Marine Fisheries Service. For several years the program supported some of my work. I also would like to thank the Department of the Navy, the research program at the David Taylor Research Lab, and the U.S. Environmental Protection Agency for supporting my work as well. Finally I appreciate the help by Kathy O’Hara (Center for Marine Conservation) in providing the photographs used in this presentation.
* About the Conference
The International Marine Debris Conference on Derelict Fishing Gear and the Ocean Environment was convened to address the Pacific-wide nature of lost and discarded fishing gear and its impacts on protected species, coral reefs, and the marine environment.
The conference attempted to address the problem of derelict fishing gear at its source. Evaluation of netting removed from coral reefs during multi-agency cleanup efforts in the Northwestern Hawaiian Islands indicated to National Marine Fisheries Service (NMFS) officials at the Honolulu Laboratory that the majority of recovered debris was not originating locally but rather from other fisheries operating in the North Pacific, including Asia and Alaska.
Funding for the conference was provided by the U.S. Congress to the National Oceanic and Atmospheric Administration's Hawaiian Islands Humpback Whale National Marine Sanctuary. Congress charged the agency with the overall organization of the conference and with the directive to bring together a diverse group of individuals from industry, government, and the public sector to assess the Pacific-wide nature of derelict fishing gear and develop specific recommendations and strategies for action.
The conference convened in Honolulu, Hawai‘i on August 6-11, 2000. Representatives from across the Pacific came together to share ideas and develop a list of recommendations and detailed strategies for action including Chile, Taiwan, Japan, Australia, New Zealand, American Samoa, and Micronesia.
Among the recommendations were calls for: u an international action plan, u greater attention to marine debris issues by members of the International Maritime Organization and various UN Regional Seas Programs, and u public and private partnerships to assist in the implementation and compliance of international agreements and guidelines.
This proceedings document is a compilation of the papers, speaker presentations, and recommendations developed by the conference participants. We hope that the recommendations will be shared amongst colleagues and that collaborative multi-agency and international efforts will continue to produce solutions to this problem.
Naomi McIntosh Conference Organizer Honolulu, Hawai‘i
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