When it rains heavily, plastic waste is washed off our streets into rivers, flowing out to the ocean. Most plastic is trapped in estuaries and coastal ecosystems, with a small fraction ending up offshore in the high seas.

In the coastal ocean, waves and tides break down plastic waste into smaller and smaller bits. These micro and nanoplastics linger in the environment indefinitely, impacting the health of marine creatures from microorganisms all the way up to seabirds and whales, which mistake them for food.

When we look at the scale of the problem of microplastics (smaller than 5mm) and nanoplastics (defined as 1 micrometer or less), we find something alarming. Our new research shows the shallow embayment of Moreton Bay, off Brisbane in Southeast Queensland now has roughly 7,000 tonnes of accumulated microplastics, the same as 700 million half-litre plastic bottles.

This bay accumulates plastics fast, as the Brisbane River funnels the city’s waste into it, along with several other urban rivers. The research hasn’t yet been done, but we would expect similar rates of microplastics in Melbourne’s Port Phillip Bay and Sydney Harbour.

Our research shows how much plastic waste from a big city makes it into its oceans.

What volume of microplastics does a large city accumulate offshore? It’s hard to measure this for cities built on open coastlines. That’s because sediments and microplastics are rapidly washed away from the original source by waves and........

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