A handful of studies have considered altered behaviour, such as motility, hiding responses and predator–prey interactions, result-ing from microplastic exposure. The predatory performance of juvenile gobies (Pomatoschistus microps) in catching prey (Artemiaspp.) was reduced by 65% and feeding efficiency by 50% in labo-ratory bioassays when fish were simultaneously exposed to poly-ethylene microspheres of a similar size and abundance to prey 69 .
Artemia are highly mobile, raising the possibility that the stationarymicroplastic reduced the discrimination of the fish for their prey.Beachhoppers show characteristic behaviours including distinctive jumping, a highly energy-dependent process, and shelter relocationpost disturbance, driven largely by hygrokinetic (favouring move-ment towards humid conditions) and intraspecific interactions 70 .Exposure of the Australian beachhopper Platorchestia smithi to beach sediments containing 3.8% by weight polyethylene micro-spheres led to reduced jumping, whilst the time taken to return to shelters post disturbance動亂後的避難所 was not changed 71 . Beach hoppers that ingested microplastic were significantly heavier, with an increasein gut retention times.
Similarly, in the freshwater crustaceanD. magna, ingestion of 1-μm polyethylene particles from the watercolumn caused immobilization in a dose- and time-dependentmanner 72 . Weight gain may contribute directly to reduced motility,but motility may also be affected indirectly as a result of reduced energy uptake from the diet. Reduced energy reserves 34,56 , for exam-ple, could influence a wide range of behaviours, including those associated with risk versus benefit decisions in feeding behaviour.Studies in social vertebrates (for example, birds and fish) show how individuals will accept a greater risk of predation to obtain food withincreasing hunger or energy deficit 73–75 . The internal state of animalscan significantly determine their choice between alternative behav-ioural tactics 76 , providing an interesting hypothetical mechanism by which microplastic ingestion may influence complex behaviour and species interactions.
The reworking of sediments by plants and animals contributes towards ecosystem functioning by modifying benthic 深海底的;水底的seascapes,increasing nutrient flux across the benthic boundary layer and altering habitat structure for other benthic organisms.
Benthic filter feeders such as mussels and sea squirts processlarge volumes of seawater per hour through their siphons. Expelled waste water and pseudofaeces could draw down microplastics from the water column to the benthic boundary layer, leading to incorporation into sediments by burrowing species.
Hence, micro-plastics may impact feeding rates of key species, whilst the same feeding activities may impact the fate of microplastics within the marine environment.
Altered feeding behaviour in zooplankton 浮遊動物in the presence of micro-plastics may contribute towards larger scale effects due to their important role within pelagic遠洋 ecosystems. For example, prey selec-tion by zooplankton can have a disproportionate impact on both
the biogeochemistry and the timing of food presence in pelagicfood webs 79 .
Microplastic ingestion reduced the energetic intake of the copepod Calanus helgolandicus by 40% in laboratory exposures,even when the abundance of microplastic was an order of magnitudeless than that of prey 56 .If similar reductions in consumption are observed across entire zooplankton communities as a result of microplastic ingestion this could have knock-on effects for pelagic ecosystems.
However, whilstzooplankton ingestion of microplastic has been reported for natu-rally caught animals 80 , we know little of the extent of microplastic consumption within communities in their natural settings, let alone how it might influence the dynamics of mixed species assemblages.
Zooplankton not only influence planktonic assemblages via their feed-ing behaviour and prey selection but contribute to carbon transport to deeper waters through excretion of ingested organic matter 81–83 .In laboratory exposure studies copepods egested micropolystyrene-laden faecal pellets of reduced density and integrity and which hada 2.25-fold reduction in sinking rate 33 . Extrapolating these results tothe average depth of the ocean would hypothetically result in fae-cal pellets taking on average 53 days longer to sink to the benthos.Polyethylene and polypropylene micro plastics, which are very com-mon in surface waters of oceanic regions, may have an even morepronounced effect on faecal pellet sinking speed, because they areless dense than the polystyrene used in these experiments. Given theimportance of zooplankton faecal material in driving carbon exportfrom surface waters, such reductions in density and sinking rates could potentially contribute to global scale alterations in carbon fluxif zooplankton across the oceans are indeed consuming microplasticparticles in sufficient quantities 25 .What emerges from this account are the varied ways in which thein flux of microplastic into the oceans could plausibly be impact-ing ecological processes.
