These numbers refer to microscopic and otherwise extremely small particle size. You're still in "a couple of kg per square km" territory in the denser regions for these other (more famous) patches.

Again, this is not a good thing and has important negative ecological effects. This needs to be studied and mitigated. But we're still not walking on top of floating islands of garbage.

Sure, I'm not trying to say they're big particles.

Most humans would have a hard time to know if they're in the garbage patches or not.

The term garbage patch is probably misleading. Soup is better, but still makes people think you'd be able to see it.

Record levels of plastic pollution found in Lake Erie.

  One sample drawn near the border of Lake Erie's central and eastern basins
 yielded 600,000 pieces of plastic per square kilometer 

-- twice the number found in the most contaminated oceanic sample on record, Mason said. - Cleveland.com