I'm curious to know what you mean by "usage is less complete". I've been growing copepods on a commercial scale for 15 years and love talking about this subject.
Also, phytoplankton, no matter the species, mode of motility, or whether it's alive or not, will stay in suspension when there is current (hence the name "plankton" or "drifter"). It takes dead phytoplankton a long time (hours) to settle out if you turn off all the pumps in your tank (of course non-motile, live phyto will also settle. Nannochloropsis falls into this category and is the algal genus most commonly found in the hobby). I'm basically disagreeing with your comments about higher percentage of live phyto gets eaten versus dead phyto due to settling. I'm also curious if you've ever tested this.
-Chad
Since it's still a live thread, I did actually run a basic test following that post as I hadn't run any specific tests before, but have definitely observed settling phyto (live and dead) as well as organisms which do not eat phyto that isn't alive (parvocalanus genus copepods, for example, haven't been successfully cultured on non-live feeds).
The test setup was basic: little vials had about 1mL of isochrysis galbana culture to them. One was a control with nothing else, one had half a drop of bleach to kill it, one was frozen for half an hour to kill it, and one was diluted with about 50% more RODI to kill it, and then I waited with them in a stable position to see if they settled. After a few hours I saw basically what I did at 7 hours and the next day, that the dead ones settled much more to the bottom (completely with bleach, mostly frozen, and little in the other two, I think perhaps the salinity shock wasn't enough to kill it), though all had some amount of sediment.
(Frozen, RODI added, Control, Bleach, left to right, 7 hours in)
I chose isochrysis because it tends to stay in suspension well in storage, both at room temperature and refridgerated, but as you've probably notice if you've worked with it, but tetraselmis (at least the species I've worked with) will settle even with modest agitation levels - if I don't heavily aerated my culture jugs (with a flat bottom), it accumulates there even when growing, and it's one of the first phytoplankton types to settle out when stored (even though it's motile). Then there are diatoms like chaetoceros gracillis which can't swim but which stay in suspension well and thalassiosira (the species I've worked with) which settle out quickly.
In any case, it was not a good sample size or a tightly controlled experiment, but I think it demonstrates the validity of the claim at surface value - a higher percentage of dead phyto settles out than live, though this may be less or negligible with nonmotile algaes or with certain species.
I don't think my comment about it not being used is inaccurate in the sense I meant, though I could have said it in a more specific manner: I, and I think most, feed phytoplankton to feed the creatures in our tanks which filter feed or which specifically eat phytoplankton. While the remainder that settles into the sediment or collects in a back chamber does still get eaten, it's detritus, and it likely will feed the same creatures as would be eating other kinds of food and animal waste that accumulates similarly.
Now to why I think you took issue with my claim: phytoplankton is pretty good at staying in suspension in the presence of flow, so it may be that the percentage of a dead phyto feed that goes unused by filter feeders and ends up as detritus vs. the percentage of live phyto that does the same is both not a huge chunk of the total amount fed to the system nor a dramatic percentage difference of the total amount that settles whether live or dead. As discussed above, with high flow situations I would expect that most of it stays in suspension for hours, at least, if not eaten until it finds some low flow nook or cranny in a tank to settle in. Even then, it's eaten by detritivores which may also be the sorts of copepods and things you were trying to feed when dosing phyto in the first place.
In the same vein of discussion, dead plankton foods also can't live in the water column (obviously), so there is no possibility of them multiplying to use up tank nutrients (though I'm not about to argue that the figure is a significant one), and when they settle out initially, there is no additional chance of getting back in the water column as with a motile algae (again, a small effect if a measurable one, probably). And it's probably not a huge difference, but I think it's perfectly reasonable to say it is a difference and that anyone inclined to try and quantify it to see if it should be an actual consideration should be encouraged to find out.
So while I don't think "The live stuff tends to stay in suspension a bit better (though this depends on the phyto species), so a higher percentage of it probably gets eaten." is inaccurate or even unintuitive, perhaps a more fully qualified version is: Live phytoplankton is more nutritious, more widely accepted by certain organisms, stays in suspension somewhat better (though perhaps not much better), and is thus is consumed in a higher portion by the filter feeding organisms we're trying to feed by offering phytoplankton to a tank (instead of the detritivores), but it may not be a significant enough benefit across the board to make it worth the extra expense and complexity over preserved phytoplankton feeds, at least for the majority of organisms we want to keep in our tanks.
My intention was never to complain about preserved phyto feeds, they are a better food source than dried versions or nothing at all from what I can tell, but I will still maintain that live phyto supplementation is the ideal option, even if it offers little additional benefit.
