That's fine, but let's try to keep the thread on topic. This isn't the place for "DSBs don't work" opinions.That would be true if you believed it was the flow of water that was important with a dsb---I don't.
I side with the other group that believe nitrates and phosphates are carried to the lower anerobic and anoxic areas by the organisms etc that live in the sand bed. This is supported by the work of Calfo, Fenner, Schemick etc