Is there really six times more plastic than plankton in the North Pacific Gyre?

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.

74 Responses to Is there really six times more plastic than plankton in the North Pacific Gyre?

  1. Aerik says:

    Thanks for explaining that. Submitted this to social bookmarks. Hope it sparks good discussion.

  2. Then we should just not worry and throw more plastic in the ocean! Phew! Thanks for that wonderful anti-Earth diatribe.

  3. #2:

    Using bad science to support a cause you believe in devalues both the cause and the science.

  4. KH says:

    I thought the point was that normally it should be a 0:1 ratio. (i.e. NO plastic). They don’t need to control every variable to tell folks there’s a lot of plastic.

  5. vanderleun says:

    web design drools:”Thanks for that wonderful anti-Earth diatribe.”

    It saddens me to read this kind of knee-jerk ignorance. The good done by responsible scientists to debunk junk science is always in the service of the Earth and the truth.

    All that the commenter above reinforces is that we live in a world of colonized minds.

  6. vanderleun says:

    I mean really now.

    A very brief scan of the archives and categories here reveals that Goldstein is an intelligent and staunch supporter of pretty much all things green.

    But let even one small caveat be written that perhaps, just perhaps, organizations should not play fast and loose with standard research criteria in order to bolster their founding myth and thirst for funds and, pow!, the true believers show up.

    And some scientists wonder why science is not as trusted as it should be.

  7. […] or None?: [Via Deep Sea News] I like Miriam, she is a lady that gets it. Go there now and read her excellent post on the story behind the 6:1 ratio of plastic to plankton that is often touted in the media and why it is […]

  8. Thanks for your interest! I wanted to clarify one point – I am completely convinced that there is lots of plastic in the North Pacific Gyre, and that this is a serious environmental problem. My issue with the plastic:plankton ratio is that it doesn’t accurately measure the amount of plastic.

  9. Mary says:

    Good, clear thinking.

    The other aspect that should be considered, I think, is area vs. volume. The manta net tows at the surface where the plastic is, but as you suggested in the time-of-day argument, plankton are distributed, well, down to the ocean floor, really, even though most of it is in the top, say, 200 m. A vertical tow from 200 m to the surface would give a much better representation of plastic vs plankton volume, imho.


  10. Aerik says:

    Good observation, Mary.

    To be blunt, a tow, if that implies a net, shouldn’t be done at all. It shouldn’t be a damn net. They should use containers that go underwater at designated depths by design of their weight, that take in a set volume of ocean, and then are retrieved. That or one big/deep column all at once, repeated.

    The study was just so poorly done all around. Do we dehydrate people befor ewe weigh them in the morgue? WTF really.

  11. Mary – I agree. Even at night, the zooplankton may not be in the first meter from the surface.

    Aerik – Tows (yes, nets) are a standard oceanographic method for estimating abundance. So are water samples of the kind that you suggest. They measure different things. A tow, properly deployed, will integrate over an area, so you’ll measure X per km2. This is important for things that aren’t very common, like fish, since if you just do a water grab you have a very small chance of getting anything. Water samples are good for small things, like phytoplankton and bacteria. As for plastic, both types of sampling might be appropriate, depending on the size of the particles.

  12. J. says:

    What the study found was that the ratio of dry weight of plastic to plankton, of a certain size, during a certain time of day, during a certain season, in one part of the ocean is about 6 to 1. And that plankton ABUNDANCE is 5 times higher than plastic.

    The abstract of Moore et al. (2001) is pretty clear:

    “The potential for ingestion of plastic particles by open ocean filter feeders was assessed by measuring the relative abundance and mass of neustonic plastic and zooplankton in surface waters under the central atmospheric high-pressure cells of the North Pacific Ocean. Neuston samples were collected at 11 random sites, using a manta trawl lined with 333 u mesh. The abundance and mass of neustonic plastic was the largest recorded anywhere in the Pacific Ocean at 334 271 pieces km2 and 5114 g km2, respectively. Plankton abundance was approximately five times higher than that of plastic, but the mass of plastic was approximately six times that of plankton. The most frequently sampled types of identifiable plastic were thin films, polypropylene/monofilament line and unidentified plastic, most of which were miscellaneous fragments. Cumulatively, these three types accounted for 98% of the total number of plastic pieces.”

    Lots of room for more research.

    The media echoes the simplified version (of most things).

    The point, though, is that there’s an increasing amount of plastic in our ocean (and inside ocean wildlife). And pretty much everywhere else.

  13. Chris J says:

    Very interesting post and just goes to show that these things are rarely clear cut.

    Thank you for shedding some extra light on the issue. I am a member of the media and yes #12 we do tend to simplify issues for public consumption but that doesn’t mean we are just looking for simplistic answers.

    Miriam, your work is excellent in reminding us to daily question what we are being told and who is doing the telling…

  14. Martha Quigley says:

    I am going far afield on this, I know. I admire the intelligence of the people that I have read here. My question is–would it be possible to mine the plastic in the ocean and recycle it?

