Life in Plastic Ain’t so Fantastic

Guest post by Louis Vassos, MEnvSci Candidate in the Professional Masters of Environmental Science program at the University of Toronto-Scarborough

Much like the Buggles’ 1980 debut album, our material preferences are well within the age of plastic. Thanks to its light weight, durability, inertness, and low manufacturing costs, our use of plastics has increased dramatically since the mid-20^th century. From bottles and toys to car parts and electronics, there is seemingly no application beyond its reach. Despite its uses and benefits, it has come under increasing scrutiny by environmentalists in recent years. In this regard, we tend to think of larger-scale and more visible environmental impacts, such as accumulation in landfills and petrochemical use in manufacturing. There has also been a significant amount of research on plastic in marine environments, usually focused on larger debris known as macroplastics. Over the past decade, however, there has been increasing concern about a new type of plastic debris in our oceans. Though its presence was first highlighted in the 1970s, we are only just beginning to realize the impact of fragments known as microplastics. As their name would suggest, they are small pieces of plastic, typically measuring less than 5mm in diameter and sorted into two distinct classifications.

Primary microplastics are manufactured to be microscopically sized and are typically used in air blasting as a paint and rust remover, as well as in personal care products as an exfoliating scrubber. This latter use has risen sharply in cosmetics and facial cleansers since the 1980s, with plastic “microbeads” replacing natural materials such as pumice and ground almonds. Regardless of application they usually enter water bodies through drainage systems, and are easily able to pass through filtration systems at sewage treatment plants due to their small size.

Microbeads in toothpaste. Retrieved from: https://blog.nationalgeographic.org/2016/04/04/pesky-plastic-the-true-harm-of-microplastics-in-the-oceans/

Secondary microplastics arise from the breakdown of larger pieces of plastic debris on both land and in water. Larger debris will typically enter marine ecosystems directly or indirectly through careless waste disposal, often being transported through river systems. Sources of transfer include coastal tourism, extreme weather events, fishing, other marine industries, and accidental spillage during transportation. Over time, a culmination of processes such as exposure to UV radiation can reduce the debris’ structural integrity, causing brittleness, cracking, and yellowing. This in turn can lead to fragmentation through abrasion and waves, and fragments will gradually become smaller over time before reaching microplastic size (Cole et al, 2011).

As Eriksen et al (2014) have estimated, there is a minimum of 5.25 trillion plastic particles weighing 268,940 tons in the world’s oceans. Microplastics account for 92.4% of this mass, and their reach has been substantial. Because of their buoyancy and durability, they have the ability to travel long distances without degrading for years. Denser plastics (such as PVC) will sink and have the potential to reach coastal sediment (Andray, 2011). Other marine microplastics will end up trapped in ocean current systems known as gyres, the most famous grouping of which is the “Great Pacific Garbage Patch” in the North Pacific Gyre. Despite what the name would suggest, it is not an island-like mass of floating debris, but is more akin to an extensive “soup” of debris difficult to see with the naked eye. At a density of 334,271 pieces/km², microplastic mass in the area was found to be 6 times that of plankton (Moore et al, 2001). 

Potential microplastic transport pathways (From Wright et al, 2013)

Densities such as this increase potential microplastic ingestion by various marine organisms, especially filter feeders, plankton, and suspension feeders. These species may mistake debris for prey based on size or colour, or passively ingest them without being selective (Wright et al, 2013). In Farrell and Nelson’s (2013) study of mussel-eating crabs, they found that it is possible for microplastics to be transferred to individuals at a higher trophic level. The large surface area to volume ratio of microplastics makes them susceptible to water-borne pollutant contamination, and can cause toxic plastic additives such as BPA and PCB to leach into the water. This debris can also act as a dispersal vector for microbial communities, including potentially pathogenic species (Jiang et al, 2018). While the ingested debris can accumulate within individuals and be transferred up the food chain, the exact effects of this are not entirely known at this point in time (Avio et al, 2017). A recent study by Lei et al (2018), however, found that microplastics can cause oxidative stress and intestinal damage in zebrafish and nematodes, and that their toxicity is closely dependent on particle size.

