I would guess it might be easy to determine if farmers are using more fertlizer than in years past. And it seems like more fields are being tiled every year. I would think running drain tiles to ditches would carry more nitrites more quickly to ditches than allowed to filter thru the soil.
And as mentioned before most municipalities dump raw sewage directly into streams at times.
I don't see how it would be easy to gauge the fertilizer run off. but I do agree that tiling fields and running the outlets to streams and drains aggrivates the issue.
One thing you need to understand. The Federal Government ended the CREPS program last year. When it was in effect, farmers were paid an amount to plant and maintain filter strips. That is all gone so now they plant to the edges of every waterway. I used to mow a couple 'set asides'. You can/could mow and bale them once yearly, in August.
Farmers don't do anything out of the goodness of their hearts and are not really 'stewards of the land'. In reality, it's a for profit business and unless compensated, they won't do it. Most of them don't give 2 shilts about fertilizer runoff and the impact on the lakes. Its out of sight, out of mind philosophy.
They need to be forced into compliance through comprehensive regulations.
Same applies to sewage outflows from municipalities. Until the government goes from hand slapping to monetary fines, nothing will change.
Bottom line is, get in their pockets and they do something about it.
Until that happens you'll be drinking green stink water every summer.
People in the Great Lakes don't realize the wonderful natural resource they have and continue to use it as a sewer.