Estuaries and coastlines capture most plastic before it gets out to sea, giving us a chance to stop ocean pollution

By Melissa Bowen, Gaoyang Li, Giovanni Coco, Zheng Chen, University of Auckland, Waipapa Taumata Rau

If you ask someone where plastic ends up, they will usually say the ocean. It’s not a surprising answer because we have known since the 1970s that plastic is accumulating in the subtropical oceans, far from land.

Most people have heard of the “great garbage patch”, a region of the North Pacific between Hawaii and California, where plastic is accumulating. Images of remote plastic-covered beaches often appear in the media.

However, the numbers don’t add up. Estimates of all the plastic drifting in the ocean show less than 10% of what enters rivers and coastlines reaches the subtropical accumulation zones.

Most must be ending up elsewhere, and the most likely places are along coastlines – either in estuaries, where rivers meet the sea, or along the open coast, where waves can push floating debris ashore.

We set out to investigate just how much plastic is retained in an estuary. The results of our research astonished us.

Using the main harbour of Auckland, the Waitematā estuary, as our study site, we made floating plastic packets using GPS receivers inside mobile phone pouches and tracked where they went over several tidal cycles.

Most ended up on the shoreline and none were able to move the relatively short distance needed to exit the estuary.

The speed of the drifters varied between tides, as shown in the animations above (spring tide) and below (neap tide).

Currents in estuaries trap plastic

When we repeated the experiment with computer simulations, trying a wide range of freshwater flows and tides, we found the same thing. Anywhere from 60% to 90% of buoyant material was retained in the estuary over ten tidal cycles.

Surprisingly, when we increased the river flow in the model, the percentage of plastic retained in the estuary was very similar. Although the buoyant material moved towards the ocean, it became trapped before it reached the mouth.

Read Full Story https://theconversation.com/estuaries-and-coastlines-capture-most-plastic-before-it-gets-out-to-sea-giving-us-a-chance-to-stop-ocean-pollution-232896