Naman,

DOC's are easy to measure and control for actually.
If you want a contro to compare the effects of DOC's on a treatment organism(Fish, shrimp, algae, plants etc)l, simply use Activated carbon to remove them, this is a standard method for many such experiments.

You need to look at rivers and lakes over the course of season/s, not just the average, or a one time reading. It's like getting to know a person based on a quick 2 minute meeting vs living with them for a year.

Chelation has been discussed here in natural systems. Quite a bit in fact. There are two good text for it as well, The Ecology of Humic Substances and the Limnology of Humic Waters.

Generally it's discussed in terms of Hg, Cd, Se etc, nasty toxic metals, less so for Fe and Mn.

Reduction is also something to really look at as it is a larger driver of metal biogeochemistry than humics are in general.

You are correct about rivers having a non limiting but very low molar concentrations. 0.5 ppm of NO3 is common in many rivers with plants, sometimes 10X less, but the plants are not stressed, actually well adapted.

Takes time to adapt to low external levels though, but once this occurs, it's pretty easy for the plants.

Actually such topics use to be discussed on the web, namely the APD, back in the mid and late 1990's, since the list has been diluted and most of the more academic minded folks have long since left.

Lots of good discussions back then, you will not see such discussions in the future except here and there by myself and very few others.

PO4 time series measurements I made some years ago in several tanks, they do not drop rapidly, but showed and fairly normal uptake response.

This suggested little preciptation and lock up from other critters/bacteria/algae/OM etc.

Lakes are good models, better actually for our purposes, but few usee tropical lakes, and even fewer used large numbers with a representative no# of lakes.
Adding to that, you need 30-50% coverage with PLANTS, which florida also has(as well as plant less lakes for the most part).

Looks at Crismann, especially Bachmann, Hoyer, etc for Trophic status for lakes in Florida, USA.
I know all these folks.
Florida has about 7800 lakes 4 hectares and larger, lots of different types.

Regards,
Tom Barr