International Coastal Cleanup 2009 Special
Lifecycle model for marine litter
A key challenge in developing guidelines for the assessment of marine litter is to identify the major processes that control the entry and / or removal of litter from the oceans and also the transformations that occur during the lifecycle of any given litter item (e.g. when floating litter sinks to become benthic litter or is cast onto a beach to become beach cast litter). In developing any sampling strategy it is necessary to establish a model of the system being investigated which makes explicit the various assumptions about how the system works. Here following we provide a systems model that describes the dynamics of marine litter from source to sink. This model can be used to visualise the “lifecycle” of marine litter by tracking the various pathways that litter can take from the point of discard and through the system until it is eventually removed or decomposed. The model, represented schematically in Figure 1, provides a simplified view of the key parameters and processes that can be measured or inferred from an appropriately configured marine litter sampling strategy. In summary, the model identifies a set of key state variables (rectangular boxes) that represent “pools” of material that are in dynamic flux within the system. These pools include Floating litter, Benthic litter and Beach cast litter. The size of the litter pools are defined in terms of quantities of material (e.g. tonnes of floating litter or the numbers of particular items) and thereby represent the sum total of material within the system under consideration. Dynamical processes (indicated in Figure 1 by arrows) illustrate the flux rates or movement of litter from one pool to another. These values are measured and reported as rate functions (e.g. tonnes of litter discarded per year or tonnes of litter being cast onto beaches per year). In general terms these flux rates can be measured either directly, by observation of amounts of material being transported, or indirectly through inferences based on changes in the amounts of litter in each pool over time. The model can be used to illustrate some simple truths about the longterm options for the management of marine litter including:
1. For as long as the input processes (Discard) exceed the removal processes (Collection and Decomposition) the amount of litter will increase through time resulting in more litter in the oceans and on the beaches.
2. Given that decomposition is slow (particularly for some of the persistent and more toxic plastic forms of litter) then this will never be a solution to the marine litter problem. In some cases material engineering may provide alternative materials that decompose more rapidly; increased rates of decomposition would then result in a reduction in the size of the litter pool.
3. The key point of control in the system is through the management of discard behaviours. If we can reduce inputs we have some chance of managing the downstream environmental consequences. Improvements are needed in waste management and reception facilities in ports and harbours, education of beach goers is essential to reduce domestic discards and improved management of rubbish dumps, particularly those in coastal catchments.
Management of marine litter can be informed by obtaining good quality data on the size of each of the pools and the rates of exchange between them. This allows us to articulate a set of useful objectives for any national or international programme of marine litter surveys: i) To provide information about the sources of different types of litter, and ii) To quantify the amount of litter in different ocean systems.
To achieve these objectives, litter assessment guidelines must explicitly incorporate an awareness of the “life-cycle” of marine litter into the design, to support quantification of the key response variables and to allow an analysis of the efficacy of various management interventions.
In the absence of better management at source, the exponential growth of litter in the marine environment is certain to continue (Barnes 2002). The need to develop and evaluate alternative management strategies is therefore central if we aim to limit the amount of litter entering marine systems. In 1975, the annual influx rate of litter to the world's oceans was estimated at six million tonnes (National Academy of Sciences 1975) ; current rates are likely to be substantially greater. Given the prolonged timeframe for decomposition (UNEP 1990, The Ocean Conservancy 2006) and the very small amounts of litter actually removed through beach clearances (The Ocean Conservancy 2006), it can be argued that the volume of marine litter in the oceans will continue to increase exponentially over the coming decades.
Source : UNEP/IOC Guidelines on Survey and Monitoring of Marine Litter, Regional Seas Reports and Studies No. 186, IOC Technical Series No. 83 by Anthony Cheshire and Ellik Adler.
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