[graemep]

Depends on what you put in, how much, and where.

I do not think moderate quantities of nutrients are a problem, and very likely has benefits.

[emilamlom]

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.

[Tagbert]

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.

[emilamlom]

That's interesting. I didn't have access to the paper, just the abstract, so didn't know it was different.

[graemep]

It will be a moderate amount compared to the amounts that produce dead zones.