Plastics are mostly carbon and hydrogen atoms, neither of which are even remotely limiting factors because autotrophs at the bottom of the food chain produce plenty of both from water and carbon dioxide.

Agricultural runoff is mostly nitrogen and phosphorus, which are limiting factors (hence why we have to supplement them in agriculture).

> Plastics are mostly carbon and hydrogen atoms

In general, this particular stuff is significantly different.

The article mentions sodium hexametaphosphate [0] and guanidinium sulfate [1], which have phosphorous and nitrogen respectively. Those are both common in fertilizers and are implicated in algal blooms.

[0] https://pubchem.ncbi.nlm.nih.gov/compound/Hexasodium-hexamet...

[1] https://pubchem.ncbi.nlm.nih.gov/compound/Guanidinium-sulpha...