The bottom layer of a DSB are not alive, they are anoxic and where facultative
denitrification take place, where NO3 is converted to N2 gas, by facultative denitrifying
bacteria. If they were alive, being mixed by microfauna or meiofauna, the water would
carry in O2, which would inhibit denitrification. It is the upper layers that are alive,
the bottom is dead just like a plenum. Microfauna can not be supported in these lower
layer as there is not enough O2 for them to survive

--
Boomer

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"Pszemol" wrote in message ...
: I would try to explain this with a dynamic nature of this anoxic layer.
: Deep sand beds are alive. They contain micro worms, micro stars, snails.
: They constantly travel through the sand mixing it a little bit all the time.
: This will help with water exchange and the transport of energy/food
: between layers of sand. This is an atempt of explanation, but it does
: not explain plenums - which are "dead" and separated from upper layers.
:
: "Phil" wrote in message
...
: Hi,
:
: I understand (I believe) how DSB and plenums work. By creating an anoxic
: zone where bacteria are forced to pull oxygen out of the nitrate molecule,
: leaving inert nitrogen behind.
:
: What baffles me is if the oxygen cant get the anoxic zone, how in blazes
: does the water and nitrate get there in order to be denitrified? Or is it
: that the anoxic zone incubates this sort of bacteria, which then finds its
: way into the water column, but still goes for the nitrate rather than
: oxygen as thats all it knows?
:
: Inquiring minds want to know.
:
: -Phil