Nobody knows for sure, but the new estimate - 315 billion pounds - posted at 5 Gyres is surely wrong.
You know that the estimate is going to be bad when the author, Stiv Wilson, starts out with this sentence: "If we were to attempt to quantify how much is out there, we need to do some big math." Watch out! Big math is coming! Danger! Danger!
Stiv starts with a measurement of 5 kg/km2, derived from drag net measurements taken in the North Atlantic Gyre, and also states that plastic pieces can be found 90 feet down. Good. Data is always a good spot to start with.
However, Stiv takes this value of 5 kg/km2, taken from a gyre, an known area of concentration and multiplies it by the entire surface area of oceans on the earth, completely overlooking that most of the areas are of lower concentration, since they are not gyres or "garbage patches".
But it get worse. 90 feet worse. Stiv further makes that assumption that the concentration above is for the first foot and then multiples it by 90, as if the plastic is uniformly distributed for all 90 feet. The assumption of uniformity is not rationalized at all. I would strongly propose that the uniform assumption is going to be inaccurate and overstates the amount of plastic. Just the fact that a number exists as a lower limit strongly suggests that it is an asymptote, a depth below which plastic is so seldom seen as to be nonexistent. If 90 feet were not an asymptote, there is no reason why 90 feet would be a cutoff at all. Plastic should be found at all depths if it were uniformly dispersed. Lacking data, all sorts of concentration-depth profiles could be proposed - linear, exponential decay, a power law fit or whatever. Regardless of the model, the end result is again that Stiv has overestimated the amount of plastic.
Now the real kicker: Stiv calls these "conservative estimates". Opps, sorry, I misquoted him. Shame on me. He doesn't call them conservative estimates afterall. Instead, he calls them"extremely conservative estimates".
So know we all know how bad "big math" can be, which in the end turned out to be merely the product of 3 numbers.
Look, plastic has no business being in the ocean, but to overinflate estimates of how much their is has no value to anybody.