Microplastic represents a novel matrix,providing an alternative surface for pollutants, bacteria and othertypes of organic matter to absorb, interact, and be transported.
Its bioavailability to marine animals appears to be rarely lethal, but chronic exposures can evidently alter feeding, energy assimilation,growth and reproductive output. Extrapolating these impacts to the ecosystem level challenges our current abilities to measure andmodel relevant processes on a global scale, but we can deduce that potential impacts include behavioural changes to predator–preyrelationships, bioturbation, and perturbations不安 to carbon cycling.
How do we respond to these observations and what can we do to mitigate them? How does microplastic compare with other anthro-pogenic stressors and can we use existing tools for monitoring and remediation?Is microplastic a persistent pollutant?A wide range of policy documents and procedures are in place to assess and restrict the release of chemical pollutants, includ-ing international treaties, for example, the Montreal Convention蒙特婁公約,Stockholm Convention關於持久性有機汙染物的斯德哥爾摩公約
, Minamata Convention蒙特婁公約共同制止臭氧層的耗竭, and diverse national legal instruments.
In general, chemicals are assessed and controlled according to their persistence, bioaccumulation potentialand toxicity and controlled accordingly 84 . It could be argued thatsince these measures have been so successful in controlling otherpersistent pollution threats, such as organochlorine pesticides andpolychlorinated biphenyls, they should also be sufficient to curtailmicroplastic pollution. An immediate problem is presented by theobservation that a microplastic is not an individual entity, but con-sists of a complex mixture of polymers, additive chemicals, absorbedorganics and living substances. The assessment of each substanceindividually is unlikely to reflect the net sum of their action or to adequately assess their bioavailability to organisms 85 .
Despite this limitation, comparison of microplastic against the criterion for clas-sification as a persistent organic pollutant under the Stockholm Convention shows the concept of including them to be worthy ofdiscussion (Table 1).
Another way of viewing microplastic could be as a planetary boundary threat. Chemical pollution has been identified as one ofthe anthropogenic impacts of such magnitude that it threatens to exceed global resilience, alongside stressors such as climate change,biodiversity and ocean acidification 86 . By identifying these science-based planetary threats, we can theoretically encourage boundariesto be set at a global scale to allow humanity to flourish without caus-ing unacceptable global change.
Assessing microplastic against thecriteria of planetary boundary threats could there fore be one way ofencourage global action towards remediation and control (Table 1).
Is microplastic a marker of the Anthropocene?Microplastic could also be viewed as a new anthropogenic material,alongside the products of mining, waste disposal and urbanization,identified as geological materials displaced by human activity withthe potential for long term persistence 3 . According to this view, the massive increase in the production and release of plastics is mir-rored by several other substances, including aluminium, concrete and synthetic fibres for which hundreds of thousands of tonnes are manufactured each year, sufficient to leave an imprint of population growth and industrialization in the fossil record. By defining these products as markers of a new geological epoch, the Anthropocene,the authors argue that this places the impetus on human society to acknowledge the consequences of its own actions.
The opportunity for change and remediation is not outside the realms of possibility. Figure 5 shows how global action has been suc-cessful in reducing the amount of spilled oil reaching the oceans each year as a result of concerted global action to improve tanker safety 87 .Statistical data for global emissions of hazardous waste is hard to come by, but systematic data gathered by the US Environmental Protection Agency on chemical waste emissions by US industriesrevealed impressive reductions, from some 278 million tonnesof hazardous waste generated by chemical plants in 1991, to just 35 million tonnes in 2009 88 . This latter improvement was brought about through an industry-led move towards green chemistry,which aimed to redesign chemical processes to make them cleaner,safer and more energy efficient.
Polymers make up around 24% ofthe output of chemical industries worldwide 89 , raising the possibil-ity that concerted action to improve current chemical managementand disposal practices for polymers is a real possibility that couldlead to a similar positive reduction in waste.
Meeting the challenges posed by microplastic requires us, as asociety, to actively engage and consider our role in patterns of con-sumption and careless disposal. Industry can play its role by reas-sessing the integrated management of chemical production. Finally,we have a golden opportunity as scientists to find innovative ways of rising to the multidisciplinary global challenge posed by the vast tide of marine microplastic debris which threatens to engulf our oceans, before it causes irreversible harm.