Plastic pollution in the ocean is a major environmental issue, but recent studies show that most plastic waste stays close to where it first enters the environment – which means we can all do our bit to clean it up.
We’ve known since the 1970s that plastic is accumulating in subtropical ocean basins far from land, including the infamous Great Pacific Garbage Patch between Hawai’i and the Californian coast. But these open-ocean patches account for a small fraction of the plastic that enters rivers. Most of the fragments flowing down a river end up trapped in estuaries where rivers meet the sea, or along the coastline, where tides and waves distribute floating debris.
“Plastic in the ocean is closer than you think,” says Melissa Bowen, a physical oceanographer at the University of Auckland who is studying how plastic travels through the upper Waitematā Harbour estuary.
International researchers have studied the transport and retention of plastic along some of the world’s major rivers, including the Seine, Rhine and Saigon. They found estuaries act as plastic retention hotspots, but those fed by long, fast-flowing rivers with a high discharge of water potentially flush more plastics into the ocean.
Less is known about the hydro-dynamics of smaller, branched urban catchments, where tides and winds complicate the transport route of plastic. This is where the Waitematā comes in. When Bowen and her colleagues set out to see just how much plastic the relatively short estuary traps, they were astonished by what they found.
They used GPS receivers wrapped in mobile phone pouches as floating trackers to follow where they went during several tidal cycles. They found that none moved the relatively short distance needed to exit the estuary out to sea. They ended up on the shoreline close by.
When the team ran computer simulations to test for a wide range of water flows and tides, they found 60-90% of floating material stayed in the estuary. Even when they raised the level of water discharge by two orders of magnitude, the percentage of debris retained by the estuary remained very similar. Most plastic material became trapped before it could reach the sea.
The GPS trackers were an innovative part of the experiments, Bowen says. “They give us evidence from the observations and [verify] the simulations. I’m still amazed that an estuary as small as the Waitematā with such vigorous tides is retaining such a high proportion of plastic.”
The team was motivated by wanting to understand, and eventually predict, where plastic is ending up. “If we don’t even know where it is going, it is difficult to know how we could clean up and what areas will face impacts first,” Bowen says.
The outcomes of the study can now be applied to similar New Zealand estuaries such as Whangārei Harbour and Otago Harbour, or even globally. And the shift in understanding where plastic waste ends up raises other issues.
Given how long plastics last in the environment, shards have likely been accumulating along estuary shorelines for decades, breaking into ever smaller micro- and nano-sized particles.
This means shoreline environments will be the first to show impacts and we should pay more attention to how these plastic “reservoirs” affect coastal ecosystems and, ultimately, human health.
The results also show that neap tides are the best time to organise clean-ups to intercept most plastic from coastlines, rivers and estuaries before it enters the ocean. “Since plastic is being kept locally, local solutions matter and local efforts do make a difference. Now we know most of the waste is along coastlines, we know that cleaning those areas is a step to removing much of it from the marine environment.”
