When I had dinoflagellate problems, I did a lot of research on the life cycle on them. Technically, dinoflagellates are zooplankton and phytoplankton. There are a number of other critters in our tanks that don't properly fit into a category like cyanobacteria or
foraminiferens. I ended up doing a lot of research about the life cycle of both phytoplanktons and zooplanktons.
I figured out what caused the dinoflagellates and how to get rid of them. Then much later, I had a little discussion with mojoreef about phytoplanktons. Finally, the research I did to get rid of the dino's made sense.
The best way I can explain it is this. I have a refugium that I keep macroalgae in. I do this because the macroalgaes will "eat" nitrates and inorganic phosphates. Both of which I want to limit as much as possible. There are a number of threads on this board why you never want to let your macroes go sexual because they will immediately release the nitrates, organic phosphates, along with some lovely toxins the macros use for protection against predation.
If your macroes went sexual the organic phosphates which were in the living macros are now called inorganic (because they aren't living in anything alive). However, they won't stay inorganic for long. Bacteria and algaes will blow the trumpets and sound the alarm.
There's inorganic phosphates!!! There's inorganic phosphates!!! We're gonna have a feast. The bacteria and algaes have now turned the inorganic phosphates back into organic phosphates. They're plump and healthy now so they reproduce like mad until there are no more inorganic phosphates to eat. Then they start starving and yep, you guessed it we now have inorganic phosphates in our tank again. (Shampoo, rinse, repeat
)
If you put dead phytoplankton into your tank, all you are doing is throwing down fertilizer for algae. Live phytoplankton must be eaten so it doesn't die and turn into inorganic phytoplankton. That is unless you like the look of nuisance algaes. Only put in enough that can be skimmed out before it dies. Frankly I try to hit the phosphate problem from all directions. I'm careful to only use RO/DI water, I skim, I soak my food in ro/di water (not saltwater) to suck the inorganic phosphates out of it prior to adding it to my tank, and I try my best not to overfeed, I drip Kalk to precipitate the phosphates, etc., etc., etc. (If you have ever had dinoflagellates in your tank, you would be diligent too). I do use phytoplankton but I use MUCH less of it than most people and it might be once per month. My tank is mostly softies at this point and some of them do eat a little of it. Actually, I think I'm going to try going a couple of months and seeing what happens.
If you have an SPS tank, I would never recommend adding phyto. Phosphates inhibit calcification.
(Oh, and by the way----I'm still at work but I'm leaving now. I've been wating on an email from an attorney that was promised by 5:00). I just got an email saying the agreement will be coming tommorrow. AAARRRGGGHHH!!!)