Phytoplankton

Phytoplankton (/ˌfaɪtoʊˈplæŋktən/) are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν (phyton), meaning “plant”, and πλαγκτός (planktos), meaning “wanderer” or “drifter”. Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells (although the actual color may vary with the species of phytoplankton present due to varying levels of chlorophyll or the presence of accessory pigments such as phycobiliproteins, xanthophylls, etc.).
Generally, 63 – 153 µm long. Some algae form large clusters, clumps and threadlike groups of considerable size (which may not be defined as plankton), but individual cells are usually 63 µm – 153 µm.