Because of all the traffic on this post, I wanted to clarify that I am completely convinced that there is lots of plastic in the North Pacific Gyre, and that it is a serious environmental problem. My issue with the plastic:plankton ratio is that it doesn’t accurately measure the amount of plastic.
The Algalita Marine Research Foundation is great at raising awareness of the problem of trash in the North Pacific Gyre. They’ve tirelessly lobbied for political change, coined terms like “plastic soup,” worked in the schools, and are sailing the Junk raft to Hawaii as we speak. However, as part of their quest to make the enormity of the plastic problem understood, they’ve been claiming that there is six time more plastic than plankton in the North Pacific Gyre. The 6:1 ratio has appeared in PBS, The Seattle Times, and has been repeated all over the internet.
Though I admire Algalita’s work, the 6:1 plastic:plankton ratio is deeply flawed. Worse, it is flawed in a direction that undermines Algalita’s credibility: It may vastly underestimate plankton and overestimate plastic. Here’s why, based off the methodology published in Moore et al’s 2001 paper in Marine Pollution Bulletin.
1) The mesh in the net was too big, and half the samples were taken at the wrong time of day.
In the Moore et al (2001) paper, the researchers use a 333 micron (millionth of a meter) manta tow. This means that the holes in the mesh are approximately 333 microns in diameter, though they may stretch somewhat depending on how the net was towed. This is a standard technique for sampling zooplankton.
Just calling all tiny marine life “plankton” and lumping it together makes as little sense as saying that a tree and a beetle are the same because they both live in a forest. So I am going to briefly digress into the difference between phytoplankton and zooplankton. Phytoplankton are essentially tiny floating plants usually with only one cell, while zooplankton are larger floating animals with tons of cells. In areas with lots of nutrients, phytoplankton are relatively big. Diatoms, for example, range from 10-150 microns. But the North Pacific Gyre has very few nutrients, and the most common phytoplankton are very small. A single type of cyanobacteria, Prochlorococcus, accounts for 50% of the total phytoplankton community, but it is so small (less than 1 micron) that it wasn’t even discovered until the 1980s (Karl 1999). The vast, vast majority of life in the North Pacific Gyre is smaller than 8 microns (Karl 1999).
A 333 micron net is way too big to sample phytoplankton; it is designed to sample tiny animals, or zooplankton. The most common types of zooplankton are tiny crustaceans, like copepods, that make a living by grazing phytoplankton. But the tiny plants have to be big enough for them to grasp and put in their mouths. That’s easy in productive waters where phytoplankton are big and the zooplankton can pop them like candy, but hard in nutrient-poor waters where the phytoplankton are very small. So there’s not very much zooplankton at all in the North Pacific Gyre – most of the life there is the very tiny phytoplankton.
To make matters more complicated, most zooplankton hang out hundreds of meters below the surface during the day, and only come to the surface at night. Otherwise, they’d be eaten in an instant by sight predators like birds or fish. (This is called vertical migration.) Sampling for zooplankton during the day is like looking for an open bar at 10 AM. If you look hard enough you’ll find one or two, but you really have to wait until full night for the party to start. The Moore et al. (2001) paper states that the samples were evenly split between daytime and nighttime hours, but that means that the daytime samples probably underestimated zooplankton abundance. Since there isn’t very much zooplankton in the Gyre anyway, sampling during the day is going to mean that you won’t get much of anything — except plastic.
2) The 6:1 ratio is based off dry weight, but plankton is 95% water.
Moore et al. (2001) calculated the ratio based off the dry weight of the stuff they scooped up in their manta trawl. That means they put everything in an oven until all of the water was evaporated. That’s not going to change the weight of plastic, but drying out a zooplankter is like drying out Jello: there’s not going to be very much left.
Therefore, comparing the dry weight of plastic to the dry weight of zooplankton is going to vastly overestimate the amount of plastic. To be fair, the ratio might accurately reflect how, for example, an albatross’s stomach might deal with the different masses; plastic just sits there, while zooplankton would be digested and the water removed. Nonetheless, the ratio is a poor reflection of how much plastic is out there. A more accurate way to measure it might have been displacement volume: How much space is taken up by plastic versus space taken up by plankton?
3. Plankton populations fluctuate wildly, and maybe plastic does too.
The 6:1 plastic:plankton ratio is based off a single moment in time — four days in August 1999, to be exact. Plankton populations often bloom and bust, depending on the season and the oceanic conditions. For example, in the winter, storms stir up the water which brings more nutrients to the surface which causes phytoplankton to bloom. There’s no way to tell from a single point in time whether this plankton is blooming or busting, whether it’s a good year or a bad year, or whether this particular moment is representative of “normal conditions.” So even if there was a 6:1 plastic:plankton ratio on those days in August 1999, the ratio could have been completely different in October 1999, or could be completely different now. There is no constant plankton amount. (There’s probably no constant plastic amount, either, depending on storm mixing.)
I don’t mean to criticize Algalita’s mission to reduce plastics in the ocean. I deeply admire it. But I see the 6:1 ratio all over the media coverage of the North Pacific Gyre, and I fear in the end it will backfire on Algalita, and consequently on the whole issue of marine plastic debris. The constant hammering on the flawed 6:1 ratio makes it easy for oceanographers to dismiss the problem, the plastic lobby to discredit it, and regular people to ignore it, which would be the worst outcome of all.
Karl, D. M. 1999. Minireviews: A Sea of Change: Biogeochemical Variability in the North Pacific Subtropical Gyre. Ecosystems 2:181-214.
Moore, C. J., S. L. Moore, M. K. Leecaster, and S. B. Weisberg. 2001. A comparison of plastic and plankton in the North Pacific central gyre. Marine Pollution Bulletin 42:1297-1300.