Intestinal damage in zebrafish caused by exposure to 1.0 mg L^-1 of different microplastic types and sizes. Photograph A shows control (top), survival (middle), and dead after exposure (bottom) zebrafish (From Jiang et al, 2018)

Fluorescent microspheres on a crab’s gill lamella transferred from ingesting mussels, each measuring 5 micrometres in diameter (From Farrell and Nelson, 2013)

          What does the future hold for microplastics? Because their effects on both marine life and humans is relatively unknown, it is important to try and prevent them from entering and accumulating within marine environments. Properly dispose of larger plastic items to prevent them from entering waterways and breaking down into secondary microplastics, and be conscious about the presence of primary microplastics in other products. Make informed decisions when buying cosmetics, and choose ones that use natural exfoliating materials. Microbead bans have already begun to be enacted in several countries, including the UK, US, Canada and New Zealand (Pfeifer, 2018). There is also the potential for future studies on topics such as the health effects of microplastic ingestion and leached additives, debris behavior within the water column, and new standardized techniques for detection and sampling (Cole et al, 2011). It is hard to say what will happen next, but the removal of these 5.25 trillion particles from our oceans will prove to be a very difficult challenge without the development of novel extraction methods.

SOURCES

Anadrady, A.L. 2011. Microplastics in the marine environment. Marine Pollution Bulletin 62:1596 – 1605

Avio, C.G., S. Gorbi, and F. Regoli. 2017. Plastics and microplastics in oceans: from emerging pollutants to emerged threat. Environmental Research 128: 2 – 11

Cole, M., P. Lindeque, C. Halsband, and T.S. Galloway. 2011. Microplastics as contaminants in the marine environment: a review. Marine Pollution Bulletin 62:2588 – 2597

Eriksen, M., L.C.M. Lebreton, H.S. Carson, M. Thiel, C.J. Moore, J.C. Borerro. F. Galgani, P.G. Ryan, and J. Reisser. 2014. Plastic pollution in the world’s oceans: more than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea. PLOS One

Farrell, P., and K. Nelson. 2013. Trophic level transfer of microplastic: Mytilus edulis (L.) to Carcinus maenas (L.). Environmental Pollution 177:1 – 3

Jiang, P., S. Zhao, L. Zhu, and L. Daoji. 2018. Microplastic-associated bacterial assemblages in the intertidal zone of the Yangtze Estuary. Science of the Total Environment 624:48 – 54

Lei, L., S. Wu, S. Lu, M. Liu, Y. Song, Z. Fu, H Shi, K. Raley-Susman, and D. He. 2018. Microplastic particles cause intestinal damage and other adverse effects in zebrafish Danio rerio and nematode Caenorhabditis elegans. Science of the Total Environment 619:1 – 8

Moore, C.J., S.L. Moore, M.K. Leecaster, and S.B. Weisberg. 2001. A comparison of plastic and plankton in the North Pacific Central Gyre. Marine Pollution Bulletin 42:1297 – 1300

Pfeifer, H. 2018. The UK now has one of the world’s toughest microbead bans. CNN. Retrieved from: https://www.cnn.com/2018/01/09/health/microbead-ban-uk-intl/index.html

Wright, S.L., R.C. Thompson, and T.S. Galloway. 2013. The physical impacts of microplastics on marine organisms: a review. Environmental Pollution 178:483 – 492

Emergent linkages in seemingly unconnected food chains

Food webs are notoriously complex, and a difficult aspect of ecology is to offer a priori model-derived predictions of food web processes. There are some ecologists, such Neo Martinez and Jordi Bascompte, who have advanced our understanding of the general mechanisms of food web properties and dynamics through tools such as network theory. Such advanced approaches rely on direct interactions among species, or at least indirect interactions that are mediated through changes in abundance of different network players. However, what is missing from our general understanding of food web interactions is the role that behavioral responses can affect patterns of consumption and network connectivity.

Washington State University ecologists, Renée Prasad and William Snyder convincingly show how behavioral responses to predation can fundamentally alter food web interactions and link previously independent predator-prey interactions. They used two spatially independent insect predator-prey links in a novel, factorially-designed experiment. The two food chains consisted of a ground-based one, where ground beetles consume fly eggs and a plant-based one, where green peach aphids feed on the plants and are consumed by lady beetles. Under the ground-based chain only, the ground-based chain plus aphids, or ground-based chain plus lady beetles, the ground beetles consume a high proportion of the fly eggs. However, when both aphids and lady beetles are present, aphids respond to lady beetles by dropping off the plants and the ground beetles switch from consuming fly eggs to aphids. Under this last treatment, very few fly eggs are consumed, fundamentally altering the strength of the linkages in the two food chains and connecting them together.

This research highlights the inherent complexity in trying to understand multispecies systems, where the actors potentially have behavioral responses to other species, changing the nature of interactions. These types of responses may also generally increase the connectedness of such networks, which may result in more stable food webs, but this would need to be empirically tested. Regardless, this type of experiment offers food-for-thought to scientists trying to work general processes into a broad understanding of food web dynamics.

