Muscatine

Is this going to include Canada as well?  And will you be able to tell whether somebody drinking Moosehead peed into the river, as opposed to somebody who drank Bud Light in Iowa?

You are truly an incredible dork.

It's time for the next step.   When we know the area of the watershed and the total annual lbs. of nitrogen contributed from it we can calculate the lbs per square mile.   We don't care if some came from Canada or Minnesota.

I'll go to the next step and hope it prevents you from acting foolish again.
With the lbs per square mile we can multiply that times the square miles of the water shed in Iowa (not including that in Canada and/or Minnesota) and get the total nitrogen contributed by that river coming from Iowa.

But of course we care if it came from Canada or Minnesota, since the whole point you're hopelessly trying to make is that we can differentiate Iowa Nitrogen from other nitrogen in the gulf. 

Now.....start again.  From the top.  I sense your spirit dropping, and we all want you to just be happy. 

I took the time to do some on-line research of the papers published over the last decade that deal with Nitrogen source tracking.  At least the few that I could find.

My conclusion:  The scientists are still working to establish methods (such as Nitrogen Isotope tracking, etc.) that can positively identify the type of Nitrogen found in the oceans, let alone the actual geographical source of that Nitrogen.

In other words, they are not quite there yet even trying to identify whether a particular sample of N came from human waste, animal waste, fertilizer, natural sources, etc.  They are getting there, but not there yet.

Actually being able to say that this particular isotope of N (for instance) came from Iowa or came from Illinois, is a pipe dream at this stage.