Interesting and complicated subject.
would these variables cause dramatic variations in the NO3: PO4 generation and consumption ratios?
Do we know if there is a roughly fixed ratio of NO3: PO4 generated by fauna?
There is none. It depends on the feed and needs of each organism. Different types of fish have dif dietary requirements and will therefore excrete different amounts of things. You may be able to find a mean value but why would a mean value apply to your aquarium?
Do we know if there is a roughly fixed ratio of NO3: PO4 consumed by plants?
Again on average yes, however in one aquarium the amounts and ratios will differ from one day to the next. Plants are able to make nutrient reserves, this means there will be high uptake at first, and then possibly even higher of some nutrients, without a good ROI.
Also do not ignore the bacterial population of your aquarium. They are major contributors in generating No3 and are also able to consume good amounts of PO4 and Fe. If conditions are right you will need an ever increasing amount of Po4 to feed the bacteria and leave something for the plants. This happens until bacteria run into a limiting factor, oxygen, space other nutrient. What does this do to the nutrient ratios?
I agree, don't worry about ratios. Dose to have enough.
Speaking in general when describing a ratio it is good practice (although largely ignored) to give units or at least what was measured, ie
10:1 N:P by volume is not the same as
10:1 N:P g:mg or
10:1 N:P by weight or
10:1 N:P mol
Believe it or not I can even say 10:1 N:P by grains of my particular salt and use the ratio adequately. A percentage % or ppm or ppt are also ratios, whereas mg/L is is not and gives you all the information needed.
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