Phytoplankton is the plant life portion of plankton. Plankton is composed of both plants and animals and is a staple in the diet of many sea creatures, such as the blue whale. Phytoplankton is composed of tiny single-celled plants that knit together and float on the ocean surface. Phytoplankton obtains nutrients from sea water and sunlight and cannot grow unless it has continuous direct sun exposure.
Algae, often called seaweed, is a rooted sea plant that occurs in three basic types: red algae (Rhodophyta, Porphyra) brown algae (Phaeophyta and Nereocystis) and green algae (Chlorophyta.) Algae is rooted to the ocean floor and can be found in shallower portions of the ocean where there is excellent sun exposure. Algae serves as both a hiding place for small fish and crustaceans and as nourishment for the smaller members of the ocean ecosystem. Algae is an integral part of the food chain in the ocean, as the small fish who eat the algae are then continually eaten by larger fish, thereby passing the algae all the way up the marine food chain.
Seagrass grows in the same areas as algae since it has similar nutritional needs. Plenty of sunlight is essential for seagrass growth. Seagrass is the only flowering plant able to sustain life underwater, according to the Sydney Aquarium. Seagrass grows in clusters called "meadows" and continually replenishes itself through asexual reproduction. Seagrass types include Cymodocea rotuntada, Enhalus acoroides and Halodule uninervis. Turtles, manatees and black swans eat seagrass as a central part of their diet.