Phyto Feast LiveWhat are phytoplankton and why are they important in a Reef System? Phytoplankton are single cell microalgae that exist primarily in the water column. Microalgae is the beginning of the marine food chain and the primary source of essential nutrients for most reef filter feeders. Filter feeders that rely directly on phytoplankton include bivalves like clams, scallops, mussels, and oysters, as well as many corals, feather dusters, sponges, ascidians and