IMAGES of vast accumulations of plastic waste floating at sea have captured the attention of the media. You’ve probably seen images of the disturbing contents of the stomachs of turtles, fish and seabirds. Even in the world’s deepest ocean trench, 11 kilometres below the surface, plastic fibres have been found in tiny crustaceans.
Yet, this plastic represents only a small fraction of all the plastic estimated to be entering the ocean.
A CSIRO study of debris along Australia’s coastline shows that huge amounts of plastic are stranded in vegetation just back from the water’s edge. And most of it is local trash.
The coastline – an obvious place to look
“If we want to be able to prevent plastic waste from entering the ocean, the coastline—or even further upstream—is an obvious place to look,” says CSIRO marine ecologist Dr Denise Hardesty who led the work.
“We need to understand how much trash is out there and what’s driving the problem—where is it coming from and why. But this hadn’t been done in a rigorous way.
“It’s not enough to say, hey, this beach is really dirty. To provide solutions, we need to ask questions in a holistic way, not just say there are x number of pieces of plastic here and y number there. It needs to be put in context of how many people live in the area, what’s the infrastructure, what are the economics and accessibility of the area, what are the local policies—these are all important to consider.”
Between 2011 and 2014, Dr Hardesty’s team surveyed the entire coastline of Australia, including Tasmania, at intervals of 100 kilometres.
They examined the strip of land between the water’s edge and the backshore vegetation, which varied from reeds, mangroves, grasses, shrubs and trees to cliffs and seawalls. They measured individual pieces of debris, mostly plastic, at various distances from the shoreline, recording size, material and colour.
From fridge freezers to fishing gear
Fridge freezers, fishing tackle, plastic bottles and bags—it turns out that the backshore vegetation is where a lot of debris ends up, especially the big stuff.
“We found a tremendous amount of debris in the backshore area and nearly two-thirds of it was plastic,” says Dr Hardesty.
In urban areas, they found more large, whole plastic items. In remote areas, such as the west coast of Tasmania and across the Top End, fishing gear was more common.
“This evidence flies in the face of where the focus has been—the open ocean,” she says.
The further back from the water’s edge, the more debris there is and the bigger it gets.
Where does the plastic come from?
How does all this plastic end up on the backshore? It’s complicated.
Sandy beaches, rocky slabs, mangroves—they all trap things differently. The shape and slope of a headland or bay influences which items get trapped and which items are spat out to sea.
The speed and direction of winds and ocean currents are also important. Some plastic is blown around by the wind. Some drifts in on the current or is carried in by waves.
And some plastic originates locally—the more accessible the location, the more people, the more plastic.
Australians may be surprised to find that most of the plastic littering our coastline originated locally.
Exceptions include Midway Atoll in the north Pacific Ocean and, closer to home, some of the small atolls/cays off the Great Barrier Reef.
The age of plastic and how weathered it is can help us figure out where it came from, but text in another language is not always a reliable indicator, says Dr Hardesty. Among the creative ways of determining whether plastic is of local origin is to look at the ratio of glass to plastic.
Glass is likely to be local trash because it sinks. If there is a lot of glass, then a lot of the plastic is likely to be local. If there’s not much glass but there’s a lot of plastic, then maybe a lot of that plastic comes from elsewhere.
“So we can use the ratio as an independent measure,” she says.
Local trash can be managed locally
The upside of our coastal debris being mostly of local origin is that we have control over it.
As part of the project, PhD student Kathy Willis is working with about 40 local councils to see what waste management policies, programs and outreach work best where, given their population density and budget. To help evaluate the effectiveness of changes put in place by councils since our baseline analysis in 2011, many sites were re-surveyed in 2018–19.
“Plastic is something we can all see,” says Dr Hardesty. “Most of the trash was in someone’s hand at one point in time. We use it and we discard it. We have the opportunity to effect change.”
But when change occurs, we may see a corresponding difference in the oceans within a short timeframe: “When plastic pellet manufacturers in Europe stopped washing pellets down the stormwater drains, we saw a decrease in the number of pellets found in seabirds within five years. That’s a quick turnaround.”
Plastic pollution – an issue uniting the world
Aside from affecting wildlife and being a potential navigation hazard at sea, plastic pollution has economic and human health consequences, and governments are united on the issue. “It’s a non-partisan issue,” says Dr Hardesty. “No matter their politics, governments agree we don’t want trash in our oceans.”
The survey methods developed in the national coastal project have been adapted and are being used in the world’s largest marine pollution project—the Global flows of plastic from land to sea project started in 2017 and involves countries from around the world.
Dr Hardesty and her team are working across Asia and beyond to quantify, country by country, the amount of litter entering the oceans and to identify where it is coming from.
“We will soon have a robust, empirical global baseline of how much plastic is actually entering our ocean,” she says.