“...happens to be my line of work.”
From other replies seems that in some cases it is entirely natural — not influenced by any human activity. Is it that the runoff from farming chemicals increases a the growth more than would normally take place?
Absolutely. In the long involved Chesapeake issue, those closely associated with it technically will readily concede that intensive agriculture is as much or more a responsible party as low intensity development. (When you have flown over the planet as I have had the fortune to do one comes to realize that intensive farming IS intense development).
Not saying there are not solutions, but quantitatively one does wonder when the technical solutions reach diminishing returns versus overwhelming population pressure, at least as far as one's desire for a naturally pleasant environment is concerned.
That is one reason, and only one of many, that I vehemently oppose the illegal Mexican invasion. How can one rationalize importing twenty million civilly irresponsible and environmentally devastating refugees who pay nothing, but press the environment which most of us struggle to protect and pay for?