Prasad, R., & Snyder, W. (2009). A non-trophic interaction chain links predators in different spatial niches Oecologia DOI: 10.1007/s00442-009-1486-7

Global warming and shifts in food web strucutre

Predicting the effects of global warming on biological systems is of critical importance for informing proactive policy decisions. Most research so far has been on trying to predict shifts in species distributions and changes in interactions within local habitats. But what many of these studies assume is that the basic biological processes and requirements of the individual species will not change -that is their biology is fixed and they simply need to find the place that best suits them. Not so, say Mary O'Connor and colleagues, in a just-released study in PLoS Biology.

O'Connor and colleagues experimentally warmed marine microcosms and tested two alternative hypotheses on food web structure: 1) that productivity increases with warming; and 2) warming increases metabolic rates, thus changing consumer-autotroph (i.e., primary producers) interactions. What they found was that warming indeed altered consumer-autotroph interactions. Warming increased base metabolic rates of consumers, as well as primary production, and the net effect was that food webs shifted towards increasing consumer control (i.e., top-down control).

What this research means is that global warming may alter food web interactions by increasing resource needs of organisms as their metabolic rates increase. This may increase the stress on communities and change diversity patterns as increased needs may shift competitive hierarchies or affect autotroph's ability to withstand consumer effects.

O'Connor, M., Piehler, M., Leech, D., Anton, A., & Bruno, J. (2009). Warming and Resource Availability Shift Food Web Structure and Metabolism PLoS Biology, 7 (8) DOI: 10.1371/journal.pbio.1000178

Grazers chew, cereal gets sick

Managing plant disease is a major part modern agricultural practice, so it is important to understand the basic ecological dynamics of plant diseases. Some theoretical studies have found that the prevalence of plant diseases can be affected by the amount of herbivory in a system. Given that human land-use and the removal of top predators from many ecosystems has fundamentally changed the abundance and distribution of many herbivores, the repercussions of herbivory can have important cascading consequences throughout foodwebs –including disease dynamics. In the first experimental study of the interaction between herbivory and plant disease, the forthcoming paper in PNAS by Elizabeth Borer and colleagues, shows that increased exposure to large herbivores (e.g., mule deer) resulted in higher disease prevalence in the plant community. The disease they studied, barley and cereal yellow dwarf virus (shown in the photo), is transmitted by aphids, so herbivory does not cause increased transfer. Rather, herbivores changed community composition resulting in higher abundances of very susceptible species, creating a feedback where higher abundances resulted in higher infection rates due to a larger pool of potential hosts near by. These results are important for two reasons. First, this particular virus is an important agricultural disease. Secondly, we need to take a whole-community approach to understanding disease dynamics because these dynamics are not only a property of host-vector-pathogen interactions, but are subject to direct and indirect effects from interactions with other community members.

E. T. Borer, C. E. Mitchell, A. G. Power, E. W. Seabloom (2009). Consumers indirectly increase infection risk in grassland food webs Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0808778106

Fisheries and food webs: a whole system approach to cod recovery

The collapse of cod fisheries around the world is a breathtaking example of over exploitation and poor planning. But with reduced fishing pressure why have cod populations shown such slow or stagnant population recovery? This has been an extremely active area of research for fisheries scientists. In a recent paper by Casini and colleagues in the Proceedings of the National Academy of Sciences, they found that over-fishing of cod in the Baltic Sea has led to a regime shift, where a small planktivorous fish called sprat now dominate the system. But its not just that there is a new dominant, sprat seem to really change how the ecosystem operates, to the detriment of cod recovery. When the ecosystem was cod dominated, zooplankton abundance was unrelated to sprat abundance but did appear to be dependent on hydrological environmental variables. In the new sprat-dominated system zooplankton numbers are negatively related to sprat abundance and the environmental controls of zooplankton abundance do not appear to be important. So why is this bad for cod recovery? Adult sprat compete with larval and juvenile cod for zooplankton and sprat consume cod eggs. The authors suggest that a good cod recovery plan will involve managing key aspects of the food web. This paper reveals how a whole food web or ecosystem approach is necessary for understanding population controls of important fisheries species.

M. Casini, J. Hjelm, J.-C. Molinero, J. Lovgren, M. Cardinale, V. Bartolino, A. Belgrano, G. Kornilovs (2009). Trophic cascades promote threshold-like shifts in pelagic marine ecosystems Proceedings of the National Academy of Sciences, 106 (1), 197-202 DOI: 10.1073/pnas.0806649105