What the other commenter is alluding to is that, if this comes into widespread use, it won't just be a moderate amount. We produce mind-boggling amounts of plastic waste and a lot of it would concentrate in rivers and estuaries.
It will be a moderate amount compared to the amounts that produce dead zones.
These would not break down into the kinds of nutrients that cause algal blooms and dead zones. That is cause by nitrogen and phosphorus runoff.
That's interesting. I didn't have access to the paper, just the abstract, so didn't know it was different.