Ain't no mountain high, ain't no ocean deep (or something like that) - the curse of Microplastics

What are they?

They are quite literally micro-plastics; tiny bits of plastic. Their size is usually defined at <5mm, but sometimes at <1mm. The problem is that they end up in oceans.

Where do they come from?

Predominantly, microplastics come from onland sources. Their provenance is usually from either

1)      Manufactured microplastics used in personal care products (PCP’s) or abrasive cleaners

2)      Via the degradation and fragmentation of larger plastic particles. This can be through natural means (wave/weathering action), or through anthropogrenic means (such as washing machines)

By and large, they enter waterways and oceans due to lack of effective waste management. In previous posts, you’ll see case examples from Bangladesh and California outlining the struggles different nations face when it comes to waste management. While Bangladesh may be a big offender on a general plastic waste basis, more developed countries such as the UK and USA are also responsible. Many waste water treatment plants (WWTP) cannot effectively filter out microplastics and microbeads that end up in sewage. As a result, and particularly after heavy rain, these particles are released intothe environment. 

So what’s the big deal- why are they so harmful to the environment?

With up to more than 240 million tonnes of plastic used annually, debris entering the sea are ever increasing. While macroplastics are a significant problem, recent focus has shifted to the prevalence of microplastic throughout the ocean, both in surface waters, but also deeper within the water column. The subsequent effects on the ecology of the ocean are wide-ranging. Not only does evidence suggest  these particles are being ingested by biota and accumulating in the food chain, but plastics can also sorb harmful pollutants onto their surfaces. 

 This contamination can be transported around the ocean via currents, but when digested can also release toxins into animal species and throughout the food chain, potentially endangering human health too.

Where can they be found?

The presence and effect of microplastic particles in the ocean has dominated research. The bulk of research has focused their investigations on shorelines and near-coastal regions in attempts to quantify plastics and address the problem. However, there is an increasing realisation that many microplastics end up in oceans via rivers, and later research has shifted toward exploring the prevalence and impact of microplastics within these alternative waterways. The sections below explore in more detail:

The Seas:

Probably the most abundant form of plastics in the seas, microplastic quantities are increasing. While 80% of plastics in the ocean are sourced on-land and would therefore be assumed to be most commonplace in coastal regions, microplastics have actually been found in remote areas that were previously regarded as pristine. 

A study found evidence of microplastics in deep-sea sediments in the Southern Ocean, off the polar front at depths of up to 4800m. The complex ocean currents at work that transport these particulates make detection of long-term trends extremely difficult. 

Possible oceanographic processes aiding their transfer include downwelling, severe storms, and saline subduction . As these processes spread microplastics further and further afield, it means increasing amounts of deep-sea water columns become a sink for plastics. This plastic ubiquity in ocean environments makes them a great threat to marine habitats and ecosystems worldwide.  

But it is the detrimental effects on ecosystems that makes their spread throughout the oceans so worrying. They accumulate harmful chemicals onto their surfaces, and release them into animals when accidentally digested, compounding toxins within the  foodchain. An independent study into 101 peer-reviewed papers shows the extent of the threat. We have the obvious problems of ingestion by animals, with the case in the Canadian Arctic, where over 80% of fulmars (bird species) had signs of plastic ingestion. But more than that, plastics have become a mode of transport for both pollutants and invasive species. In the western Atlantic, 24% had eggs attached by insects. As these previously buoyant plastics get heavier, they sink and transport these attachments to deeper mobilities within the ocean. 

Not only are the impacts of plastic pollution transferred to deeper and deeper ocean levels, but they also recycled back on to land. The very fish we eat is contaminated, and a recent study also estimated the microplastic content of sea salt at 550-681 particles per kilogram. This is the salt we use on our food. In this way, the risk of ingestion is transferred to our dinner tables. 

 The Rivers

The pervasiveness of plastic pollution in rivers has been significantly overlooked in recent years, leading to a deficiency of data on riverine environments, particularly in regard to microplastics  . However, rivers and terrestrial waterways provide the main pathway of plastics into the ocean. A number of separate studies have found microplastics in a range of freshwater environments, from urban rivers (where we would expect to see micro and macro plastic particles), but also found them in remote and isolated lakes.

Measurements from an urban river in Chicago, USA, found that microplastic concentrations met or exceeded those within marine environments. However, their quantity is not the main problem – it is the consequences of this. The table below shows increased microplastic concentrations downstream of the waste water treatment plant (WWTP), proving the inefficiency of sewage systems in filtering out these particles. Upon the microplastics were a ‘biofilm’ consisting of a bacterial assemblage unique to microplastics. These colonising bacteria were associated with wastewater organisms. Not only is this gross, but it also ‘indicates that microplastic may be a novel pathway for transporting disease-causing bacteria into waterways’ and marine environments.

Source: McCormick et al, 2014

Their presence in remote Lake Hovsgol, Mongolia, was discovered in a 2014 study by Free et al. As this mountainous lake is not connected to any sewage systems, possible sources of these particles stem from degradation of larger littered plastic particles, but also transport via prevailing winds. This correlated with increased microplastic concentrations of the southwestern shore, which bore the brunt of the wind.  The average microplastic density was 20,264 particles per km². The long residence time of this lake possibly suggests the higher quantities of plastic pollution, as neighbouring Lake Huron had lower microplastic densities and a relatively quick residence time of approximately 20 years, acting to displace pollutant particles.

Source: Free et al, 2014

But despite the high concentrations of microplastics in freshwater environments, the lack of attention on them is astounding. In fact, the discharge of microplastics into rivers is actually legally permitted in some places. A case study of the Austrian Danube revealed that industrial microplastic (IMP) consisting of pellets and flakes were classified as a filterable substance, and therefore up to 30 mg l⁻¹ would be upper limit for legal plastic discharge into the Danube. Austria is not alone – other nations continue to class plastics as harmless solid waste.

This lack of investigation into microplastics both within freshwater and marine environments isn’t good. There needs to be more so we can solidly appreciate the impacts they are having, and effectively engage with solutions to this problem.