"Richard Reynolds" wrote in message news:MhR6b.47296$Qy4.13076@fed1read05...
nothing stoped it

if it did then you would be in trouble

we want the process to continue, its slow, very slow and with its slow speed the low
oxygen water and nitrogen is moved out, and other oxygen rich water is slowly moved in.
allowing bacteria to process the oxygen rich water, then the oxygen poor water, first
converting ammonia to nitrite removing some oxygen, then nitrite to ammonia removing more
oxygen, then finally the anoxic zone where nitrate is converted and the low oxygen water
is moved out, nitrogen is moved out, and the process repeates.

ok, assuming it happens like you said, and that below a couple of centimeters
there is no oxygen - do we care how deep the anoxic layer is? In other words:
Why deep sand beds are so deep? Is there any role to play for the most bottom
layer of the sand, like 4-5 inches below the sand surface? I would expect it
to be completely dead and inactive if we eliminate the work of burrowing worms
and other critters breathing with oxygen but making short trips there "for food".