dissolved nutrients do accumulate, they rise up from sandbes or LR to the water suface, but not limited to it. ever see that layer of film on the surface? many get it in fuge but not the DT bacuse of surface skimming those are orangics. lots of dynamics here.
ok here it is in a nut shell
at the water surface ammonia and O2 other organics are plentyful and together with the lights provided energy, phytoplankton/zooplankton use it eat it and attach to things or die sink and get eaten themselves. this make for bigger heavier things, weighing them down until they settle somewhere. keeping higher flow rate in teh DT keeps these things in the water column even though bacteria attach to things and weight them down. in the ocean they accumulate on the bottom in pockets where the flow has taken them. in our tanks more flow in the DT keeps these thing from settling in the DT and keeps them in the water column. thus, surface skimming and less flow in the fuge, allows them to settle in the sump.
here is a link about what i am talking about here. its not about flow in the refugium. its about more about how bacteria, protozoans and things coagulate the organics and things. sorry trying not to get all tech on ya. i think this link will help you get it.
http://drs.nio.org/drs/bitstream/226...ith_chap06.pdf