@hugovdberg that's what I meant. It will work if the other aspects could be considered 'constant', but are they? is there any time-development of the earth's total biomass? But you're right in that I used ratio rather than sum - still the same problem though.

@milancurcic this would only work if the ratio fresh:sea water is (about) constant, wouldn't it? But water is also found in the atmosphere and "stored" in the biomass.