One of the many many "ghost nets" floating around the ocean. They are abandoned fishing nets, which continue to catch and trap fish and other marine life. We don't know how long these take to break down.
One of the many many “ghost nets” floating around the ocean. They are abandoned fishing nets, which continue to catch and trap fish and other marine life. We don’t know how long these take to break down.

By Claire

We’ve posted a couple of times about the giant garbage patches sitting in the middle of the world’s oceans, but I want to re-visit this topic again today.

Earlier this week, I attended a really great seminar, given by Erik van Sebille, from UNSW, titled, “Pathways of Marine Plastic into the Garbage Patches”.

Erik is a physical oceanographer, and so came to the issue of the ocean’s garbage patches from a different direction, “where does all the plastic come from?”

Now, I have to admit, that I hadn’t really thought a lot about this question. I guess, I had just assumed that the plastic was dumped overboard from vessels already in the ocean. But, given the volume of plastic in the world’s oceans, this isn’t the case. The vast majority of the plastic is washed down drains and into the ocean from the litter you see on the street.

So where does plastic, put into the ocean at, for example, Bondi Beach, end up?

That’s where physical oceanography steps in. A plastic bottle thrown into the water at Bondi Beach makes its way out into the ocean, and then basically becomes a tracer for ocean currents.

A “tracer” is simply something that we can use to track something that’s difficult to observe (more or less). Ocean currents are difficult to observe because you can’t actually see them. That’s where tracers come in. If we threw a plastic bottle into Bondi Beach, with, say, a GPS unit attached, we could track the movement of the bottle through the world’s ocean currents.

This is exactly what they have done.

I found it particularly interesting that all of the ocean’s garbage patches are interconnected. Countries surrounding the Pacific aren’t solely responsible for the Pacific garbage patch, for example. The ocean’s current shift plastic between ocean basins, so that a Coke bottle thrown into the Ganges can actually eventually end up off the coast of Hawaii.

If you’re interested, you can set up your own tracer experiment at http://adrift.org.au/. Pick a point in the ocean, and let lose your own piece of plastic and see where it will end up over the next 10 years.

Screen shot from adrift.org.au. Pick a spot in the ocean, and see where your plastic bottle might end up. This example releases the tracer from Sydney.
Screen shot from adrift.org.au. Pick a spot in the ocean, and see where your plastic bottle might end up. This example releases the tracer from Sydney.

If you’re interested in finding out more, you can check out Erik’s paper in Environmental Research Letters (hooray for open access!), or check out his website.