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> > Karen wrote:
> > << But isn't it possible to limit N in the _water column_, while still not
> > allowing N to be limited in the substrate? >>
> Bob Dixon replied:
> > Nitrogen is the single most common element in the atmosphere. It will be
> > almost four times more common in the water column than oxygen, because
> > of the forces that cause exchange of gases at the surface.
> True, but this is merely N2, and entirely inaccessible to our aquatic
> plants. Even though it is so common in the atmosphere, nitrogen
> is almost always the #1 limiting factor in terrestrial plant growth
> because N2 is also inaccessible by almost all terrestrial plants.
I think it begs mentioning here that while N2 is inaccessible to plants
and higher algae, cyanobacteria fix it quite readily. While they will not
add any meaningful amount of fixed N to the water column, they will quite
happily add it to themselves and grow into what might be considered an
algae problem <g>. Theoretically, a tank with plenty of phosphate and
iron but not one molecule of fixed N in the water column could still grow
loads of cyanobacteria.
At the same time, however, the process of nitrogen fixation is VERY energy
expensive, and I'm sure that cyanobacteria would prefer to get their N in
the cheap "pre-fixed" form when they can. So if there are plants
competing with them for other nutrients and the plants aren't N limited,
the cyanos probably won't be able to keep up because they will have to
expend much of their energy pulling N out of the atmosphere.