How we're eating the plastic we put in the ocean

Nearly 13 million tonnes of plastic waste ended up in the world's marine environment every year. Photo / 123RF

Broken-down plastics in the ocean are finding their way into our diets – while also harming the marine systems that are crucial for our survival.

Nearly 13 million tonnes of plastic waste ended up in the world's marine environment every year and the volume of microplastics appeared to be on the rise.

Environmentalists were recently alarmed to find the problem had reached the pristine waters of Antarctica.

"We are living in an age of plastic," the University of Auckland's Dr Julie Hope said.

"We increasingly see in the news that we are really struggling to deal with the plastic waste we produce globally."

While research around microplastics had typically focused on fish, shellfish and larger animals in the ocean, less was known about how they affected a group of even tinier organisms.

These were the microalgae that live on the surface of coastal sediments, and which had a disproportionately huge influence.

They were responsible for producing sticky sugars that bound seabed sediment together – effectively influencing how it moved about our coasts.

We could also thank microalgae for helping regulate our climate, as they captured CO2 from the atmosphere, and, as a byproduct, produced up to half of the oxygen that we breathe.

"Coastal soft sediment ecosystems may look boring and just 'muddy' but they are extremely complex and critical for our society."

The carbon they fixed from CO2 and nutrients they captured channelled up through the food web - from microalgae to larger marine organisms.

Therefore, Hope said, if the smaller life at the bottom was affected by microplastics, our marine systems might not be as productive or healthy.

Vital processes and functions could change, lowering water quality and clarity.

"Microplastics may not only be affecting animals through ingestion but may in fact be affecting the way our whole coastal marine systems function," she said.

"This will ultimately impact our lives as a society as these systems and the functions they carry out are critical for our survival."

Microalgae are responsible for producing sticky sugars that bind seabed sediment together - effectively influencing how it moves about our coasts. Photo / Supplied

Yet we still knew little about what was really happening in this muddy corner of our environment.

"Microalgae are truly the unsung heroes of our marine systems but we are stressing them and the small animals in the sediment from what we do on land," she said.

"We are increasing the inputs of nutrient and sediment from land use changes - and we are starting to understand the scale at which we are polluting them with microplastics."

Hope suspected that microplastics might even be accumulating in the sticky substances these organisms produce.

"If microplastics accumulate where microalgae densities are higher, they may be more likely to be ingested and accumulate up the food web," she said.

"For example, if microplastics are sticking to the microalgae or biofilms on the sediment surface, they will be more likely to be ingested by bivalves and worms that live in the sediment and feed on microalgae."

This image shows ground up Polyethylepe terephthalate (PET) - of which soft drink bottles are made. Such products eventually become microplastics after entering the ocean. Photo / Supplied

These grazers were the primary food source for fish, larger shellfish and birds on our shores - including commercial species that humans consume.

Microplastics could also carry organic contaminants and may release toxins, such as plasticisers and chemicals, to the surrounding environment.

"This may affect the 'health' of the system or the way it works in many ways such as the primary productivity of the microalgae, the behaviour of animals living in the sediment and a whole suite of processes."

In a new study, which recently received a $300,000 grant from the Marsden Fund, Hope and colleagues seek to look at these effects in unprecedented detail.

Auckland's Waitemata Harbour will be a focus of the new study. Photo / Supplied

"We can't add microplastics to the environment without exacerbating the problem we already have with this pollutant so instead, we will compliment field surveys with experiments in the lab, and work across natural gradients in the field," she said.

"We will also be looking at the effects of microplastics alongside other stressors as these systems are dealing with multiple pressures at any one time."

It was expected the study's findings would shed some light on the magnitude of this issue for New Zealand – and they would be relevant for studies across the world.

The researchers also hoped their work would encourage agencies to monitor this contaminant – and for people and companies to make better choices.

"Single-use items such as coffee lids, plastic cutlery, cling wrap on vegetables are often unnecessary, and I am encouraged by the number of bars and restaurants in New Zealand that are now swapping plastic straws for paper or metal ones," she said.

"We also need to communicate the effects of microplastics to the public as this is a great way to change the decisions we make.

"As we increase public knowledge of the problem, we can put pressure on companies to change packaging, make wiser choices about our own purchases and disposal, as well as begin to explore ways to clean up this mess."