This isn't nuclear physics at all Theo. It's materials balance, what is taught in Jr High science classes across the country. Here are some examples

Water in Pipe = leaks + water at exit

Equity = Assets - liabilities

Food Eaten = Food assimilated + Food waste products

which is equivalent to:

Food waste products = Food Eaten - Food assimilated

No practical means to know how much is wasted and how it affects the nutrient pool? Rather than this response I would expect something more along the lines of "Hey I never thought of it that way" or "Ahh yes, we can know how much waste there is".

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers