Phytoplankton are autotrophic creatures, which means they create their own energy through photosynthesis. Phytoplankton usually reproduce though asexual reproduction and form the base of the aquatic food web. The generational time of phytoplankton is in the order of a few days to a few hours. Marine phytoplankton live in the ocean's photic zone, or epipelagic zone. The light, temperature and nutrient levels in the epipelagic zone are optimal for photosynthesis. Phytoplankton are eaten by zooplankton or are skimmed up by filter-feeders.

Phytoplankton account for over 99% of the plants in the ocean. Large plants, like kelps and seaweeds that inhabit near-shore areas account for the remaining one percent.

    Silicate-based organisms
  • Diatoms: Diatoms are important geologically because the accumulation on the ocean floor causes a siliceous sediment, or opal deposits. Diatoms are made of frustules, silicate casings that resemble a pill-box. When reproducing by cell division, the parts of the frustule (the epitheca, the larger part, and the hypotheca, the smaller part) separate and each becomes the larger casing on the new cell. As the hypotheca divides, down the generational time, the cell will become too small to be viable. When this occurs, the diatom produces an auxospore. An auxospore is a diatom that sheds its frustule to allow growth to a full size diatom, so that normal reproduction by shedding of the frustule can recommence.

  • Calcium carbonate-based organisms
  • Coccolithophores: these phytoplankton have flagellum, little whip-like locomotive structures. Coccolithophores sediment out into large calcium carbonate deposits in the temperate and warm parts of the oceans.

  • Dinoflagellates: These phytoplankton possess flagella for movement, and are covered in cellulose (so no mineral deposits occur when dinoflagellates sink). Red tide is caused by two genera of dinoflagellate, Ptychodiscus and Gonyaulax.