Nobody ever told me that becoming a marine biologist would involve spending four years figuring out how to count. Because, seriously, how hard can counting be? Well, it turns out, when you’re trying to count tiny pieces of plastic in the ocean, it gets complicated really fast.

When I went out to the North Pacific Subtropical Gyre in 2009 and 2010, part of the goal was to figure out how much plastic debris was actually there. That’s the first step to understanding what impact it might be having the ecosystem, after all. So we towed a net around on the surface, and towed a net underwater, and made visual counts of the plastic floating by on the ocean’s surface. Between the two cruises, we had measurements of plastic quantity over 6,000 miles of ocean – we were all set, right?

But when we started to analyze the data, things got complicated. The quantity of trash was hugely variable. Tows taken right next to each other, or taken in around the same location a year apart, had very different quantities of plastic. In order to get a handle on why this was, I teamed up with Andrew Titmus, an ornithologist who did the visual counts of floating plastic on our 2009 cruise, and Mike Ford, a NOAA oceanographer who was Chief Scientist on the 2010 cruise. The results were published in PLOS ONE last week, and NOAA has a brief writeup here.

The paper was hard to write, because it’s essentially “here’s a bunch of things that you should know about where plastic is in the quote-unquote garbage patch,” or, as I very scientifically referred to it on Twitter, a giant BLORT of data. I’m going to highlight a couple major points, but feel free to check out the paper yourself and ask more questions below.

1. Wind matters.

When the ocean is really calm, the plastic bobs to the surface and there’s a lot of it. When the wind kicks up and the ocean gets choppy, the plastic gets mixed below the surface, and you can’t capture it in a surface-towed net (which is the standard way to measure plastic). Our plastic counts go way down once the wind gets to a certain point, regardless of where we are in the ocean. Giora Proskurowski & colleagues found a similar phenomenom in the Atlantic.

2. Filtering tiny amounts of plastic out of the ocean takes out a lot of life, too.

For every 1000 grams (2 lbs) of plastic bits we removed from the water, we took out 731 grams (1.6 lbs) of ocean life, primarily zooplankton and baby fish. That’s a lot of critters, particularly since life is relatively sparse in the North Pacific Gyre. Remediation schemes will have to be sure that they are not causing more damage than they’re solving. For more on that, check out the Open Ocean Cleanup Guidelines.

3. Since plastic varies so much, it’s going to take a lot of work to figure out whether it’s increasing or decreasing.

We used our data to create an imaginary future where plastic had increased between 10% and 100%. It turns out that it’s really hard to detect even relatively large increases in plastic with reasonable certainty. On the 2009 cruise, we worked our butts off for three weeks to take 119 surface samples (and it took me over a year, a lot of bad R code, and the help of awesome volunteers to convert jars of plankton and plastic into data). Unfortunately, it would take 250 surface samples to detect a 50% increase in microplastic with 80% probability. We’re going to have to figure out a better way to do that, or we won’t be able to tell if the problem is getting better, or getting worse.

So, what’s the take-home of this paper? We can’t go waltzing into the Gyre wanting to do everything at once (like I did in 2009 *cough cough*). To be effective, expeditions on the science of plastic debris need to think about what their specific objectives are. Want to study the animals growing right on the plastic? Target the rarer large floating objects. Want to get a glimpse of how the widest array of ocean life is interacting with plastic? Seek out trash stuck in eddies, where temporary pulses of high-nutrient water cause plankton to grow and attract fish.

If you want more, data from this paper is archived online at the CCE LTER Datazoo, and figures that didn’t quite fit into the paper, such as the types of plastic we collected, can be found over on Figshare. Want to know more about what all this plastic is doing to marine life? Check out Chelsea’s guest post and new paper on what happens when fish eat plastic, and Mark Browne’s new paper (with BBC article!) on lugworms. And as always, I’m happy to answer your questions in the comment thread.