  15. Hi Martha – Here are three reasons that it would be very difficult to “mine” the plastic.
    1) The plastic is very small, mostly less than 2 mm. A net fine enough to catch it would also catch and kill most marine life in the area.
    2) It would be crushingly expensive. A vessel big enough to tow the net costs about $30,000 per DAY to run. The plastic has no value to offset this cost.
    3) The plastic is probably not all on the surface – it may be distributed in the first 600 feet or so of water. Thus, the area would need to be trawled many times or with an unbelievable huge net. This would be very very very expensive and would damage or kill hundreds of tons of marine life.

    Hope this helps!

  16. […] like Miriam, she is a lady that gets it. Go there now and read her excellent post on the story behind the 6:1 ratio of plastic to plankton that is often touted in the media and why it is flawed. “Though I […]

  17. Shawn Tisdell says:

    Seems a very reasonable explanation. Has there been additional follow up studies done on the degradation and abundance and distribution of plastics in the worlds oceans? I have a growing concern not only for the effects of direct ingestion of the plastic but that it seems that the plastic is absorbing PCB’s out of the water and this is leading to biomagification up the food chain. This may be a large reason why so many fish are now recommended for us not to eat at all. Seems a pretty big problem to be ignorant about, if only for the selfish reason that people are being recommended not to eat fish.

  18. Apparently when plastic breaks down into tiny particles and is absorbed by the micro flora and fauna it becomes very toxic.

  19. […] than others—the dragonfly-shaped area on this map, for example, has high amounts of fish, phytoplankton and zooplankton, whereas the Garbage Patch itself (the pink area between California and Hawaii) is a relative dead […]

  20. […] -cette zone qui ressemble à une libellule sur cette carte, par exemple, est riche en poissons, en phytoplancton et en zooplancton, alors que la Plaque de déchets, en elle-même (la zone rose entre la Californie et Hawaï) est […]

  21. Emma says:

    Exactly as you are saying, here is a WSJ story (March 25, 2009) that emphasizes the confusion about precise numbers and portrays the garbage patch as another over-hyped “environmental catastrophe”. They even say that plastic might be good for the environment!

  22. As a marine ecologist, I appreciated the discussion presented here about the potential flaws in the finding of a 6:1 ratio by Algalita. I support Algalita in its activities to address this problem. I also value peer review that addresses flaws in the methodology… this is a critical part of science and therefore this piece serves the scientific community and the general population well.

    There is little dispute that plastics are a BIG problem regardless of what the actual ration is.

  23. mailace says:

    Very interesting posts. It is indeed important that scientific material that gets into the general press be as accurate as possible and I am very glad to learn that the ratio of plastic nurdles to plankton is probably not 6:1 in the N.Pacific gyre. Similarly I think it is not very accurate to say that the infected area of the N.Pacific gyre is twice as big as Texas or twice as big as the U.S.A. but I understand that these descriptive types of comparison have more effect on the general public. It is extremely important I believe to educate the general public as to the real harm being done to our environment by the careless use of plastics. I became aware of the so called ‘North Pacific Garbage Patch’ about 3 or 4 years ago when I didn’t know what a gyre was and set out to learn as much as I could about it without actually going and seeing for myself – something I would like to do as I don’t feel I really can know the problem without going there. I now lecture to any group I can about the danger to life on this planet from the virtually uncontrolled disposal of plastics in all forms. Less than 20% of plastic waste is recycled worldwide. In time almost all waste plastics wind up in the lakes and rivers and mainly the ocean gyres. We already know that there are immense amounts of plastic, both the small nurdles as well as larger pieces, floating in the North Pacific and the North Atlantic gyres. I’m certain that in time similar ‘garbage patches’ will be discovered in the other three gyres. Because plankton is a major basic food for a large percentage of ocean life forms we need to be very concerned about how plastic is affecting the plankton itself. More study needs to be done on this. In the meantime I think it is incumbent upon all concerned people to spread the word and make every effort possible to reduce plastic usage and therefore reduce manufacture. I will be presenting as a start, a proposal to my local borough council to ban the distribution of plastic bags in retail outlets. Such efforts have so far been successful in 55 cities, towns and villages in the U.S. and a similar amount world wide. Not much but it’s a start and the movement to ban the use of plastic bags and the idiot use of plastic bottles for water containment is growing rapidly.

  24. […] going on in the ocean. The reasons for this are somewhat technical, but you can read about them in this blog entry, which I wrote a year before I starting doing my own research on plastic in the North Pacific. I […]

  25. […] going on in the ocean. The reasons for this are somewhat technical, but you can read about them in this blog entry, which I wrote a year before I starting doing my own research on plastic in the North Pacific. I […]

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    It’s true that the holes in the net were to big, and that most of the plankton wight is water.
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