Hey Terence,
My quick answer is that phytoplankton serves as a food source, and that's it. The most important thing to keep in mind is that phytoplankton is a source of nutrition that is unique. Feeding multiple species of phytoplankton offers a variety of nutritional types. In the hobby, the mantra is variety, variety, variety. We all do this with our fish, always providing a variety of nutrition. Some animals benefit tremendously from phytoplankton feedings: NPS corals, bivalves, crustaceans, basket and feather stars, gorgonians and sea fans, and Sabellid feather dusters. Can the results be quantified? Well, some would say that the fact that the animal, or colony, is alive is enough proof for them. For instance, non-tridacnid bivalves live longer when phytoplankton is incorporated into the diet. We have a flame scallop going on 2 years in our office tank. I have heard from a number of people that they die off within 6 months to a year due to starvation. We have shellfish customers that use our phytoplankton blends and single species products because it is an absolute requirement for their delicate larvae and for conditioning broodstock. There is no doubt that bivalves grow faster when fed phytoplankton.
If your tank has inhabitants that qualify as filter feeders and are known to consume phytoplankton, then the answer should be easy: feed them microalgae. I feed microalgae to our office tank, along with our Oyster-Feast. Microalgae is in the small particle feed category (1 to 20 microns roughly), so animals that rely on this size range will benefit.
We would be happy to send you a free sample of one of our blends. Let me know.
Chad