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In oligotrophic waters, total N concentrations are in the range of 400
ppb or less. In eutrophic waters total N can be 3 ppm or higher... so yes
10 ppm would be considered 'polluted' under natural conditions. What you
have to consider, however, is that in an aquarium the ratio of the total
amount of N to plant biomass is orders of magnitude smaller than in a
natural system. This means that you have to keep the N at higher levels
to ensure an adequate supply. Another consideration is the huge amount of
N in the sediment of natural systems which aquatic plants can utilize...
Wetzel lists amounts of 167 ppm exchangeable ammonia as an average amount
in some Wisconsin lakes. These levels just arent attainable in most aquaria.
A third factor is the large amount of N2 being fixed in natural systems,
so that even though the absolute amount of free N is low, the flux
of N is huge.